draftExample of SCAP Security GuidanceThis example security guidance has been created to demonstrate SCAP functionality
on Linux.0.1Default install settingsThis profile is an example policy that simply checks if some of Fedora 14 default
install settings have been modified. It is not comprehensive nor checks security hardening. It is just for testing
purposes.Introduction
The purpose of this guide is to provide security configuration
recommendations for Fedora Linux. Recommended settings for the basic
operating system are provided, as well as for many commonly-used services
that the system can host in a network environment.
The guide is intended for system administrators. Readers are
assumed to possess basic system administration skills for Unix-like systems, as well as some
familiarity with Red Hat's documentation and administration conventions. Some instructions
within this guide are complex. All directions should be followed completely and with
understanding of their effects in order to avoid serious adverse effects on the system and its
security.
General Principles
The following general principles motivate much of the advice in
this guide and should also influence any configuration decisions that are not explicitly
covered.Encrypt Transmitted Data Whenever Possible
Data transmitted over a network, whether wired or wireless, is
susceptible to passive monitoring. Whenever practical solutions for encrypting such data
exist, they should be applied. Even if data is expected to be transmitted only over a
local network, it should still be encrypted. Encrypting authentication data, such as
passwords, is particularly important. Networks of machines can and should be
configured so that no unencrypted authentication data is ever transmitted between
machines.Minimize Software to Minimize Vulnerability
The simplest way to avoid vulnerabilities in software is to avoid
installing that software. The RPM Package Manager allows for careful management of the
set of software packages installed on a system. Installed software contributes to system
vulnerability in several ways. Packages that include setuid programs may provide local
attackers a potential path to privilege escalation. Packages that include network services
may give this opportunity to network-based attackers. Packages that include programs
which are predictably executed by local users (e.g. after graphical login) may provide
opportunities for trojan horses or other attack code to be run undetected. The number of
software packages installed on a system can almost always be significantly pruned to include only
the software for which there is an environmental or operational need.Run Different Network Services on Separate Systems
Whenever possible, a server should be dedicated to serving
exactly one network service. This limits the number of other services that can be
compromised in the event that an attacker is able to successfully exploit a software flaw
in one network service.Configure Security Tools to Improve System Robustness
Several tools exist which can be effectively used to improve a
system's resistance to and detection of unknown attacks. These tools can improve
robustness against attack at the cost of relatively little configuration effort. In
particular, this guide recommends and discusses the use of Iptables for host-based
firewalling, SELinux for protection against vulnerable services, and a logging and
auditing infrastructure for detection of problems.Least Privilege
Grant the least privilege necessary for user accounts and
software to perform tasks. For example, do not allow users except those that need
administrator access to use sudo. Another example is to limit logins on server
systems to only those administrators who need to log into them in order to perform
administration tasks. Using SELinux also follows the principle of least privilege:
SELinux policy can confine software to perform only actions on the system that are
specifically allowed. This can be far more restrictive than the actions permissible
by the traditional Unix permissions model.How to Use This GuideReaders should heed the following points when using the guide.Read Sections Completely and in Order
Each section may build on information and recommendations
discussed in prior sections. Each section should be read and understood completely;
instructions should never be blindly applied. Relevant discussion will occur after
instructions for an action. The system-level configuration guidance in Chapter 2 must be
applied to all machines. The guidance for individual services in Chapter 3 must be
considered for all machines as well: apply the guidance if the machine is either a server
or a client for that service, and ensure that the service is disabled according to the
instructions provided if the machine is neither a server nor a client.Test in Non-Production Environment
This guidance should always be tested in a non-production
environment before deployment. This test environment should simulate the setup in which
the system will be deployed as closely as possible.Root Shell Environment Assumed
Most of the actions listed in this document are written with the
assumption that they will be executed by the root user running the /bin/bash shell. Any
commands preceded with a hash mark (#) assume that the administrator will execute the
commands as root, i.e. apply the command via sudo whenever possible, or use su to gain
root privileges if sudo cannot be used.Formatting Conventions
Commands intended for shell execution, as well as configuration
file text, are featured in a monospace font. Italics are used to indicate instances where
the system administrator must substitute the appropriate information into a command or
configuration file.Reboot Required
A system reboot is implicitly required after some actions in
order to complete the reconfiguration of the system. In many cases, the changes will not
take effect until a reboot is performed. In order to ensure that changes are applied
properly and to test functionality, always reboot the system after applying a set of
recommendations from this guide.System-wide ConfigurationInstalling and Maintaining Software
The following sections contain information on security-relevant
choices during the initial operating system installation process and the setup of software
updates.Initial Installation Recommendations
The recommendations here apply to a clean installation of the
system, where any previous installations are wiped out. The sections presented here are in
the same order that the installer presents, but only installation choices with security
implications are covered. Many of the configuration choices presented here can also be
applied after the system is installed. The choices can also be automatically applied via
Kickstart files.Disk Partitioning
Some system directories should be placed on their own partitions
(or logical volumes). This allows for better separation and protection of data.
The installer’s default partitioning scheme creates separate partitions (or logical volumes)
for /, /boot, and swap.If starting with any of the default layouts, check the box to “Review and modify
partitioning.” This allows for the easy creation of additional logical volumes inside
the volume group already created, though it may require making /’s logical volume smaller
to create space. In general, using logical volumes is preferable to using partitions
because they can be more easily adjusted later.If creating a custom layout, create the partitions mentioned in the previous paragraph
(which the installer will require anyway), as well as separate ones described in the
following sections.
If a system has already been installed, and the default partitioning scheme was
used, it is possible but nontrivial to modify it to create separate logical volumes for the
directories listed above. The Logical Volume Manager (LVM) makes this possible. See the LVM
HOWTO at http://tldp.org/HOWTO/LVM-HOWTO/ for more detailed information on LVM.
Create Separate Partition or Logical Volume for /tmp
The /tmp directory is a world-writable directory used for
temporary file storage. Ensure that it has its own partition or logical volume.
Because software may need to use /tmp to temporarily store large files, ensure
that it is of adequate size. For a modern, general-purpose system, 10GB should be adequate. Smaller or larger sizes could be used, depending on
the availability of space on the drive and the system’s operating requirements
Minimum size for /tmpChoose minimum size of /tmp2G125M500M2G10G40G^[\d]+[KMGkmg]?$Ensure that /tmp has its own partition or logical volumeThe /tmp directory is a world-writable directory used for temporary file storage. Ensure that it has its own partition or logical volume.Ensure that /tmp is of adequate sizeBecause software may need to use /tmp to temporarily store large files, ensure that it is of adequate size.Create Separate Partition or Logical Volume for /var
The /var directory is used by daemons and other system
services to store frequently-changing data. It is not uncommon for the /var directory
to contain world-writable directories, installed by other software packages.
Ensure that /var has its own partition or logical volume.
Because the yum package manager and other software uses /var to temporarily store
large files, ensure that it is of adequate size. For a modern, general-purpose system,
10GB should be adequate.
Minimum size of /varChoose minimum size of /varChoose minimum size of /var5G500K1G5G10G15G20G^[\d]+[KMGkmg]?$Ensure that /var has its own partition or logical volumeThe /var directory is used by daemons and other system services to store frequently-changing data. It is not uncommon for the /var directory to contain world-writable directories, installed by other software packages. Ensure that /var has its own partition or logical volume.Ensure that /var is of adequate sizeBecause the yum package manager and other software uses /var to temporarily store large files, ensure that it is of adequate size. For a modern, general-purpose system, 10GB should be adequate.Create Separate Partition or Logical Volume for /var/log
System logs are stored in the /var/log directory.
Ensure that it has its own partition or logical volume.
See 2.6 for more information about logging and auditing.Ensure that /var/log has its own partition or logical volume
System logs are stored in the /var/log directory.
Ensure that it has its own partition or logical volume.Create Separate Partition or Logical Volume for /var/log/audit
Audit logs are stored in the /var/log/audit directory.
Ensure that it has its own partition or logical volume. Make absolutely certain
that it is large enough to store all audit logs that will be created by the auditing
daemon.
See 2.6.2.2 for discussion on deciding on an appropriate size for the volume.Ensure that /var/log/audit has its own partition or logical volume
Audit logs are stored in the /var/log/audit directory.
Ensure that it has its own partition or logical volume.
Make absolutely certain that it is large enough to store
all audit logs that will be created by the auditing daemon.Create Separate Partition or Logical Volume for /home if Using Local Home Directories
If user home directories will be stored locally, create a separate
partition for /home. If /home will be mounted from another system such as an NFS server, then
creating a separate partition is not necessary at this time, and the mountpoint can
instead be configured later.Ensure that /home has its own partition or logical volume
If user home directories will be stored locally, create a separate partition for /home.
If /home will be mounted from another system such as an NFS server, then creating a
separate partition is not necessary at this time, and the mountpoint can instead be
configured later.Boot Loader Configuration
Check the box to "Use a boot loader password" and create a
password. Once this password is set, anyone who wishes to change the boot loader
configuration will need to enter it. More information is available in Section
2.3.5.2.
Assigning a boot loader password prevents a local user
with physical access from altering the boot loader configuration at system startup.
Network Devices
The default network device configuration uses DHCP, which is
not recommended.
Unless use of DHCP is absolutely necessary, click
the "Edit" button and:
Uncheck "Use Dynamic IP configuration
(DHCP)".Uncheck "Enable IPv4 Support" if the system does not require IPv4. (This is
uncommon.)
Uncheck "Enable IPv6 Support" if the system does not require
IPv6.
Enter appropriate IPv4 and IPv6 addresses and prefixes as
required.
With the DHCP setting disabled, the hostname, gateway, and DNS
servers should then be assigned on the main screen.
Sections 3.9.1
and 3.9.2 contain more information on network configuration and the use of DHCP.
Root Password
The security of the entire system depends on the strength of
the root password. The password should be at least 12 characters long, and should
include a mix of capitalized and lowercase letters, special characters, and numbers. It
should also not be based on any dictionary word.Software Packages
Uncheck all package groups, including the package groups
"Software Development" and "Web Server", unless there is a specific requirement to
install software using the system installer. If the machine will be used as a web
server, it is preferable to manually install the necessary RPMs instead of installing
the full "Web Server" package group. See Section 3.16 for installation and configuration
details.
Use the "Customize now" radio box to prune package groups
as much as possible. This brings up a two-column view of categories and package groups.
If appropriate, uncheck "X Window System" in the "Base System" category to avoid
installing X entirely. Any other package groups not necessary for system operation
should also be unchecked.
First-boot Configuration
The system presents more configuration options during the first
boot after installation. For the screens listed, implement the security-related
recommendations:
Firewall - Leave set to
'Enabled.' Only check the 'Trusted Services' that this system needs to serve. Uncheck
the default selection of SSH if the system does not need to serve
SSH.
SELinux - Leave SELinux set to 'Enforcing' mode.
Kdump -
Leave Kdump off unless the feature is required, such as for kernel development and
testing.
Set Up Software Updates - If the system is connected to the
Internet now, click 'Yes, I'd like to register now.' This will require a connection to
either the Red Hat Network servers or their proxies or satellites. This can also be
configured later as described in Section 2.1.2.1.
Create User - If the
system will require a local user account, it can be created here. Even if the system
will be using a network-wide authentication system as described in Section 2.3.6, do
not click on the 'Use Network Login...' button. Manually applying configuration later
is preferable.
Security Updates
As security vulnerabilities are discovered, the affected software must be updated in order
to limit any potential security risks. If the software is part of a package within a Fedora
distribution that is currently supported, Fedora is committed to releasing updated packages
that fix the vulnerability as soon as is possible. Often, announcements about a given
security exploit are accompanied with a patch (or source code that fixes the problem).
This patch is then applied to the Fedora package and tested and released as an errata update.
However, if an announcement does not include a patch, a developer first works with the maintainer
of the software to fix the problem. Once the problem is fixed, the package is tested
and released as an errata update.
Updating Software
The yum command line tool is used to install and update software
packages. The system also provides package management service called PackageKit
that allows the session users to manage packages in a secure way. There are several
graphical utilities designed for installing, updating and removing packages on your
system that use PackageKit API.
It is recommended to use these mechanisms to keep systems up to date with the latest
security patches.
Ensure Fedora GPG Key is Installed
To ensure that the system can cryptographically verify update packages run the following command to verify
that the system has the Fedora GPG properly installed:$ rpm -q --queryformat "%{SUMMARY}\n" gpg-pubkey
The command should return the string:
gpg(Fedora (14) <fedora@fedoraproject.org>)Ensure Fedora GPG Key is InstalledThe GPG key should be installed.Obtain Software Package Updates with yum
The yum update utility can be run by hand from the command
line, called through one of the provided front-end tools, or configured to run
automatically at specified intervals.Configure Automatic Update Retrieval and Installation with Cron
The yum-updatesd service is not mature enough for an
enterprise environment, and the service may introduce unnecessary overhead. When
possible, replace this service with a cron job that calls yum
directly.
Create the file yum.cron, make it executable, and place it in
/etc/cron.daily:#!/bin/sh
/usr/bin/yum -R 120 -e 0 -d 0 -y update yum
/usr/bin/yum -R 10 -e 0 -d 0 -y update
This particular script instructs yum to update any
packages it finds. Placing the script in /etc/cron.daily ensures its daily execution.
To only apply updates once a week, place the script in /etc/cron.weekly instead.
Schedule yum update using cronEnter frequency of with which to invoke yum updateSelect frequency of yum updatedailyhourlydailyweeklymonthlyhourly|daily|weekly|monthlyhourlydailyweeklymonthlyyum-updatesd service should be disabledThe yum-updatesd service should be disabledCCE-4218-4# chkconfig yum-updatesd offAutomatic Update Retrieval should be scheduled with CronPlace the yum.cron script somewhere in /etc/cron.*/echo -e "/usr/bin/yum -R 120 -e 0 -d 0 -y update yum\n/usr/bin/yum -R 10 -e 0 -d 0 -y update" > /etc/cron.weekly/yum.cronEnsure Package Signature Checking is Globally Activated
The gpgcheck option should be used to ensure that checking of an RPM package’s signature always occurs prior
to its installation.
To force yum to check package signatures before installing them, ensure that the following line appears in
/etc/yum.conf in the [main] section:
gpgcheck=1
Ensure gpgcheck is Globally Activated
The gpgcheck option should be used to ensure that checking of an RPM package’s signature always occurs prior to its installation.To force yum to check package signatures before installing them, ensure that the following line appears in /etc/yum.conf in the [main] section:
gpgcheck=1Ensure Package Signature Checking is Not Disabled For Any Repos
To ensure that signature checking is not disabled for any repos, ensure that the following line DOES NOT
appear in any repo configuration files in /etc/yum.repos.d or elsewhere:
gpgcheck=0
Ensure Package Signature Checking is Not Disabled For Any Repos
To ensure that signature checking is not disabled for any repos, ensure that the following line DOES NOT appear in any repo configuration files in /etc/yum.repos.d or elsewhere:gpgcheck=0Ensure Repodata Signature Checking is Globally Activated
The repo_gpgcheck option should be used to ensure that checking of a signature on repodata is performed prior
to using it.
To force yum to check the signature on repodata sent by a repository prior to using it, ensure that the
following line appears in /etc/yum.conf in the [main] section:
repo_gpgcheck=1
Ensure Repodata Signature Checking is Globally Activated
The repo_gpgcheck option should be used to ensure that checking of a signature on repodata is performed prior to using it.To force yum to check the signature on repodata sent by a repository prior to using it, ensure that the following line appears in /etc/yum.conf in the [main] section:repo_gpgcheck=1Ensure Repodata Signature Checking is Not Disabled For Any Repos
To ensure that signature checking is not disabled for any repos, ensure that the following line DOES NOT
appear in any repo configuration files in /etc/yum.repos.d or elsewhere:
gpgcheck=0
Note: Red Hat’s repositories support signatures on repodata, but some public repositories do not. If a repository
does not support signature checking on repodata, then this risk must be weighed against the value of using the
repository.
Ensure Repodata Signature Checking is Not Disabled For Any Repos
To ensure that signature checking is not disabled for any repos, ensure that the following line DOES NOT appear in any repo configuration files in /etc/yum.repos.d or elsewhere: gpgcheck=0Software Integrity Checking
The AIDE (Advanced Intrusion Detection Environment) software is
included with the system to provide software integrity checking. It is designed to be a
replacement for the well-known Tripwire integrity checker. Integrity checking cannot
prevent
intrusions into your system, but can detect that they have occurred.
Any integrity checking software should be configured before
the system is deployed and able to provides services to users. Ideally, the integrity
checking database would be built before the system is connected to any network, though
this may prove impractical due to registration and software updates.
Configure AIDE
Requirements for software integrity checking should be defined
by policy, and this is highly dependent on the environment in which the system will be
used. As such, a general strategy for implementing integrity checking is provided, but
precise recommendations (such as to check a particular file) cannot be. Documentation
for AIDE, including the quick-start on which this advice is based, is available in
/usr/share/doc/aide-0.12.Install AIDEAIDE is not installed by default.Install AIDEThe AIDE package should be installedCCE-4209-3yum install aideCustomize Configuration File
Customize /etc/aide.conf to meet your requirements. The
default configuration is acceptable for many environments.
The man page aide.conf(5)
provides detailed information about the configuration file format.
Build, Store, and Test Database
Generate a new database:# /usr/sbin/aide --init
By default, the database will be written to
the file /var/lib/aide/aide.db.new.gz. The database, as well as the configuration file
/etc/aide.conf and the binary /usr/sbin/aide (or hashes of these files) should be
copied and stored in a secure location. Storing these copies or hashes on read-only
media may provide further confidence that they will not be
altered.
Install the newly-generated database:# cp /var/lib/aide/aide.db.new.gz /var/lib/aide/aide.db.gz
Run a manual check:
# /usr/sbin/aide --check
If this check produces any unexpected output, investigate.
Implement Periodic Execution of Integrity Checking
By default, AIDE does not install itself for periodic execution.
Implement checking with whatever frequency is required
by your security policy. A once-daily check may be suitable for many environments. For
example, to implement a daily execution of AIDE at 4:05am, add the following line to
/etc/crontab:
05 4 * * * root /usr/sbin/aide --check
AIDE output may be an indication of an attack against
your system, or it may be the result of something innocuous such as an administrator's
configuration change or a software update. The steps in Section 2.1.3.1.3 should be
repeated when configuration changes or software updates necessitate. This will
certainly be necessary after applying guidance later in this guide.
Schedule AIDE check using cronFrequency with which to run AIDE checkSelect frequency with which to run AIDE checkdailyhourlydailyweeklymonthlyhourly|daily|weekly|monthlyhourlydailyweeklymonthlyRun AIDE periodicallySetup cron to run AIDE periodically using cron.echo -e "/usr/sbin/aide --check" > /etc/cron.daily/aide.cronManually Verify Integrity of AIDE
Because integrity checking is a means of intrusion detection
and not intrusion prevention, it cannot be guaranteed that the AIDE binaries,
configuration files, or database have not been tampered with. An attacker could
disable or alter these files after a successful intrusion. Because of this, manual and
frequent checks on these files is recommended. The safely stored copies (or hashes) of
the database, binary, and configuration file were created earlier for this
purpose.
Manually verify the integrity of the AIDE binaries,
configuration file, and database. Possibilities for doing so include:
Use sha1sum or md5sum to generate checksums on the
files and then visually compare them to those generated from the safely stored
versions. This does not, of course, preclude the possibility that such output could
also be faked.Mount the stored versions on read-only media and run
/bin/diff to verify that there are no differences between the
files.Copying the files to another system and performing the hash or file
comparisons there may impart additional confidence that the manual verification
process is not being interfered with.Verify Package Integrity Using RPM
The RPM package management system includes the ability to
verify the integrity of installed packages by comparing the installed files with
information about the files taken from the package metadata stored in the RPM
database. Although an attacker could corrupt the RPM database (analogous to
attacking the AIDE database as described above), this check can still reveal
modification of important files.
To determine which files on the system differ from what is expected by the RPM
database:# rpm -qVa
A “c” in the second column indicates that a file is a configuration file (and may be
expected to change). In order to exclude configuration files from this list, run:
# rpm -qVa | awk '$2!="c" {print $0}'
The man page rpm(8) describes the format of the output. Any files that do not
match the expected output demand further investigation if the system is being
seriously examined. This check could also be run as a cron job.
Verify Package Integrity Using RPMVerify the integrity of installed packages by comparing the installed files with information about the files taken from the package metadata stored in the RPM database.File Permissions and Masks
Traditional Unix security relies heavily on file and directory
permissions to prevent unauthorized users from reading or modifying files to which they
should not have access. Adhere to the principle of least privilege — configure each file,
directory, and filesystem to allow only the access needed in order for that file to serve
its purpose.
However, Linux systems contain a large number of files, so
it is often prohibitively time-consuming to ensure that every file on a machine has exactly
the permissions needed. This section introduces several permission restrictions which are
almost always appropriate for system security, and which are easy to test and
correct.
Note: Several of the commands in this section search
filesystems for files or directories with certain characteristics, and are intended to be
run on every local ext2, ext3 and ext4 partition on a given machine. When the variable
PART
appears in one of the commands below, it means that the command
is intended to be run repeatedly, with the name of each local partition substituted for
PART
in turn.
The following command prints a
list of ext2, ext3 and ext4 partitions on a given machine:$ mount -t ext2,ext3,ext4 | awk '{print $3}'
If your site uses a local filesystem type other than ext{234}, you will need to modify
this command.
Restrict Partition Mount Options
System partitions can be mounted with certain options which limit
what files on those partitions can do. These options are set in the file /etc/fstab, and
can be used to make certain types of malicious behavior more difficult.Add nodev Option to Non-Root Local Partitions
The nodev option prevents users from mounting unauthorized
devices on any partition which is known not to contain any authorized devices. The root
partition typically contains the /dev partition, which is the primary location for
authorized devices, so this option should not be set on /.
However, if system programs are being run in chroot jails, this advice may need to be
modified further, since it is often necessary to create device files inside the chroot
directory for use by the restricted program.
Add nodev Option to Non-Root Local PartitionsThe nodev option should be disabled as appropriate for all non-root partitions.CCE-4249-9
Edit the file /etc/fstab. The important columns for purposes of
this section are column 2 (mount point), column 3 (filesystem type), and column 4 (mount
options). For any line which satisfies all of the conditions:
The filesystem type is ext2, ext3 or ext4The mount point is not /
add the text “,nodev” to the list of mount options in column 4. Add nodev, nosuid, and noexec Options to Removable Media Partitions
Users should not be allowed to introduce arbitrary devices or
setuid programs to a system. These options are used to prevent that. In addition, while
users are usually allowed to add executable programs to a system, the noexec option
prevents code from being executed directly from the media itself, and may therefore
provide a line of defense against certain types of worms or malicious code.Add nodev Option to Removable Media PartitionsThe nodev option should be disabled for all removable media.CCE-3522-0Edit the file /etc/fstab. Filesystems which represent removable media can be
located by finding lines whose mount points contain strings like floppy or cdrom, or
whose types are iso9660, vfat, or msdos. For each line representing a removable media
mountpoint, add the text ',nodev' to the list of mount options in column 4.Add noexec Option to Removable Media PartitionsThe noexec option should be disabled for all removable media.CCE-4275-4Edit the file /etc/fstab. Filesystems which represent removable media can be
located by finding lines whose mount points contain strings like floppy or cdrom, or
whose types are iso9660, vfat, or msdos. For each line representing a removable media
mountpoint, add the text ',noexec' to the list of mount options in column 4.Add nosuid Option to Removable Media PartitionsThe nosuid option should be disabled for all removable media.CCE-4042-8Edit the file /etc/fstab. Filesystems which represent removable media can be
located by finding lines whose mount points contain strings like floppy or cdrom, or
whose types are iso9660, vfat, or msdos. For each line representing a removable media
mountpoint, add the text ',nosuid' to the list of mount options in column 4.Restrict Dynamic Mounting and Unmounting of Filesystems
Linux includes a number of facilities for the automated addition
and removal of filesystems on a running system. These facilities may increase convenience,
but they all bring some risk, whether direct risk from allowing unprivileged users to
introduce arbitrary filesystems to a machine, or risk that software flaws in the automated
mount facility itself will allow an attacker to compromise the
system.
Use caution when enabling any such facility, and find out
whether better configuration management or user education might solve the same problem
with less risk.
Disable USB Device SupportUSB flash or hard drives allow an attacker with physical access to a system to quickly copy an enormous amount of data from it.Disable Modprobe Loading of USB Storage Driver
If USB storage devices should not be used, the modprobe
program used for automatic kernel module loading should be configured to not load the
USB storage driver upon demand.
This will prevent the modprobe program from loading the usb-storage module, but will
not prevent an administrator (or another program) from using the insmod program to
load the module manually.
Disable Modprobe Loading of USB Storage DriverThe USB device support module should not be loadedCCE-4187-1echo -e "\nblacklist usb_storage" >> /etc/modprobe.d/blacklist.confRemove USB Storage Driver
If your system never requires the use of USB storage devices,
then the supporting driver can be removed. Though more effective (as USB storage
certainly cannot be used if the driver is not available at all), this is less elegant
than the method described in Section 2.2.2.1.1.
Note that this guidance will not prevent USB storage devices from being mounted if a
custom kernel (i.e., not the one supplied with the system) with built-in USB support
is used.
Remove USB Storage Driver
The USB device support module should not be installed. The command in
the FIX will need to be repeated every time the kernel is updated. This command
will also cause the command rpm -q --verify kernel to fail, which may be an
undesirable side effect.CCE-4006-3rm /lib/modules/2.6.*/kernel/drivers/usb/storage/usb-storage.koDisable Kernel Support for USB via Bootloader Configuration
Another means of disabling USB storage is to disable all USB
support provided by the operating system. This can be accomplished by adding the
'nousb' argument to the kernel's boot loader configuration.
NOTE
- Disabling all kernel support for USB will cause problems for systems with USB-based
keyboards, mice, or printers. This guidance is inappropriate for systems which require
USB connectivity.
Disable Kernel Support for USB via Bootloader ConfigurationUSB kernel support should be disabled.CCE-4173-1To disable kernel support for USB, append 'nousb' to the kernel line in
/etc/grub.conf as follows: kernel /vmlinuz-version ro vga=ext
root=/dev/VolGroup00/LogVol00 rhgb quiet nousbDisable Booting from USB Devices
An attacker with physical access could try to boot the system
from a USB flash drive and then access any data on the system's hard drive,
circumventing the normal operating system's access controls. To prevent this,
configure the BIOS to disallow booting from USB drives. Also configure the BIOS or
firmware password as described in Section 2.3.5.1 to prevent unauthorized
configuration changes.Disable Booting from USB Devices in the BIOSThe ability to boot from USB devices should be disabledCCE-3944-6BIOS settingsDisable the Automounter if Possible
If the autofs service is not needed to dynamically mount NFS
filesystems or removable media, disable the service.
The autofs daemon mounts and unmounts filesystems, such as user home directories shared
via NFS, on demand. In addition, autofs can be used to handle removable media, and the
default configuration provides the cdrom device as /misc/cd. However, this method of
providing access to removable media is not common, so autofs can almost always be
disabled if NFS is not in use.
Even if NFS is required, it is almost always
possible to configure filesystem mounts statically by editing /etc/fstab rather than
relying on the automounter.
Disable the Automounter if PossibleThe autofs service should be disabled.CCE-4072-5chkconfig autofs offDisable GNOME Automounting if Possible
The system's default desktop environment, GNOME, runs the
program gnome-volume-manager to mount devices and removable media (such as DVDs, CDs and
USB flash drives) whenever they are inserted into the system.
The system's capabilities for automatic mounting should be configured to match whatever
is defined by security policy. Disabling USB storage as described in Section 2.2.2.2.1
will prevent the use of USB storage devices, but this step can also be taken as an
additional layer of prevention and to prevent automatic mounting of CDs and DVDs if
required.
Particularly for kiosk-style systems, where users should
have extremely limited access to the system, more detailed information can be found in
Red Hat Desktop: Deployment Guide. The gconf-editor program, available in an RPM of the
same name, can be used to explore other settings available in the GNOME environment.
Disable GNOME Automounting if PossibleThe GNOME automounter (gnome-volume-manager) should be disabled if possibleCCE-4231-7Execute the following commands to prevent gnome-volume-manager from automatically
mounting devices and media:
# gconftool-2 --direct --config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory
--type bool --set /desktop/gnome/volume_manager/automount_media false
# gconftool-2 --direct
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory
--type bool
--set /desktop/gnome/volume_manager/automount_drives false
Verify the changes by executing
the following command, which should return a list of settings: # gconftool-2 -R /desktop/gnome/volume_manager
The automount drives and automount media settings should
be set to false. Survey the list for any other options that should be adjusted.Disable Mounting of Uncommon Filesystem Types
Specifying kernel module in /etc/modprobe.d/blacklist.conf will prevent
kernel module loading system from inserting the modele into the kernel.
This mechanism effectively prevents usage of these uncommon filesystems.Disable Mounting of cramfscramfs is uncommon filesystemsecho "blacklist cramfs" >> /etc/modprobe.d/blacklist.confDisable Mounting of freevxfsfreevxfs is uncommon filesystemsecho "blacklist freevxfs" >> /etc/modprobe.d/blacklist.confDisable Mounting of jffs2jffs2 is uncommon filesystemsecho "blacklist jffs2" >> /etc/modprobe.d/blacklist.confDisable Mounting of hfshfs is uncommon filesystemsecho "blacklist hfs" >> /etc/modprobe.d/blacklist.confDisable Mounting of hfsplushfsplus is uncommon filesystemsecho "blacklist hfsplus" >> /etc/modprobe.d/blacklist.confDisable Mounting of squashfssquashfs is uncommon filesystemsecho "blacklist squashfs" >> /etc/modprobe.d/blacklist.confDisable Mounting of udfudf is uncommon filesystemsecho "blacklist udf" >> /etc/modprobe.d/blacklist.confVerify Permissions on Important Files and Directories
Permissions for many files on a system should be set to conform
to system policy. This section discusses important permission restrictions which
should be checked on a regular basis to ensure that no harmful discrepancies have arisen.
Verify Permissions on passwd, shadow, group and gshadow Files
These are the default permissions for these files. Many
utilities need read access to the passwd file in order to function properly, but read
access to the shadow file allows malicious attacks against system passwords, and should
never be enabled.Permissions for shadowFile permissions for /etc/shadowSelect permissions for /etc/shadow000000000000000000100000000110100100^[10]+$Permissions for groupFile permissions for /etc/groupSelect permissions for /etc/group110100100100000000110100100111000000^[10]+$Permissions for gshadowFile permissions for /etc/gshadowSelect permissions for /etc/gshadow000000000000000000100000000110100100^[10]+$Permissions for passwdFile permissions for /etc/passwordSelect permissions for /etc/password110100100100000000110100100111000000^[10]+$Verify user who owns 'shadow' fileThe /etc/shadow file should be owned by root.CCE-3918-0Verify group who owns 'shadow' fileThe /etc/shadow file should be owned by root.CCE-3988-3Verify user who owns 'group' fileThe /etc/group file should be owned by root.CCE-3276-3Verify group who owns 'group' fileThe /etc/group file should be owned by root.CCE-3883-6Verify user who owns 'gshadow' fileThe /etc/gshadow file should be owned by root.CCE-4210-1Verify group who owns 'gshadow' fileThe /etc/gshadow file should be owned by root.CCE-4064-2Verify user who owns 'passwd' fileThe /etc/passwd file should be owned by root.CCE-3958-6Verify group who owns 'passwd' fileThe /etc/passwd file should be owned by root.CCE-3495-9Verify permissions on 'shadow' fileFile permissions for /etc/shadow should be set correctly.CCE-4130-1Verify permissions on 'group' fileFile permissions for /etc/group should be set correctly.CCE-3967-7Verify permissions on 'gshadow' fileFile permissions for /etc/gshadow should be set correctly.CCE-3932-1Verify permissions on 'passwd' fileFile permissions for /etc/passwd should be set correctly.CCE-3566-7Verify that All World-Writable Directories Have Sticky Bits Set
When the so-called 'sticky bit' is set on a directory, only the
owner of a given file may remove that file from the directory. Without the sticky bit,
any user with write access to a directory may remove any file in the directory. Setting
the sticky bit prevents users from removing each other's files. In cases where there is
no reason for a directory to be world-writable, a better solution is to remove that
permission rather than to set the sticky bit. However, if a directory is used by a
particular application, consult that application's documentation instead of blindly
changing modes.Verify that All World-Writable Directories Have Sticky Bits SetThe sticky bit should be set for all world-writable directories.CCE-3399-3Locate any directories in local partitions which are world-writable and do not have
their sticky bits set. The following command will discover and print these. Run it
once for each local partition PART: # find PART -xdev -type d \( -perm -0002 -a !
-perm -1000 \) -print
If this command produces any output, fix each reported directory
/dir using the command: # chmod +t /dirFind Unauthorized World-Writable Files
Data in world-writable files can be modified by any user on the
system. In almost all circumstances, files can be configured using a combination of user
and group permissions to support whatever legitimate access is needed without the risk
caused by world-writable files.
It is generally a good idea to
remove global (other) write access to a file when it is discovered. However, check with
documentation for specific applications before making changes. Also, monitor for
recurring world-writable files, as these may be symptoms of a misconfigured application
or user account.
Find Unauthorized World-Writable FilesThe world-write permission should be disabled for all files.CCE-3795-2The following command discovers and prints any world-writable files in local
partitions. Run it once for each local partition PART: find PART -xdev -type f -perm -0002 -print | xargs chmod o-wFind Unauthorized SUID/SGID System Executables
The following command discovers and prints any setuid or setgid
files on local partitions. Run it once for each local partition : # for PART in `mount -t ext2,ext3,ext4 | awk '{print $3}'`;
do find $PART -xdev \( -perm -4000 -o -perm -2000 \) -type f -print;
done
If the file does not require a setuid or
setgid bit as discussed below, then these bits can be removed with the command: # chmod -s file
The following table contains all setuid and setgid files which are expected to
be on a stock system. The setuid or setgid bit on these files may be disabled to reduce
system risk if only an administrator requires their functionality. The table indicates
those files which may not be needed.
Note: Several of these files are used for applications which are unlikely to be
relevant to most production environments, such as ISDN networking, SSH hostbased
authentication, or modification of network interfaces by unprivileged users. It is
extremely likely that your site can disable a subset of these files with no loss of
functionality.
Any files found by the above command which are not in the table should be examined.
If the files are not authorized, they should have permissions removed, and further
investigation may be warranted.
FileSet-IDPackage/bin/mountuid rootutil-linux-ng/bin/pinguid rootiputils/bin/ping6uid rootiputils/bin/suuid rootcoreutils/bin/umountuid rootutil-linux-ng/bin/fusermountuid rootfuse/bin/cgexecuid rootlibcgroup/sbin/mount.nfsuid rootnfs-utils/sbin/umount.nfsuid rootnfs-utils/sbin/netreportuid rootinitscripts/sbin/pam_timestamp_checkuid rootpam/sbin/unix_chkpwduid rootpam/usr/bin/atuid rootat/usr/bin/chageuid rootshadow-utils/usr/bin/chfnuid rootutil-linux-ng/usr/bin/chshuid rootutil-linux-ng/usr/bin/crontabuid/gid rootcronie/usr/bin/gpasswduid rootshadow-utils/usr/bin/locategid slocatemlocate/usr/bin/lockfilegid mailprocmail/usr/bin/gnominegid gamesgnome-games/usr/bin/iagnogid gamesgnome-games/usr/bin/newgrpuid rootshadow-utils/usr/bin/passwduid rootpasswd/usr/bin/pkexecuid rootpolkit/usr/bin/rcpuid rootrsh/usr/bin/rloginuid rootrsh/usr/bin/rshuid rootrsh/usr/bin/staprunuid rootsystemtap-runtime/usr/bin/ssh-agentgid nobodyopenssh-clients/usr/bin/sudouid rootsudo/usr/bin/sudoedituid rootsudo/usr/bin/wallgid ttysysvinit-tools/usr/bin/writegid ttyutil-linux-ng/usr/bin/screengid screenscreen/usr/bin/jwhoisgid jwhoisjwhois/usr/bin/Xorguid rootxorg-x11-server-Xorg/usr/bin/ksuuid rootkrb5-workstation/usr/sbin/lockdevgid locklockdev/usr/sbin/sendmail.sendmailgid smmspsendmail/usr/sbin/suexecuid roothttpd/usr/sbin/seunshareuid rootpolicycoreutils/usr/sbin/userhelperuid rootusermode/usr/sbin/userisdnctluid rootisdn4k-utils/usr/sbin/mtruid rootmtr/usr/sbin/usernetctluid rootinitscripts/usr/sbin/ccreds_chkpwduid rootpam_ccreds/usr/libexec/openssh/ssh-keysignuid rootssh/usr/libexec/kde4/kpac_dhcp_helperuid rootkdelibs/usr/libexec/polkit-1/polkit-agent-helper-1uid rootpolkit/usr/libexec/pt_chownuid rootglibc-common/usr/libexec/pulse/proximity-helperuid rootpulseaudio-module-bluetooth/usr/libexec/news/innbinduid rootinn/usr/libexec/news/rnewsuid uucpinn/usr/libexec/utempter/utemptergid utmplibutempter/usr/lib/nspluginwrapper/plugin-configuid rootnspluginwrapper/usr/lib/vte/gnome-pty-helpergid utmpvte/usr/share/BackupPC/sbin/BackupPC_Adminuid backuppcBackupPC/var/cache/jwhois/jwhois.dbgid jwhoisjwhois/lib/dbus-1/dbus-daemon-launch-helperuid rootdbusFind Unauthorized SGID System ExecutablesThe sgid bit should not be set for all files.CCE-4178-0Find Unauthorized SUID System ExecutablesThe suid bit should not be set for all files.CCE-3324-1Find and Repair Unowned Files
The following command will discover and print any files on
local partitions which do not belong to a valid user and a valid group. Run it once for
each local partition PART: # find PART -xdev \( -nouser -o -nogroup \) -print
If this command prints any results, investigate each reported file and either assign it to an
appropriate user and group or remove it. Unowned files are not directly exploitable, but
they are generally a sign that something is wrong with some system process. They may be
caused by an intruder, by incorrect software installation or incomplete software
removal, or by failure to remove all files belonging to a deleted account. The files
should be repaired so that they will not cause problems when accounts are created in the
future, and the problem which led to unowned files should be discovered and addressed.Find files unowned by a userAll files should be owned by a userCCE-4223-4Find files unowned by a groupAll files should be owned by a groupCCE-3573-3Verify that All World-Writable Directories Have Proper Ownership
Locate any directories in local partitions which are world-writable and
ensure that they are owned by root or another system account. The following command will discover
and print these (assuming only system accounts have a uid lower than 500). Run it once for each
local partition PART:# find PART -xdev -type d -perm -0002 -uid +500 -print
If this command produces any output, investigate why the current owner is not root or another
system account.
Allowing a user account to own a world-writeable directory is undesirable because it allows the
owner of that directory to remove or replace any files that may be placed in the directory by
other users.Find world writable directories not owned by a system accountAll world writable directories should be owned by a system userRestrict Programs from Dangerous Execution Patterns
The recommendations in this section provide broad protection
against information disclosure or other misbehavior. These protections are applied at the
system initialization or kernel level, and defend against certain types of
badly-configured or compromised programs.Set Daemon umask
The system umask for scripts in /etc/init.d must be set to at least 022, or daemon
processes may create world-writable files. The more restrictive setting
027 protects files, including temporary files and log files, from unauthorized reading
by unprivileged users on the system.
If a particular daemon needs a
less restrictive umask, consider editing the startup script or sysconfig file of that
daemon to make a specific exception.
daemon umaskEnter umask for daemonsEnter umask which will be used for new files created by daemons022022027^0?[0-7][0-7][0-7]?$Set Daemon umaskThe daemon umask should be set to profile valueCCE-4220-0Edit the file /etc/rc.d/init.d/functions, and add or correct the following line: umask
Disable Core Dumps
A core dump file is the memory image of an executable program
when it was terminated by the operating system due to errant behavior. In most cases,
only software developers would legitimately need to access these files. The core dump
files may also contain sensitive information, or unnecessarily occupy large amounts of
disk space.
By default, the system sets a soft limit to stop the
creation of core dump files for all users. This is accomplished in /etc/profile with the
line: ulimit -S -c 0 > /dev/null 2>&1 However, compliance with this
limit is voluntary; it is a default intended only to protect users from the annoyance of
generating unwanted core files. Users can increase the allowed core file size up to the
hard limit, which is unlimited by default.
Once a hard limit is set
in /etc/security/limits.conf, the user cannot increase that limit within his own
session. If access to core dumps is required, consider restricting them to only certain
users or groups. See the limits.conf man page for more
information.
The core dumps of setuid programs are further
protected. The sysctl variable fs.suid_dumpable controls whether the kernel allows core
dumps from these programs at all. The default value of 0 is recommended.
Disable Core Dumps for all usersCore dumps for all users should be disabledCCE-4225-9To disable core dumps for all users, add or correct the following line in
/etc/security/limits.conf: * hard core 0Disable Core Dumps for SUID programsCore dumps for setuid programs should be disabledCCE-4247-3To ensure that core dumps can never be made by setuid programs, edit
/etc/sysctl.conf and add or correct the line: fs.suid_dumpable = 0Enable ExecShield
ExecShield comprises a number of kernel features to provide
protection against buffer overflows. These features include random placement of the
stack and other memory regions, prevention of execution in memory that should only hold
data, and special handling of text buffers. This protection is enabled by default, but
the sysctl variables kernel.exec-shield and kernel.randomize va space should be checked
to ensure that it has not been disabled.
ExecShield uses the
segmentation feature on all x86 systems to prevent execution in memory higher than a
certain address. It writes an address as a limit in the code segment descriptor, to
control where code can be executed, on a per-process basis. When the kernel places a
process's memory regions such as the stack and heap higher than this address, the
hardware prevents execution there. However, this cannot always be done for all memory
regions in which execution should not occur, so follow guidance in Section 2.2.4.4 to
further protect the system.
Enable ExecShieldExecShield should be enabledCCE-4168-1To ensure ExecShield (including random placement of virtual memory regions) is
activated at boot, add or correct the following settings in /etc/sysctl.conf:
kernel.exec-shield = 1Enable ExecShield randomized placement of virtual memory regionsExecShield randomized placement of virtual memory regions should be enabledCCE-4146-7To ensure ExecShield (including random placement of virtual memory regions) is
activated at boot, add or correct the following settings in /etc/sysctl.conf:
kernel.randomize_va_space = 2Enable Execute Disable (XD) or No Execute (NX) Support on x86 Systems
Recent processors in the x86 family support the ability to
prevent code execution on a per memory page basis. Generically and on AMD processors,
this ability is called No Execute (NX), while on Intel processors it is called Execute
Disable (XD). This ability can help prevent exploitation of buffer overflow
vulnerabilities and should be activated whenever possible. Extra steps must be taken to
ensure that this protection is enabled, particularly on 32-bit x86 systems. Other
processors, such as Itanium and POWER, have included such support since inception and
the standard kernel for those platforms supports the feature.Check for Processor Support on x86 Systems
Check to see if the processor supports the PAE and NX
features: $ cat /proc/cpuinfo If supported, the flags field will contain pae and nx.Enable NX or XD Support in the BIOS
Computers with the ability to prevent this type of code
execution frequently put an option in the BIOS that will allow users to turn the
feature on or off at will.
See Section 2.3.5.1 for information on protecting this and
other BIOS settings.Enable NX or XD Support in the BIOSThe XD/NX processor feature should be enabled in the BIOSCCE-4177-2Reboot the system and enter the BIOS or 'Setup' configuration menu. Navigate the
BIOS configuration menu and make sure that the option is enabled. The setting may be
located under a 'Security' section. Look for Execute Disable (XD) on Intel-based
systems and No Execute (NX) on AMD-based systems.Account and Access Control
In traditional Unix security, if an attacker gains shell access to
a certain login account, he can perform any action or access any file to which that account
has access. Therefore, making it more difficult for unauthorized people to gain shell access
to accounts, particularly to privileged accounts, is a necessary part of securing a system.
This section introduces mechanisms for restricting access to login accounts.Protect Accounts by Restricting Password-Based Login
Conventionally, Unix shell accounts are accessed by providing a
username and password to a login program, which tests these values for correctness using
the /etc/passwd and /etc/shadow files. Password-based login is vulnerable to guessing of
weak passwords, and to sniffing and man-in-the-middle attacks against passwords entered
over a network or at an insecure console. Therefore, mechanisms for accessing accounts by
entering usernames and passwords should be restricted to those which are operationally
necessary.Restrict Root Logins to System Console
Edit the file /etc/securetty. Ensure that the file contains
only the following lines: The primary system console device: consoleThe virtual console devices: tty1 tty2 tty3 tty4 tty5
tty6 ... If required by your organization, the deprecated virtual console interface
may be retained for backwards compatibility:vc/1 vc/2 vc/3 vc/4 vc/5
vc/6 ...If required by your organization, the serial consoles may be added:
ttyS0 ttyS1
Direct root logins should be allowed only for
emergency use. In normal situations, the administrator should access the system via a
unique unprivileged account, and use su or sudo to execute privileged commands.
Discouraging administrators from accessing the root account directly ensures an audit
trail in organizations with multiple administrators. Locking down the channels through
which root can connect directly reduces opportunities for password-guessing against the
root account. The login program uses the file /etc/securetty to determine which
interfaces should allow root logins. The virtual devices /dev/console and /dev/tty*
represent the system consoles (accessible via the Ctrl-Alt-F1 through Ctrl-Alt-F6
keyboard sequences on a default installation). The default securetty file also contains
/dev/vc/*. These are likely to be deprecated in most environments, but may be retained
for compatibility. Root should also be prohibited from connecting via network protocols.
See Section 3.5 for instructions on preventing root from logging in via SSH.Restrict Root Logins to System ConsoleLogins through the specified virtual console interface should be disabled
CCE-3820-8Edit /etc/securettyRestrict Root Logins to System ConsoleLogins through the specified virtual console device should be disabledCCE-3485-0 Edit /etc/securettyRestrict virtual console Root LoginsLogins through the virtual console devices should be disabledCCE-4111-1 Edit /etc/securettyRestrict serial port Root LoginsLogin prompts on serial ports should be disabled.CCE-4256-4Edit /etc/securettyLimit su Access to the Root Account
The su command allows a user to gain the privileges of another user by entering the
password for that user's account. It is desirable to restrict the root user so that only
known administrators are ever allowed to access the root account. This restricts
password-guessing against the root account by unauthorized users or by accounts which
have been compromised.
By convention, the group wheel contains all users who are allowed to run privileged
commands. The PAM module pam_wheel.so is used to restrict root access to this set of
users.Limit su Access to the Root AccountThe wheel group should exist Ensure that the group wheel exists, and that the usernames of all administrators
who should be allowed to execute commands as root are members of that group.
Limit su Access to the wheel groupCommand access to the root account should be restricted to the wheel group. Edit the file /etc/pam.d/su. Add, uncomment, or correct the line: auth required pam_wheel.so use_uidConfigure sudo to Improve Auditing of Root AccessEnsure that the group wheel exists, and that the usernames
of all administrators who should be allowed to execute commands as root are members of
that group. # grep ^wheel /etc/groupEdit the file /etc/sudoers. Add, uncomment, or
correct the line:
%wheel ALL=(ALL) ALL
The sudo command allows fine-grained control over
which users can execute commands using other accounts. The primary benefit of sudo when
configured as above is that it provides an audit trail of every command run by a
privileged user. It is possible for a malicious administrator to circumvent this
restriction, but, if there is an established procedure that all root commands are run
using sudo, then it is easy for an auditor to detect unusual behavior when this
procedure is not followed.
Editing /etc/sudoers by hand can be dangerous, since a configuration error may make it
impossible to access the root account remotely. The recommended means of editing this
file is using the visudo command, which checks the file's syntax for correctness before
allowing it to be saved.
Note that sudo allows any attacker who gains access to the password of an administrator
account to run commands as root. This is a downside which must be weighed against the
benefits of increased audit capability and of being able to heavily restrict the use of
the high-value root password (which can be logistically difficult to change often). As
a basic precaution, never use the NOPASSWD directive, which would allow anyone with
access to an administrator account to execute commands as root without knowing the
administrator's password.
The sudo command has many options which can be used to further customize its behavior.
See the sudoers(5) man page for details.Configure sudo to Improve Auditing of Root AccessSudo privileges should granted to the wheel groupCCE-4044-4echo "%wheel ALL=(ALL) ALL" >> /etc/sudoersBlock Shell and Login Access for Non-Root System Accounts
Using /etc/passwd, obtain a listing of all users, their UIDs,
and their shells, for instance by running: # awk -F: '{print $1 ":" $3 ":" $7}' /etc/passwd
Identify the system accounts from this listing. These will primarily be the accounts
with UID numbers less than 500, other than root.
For each identified system account SYSACCT , lock the account: # usermod -L SYSACCT
and disable its shell: # usermod -s /sbin/nologin SYSACCT
These are the accounts which are
not associated with a human user of the system, but which exist to perform some
administrative function. Make it more difficult for an attacker to use these accounts by
locking their passwords and by setting their shells to some non-valid shell. The Fedora
default non-valid shell is /sbin/nologin, but any command which will exit with a failure
status and disallow execution of any further commands, such as /bin/false or /dev/null,
will work.Do not perform the steps in this section on the root account.
Doing so might cause the system to become inaccessible.Block Shell and Login Access for Non-Root System AccountsLogin access to non-root system accounts should be disabledCCE-3987-5Edit /etc/passwdVerify Proper Storage and Existence of Password HashesVerify that No Accounts Have Empty Password Fields
Run the command: # awk -F: '($2 == "") {print}' /etc/shadow
If this produces any output, fix the problem by locking each account
(see Section 2.3.1.4 above) or by setting a password.
If an account has an empty password, anybody may log in and run commands with the
privileges of that account. Accounts with empty passwords should never be used in
operational environments.Verify that No Accounts Have Empty Password FieldsLogin access to accounts without passwords should be disabledCCE-4238-2Verify that All Account Password Hashes are Shadowed
To ensure that no password hashes are stored in /etc/passwd, the following command should have no output:# awk -F: '($2 != "x") {print}' /etc/passwd
The hashes for all user account passwords should be stored in the file /etc/shadow and never in /etc/passwd,
which is readable by all users.
Verify that All Account Password Hashes are ShadowedCheck that passwords are shadowedVerify that No Non-Root Accounts Have UID 0
This command will print all password file entries for accounts
with UID 0: # awk -F: '($3 == "0") {print}' /etc/passwd
This should print only one line, for the user root. If any other lines appear, ensure
that these additional UID-0 accounts are authorized, and that there is a good reason for
them to exist.
In general, the best practice solution for auditing use of the root account is to restrict
the set of cases in which root must be accessed anonymously by requiring use of su or sudo
in almost all cases. Some sites choose to have more than one account with UID 0 in order
to differentiate between administrators, but this practice may have unexpected side
effects, and is therefore not recommended.Verify that No Non-Root Accounts Have UID 0Anonymous root logins should be disabledCCE-4009-7Set Password Expiration Parameters
Edit the file /etc/login.defs to specify password expiration
settings for new accounts. Add or correct the following lines:
PASS_MAX_DAYS=180
PASS_MIN_DAYS=7
PASS_MIN_LEN=8
PASS_WARN_AGE=7
For each existing human user USER , modify the current expiration settings to match
these: # chage -M 180 -m 7 -W 7 USER
Users should be forced to change their passwords, in order to decrease the utility of
compromised passwords. However, the need to change passwords often should be balanced
against the risk that users will reuse or write down passwords if forced to change them
too often. Forcing password changes every 90-360 days, depending on the environment, is
recommended. Set the appropriate value as PASS_MAX_DAYS and apply it to existing
accounts with the -M flag.
The PASS_MIN_DAYS (-m) setting prevents password changes for 7 days after the first
change, to discourage password cycling. If you use this setting, train users to contact
an administrator for an emergency password change in case a new password becomes
compromised. The PASS_WARN_AGE (-W) setting gives users 7 days of warnings at login time
that their passwords are about to expire.
The PASS_MIN_LEN setting, which controls minimum password length, should be set to
whatever is required by your site or organization security policy. The example value of
8 provided here may be inadequate for many environments. See Section 2.3.3 for
information on how to enforce more sophisticated requirements on password length and
quality
minimum password lengthMinimum number of characters in passwordThis will only check new passwordsSelect minimum number of characters in password145681014^[\d]+$minimum password ageEnter minimum duration before allowing a password changeSelect minimum duration (in days) before allowing a password change117^[\d]+$maximum password ageEnter age before which a password must be changedSelect age (in days) before which a password must be changed600306090120150180^[\d]+$password warn age
The number of days warning given before a password expires. A zero
means warning is given only upon the day of expiration, a negative
value means no warning is given. If not specified, no warning will
be provided.Select number of days warning is given before a password expires147814^[\d]+$Set password minimum lengthThe password minimum length should be set to:
CCE-4154-1Set minimum password ageThe minimum password age should be set to:
CCE-4180-6Set maximum password ageThe maximum password age should be set to:
CCE-4092-3Set password warn ageThe password warn age should be set to:
CCE-4097-2Remove Legacy + Entries from Password Files
The command: # grep "^+:" /etc/passwd /etc/shadow /etc/group
should produce no output.
The + symbol was used by systems to include data from NIS maps
into existing files. However, a certain configuration error in which a NIS inclusion
line appears in /etc/passwd, but NIS is not running, could lead to anyone being able to
access the system with the username + and no password. Therefore, it is important to
verify that no such line appears in any of the relevant system files.
The correct way to
tell the local system to consult network databases such as LDAP or NIS for user
information is to make appropriate modifications to /etc/nsswitch.conf.Remove Legacy + Entries from /etc/shadowNIS file inclusions should be set appropriately in the /etc/shadow file(1) via /etc/shadowRemove Legacy + Entries from /etc/groupNIS file inclusions should be set appropriately in the /etc/group file(1) via /etc/groupRemove Legacy + Entries from /etc/passwdNIS file inclusions should be set appropriately in the /etc/passwd fileCCE-4114-5(1) via /etc/passwdUse Unix Groups to Enhance Security
The access control policies which can be enforced by standard
Unix permissions are limited, and configuring SELinux (Section 2.4) is frequently a better
choice. However, this guide recommends that security be enhanced to the extent possible by
enforcing the Unix group policies outlined in this section.Create a Unique Default Group for Each User
When running useradd, do not use the -g flag or otherwise
override the default group.
The Red Hat default is that each new user account should
have a unique primary group whose name is the same as that of the account. This default
is recommended, in order to provide additional protection against files which are
created with group write permission enabled.Create and Maintain a Group Containing All Human Users
Identify all user accounts on the system which correspond to
human users. Depending on your system configuration, this may be all entries in
/etc/passwd with UID values of at least 500. Once, you have identified such a set of
users, create a group named usergroup (substitute some name appropriate to your
environment) and populate it with each human user: # groupadd usergroup
# usermod -G usergroup human1
# usermod -G usergroup human2 ...
# usermod -G usergroup humanN
Then modify your procedure for creating new user accounts by adding -G usergroup to the
set of flags with which useradd is invoked, so that new human users will be placed in
the correct group by default.
Creating a group of human users does not, by itself, enhance
system security. However, as you work on securing your system, you will often find
commands which never need to be run by system accounts, or which are only ever needed by
users logged into the graphical console (which should only ever be available to human
users, even on workstations). Once a group of users has been created, it is easy to
restrict access to a given command, for instance /path/to/graphical/command , to
authorized users: # chgrp usergroup /path/to/graphical/command
# chmod 750 /path/graphical/command
Without a group of human users, it is necessary to restrict
access by somehow preventing each system account from running the command, which is an
error-prone process even when it is possible at all.Protect Accounts by Configuring PAM
PAM, or Pluggable Authentication Modules, is a system which
implements modular authentication for Linux programs. PAM is well-integrated into Linux's
authentication architecture, making it difficult to remove, but it can be configured to
minimize your system's exposure to unnecessary risk. This section contains guidance on how
to accomplish that, and how to ensure that the modules used by your PAM configuration do
what they are supposed to do.
PAM is implemented as a set of shared objects which are
loaded and invoked whenever an application wishes to authenticate a user. Typically, the
application must be running as root in order to take advantage of PAM. Traditional
privileged network listeners (e.g. sshd) or SUID programs (e.g. sudo) already meet this
requirement. An SUID root application, userhelper, is provided so that programs which are
not SUID or privileged themselves can still take advantage of PAM.
PAM looks in the
directory /etc/pam.d for application-specific configuration information. For instance, if
the program login attempts to authenticate a user, then PAM's libraries follow the
instructions in the file /etc/ pam.d/login to determine what actions should be taken.
One
very important file in /etc/pam.d is /etc/pam.d/system-auth. This file, which is included
by many other PAM configuration files, defines 'default' system authentication measures.
Modifying this file is a good way to make far-reaching authentication changes, for
instance when implementing a centralized authentication service.
Be careful when making changes to PAM's configuration files. The syntax for these files
is complex, and modifications can have unexpected consequences.1 The default
configurations shipped with applications should be sufficient for most users.
Running authconfig or system-config-authentication will re-write the PAM configuration
files, destroying any manually made changes and replacing them with a series of system
defaults. 1One reference to the configuration file syntax can be found at
http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/ sag-configuration-file.html.
Set Password Quality Requirements
The default pam_cracklib PAM module provides strength checking
for passwords. It performs a number of checks, such as making sure passwords are not
similar to dictionary words, are of at least a certain length, are not the previous
password reversed, and are not simply a change of case from the previous password. It
can also require passwords to be in certain character classes.
The pam_passwdqc PAM module provides the ability to enforce even more stringent
password strength requirements. It is provided in an RPM of the same name.
The man pages pam_cracklib(8) and pam_passwdqc(8) provide information on the
capabilities and configuration of each.
Set Password Quality Requirements, if using pam_cracklib
The pam_cracklib PAM module can be configured to meet
recommendations for DoD systems as stated in [12].
To configure pam_cracklib to require at least one uppercase character, lowercase
character, digit, and other (special) character, locate the following line in
/etc/pam.d/system-auth:
password requisite pam_cracklib.so try_first_pass retry=3
and then alter it to read:
password required pam_cracklib.so try_first_pass retry=3 minlen=14 dcredit=-1 /
ucredit=-1 ocredit=-1 lcredit=0
If necessary, modify the arguments to ensure compliance with your organization’s
security policy.
Note that the password quality requirements are not enforced
for the root account for some reason.
retryNumber of retry attempts before erroring outSelect number of password retry attempts before erroring out3123^[\d]+$difokMininum number of characters not present in old passwordKeep this high for short passwordsSelect minimum number of characters not present in old password52345^[\d]+$minlenMinimum number of characters in passwordSelect minimum number of characters in pasword1468101415^[\d]+$dcreditMininum number of digits in passwordSelect number of digits in password-2-2-10^-?[\d]+$ocreditMininum number of other (special characters) in passwordSelect number of special characters in password-2-2-10^-?[\d]+$lcreditMininum number of lower case in passwordSelect minimum number of lower case in password-2-2-10^-?[\d]+$ucreditMininum number of upper case in passwordSelect minimum number of upper case in password-2-2-10^-?[\d]+$Set Password Quality RequirementsThe password strength should meet minimum requirementsCCE-3762-2(1) via PAMSet Password Quality Requirements, if using pam_passwdqc
If password strength stronger than that guaranteed by
pam_cracklib is required, configure PAM to use pam_passwdqc.
To activate pam_passwdqc, locate the following line in /etc/pam.d/system-auth:
password requisite pam_cracklib.so try_first_pass retry=3
and then replace it with the line:
password requisite pam_passwdqc.so min=disabled,disabled,16,12,8
If necessary, modify the arguments (min=disabled,disabled,16,12,8) to ensure
compliance with your organization’s security policy. Configuration options are
described in the man page pam_passwdqc(8) and also in /usr/share/doc/pam_passwdqc-version.
The minimum lengths provided here supercede that specified
by the argument PASS MIN LEN as described in Section 2.3.1.7.
The options given in the example above set a minimum length for each of the
password “classes” that pam_passwdqc recognizes. Setting a particular minimum
value to disabled will stop users from choosing a password that falls into
that category alone.
N0
N0 is used for passwords consisting of characters
from one character class only. The character classes are: digits,
lower-case letters, upper-case letters, and other characters. There is
also a special class for non-ASCII characters which could not be
classified, but are assumed to be non-digits. 24disabled2430N1
N1 is used for passwords consisting of characters
from two character classes which do not meet the requirements for a
passphrase.16disabled1824N2
N2 is used for passphrases. Note that besides
meeting this length requirement, a passphrase must also consist of a
sufficient number of words (see the "passphrase" option below). 16disabled161718N3N3 is the number of characters required for a password that uses characters from 3 character classes.Select the number of characters required for a password that uses characters from 3 character classes16disabled141516N4N4 is the number of characters required for a password that uses characters from 4 character classes.Select the number of characters required for a password that uses characters from 4 character classes14101214passphraseThe number of words required for a passphrase, or 0 to disable the support for user-chosen passphrases. Select the number of words required for a passphrase3035^[\d]+$match
The length of common substring required to
conclude that a password is at least partially based on information
found in a character string, or 0 to disable the substring search.
Note that the password will not be rejected once a weak substring is
found; it will instead be subjected to the usual strength requirements
with the weak substring removed.Enter the length of common substring required to conclude that a password is at least partially based on information found in a character string50345^[\d]+$retry
The number of times the module will ask for a
new password if the user fails to provide a sufficiently strong
password and enter it twice the first time. Enter the number of times the module will ask for a new password if user fail to provide a sufficiently strong password3234^[\d]+$Set Password Quality Requirements using pam_passwdqcThe password strength should meet minimum requirementsCCE-3762-2(1) via PAMSet Lockouts for Failed Password Attempts
The pam_tally2 PAM module provides the capability to lock out
user accounts after a number of failed login attempts. Its documentation is available in
/usr/share/doc/pam-version/txts/README.pam_tally2.
If locking out accounts after a number of incorrect login attempts is required by your
security policy, implement use of pam_tally2.so for the relevant PAM-aware programs
such as login, sshd, and vsftpd.
Find the following line in /etc/pam.d/system-auth:
auth sufficient pam_unix.so nullok try_first_pass
and then change it so that it reads as follows:
auth required pam_unix.so nullok try_first_pass
In the same file, comment out or delete the lines:
auth requisite pam_succeed_if.so uid >= 500 quiet
auth required pam_deny.so
To enforce password lockout, add the following to the individual programs'
configuration files in /etc/pam.d. First, add to end of the auth lines:
auth required pam_tally2.so deny=5 onerr=fail
Second, add to the end of the account lines:
account required pam_tally2.so
Adjust the deny argument to conform to your system security policy. The pam_tally2
utility can be used to unlock user accounts as follows: # /sbin/pam_tally2 --user username --reset
Locking out user accounts presents the risk of a denial-of-service attack. The security
policy regarding system lockout must weigh whether the risk of such a denial-of-service
attack outweighs the benefits of thwarting password guessing attacks. The pam_tally2
utility can be run from a cron job on a hourly or daily basis to try and offset this
risk.
denyDeny access if tally for this user exceeds n.313510^[\d]+$lock_timeAlways deny for n seconds after failed attempt.513510^[\d]+$unlock_time
Allow access after n seconds after failed attempt. If this
option is used the user will be locked out for the specified amount of time after
he exceeded his maximum allowed attempts. Otherwise the account is locked until the
lock is removed by a manual intervention of the system administrator.Select time (in seconds) user will be locked out after he exceeded his maximum allowed attempts0190018003600^[\d]+$Set Lockouts for Failed Password AttemptsThe "account lockout threshold" policy should meet minimum requirements.CCE-3410-8(1) via PAMDo not leak information on authorization failureAuthorization failures should not alert attackers as to what went wrong.(1) via /etc/pam.d/system-authDo not log authorization failures and successesRemove pam_succeed_if module with quiet option and remove auth pam_deny line.(1) via /etc/pam.d/system-authUse pam_deny.so to Quickly Deny Access to a Service
In order to deny access to a service SVCNAME via PAM, edit the
file /etc/pam.d/SVCNAME . Prepend this line to the beginning of the file:
auth requisite pam_deny.so
Under most circumstances, there are better ways to disable a service than to
deny access via PAM. However, this should suffice as a way to quickly make a service
unavailable to future users (existing sessions which have already been authenticated,
are not affected). The requisite tag tells PAM that, if the named module returns
failure, authentication should fail, and PAM should immediately stop processing the
configuration file. The pam_deny.so module always returns failure regardless of its
input.Restrict Execution of userhelper to Console Users
If your environment has defined a group, usergroup containing
all the human users of your system, restrict execution of the userhelper program to only
that group: # chgrp usergroup /usr/sbin/userhelper
# chmod 4710 /usr/sbin/userhelper
The userhelper program provides authentication for graphical services which must run
with root privileges, such as the system-config- family of graphical configuration
utilities. Only human users logged into the system console are likely to ever have a
legitimate need to run these utilities. This step provides some protection against
possible flaws in userhelper's implementation, and against further privilege escalation
when system accounts are compromised. See Section 2.3.2.2 for more information on
creating a group of human users.
The userhelper program is configured by the files in /etc/security/console.apps/. Each
file specifies, for some program, what user the program should run as, and what program
should be executed after successful authentication.
Note: The configuration in /etc/security/console.apps/ is applied in
combination with the PAM configuration of the service defined in /etc/pam.d/. First,
userhelper determines what user the service should run as. (Typically, this will be
root.) Next, userhelper uses the PAM API to allow the user who ran the program to
attempt to authenticate as the desired user. The PAM API exchange is wrapped in a GUI if
the application's configuration requests one.Name of group containing human usersEnter group to aggregate human usersusergroupusergroupuserhelper file permissionsEnter file permissions for /usr/sbin/userhelperEnter file permission for /usr/bin/userhelper100111001000100111001000^[10]+$Restrict Execution of userhelper to Console UsersThe /usr/sbin/userhelper file should be owned by the appropriate group.CCE-4185-5# chgrp usergroup /usr/sbin/userhelperRestrict File permissions of userhelperFile permissions for /usr/sbin/userhelper should be set correctly.CCE-3952-9# chmod 4710 /usr/sbin/userhelperPassword Hashing Algorithm
The default algorithm for storing password hashes should be SHA-512.
Password hashing algorithmEnter /etc/shadow password hashing algorithmEnter /etc/shadow password hashing algorithmsha512md5sha256sha512md5sha256sha512Password hashing algorithmThe password hashing algorithm should be set to SHA-512/usr/sbin/authconfig --passalgo=sha512 --updateLimit Password Reuse
Do not allow users to reuse recent passwords. This can be
accomplished by using the remember option for the pam_unix PAM module. In order to
prevent a user from re-using any of his or her last passwords,
append remember= to the password line which uses the
pam_unix module in the file /etc/pam.d/system-auth, as shown:
password sufficient pam_unix.so existing_options remember=
Old (and thus no longer valid) passwords are stored in the file /etc/security/opasswd.
remember
The last n passwords for each user are saved in
/etc/security/opasswd in order to force password change history and keep the user from
alternating between the same password too frequently. Enter how many last passwords will be saved to keep the user from alternating between the same password too frequently5510^[\d]+$Limit password reuseThe passwords to remember should be set to: Secure Session Configuration Files for Login Accounts
When a user logs into a Unix account, the system configures the
user's session by reading a number of files. Many of these files are located in the user's
home directory, and may have weak permissions as a result of user error or
misconfiguration. If an attacker can modify or even read certain types of account
configuration information, he can often gain full access to the affected user's account.
Therefore, it is important to test and correct configuration file permissions for
interactive accounts, particularly those of privileged users such as root or system
administrators.Ensure that No Dangerous Directories Exist in Roots Path '
The active path of the root account can be obtained by starting
a new root shell and running: # echo $PATH
This will produce a colon-separated list of directories in the path. For each directory
DIR in the path, ensure that DIR is not equal to a single . character. Also ensure that
there are no 'empty' elements in the path, such as in these examples: PATH=:/bin
PATH=/bin:
PATH=/bin::/sbin
These empty elements have the same effect as a single . character.
For each element in the path, run: # ls -ld DIR
and ensure that write permissions are disabled for group and other.
It is important to prevent root from executing unknown or untrusted programs, since such
programs could contain malicious code. Therefore, root should not run programs installed
by unprivileged users. Since root may often be working inside untrusted directories, the
. character, which represents the current directory, should never be in the root path,
nor should any directory which can be written to by an unprivileged or semi-privileged
(system) user.
It is a good practice for administrators to always execute privileged
commands by typing the full path to the command.Ensure that No Dangerous Directories Exist in Root's PathThe PATH variable should be set correctly for user rootCCE-3301-9Write permissions are disabled for group and other in all directories in Root's PathCheck each directory in root's path and make use it does not grant write permission to group and otherEnsure that User Home Directories are not Group-Writable or
World-Readable
For each human user USER of the system, view the permissions of the
user's home directory: # ls -ld /home/USER
Ensure that the directory is not group-writable and that it is not world-readable. If
necessary, repair the permissions:# chmod g-w /home/USER
# chmod o-rwx /home/USER
User home directories contain many
configuration files which affect the behavior of a user's account. No user should ever
have write permission to another user's home directory. Group shared directories can be
configured in subdirectories or elsewhere in the filesystem if they are needed.
Typically, user home directories should not be world-readable. If a subset of users need
read access to one another's home directories, this can be provided using groups.Sections 2.3.4.2–2.3.4.5 recommend modifying user home
directories. Notify your user community, and solicit input if appropriate, before making
this type of change. Ensure that User Home Directories are not Group-Writable or World-ReadableFile permissions should be set correctly for the home directories for all user accounts.CCE-4090-7Ensure that User Dot-Files are not World-writable
For each human user USER of the system, view the permissions of
all dot-files in the user's home directory: # ls -ld /home/USER /.[A-Za-z0-9]*
Ensure that none of these files are group- or world-writable. Correct each misconfigured file
FILE by executing: # chmod go-w /home/USER /FILE
A user who can modify another user's configuration files can likely execute commands
with the other user's privileges, including stealing data, destroying files, or
launching further attacks on the system.Ensure that Users Have Sensible Umask ValuesEdit the global configuration files /etc/bashrc and /etc/csh.cshrc.
Add or correct the line: umask View the additional configuration files /etc/csh.login and /etc/profile.d/*,
and ensure that none of these files redefine the umask to a more permissive value
unless there is a good reason for it.
With a default umask setting of 077, files and directories created by users will not be
readable by any other user on the system. Users who wish to make specific files group-
or world-readable can accomplish this using the chmod command. Additionally, users can
make all their files readable to their group by default by setting a umask of 027 in
their shell configuration files. If default per-user groups exist (that is, if every
user has a default group whose name is the same as that user's username and whose only
member is the user), then it may even be safe for users to select a umask of 007, making
it very easy to intentionally share files with group s of which the user is a member.
In addition, it may be necessary to change root's umask temporarily in order to install
software or files which must be readable by other users, or to change the default umasks
of certain service accounts such as the FTP user. However, setting a restrictive default
protects the files of users who have not taken steps to make their files more available,
and preventing files from being inadvertently shared.Sensible umaskEnter default user umaskEnter default user umask002002007022027077^0?[0-7][0-7][0-7]?$Ensure that Users Have Sensible Umask Values in /etc/bashrcThe default umask for all users for the bash shell should be set to:
CCE-3844-8Ensure that Users Have Sensible Umask Values in /etc/csh.cshrcThe default umask for all users for the csh shell should be set to:
CCE-4227-5Ensure that Users do not Have .netrc Files
For each human user USER of the system, ensure that the user
has no .netrc file. The command: # ls -l /home/USER /.netrc
should return the error 'No such file or directory'. If any user has such a file,
approach that user to discuss removing this file.
The .netrc file is a configuration file used to make unattended
logins to other systems via FTP. When this file exists, it frequently contains
unencrypted passwords which may be used to attack other systems.Check for existance of .netrc fileNo user directory should contain file .netrcrm .netrcProtect Physical Console Access
It is impossible to fully protect a system from an attacker with
physical access, so securing the space in which the system is located should be considered
a necessary step. However, there are some steps which, if taken, make it more difficult
for an attacker to quickly or undetectably modify a system from its console.Set BIOS Password
The BIOS (on x86 systems) is the first code to execute during
system startup and controls many important system parameters, including which devices
the system will try to boot from, and in which order. Assign a password to prevent any
unauthorized changes to the BIOS configuration. The exact steps will vary depending on
your machine, but are likely to include:Reboot the machine.Press the appropriate key during the initial boot screen (F2 is typical)Navigate the BIOS configuration menu to add a password.
The exact process will be system-specific and the system's
hardware manual may provide detailed instructions. This password should prevent
attackers with physical access from attempting to change important parameters, such as
those described in Sections 2.5.2.2.1 and 2.2.2.2.4. However, an attacker with physical
access can usually clear the BIOS password. The password should be written down and
stored in a physically-secure location, such as a safe, in the event that it is
forgotten and must be retrieved.Set Boot Loader Password
During the boot process, the boot loader is responsible for
starting the execution of the kernel and passing options to it. The boot loader allows
for the selection of different kernels – possibly on different partitions or media.
Options it can pass to the kernel include 'single-user mode,' which provides root access
without any authentication, and the ability to disable SELinux. To prevent local users
from modifying the boot parameters and endangering security, the boot loader
configuration should be protected with a password.
The default Fedora boot loader for x86 systems is called GRUB. To protect its
configuration: Select a password and then generate a hash from it by running: # grub-md5-crypt Insert the following line into /etc/grub.conf immediately after the header
comments. (Use the output from grub-md5-crypt as the value of password-hash ): password --md5 password-hash Verify the permissions on /etc/grub.conf (which is a symlink to ../boot/grub/grub.conf):
# chown root:root /boot/grub/grub.conf
# chmod 600 /boot/grub/grub.conf
Boot loaders for other platforms should offer a similar password protection feature.User that owns /boot/grub/grub.confChoose user that should own /boot/grub/grub.confrootrootGroup that owns /boot/grub/grub.confChoose group that should own /boot/grub/grub.confrootrootpermissions on /boot/grub/grub.confChoose file permissions on /boot/grub/grub.conf110000000110000000^[01]+$Set Boot Loader user ownerThe /boot/grub/grub.conf file should be owned by root.CCE-4144-2chown root /boot/grub/grub.confSet Boot Loader group ownerThe /boot/grub/grub.conf file should be owned by group root.CCE-4197-0chown :root /boot/grub/grub.confSet permission on /boot/grub/grub.confFile permissions for /boot/grub/grub.conf should be set correctly.CCE-3923-0chmod 600 /boot/grub/grub.confSet Boot Loader PasswordThe grub boot loader should have password protection enabledCCE-3818-2Edit /boot/grub/grub.confRequire Authentication for Single-User Mode
Single-user mode is intended as a system recovery method,
providing a single user root access to the system by providing a boot option at startup.
By default, no authentication is performed if single-user mode is selected. This
provides a trivial mechanism of bypassing security on the machine and gaining root
access.
To require entry of the root password even if the system is started in
single-user mode, add the following line to the /etc/inittab file:
~~:S:wait:/sbin/suloginRequire Authentication for Single-User ModeThe requirement for a password to boot into single-user mode should be enabled.CCE-4241-6(1) via /etc/inittabDisable Interactive Boot
Edit the file /etc/sysconfig/init. Add or correct the setting:
PROMPT=no
The PROMPT option allows the console user to perform an interactive system
startup, in which it is possible to select the set of services which are started on
boot. Using interactive boot, the console user could disable auditing, firewalls, or
other services, weakening system security.Disable Interactive BootThe ability for users to perform interactive startups should be disabled.CCE-4245-7(1) via /etc/sysconfig/initImplement Inactivity Time-out for Login Shells
If the system does not run X Windows, then the login shells can
be configured to automatically log users out after a period of inactivity. The following
instructions are not practical for systems which run X Windows, as they will close
terminal windows in the X environment. For information on how to automatically lock
those systems, see Section 2.3.5.6.
To implement a 15-minute idle time-out for the
default /bin/bash shell, create a new file tmout.sh in the directory /etc/profile.d with
the following lines:
TMOUT=900
readonly TMOUT
export TMOUT
To implement a 15-minute idle
time-out for the tcsh shell, create a new file autologout.csh in the directory
/etc/profile.d with the following line:
set -r autologout 15
Similar actions should be taken for any other login shells used.
The example time-out here of 15 minutes should be
adjusted to whatever your security policy requires. The readonly line for bash and the
-r option for tcsh can be omitted if policy allows users to override the value.
The automatic shell logout only occurs when the shell is the foreground process. If, for
example, a vi session is left idle, then automatic logout would not occur.
When logging in through a remote connection, as with SSH, it may be more effective to set
the timeout value directly through that service. To learn how to set automatic timeout
intervals for SSH, see Section 3.5.2.3.Inactivity timoutChoose allowed duration of inactive SSH connections, shells, and X sessionsChoose allowed duration of inactive SSH connections, shells and X sessions in minutes1501015^[\d]+$Implement Inactivity Time-out for Login ShellsThe idle time-out value for the default /bin/tcsh shell should be:
CCE-3689-7(1) via /etc/profile.d/autologout.cshImplement Inactivity Time-out for Login ShellsThe idle time-out value for the default /bin/bash shell should be:
Time out is in secondsCCE-3707-7(1) via /etc/profile.d/tmout.shConfigure Screen Locking
When a user must temporarily leave an account logged-in, screen
locking should be employed to prevent passersby from abusing the account. User education
and training is particularly important for screen locking to be effective.
A policy should be implemented that trains all users to lock the screen when they plan to
temporarily step away from a logged-in account. Automatic screen locking is only meant
as a safeguard for those cases where a user forgot to lock the screen.Configure GUI Screen Locking
In the default GNOME desktop, the screen can be locked by
choosing Lock Screen from the System menu.
The gconftool-2 program can be used to
enforce mandatory screen locking settings for the default GNOME environment. Run the
following commands to enforce idle activation of the screen saver, screen locking, a
blank-screen screensaver, and 15-minute idle activation time:
# gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type bool \
--set /apps/gnome-screensaver/idle_activation_enabled true
# gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type bool \
--set /apps/gnome-screensaver/lock_enabled true
# gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type string \
--set /apps/gnome-screensaver/mode blank-only
# gconftool-2 --direct \
--config-source xml:readwrite:/etc/gconf/gconf.xml.mandatory \
--type int \
--set /apps/gnome-screensaver/idle_delay 15
The default setting of 15 minutes for idle
activation is reasonable for many office environments, but the setting should conform
to whatever policy is defined. The screensaver mode blank-only is selected to conceal
the contents of the display from passersby.
Because users should be trained to lock
the screen when they step away from the computer, the automatic locking feature is
only meant as a backup. The Lock Screen icon from the System menu can also be dragged
to the taskbar in order to facilitate even more convenient screen-locking.
The root
account cannot be screen-locked, but this should have no practical effect as the root
account should never be used to log into an X Windows environment, and should only be
used to for direct login via console in emergency circumstances.
For more information
about configuring GNOME screensaver, see http://live.gnome.org/GnomeScreensaver. For
more information about enforcing preferences in the GNOME environment using the GConf
configuration system, see http://www.gnome.org/projects/gconf and the man page
gconftool-2(1).Implement Inactivity Time-out for Login ShellsThe idle time-out value for period of inactivity gnome desktop lockout should be 15 minutesCCE-3315-9(1) via gconftool-2Implement idle activation of screen saverIdle activation of the screen saver should be enabled(1) via gconftool-2Implement idle activation of screen lockIdle activation of the screen lock should be enabled(1) via gconftool-2Implement blank screen saverThe screen saver should be blank(1) via gconftool-2Configure Console Screen Locking
A console screen locking mechanism is provided in the vlock
package, which is not installed by default. If the ability to lock console screens is
necessary, install the vlock package: # yum install vlock
Instruct users to invoke the
program when necessary, in order to prevent passersby from abusing their login: $ vlock
The -a option can be used to prevent switching to other virtual consoles.Configure console screen lockingThe vlock package should be installedCCE-3910-7yum install vlockDisable Unnecessary Ports
Though unusual, some systems may be managed only remotely and yet
also exposed to risk from attackers with direct physical access to them. In these cases,
reduce an attacker’s access to the system by disabling unnecessary external ports (e.g.
USB, FireWire, NIC) in the system’s BIOS.
Disable ports on the system which are not necessary for normal system operation. The exact
steps will vary depending on your machine, but are likely to include:
Reboot the machine.Press the appropriate key during the initial boot screen (F2 is typical). Navigate the BIOS conguration menu to disable ports, such as USB, FireWire, and NIC.Disabling USB ports is particularly unusual and will cause problems
for important input devices such as keyboards or mice attached to the system.Use a Centralized Authentication Service
A centralized authentication service is any method of maintaining
central control over account and authentication data and of keeping this data synchronized
between machines. Such services can range in complexity from a script which pushes
centrally-generated password files out to all machines, to a managed scheme such as LDAP
or Kerberos.
If authentication information is not centrally managed, it quickly becomes
inconsistent, leading to out-of-date credentials and forgotten accounts which should have
been deleted. In addition, many older protocols (such as NFS) make use of the UID to
identify users over a network. This is not a good practice, and these protocols should be
avoided if possible. However, since most sites must still make use of some older
protocols, having consistent UIDs and GIDs site-wide is a significant benefit.
Centralized
authentication services do have the disadvantage that authentication information must be
transmitted over a network, leading to a risk that credentials may be intercepted or
manipulated. Therefore, these services must be deployed carefully. The following
precautions should be taken when configuring any authentication service: Ensure that authentication information and any sensitive account information
are never sent over the network unencrypted.Ensure that the root account has a local password, to allow recovery in case
of network outage or authentication server failure.
This guide recommends
the use of LDAP. Secure configuration of OpenLDAP for clients and servers is described in
Section 3.12. Kerberos is also a good choice for a centralized authentication service, but
a description of its configuration is beyond the scope of this guide. The NIS service is
not recommended, and should be considered obsolete. (See Section 3.2.4.)Warning Banners for System Accesses
Each system should expose as little information about itself as
possible.
System banners, which are typically displayed just before a login prompt, give
out information about the service or the host's operating system. This might include the
distribution name and the system kernel version, and the particular version of a network
service. This information can assist intruders in gaining access to the system as it can
reveal whether the system is running vulnerable software. Most network services can be
configured to limit what information is displayed.
Many organizations implement security
policies that require a system banner provide notice of the system's ownership, provide
warning to unauthorized users, and remind authorized users of their consent to monitoring.login banner verbiageEnter an appropriate login banner for your organizationEnter an appropriate login banner for your organizationModify the System Login Banner
The contents of the file /etc/issue are displayed on the screen
just above the login prompt for users logging directly into a terminal. Remote login
programs such as SSH or FTP can be configured to display /etc/issue as well.
Instructions for configuring each server daemon to show this file can be found in the
relevant sections of Chapter 3.
By default, the system will display the version of the
OS, the kernel version, and the host name.
Edit /etc/issue. Replace the default text
with a message compliant with the local site policy or a legal disclaimer.Modify the System Login BannerThe system login banner text should be: ""CCE-4060-0Take value of DOD_text and put it in /etc/issueImplement a GUI Warning Banner
In the default graphical environment, users logging directly
into the system are greeted with a login screen provided by the GNOME display manager.
The warning banner should be displayed in this graphical environment for these
users.
The files for the default RHEL theme can be found in
/usr/share/gdm/themes/RHEL. Add the following sample block of XML to
/usr/share/gdm/themes/RHEL/RHEL.xml after the first two "pixmap"
entries:
<item type="rect">
<pos anchor="n" x="50%" y="10" width="box" height="box"/>
<box>
<item type="label">
<normal font="Sans 14" color="#ffffff"/>
<text>Insert the text of your warning banner here.</text>
</item>
</box>
</item>
The
full syntax that GDM theme files expect is documented elsewhere, but the above XML will
create a text box centered at the top of the screen. The font, text color, and exact
positioning can all be easily modified by editing the appropriate values. The latest
current GDM theme manual can be found at http://www.gnome.org/
projects/gdm/docs/thememanual.html.
Implement a GUI Warning BannerThe direct gnome login warning banner text should be: ""CCE-4188-9(1) via RHEL.xmlSELinux
SELinux is a feature of the Linux kernel which can be used to guard
against misconfigured or compromised programs. SELinux enforces the idea that programs
should be limited in what files they can access and what actions they can take.
The default
SELinux policy, as configured on RHEL5, has been sufficiently developed and debugged that it
should be usable on almost any Red Hat machine with minimal configuration and a small amount
of system administrator training. This policy prevents system services — including most of
the common network-visible services such as mail servers, ftp servers, and DNS servers —
from accessing files which those services have no valid reason to access. This action alone
prevents a huge amount of possible damage from network attacks against services, from
trojaned software, and so forth.
This guide recommends that SELinux be enabled using the
default (targeted) policy on every Red Hat system, unless that system has requirements which
make a stronger policy appropriate.Frank Mayer, K. M., and Caplan, D. SELinux by Example: Using Security Enhanced LinuxHow SELinux Works
In the traditional Linux/Unix security model, known as
Discretionary Access Control (DAC), processes run under a user and group identity, and
enjoy that user and group's access rights to all files and other objects on the system.
This system brings with it a number of security problems, most notably: that processes
frequently do not need and should not have the full rights of the user who ran them; that
user and group access rights are not very granular, and may require administrators to
allow too much access in order to allow the access that is needed; that the Unix
filesystem contains many resources (such as temporary directories and world-readable
files) which are accessible to users who have no legitimate reason to access them; and
that legitimate users can easily provide open access to their own resources through
confusion or carelessness.
SELinux provides a Mandatory Access Control (MAC) system that
greatly augments the DAC model. Under SELinux, every process and every object (e.g. file,
socket, pipe) on the system is given a security context, a label which include detailed
type information about the object. The kernel allows processes to access objects only if
that access is explicitly allowed by the policy in effect. The policy defines transitions,
so that a user can be allowed to run software, but the software can run under a different
context than the user's default. This automatically limits the damage that the software
can do to files accessible by the calling user — the user does not need to take any action
to gain this benefit.
For an action to occur, both the traditional DAC permissions must be
satisifed as well as SELinux's MAC rules. If either do not permit the action, then it will
not be allowed. In this way, SELinux rules can only make a system's permissions more
restrictive and secure.
SELinux requires a complex policy in order to allow all the
actions required of a system under normal operation. Three such policies have been
designed for use with RHEL5, and are included with the system. In increasing order of
power and complexity, they are: targeted, strict, and mls. The targeted SELinux policy
consists mostly of Type Enforcement (TE) rules, and a small number of Role-Based Access
Control (RBAC) rules. It restricts the actions of many types of programs, but leaves
interactive users largely unaffected. The strict policy also uses TE and RBAC rules, but
on more programs and more aggressively. The mls policy implements Multi-Level Security
(MLS), which introduces even more kinds of labels — sensitivity and category — and rules
that govern access based on these.
The remainder of this section provides guidance for the
configuration of the targeted policy and the administration of systems under this policy.
Some pointers will be provided for readers who are interested in further strengthening
their systems by using one of the stricter policies provided with RHEL5 or in writing
their own policy.Enable SELinux
Edit the file /etc/selinux/config. Add or correct the following
lines: SELINUX=enforcing
SELINUXTYPE=targeted
Edit the file /etc/grub.conf. Ensure that
the following arguments DO NOT appear on any kernel command line in the file: selinux=0
enforcing=0
The directive SELINUX=enforcing enables SELinux at boot time. If SELinux is
causing a lot of problems or preventing the system from booting, it is possible to boot
into the warning-only mode SELINUX=permissive for debugging purposes. Make certain to
change the mode back to enforcing after debugging, set the filesystems to be relabelled
for consistency using the command touch /.autorelabel, and reboot.
However, the RHEL5
default SELinux configuration should be sufficiently reasonable that most systems will
boot without serious problems. Some applications that require deep or unusual system
privileges, such as virtual machine software, may not be compatible with SELinux in its
default configuration. However, this should be uncommon, and SELinux's application support
continues to improve. In other cases, SELinux may reveal unusual or insecure program
behavior by design.
The directive SELINUXTYPE=targeted configures SELinux to use the
default targeted policy. See Section 2.4.6 if a stricter policy is appropriate for your
site.
The SELinux boot mode specified in /etc/selinux/config can be overridden by
command-line arguments passed to the kernel. It is necessary to check grub.conf to ensure
that this has not been done and to protect the bootloader as described in Section 2.3.5.2.SELinux state
enforcing - SELinux security policy is enforced.
permissive - SELinux prints warnings instead of enforcing.
disabled - SELinux is fully disabled.
Set the SELinux stateenforcingenforcingpermissivedisabledenforcing|permissive|disabledenforcingpermissivedisabledSELinux policy
Type of policy in use. Possible values are:
targeted - Only targeted network daemons are protected.
strict - Full SELinux protection.
mls - Multiple levels of securitySet the SELinux policytargetedtargetedstrictmlstargeted|strict|mlstargetedstrictmlsEnsure SELinux is Properly Enabled
Run the command:$ /usr/sbin/sestatus
If the system is properly configured, the output should indicate:SELinux status: enabledCurrent mode: enforcingMode from config file: enforcingPolicy from config file: targetedEnsure SELinux is Properly EnabledCheck output of /usr/sbin/sestatusEnable SELinux in /etc/grub.confSELinux should NOT be disabled in /etc/grub.conf. Check that selinux=0 is not foundCCE-3977-6Remove offending line from /etc/grub.confEnable SELinux enforcement in /etc/grub.confSELinux enforcement should NOT be disabled in /etc/grub.conf. Check that enforcing=0 is not found.Remove offending line from /etc/grub.confSet the SELinux stateThe SELinux state should be: Edit /etc/selinux/configSet the SELinux policyThe SELinux policy should be set appropriately.CCE-3624-4Edit /etc/selinux/configDisable Unnecessary SELinux Daemons
Several daemons are installed by default as part of the RHEL5
SELinux support mechanism. These daemons may improve the system's ability to enforce
SELinux policy in a useful fashion, but may also represent unnecessary code running on the
machine, increasing system risk. If these daemons are not needed on your system, they
should be disabled.Disable and Remove SETroubleshoot if Possible
Is there a mission-critical reason to allow users to view
SELinux denial information using the sealert GUI? If not, disable the service and remove
the RPM: # chkconfig setroubleshoot off
# yum erase setroubleshoot
The setroubleshoot
service is a facility for notifying the desktop user of SELinux denials in a
user-friendly fashion. SELinux errors may provide important information about intrusion
attempts in progress, or may give information about SELinux configuration problems which
are preventing correct system operation. In order to maintain a secure and usable
SELinux installation, error logging and notification is necessary.
However,
setroubleshoot is a service which has complex functionality, which runs a daemon and
uses IPC to distribute information which may be sensitive, or even to allow users to
modify SELinux settings, and which does not yet implement real authentication
mechanisms. This guide recommends disabling setroubleshoot and using the kernel audit
functionality to monitor SELinux's behavior.
In addition, since setroubleshoot
automatically runs client-side code whenever a denial occurs, regardless of whether the
setroubleshootd daemon is running, it is recommended that the program be removed
entirely unless it is needed.Remove SETroubleshoot if PossibleThe setroubleshoot package should be uninstalled.CCE-4148-3(1) via yumDisable SETroubleshoot if PossibleThe setroubleshoot service should be disabled.CCE-4254-9(1) via chkconfigDisable MCS Translation Service (mcstrans) if Possible
Unless there is some overriding need for the convenience of
category label translation, disable the MCS translation service: # chkconfig mcstrans off
The mcstransd daemon provides the category label translation information defined in
/etc/selinux/targeted/ setrans.conf to client processes which request this information.
Category labelling is unlikely to be used except in sites with special requirements.
Therefore, it should be disabled in order to reduce the amount of potentially vulnerable
code running on the system. See Section 2.4.6 for more information about systems which
use category labelling.Disable MCS Translation Service (mcstrans) if PossibleThe mcstrans service should be disabled.CCE-3668-1(1) via chkconfigRestorecon Service (restorecond)
The restorecond daemon monitors a list of files which are
frequently created or modified on running systems, and whose SELinux contexts are not
set correctly. It looks for creation events related to files listed in /etc/
selinux/restorecond.conf, and sets the contexts of those files when they are discovered.
The restorecond program is fairly simple, so it brings low risk, but, in its default
configuration, does not add much value to a system. An automated program such as
restorecond may be used to monitor problematic files for context problems, or system
administrators may be trained to check file contexts of newly-created files using the
command ls -lZ, and to repair contexts manually using the restorecon command.
This guide
makes no recommendation either for or against the use of restorecond.Disable restorecon Service (restorecond)The restorecond service should be disabled.CCE-4129-3(1) via chkconfigCheck for Unconfined Daemons
Daemons that SELinux policy does not know about will inherit the
context of the parent process. Because daemons are launched during startup and descend
from the init process, they inherit the initrc t context. This is a problem because it may
cause AVC denials, or it could allow privileges that the daemon does not require.
To check for unconfined daemons, run the following command: # ps -eZ | egrep "initrc" | egrep -vw "tr|ps|egrep|bash|awk" | tr ':' ' ' | awk '{ print $NF }'
It should produce no output in a well-configured system.Check for Unconfined Daemons
Device files are used for communication with important system
resources. SELinux contexts should exist for these. If a device file is not labeled, then
misconfiguration is likely.
To check for unlabeled device files, run the following command:# ls -Z | grep unlabeled_t
It should produce no output in a well-configured system.Check for Unconfined DaemonsCheck for device file that is not labeled.(1) via chkconfigDebugging SELinux Policy Errors
SELinux's default policies have improved significantly over time,
and most systems should have few problems using the targeted SELinux policy. However,
policy problems may still occasionally prevent accesses which should be allowed. This is
especially true if your site runs any custom or heavily modified applications.
This section gives some brief guidance on discovering and repairing SELinux-related access
problems. Guidance given here is necessarily incomplete, but should provide a starting
point for debugging.
If you suspect that a permission error or other failure may be caused
by SELinux (and are certain that misconfiguration of the traditional Unix permissions are
not the cause of the problem), search the audit logs for AVC events: # ausearch -m AVC,USER_AVC -sv no
The output of this command will be a set of events. The timestamp,
along with the comm and pid fields, should indicate which line describes the problem.
Look
up the context under which the process is running. Assuming the process ID is PID , find
the context by running: # ps -p PID -Z
The AVC denial message should identify the
offending file or directory. The name field should contain the filename (not the full
pathname by default), and the ino field can be used to search by inode, if necessary.
Assuming the file is FILE , find its SELinux context: # ls -Z FILE
An administrator should
suspect an SELinux misconfiguration whenever a program gets a 'permission denied' error
but the standard Unix permissions appear to be correct, or a program fails mysteriously on
a task which seems to involve file access or network communication.
As described in
Section 2.4.1, SELinux augments each process with a context providing detailed type
information about that process. The contexts under which processes run may be referred to
as subject contexts. Similarly, each filesystem object is given a context.
The targeted
policy consists of a set of rules, each of which allows a subject type to perform some
operation on a given object type. The kernel stores information about these access
decisions in an structure known as an Access Vector Cache (AVC), so authorization
decisions made by the system are audited with the type AVC. It is also possible for
userspace modules to implement their own policies based on SELinux, and these decisions
are audited with the type USER_AVC.
AVC denials are logged by the kernel audit facility
(see Section 2.6.2 for configuration guidance on this subsystem) and may also be visible
via setroubleshoot. This guide recommends the use of the audit userspace utilities to find
AVC errors. It is possible to manually locate these errors by looking in the file
/var/log/audit/audit.log or in /var/log/messages (depending on the syslog configuration in
effect), but the ausearch tool allows finegrained searching on audit event types, which
may be necessary if system call auditing is enabled as well. The command line above tells
ausearch to look for kernel or userspace AVC messages (-m AVC,USER AVC) where the access
attempt did not succeed (-sv no).
If an AVC denial occurs when it should not have, the
problem is generally one of the following: The program is running with the wrong subject
context. This could happen as a result of an incorrect context on the program's executable
file, which could happen if 3rd party software is installed and not given appropriate
SELinux file contexts. The file has the wrong object context because the current file's
context does not match the specification. This can occur when files are created or
modified in certain ways. It is not atypical for configuration files to get the wrong
contexts after a system configuration change performed by an administrator. To repair the
file, use the command: # restorecon -v FILE
This should produce output indicating that the
file's context has been changed. The /usr/bin/chcon program can be used to manually change
a file's context, but this is problematic because the change will not persist if it does
not agree with the policy-defined contexts applied by restorecon.The file has the wrong
object context because the specification is either incorrect or does not match the way the
file is being used on this system. In this case, it will be necessary to change the system
file contexts.
Run the system-config-selinux tool, and go to the 'File Labeling' menu.
This will give a list of files and wildcards corresponding to file labelling rules on the
system. Add a rule which maps the file in question to the desired context. As an
alternative, file contexts can be modified from the command line using the semanage(8)
tool.The program and file have the correct contexts, but the policy should allow some
operation between those two contexts which is currently not allowed. In this case, it will
be necessary to modify the SELinux policy.
Run the system-config-selinux tool, and go to
the 'Boolean' menu. If your configuration is supported, but is not the Red Hat default,
then there will be a boolean allowing real-time modification of the SELinux policy to fix
the problem. Browse through the items in this menu, looking for one which is related to
the service which is not working. As an alternative, SELinux booleans can be modified from
the command line using the getsebool(8) and setsebool(8) tools.
If there is no boolean, it
will be necessary to create and load a policy module. A simple way to build a policy
module is to use the audit2allow tool. This tool can take input in the format of AVC
denial messages, and generate syntactically correct Type Enforcement rules which would be
sufficient to prevent those denials. For example, to generate and display rules which
would allow all kernel denials seen in the past five minutes, run: # ausearch -m AVC -sv no -ts recent | audit2allow
It is possible to use audit2allow to directly create a module
package suitable for loading into the kernel policy. To do this, invoke audit2allow with
the -M flag: # ausearch -m AVC -sv no -ts recent | audit2allow -M localmodule
If this is
successful, several lines of output should appear. Review the generated TE rules in the
file localmodule .te and ensure that they express what you wish to allow.
The file
localmodule .pp should also have been created. This file is a policy module package that
can be loaded into the kernel. To do so, use system-config-selinux, go to the 'Policy
Module' menu and use the 'Add' button to enable your module package in SELinux, or load it
from the command line using semodule(8): # semodule -i localmodule .pp
Section 45.2 of [9] covers this procedure in detail.Further Strengthening
The recommendations up to this point have discussed how to
configure and maintain a system under the default configuration of the targeted policy,
which constrains only the actions of daemons and system software. This guide strongly
recommends that any site which is not currently using SELinux at all transition to the
targeted policy, to gain the substantial security benefits provided by that policy.
However, the default policy provides only a subset of the full security gains available
from using SELinux. In particular, the SELinux policy is also capable of constraining the
actions of interactive users, of providing compartmented access by sensitivity level (MLS)
and/or category (MCS), and of restricting certain types of system actions using booleans
beyond the RHEL5 defaults.
This section introduces other uses of SELinux which may be
possible, and provides links to some outside resources about their use. Detailed
description of how to implement these steps is beyond the scope of this guide.Strengthen the Default SELinux Boolean Configuration
SELinux booleans are used to enable or disable segments of
policy to comply with site policy. Booleans may apply to the entire system or to an
individual daemon. For instance, the boolean allow execstack, if enabled, allows
programs to make part of their stack memory region executable. This would apply to all
programs on the system. The boolean ftp home dir allows ftpd processes to access user
home directories, and applies only to daemons which implement FTP.
The command $ getsebool -a
lists the values of all SELinux booleans on the system. Section 2.4.5
discussed loosening boolean values in order to debug functionality problems which occur
under more restrictive defaults. It is also useful to examine and strengthen the boolean
settings, to disable functionality which is not required by legitimate programs on your
system, but which might be symptomatic of an attack.
See the manpages booleans(8),
getsebool(8), and setsebool(8) for general information about booleans. There are also
manual pages for several subsystems which discuss the use of SELinux with those systems.
Examples include ftpd selinux(8), httpd selinux(8), and nfs selinux(8). Another good
reference is the html documentation distributed with the selinux-policy RPM. This
documentation is stored under
/usr/share/doc/selinux-policy-version/html/
The pages
global tunables.html and global booleans.html may be useful when examining booleans.Use a Stronger Policy
Using a stronger policy can greatly enhance security, but will
generally require customization to be compatible with the particular system's purpose,
and this may be costly or time consuming. Under the targeted policy, interactive
processes are given the type unconfined t, so interactive users are not constrained by
SELinux even if they attempt to take strange or malicious actions. The first alternative
policy available with RHEL5's SELinux distribution, called strict, extends the
protections offered by the default policy from daemons and system processes to all
processes. To use the strict policy, first ensure that the policy module is installed: # yum install selinux-policy-strict
Then edit /etc/selinux/config and correct the line:SELINUXTYPE=strict
The mls policy type can be used to enforce sensitivity or category
labelling, and requires site-specific configuration of these labels in order to be
useful. To use this policy, install the appropriate policy module: # yum install selinux-policy-mls
Then edit /etc/selinux/config and correct the line: SELINUXTYPE=mls
Note: Switching between policies typically requires the entire disk to be relabelled, so
that files get the appropriate SELinux contexts under the new policy. Boot with the
additional grub command-line options enforcing=0 single autorelabel
to relabel the disk in single-user mode, then reboot normally.SELinux ReferencesNSA SELinux resources:Web page: http://www.nsa.gov/selinux/Mailing list: selinux@tycho.nsa.gov
List information at: http://www.nsa.gov/selinux/info/list.cfmFedora SELinux resources:FAQ: http://docs.fedoraproject.org/selinux-faq/Wiki: http://fedoraproject.org/wiki/SELinux/Mailing list: fedora-selinux-list@redhat.com
List information at:
https://www.redhat.com/mailman/listinfo/fedora-selinux-listChapters 43–45 of Red Hat Enterprise Linux 5: Deployment Guide [9]The book SELinux by Example: Using Security Enhanced Linux [13]Network Configuration and Firewalls
Most machines must be connected to a network of some sort, and this
brings with it the substantial risk of network attack. This section discusses the security
impact of decisions about networking which must be made when configuring a system.
This section also discusses firewalls, network access controls, and other network security
frameworks, which allow system-level rules to be written that can limit attackers' ability
to connect to your system. These rules can specify that network traffic should be allowed or
denied from certain IP addresses, hosts, and networks. The rules can also specify which of
the system's network services are available to particular hosts or networks.Kernel Parameters which Affect Networking
The sysctl utility is used to set a number of parameters which
affect the operation of the Linux kernel. Several of these parameters are specific to
networking, and the configuration options in this section are recommended.Network Parameters for Hosts Only
Is this system going to be used as a firewall or gateway to
pass IP traffic between different networks?
If not, edit the file /etc/sysctl.conf and add or correct the following lines: net.ipv4.ip_forward = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
These settings disable hosts from
performing network functionality which is only appropriate for routers.Disable net.ipv4.conf.default.send_redirects for Hosts OnlyThe default setting for sending ICMP redirects should be disabled for network interfaces.CCE-4151-7(1) via sysctl - net.ipv4.conf.default.send_redirectsDisable net.ipv4.conf.all.send_redirects for Hosts OnlySending ICMP redirects should be disabled for all interfaces.CCE-4155-8(1) via sysctl - net.ipv4.conf.all.send_redirectsDisable net.ipv4.ip forward for Hosts OnlyIP forwarding should be disabled.CCE-3561-8(1) via sysctl - net.ipv4.ip_forwardNetwork Parameters for Hosts and Routers
Edit the file /etc/sysctl.conf and add or correct the following
lines:
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_messages = 1
net.ipv4.tcp_syncookies = 1
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
These options
improve Linux's ability to defend against certain types of IPv4 protocol attacks.
The
accept source route, accept redirects, and secure redirects options are turned off to
disable IPv4 protocol features which are considered to have few legitimate uses and to
be easy to abuse.
The net.ipv4.conf.all.log martians option logs several types of
suspicious packets, such as spoofed packets, source-routed packets, and redirects.
The icmp echo ignore broadcasts icmp ignore bogus error messages options protect against
ICMP attacks.
The tcp syncookies option uses a cryptographic feature called SYN cookies
to allow machines to continue to accept legitimate connections when faced with a SYN
flood attack. See [12] for further information on this option.
The rp filter option
enables RFC-recommended source validation. It should not be used on machines which are
routers for very complicated networks, but is helpful for end hosts and routers serving
small networks.
For more information on any of these, see the kernel source
documentation file /Documentation/networking/ip-sysctl.txt.2Deactivating "source routed packets"Trackers could be using source-routed packets to generate traffic that seems to be intra-net, but actually was created outside and has been redirected.Enable/Disable source routed packets010ICMP redirect messagesDisable ICMP Redirect Acceptance?Enable/Disable ICMP redirect messages010net.ipv4.conf.all.secure_redirectsEnable to prevent hijacking of routing path by only allowing redirects from gateways known in routing table. Enable/Disable IPv4 prevent hijacking of routing paths110net.ipv4.conf.all.log_martiansDisable so you don't Log Spoofed Packets, Source Routed Packets, Redirect Packets Enable/Disable IPv4 logging Spoofed packets, source routed packets and redirect packets010net.ipv4.conf.default.accept_source_routeDisable IP source routing?Enable/Disable IPv4 source routing010net.ipv4.conf.default.accept_redirectsDisable ICMP Redirect Acceptance?Enable/Disable default IPv4 ICMP Redirect Acceptance010net.ipv4.conf.default.secure_redirectsLog packets with impossible addresses to kernel log?Enable/Disable IPv4 logging packets with impossible addresses to kernel log110net.ipv4.icmp_echo_ignore_broadcastIgnore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicastEnable/Disable IPv4 ignoring ICMP ECHO and TIMESTAMP requests from broadcast/multicast110net.ipv4.icmp_ignore_bogus_error_messagesEnable to prevent certain types of attacks110net.ipv4.tcp_syncookieEnable to turn on TCP SYN Cookie ProtectionEnable/Disable TCP SYN Cookie Protection110net.ipv4.conf.all.rp_filterEnable to enforce sanity checking, also called ingress filtering or egress filtering. The point is to drop a packet if the source and destination IP addresses in the IP header do not make sense when considered in light of the physical interface on which it arrived. Enable/Disable all enforcing sanity checks110net.ipv4.conf.default.rp_filterEnables source route verificationEnable/Disable default source route verification110Set net.ipv4.conf.all.accept_source_route for Hosts and RoutersAccepting source routed packets should be: for all interfaces as appropriate.CCE-4236-6(1) via sysctl - net.ipv4.conf.all.accept_source_routeSet net.ipv4.conf.all.accept_redirects for Hosts and RoutersAccepting ICMP redirects should be: for all interfaces as appropriate.CCE-4217-6(1) via sysctl - net.ipv4.conf.all.accept_redirectsSet net.ipv4.conf.all.secure_redirects for Hosts and RoutersAccepting "secure" ICMP redirects (those from gateways listed in the default gateways list) should be: for all interfaces as appropriate.CCE-3472-8(1) via sysctl - net.ipv4.conf.all.secure_redirectsSet net.ipv4.conf.all.log_martians for Hosts and RoutersLogging of "martian" packets (those with impossible addresses) should be: for all interfaces as appropriate.CCE-4320-8(1) via sysctl - net.ipv4.conf.all.log_martiansSet net.ipv4.conf.default.accept_source_route for Hosts and RoutersThe default setting for accepting source routed packets should be: for all interfaces as appropriate.CCE-4091-5(1) via sysctl - net.ipv4.conf.default.accept_source_routeSet net.ipv4.conf.default.accept_redirects for Hosts and RoutersThe default setting for accepting ICMP redirects should be: for all interfaces as appropriate.CCE-4186-3(1) via sysctl - net.ipv4.conf.default.accept_redirectsSet net.ipv4.conf.default.secure_redirects for Hosts and RoutersThe default setting for accepting "secure" ICMP redirects (those from gateways listed in the default gateways list) should be: for all interfaces as appropriate.CCE-3339-9(1) via sysctl - net.ipv4.conf.default.secure_redirectsSet net.ipv4.icmp_echo_ignore_broadcasts for Hosts and RoutersIgnoring ICMP echo requests (pings) sent to broadcast / multicast addresses should be: for all interfaces as appropriate.CCE-3644-2(1) via sysctl - net.ipv4.icmp_echo_ignore_broadcastsSet net.ipv4.icmp_ignore_bogus_error_messages for Hosts and RoutersIgnoring bogus ICMP responses to broadcasts should be: for all interfaces as appropriate.CCE-4133-5(1) via sysctl - net.ipv4.icmp_ignore_bogus_error_messagesSet net.ipv4.tcp_syncookies for Hosts and RoutersSending TCP syncookies should be: for all interfaces as appropriate.CCE-4265-5(1) via sysctl - net.ipv4.tcp_syncookiesSet net.ipv4.conf.all.rp_filter for Hosts and RoutersPerforming source validation by reverse path should be: for all interfaces as appropriate.CCE-4080-8(1) via sysctl - net.ipv4.conf.all.rp_filterSet net.ipv4.conf.default.rp_filter for Hosts and RoutersThe default setting for performing source validation by reverse path should be: for all interfaces as appropriate.CCE-3840-6(1) via sysctl - net.ipv4.conf.default.rp_filterWireless Networking
Wireless networking (sometimes referred to as 802.11 or Wi-Fi)
presents a serious security risk to sensitive or classified systems and networks. Wireless
networking hardware is much more likely to be included in laptop or portable systems than
desktops or servers. See Section 3.3.14 for information on Bluetooth wireless support.
Bluetooth serves a different purpose and possesses a much shorter range, but it still
presents serious security risks.
Removal of hardware is the only way to absolutely ensure
that the wireless capability remains disabled. If it is completely impractical to remove
the wireless hardware, and site policy still allows the device to enter sensitive spaces,
every effort to disable the capability via software should be made. In general,
acquisition policy should include provisions to prevent the purchase of equipment that
will be used in sensitive spaces and includes wireless capabilities.Remove Wireless Hardware if Possible
Identifying the wireless hardware is the first step in removing
it. The system's hardware manual should contain information on its wireless
capabilities.
Wireless hardware included with a laptop typically takes the form of a
mini-PCI card or PC card. Other forms include devices which plug into USB or Ethernet
ports, but these should be readily apparent and easy to remove from the base system.
A PC Card (originally called a PCMCIA card) is designed to be easy to remove, though it
may be hidden when inserted into the system. Frequently, there will be one or more
buttons near the card slot that, when pressed, eject the card from the system. If no
card is ejected, the slot is empty.
A mini-PCI card is approximately credit-card sized
and typically accessible via a removable panel on the underside of the laptop. Removing
the panel may require simple tools.
In addition to manually inspecting the hardware, it
is also possible to query the system for its installed hardware devices. The commands
/sbin/lspci and /sbin/lsusb will show a list of all recognized devices on their
respective buses, and this may indicate the presence of a wireless device.Disable Wireless Through Software Configuration
If it is impossible to remove the wireless hardware from the
device in question, disable as much of it as possible through software. The following
methods can disable software support for wireless networking, but note that these
methods do not prevent malicious software or careless users from re-activating the
devices.Disable Wireless in BIOS
Some laptops that include built-in wireless support offer the
ability to disable the device through the BIOS. This is system-specific; consult your
hardware manual or explore the BIOS setup during boot. 2A recent version of this file
can be found online at
http://lxr.linux.no/source/Documentation/networking/ip-sysctl.txt.Disable Wireless in BIOSAll wireless devices should be disabled in the BIOS.CCE-3628-5(1) via BIOS menusDeactivate Wireless Interfaces
Deactivating the wireless interfaces should prevent normal
usage of the wireless capability.
First, identify the interfaces available with the command: # ifconfig -a
Additionally,the following command may also be used to
determine whether wireless support ('extensions') is included for a particular
interface, though this may not always be a clear indicator: # iwconfig
After
identifying any wireless interfaces (which may have names like wlan0, ath0, wifi0, or
eth0), deactivate the interface with the command: # ifdown interface
These changes
will only last until the next reboot. To disable the interface for future boots,
remove the appropriate interface file from /etc/sysconfig/network-scripts: # rm /etc/sysconfig/network-scripts/ifcfg-interfaceDeactivate Wireless InterfacesAll wireless interfaces should be disabled.CCE-4276-2rm /etc/sysconfig/network-scripts/ifcfg-interfaceifdown interfaceDisable Wireless Drivers
Removing the kernel drivers that provide support for wireless
Ethernet devices will prevent users from easily activating the devices.
To remove the wireless drivers from the system: # rm -r /lib/modules/kernelversion(s)/kernel/drivers/net/wireless
This command must also be repeated every time the kernel is upgraded.Disable Wireless DriversDevice drivers for wireless devices should be excluded from the kernel.CCE-4170-7(1) via modprobeIPv6
The system includes support for Internet Protocol version 6. A
major and often-mentioned improvement over IPv4 is its enormous increase in the number of
available addresses. Another important feature is its support for automatic configuration
of many network settings.Disable Support for IPv6 unless Needed
Because the IPv6 networking code is relatively new and complex,
it is particularly important that it be disabled unless needed. Despite configuration
that suggests support for IPv6 has been disabled, link-local IPv6 address
autoconfiguration occurs even when only an IPv4 address is assigned. The only way to
effectively prevent execution of the IPv6 networking stack is to prevent the kernel from
loading the IPv6 kernel module.MO3:S0-C1-1Disable Automatic Loading of IPv6 Kernel Module
To prevent the IPv6 kernel module (ipv6) from being loaded,
add the following line to /etc/modprobe.conf:
install ipv6 /bin/true
When the kernel requests the ipv6 module, this line will direct the system to run the
program /bin/true instead.Disable Automatic Loading of IPv6 Kernel ModuleAutomatic loading of the IPv6 kernel module should be disabled.MO3:S0-C1-1 MO3:S0-C1-2CCE-3562-6(1) via /etc/modprobe.confDisable Interface Usage of IPv6
To prevent configuration of IPv6 for all interfaces, add or
correct the following lines in /etc/sysconfig/network:
NETWORKING_IPV6=no
IPV6INIT=no
For each network interface IFACE , add or correct the following lines in
/etc/sysconfig/network-scripts/ifcfg-IFACE as an additional prevention mechanism:
IPV6INIT=no
If it becomes necessary later to configure IPv6, only the interfaces
requiring it should be enabled.Disable NETWORKING_IPV6 in /etc/sysconfig/networkThe default setting for IPv6 configuration should be disabledCCE-3381-1(1) via /etc/sysconfig/networkDisable IPV6INIT in /etc/sysconfig/networkGlobal IPv6 initialization should be disabledCCE-3377-9(1) via /etc/sysconfig/networkDisable IPV6INIT in /etc/sysconfig/network-scripts/ifcfg-*IPv6 configuration should be disabled for all interfaces.CCE-4296-0(1) via /etc/sysconfig/network-scripts/ifcfg-*Configure IPv6 Settings if Necessary
A major feature of IPv6 is the extent to which systems
implementing it can automatically configure their networking devices using information
from the network. From a security perspective, manually configuring important
configuration information is always preferable to accepting it from the network in an
unauthenticated fashion.Disable Automatic Configuration
Disable the system's acceptance of router advertisements and
redirects by adding or correcting the following line in /etc/sysconfig/network (note
that this does not disable sending router solicitations):
IPV6_AUTOCONF=noIPV6_AUTOCONFToggle global IPv6 autoconfiguration (only, if global forwarding is disabled)Enable/Disable global IPv6 autoconfigurationdisabledenableddisabledenabled|disablednet.ipv6.conf.default.accept_raaccept default router advertisementsEnable/Disable IPv6 accepting default router advertisementsnoyesnoyes|nonet.ipv6.conf.default.accept_redirectsToggle ICMP Redirect AcceptanceEnable/Disable IPv6 default ICMP Redirect Acceptancedisabledenableddisabledenabled|disablednet.ipv6.conf.all.accept_redirectsToggle ICMP Redirect AcceptanceEnable/Disable all IPv6 ICMP Redirect Acceptancedisabledenableddisabledenabled|disabledDisable IPV6_AUTOCONF in /etc/sysconfig/networkAccepting IPv6 router advertisements should be disabled for all interfaces.CCE-4269-7(1) via /etc/sysconfig/networkDisable accepting IPv6 router advertisements (net.ipv6.conf.default.accept_ra)The default setting for accepting IPv6 router advertisements should be: for all interfaces.CCE-4291-1(1) via sysctl (2) via IPV6_AUTOCONF in /etc/sysconfig/networkDisable accepting redirects from IPv6 routers (net.ipv6.conf.default.accept_redirects)Accepting redirects from IPv6 routers should be: for all interfaces.CCE-4313-3(1) via sysctl (2) via IPV6_AUTOCONF in /etc/sysconfig/networkDisable accepting redirects from IPv6 routers (net.ipv6.conf.all.accept_redirects)The default setting for accepting redirects from IPv6 routers should be: for all interfaces.CCE-4198-8(1) via sysctl (2) via IPV6_AUTOCONF in /etc/sysconfig/networkManually Assign Global IPv6 Address
To manually assign an IP address for an interface IFACE, edit
the file /etc/sysconfig/network-scripts/ ifcfg-IFACE. Add or correct the following
line (substituting the correct IPv6 address):
IPV6ADDR=2001:0DB8::ABCD/64
Manually
assigning an IP address is preferable to accepting one from routers or from the
network otherwise. The example address here is an IPv6 address reserved for
documentation purposes, as defined by RFC3849.Use Privacy Extensions for Address if Necessary
To introduce randomness into the automatic generation of IPv6
addresses, add or correct the following line in
/etc/sysconfig/network-scripts/ifcfg-IFACE:
IPV6_PRIVACY=rfc3041
Automatically-generated IPv6 addresses are based on the underlying hardware (e.g.
Ethernet) address, and so it becomes possible to track a piece of hardware over its
lifetime using its traffic. If it is important for a system's IP address to not
trivially reveal its hardware address, this setting should be applied.IPV6_PRIVACY in /etc/sysconfig/network-scripts/ifcfg-IFACEControl IPv6 privacy.Select control of IPv6 address creation privacyrfc3041disabledlightweightrfc3041Use Privacy Extensions for Address if NecessaryIPv6 privacy extensions should be: for all interfaces.CCE-3842-2(1) via IPV6_PRIVACY in
/etc/sysconfig/network-scripts/ifcfg-<interface>Manually Assign IPv6 Router Address
Edit the file /etc/sysconfig/network-scripts/ifcfg-IFACE ,
and add or correct the following line (substituting your gateway IP as appropriate):
IPV6_DEFAULTGW=2001:0DB8::0001
Router addresses should be manually set and not
accepted via any autoconfiguration or router advertisement.Limit Network-Transmitted Configuration
Add the following lines to /etc/sysctl.conf to limit the
configuration information requested from other systems, and accepted from the network:
net.ipv6.conf.default.router_solicitations = 0
net.ipv6.conf.default.accept_ra_rtr_pref = 0
net.ipv6.conf.default.accept_ra_pinfo = 0
net.ipv6.conf.default.accept_ra_defrtr = 0
net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.default.dad_transmits = 0
net.ipv6.conf.default.max_addresses = 1
The router solicitations setting determines how many router solicitations are sent
when bringing up the interface. If addresses are statically assigned, there is no need
to send any solicitations.
The accept_ra_pinfo setting controls whether the system will
accept prefix info from the router.
The accept_ra_defrtr setting controls whether the
system will accept Hop Limit settings from a router advertisement. Setting it to 0
prevents a router from changing your default IPv6 Hop Limit for outgoing packets.
The autoconf setting controls whether router advertisements can cause the system to
assign a global unicast address to an interface.
The dad_transmits setting determines how
many neighbor solicitations to send out per address (global and link-local) when
bringing up an interface to ensure the desired address is unique on the network.
The max_addresses setting determines how many global unicast IPv6 addresses can be
assigned to each interface. The default is 16, but it should be set to exactly the
number of statically configured global addresses required. net.ipv6.conf.default.router_solicitations
Setting determines how many router solicitations are
sent when bringing up the interface. If addresses are statically assigned, there
is no need to send any solicitationSelect how many router solicitations are sent when bringing up the interface001Accept Router Preference in Router Advertisements?Control IPv6 privacy.Enable/Disable IPv6 router advertisements010net.ipv6.conf.default.accept_ra_pinfoSetting controls whether the system will accept prefix info from the routerEnable/Disable IPv6 acceptance of router prefix info010net.ipv6.conf.default.accept_ra_defrtr
Setting controls whether the system will accept Hop Limit
settings from a router advertisement. Setting it to 0 prevents a router from
changing your default IPv6 Hop Limit for outgoing packets.Enable/Disable IPv6 acceptance of Hop limits from router advertisement010net.ipv6.conf.default.autoconfSetting controls whether router advertisements can cause the system to assign a global unicast address to an interface.Enable/Disable IPv6 acceptance of global unicast address from router advertisement010net.ipv6.conf.default.dad_transmits
Setting determines how many neighbor solicitations to
send out per address (global and link-local) when bringing up an interface to
ensure the desired address is unique on the networkSelect how many neighbor solicitations send out per address to ensure uniqueness of desired address for IPv6001net.ipv6.conf.default.max_addresses
Setting determines how many global unicast IPv6 addresses can be
assigned to each interface. The default is 16, but it should be set to exactly
the number of statically configured global addresses required.Select how many global unicast IPv6 addresses can be assigned to each interface160124816Limit Network-Transmitted Configuration via net.ipv6.conf.default.router_solicitationsThe default number of IPv6 router solicitations for network interfaces to send should be: CCE-4159-0(1) via sysctl - net.ipv6.conf.default.router_solicitationsLimit Network-Transmitted Configuration via net.ipv6.conf.default.accept_ra_rtr_prefThe default setting for accepting router preference via IPv6 router advertisement should be: for interfaces.CCE-4221-8(1) via sysctl - net.ipv6.conf.default.accept_ra_rtr_prefLimit Network-Transmitted Configuration via net.ipv6.conf.default.accept_ra_pinfoThe default setting for accepting prefix information via IPv6 router advertisement should be: for interfaces.CCE-4058-4(1) via sysctl - net.ipv6.conf.default.accept_ra_pinfoLimit Network-Transmitted Configuration via net.ipv6.conf.default.accept_ra_defrtrThe default setting for accepting a default router via IPv6 router advertisement should be: for interfaces.CCE-4128-5(1) via sysctl - net.ipv6.conf.default.accept_ra_defrtrLimit Network-Transmitted Configuration via net.ipv6.conf.default.autoconfThe default setting for autoconfiguring network interfaces using prefix information in IPv6 router advertisements should be: .CCE-4287-9(1) via sysctl - net.ipv6.conf.default.autoconfLimit Network-Transmitted Configuration via net.ipv6.conf.default.dad_transmitsThe default number of IPv6 duplicate address detection solicitations for network interfaces to send per configured address should be: .CCE-3895-0(1) via sysctl - net.ipv6.conf.default.dad_transmitsLimit Network-Transmitted Configuration via net.ipv6.conf.default.max_addressesThe default number of global unicast IPv6 addresses allowed per network interface should be: .CCE-4137-6(1) via sysctl - net.ipv6.conf.default.max_addressesTCP Wrapper
TCP Wrapper is a library which provides simple access control and
standardized logging for supported applications which accept connections over a network.
Historically, TCP Wrapper was used to support inetd services. Now that inetd is deprecated
(see Section 3.2.1), TCP Wrapper supports only services which were built to make use of
the libwrap library. To determine whether a given executable daemon /path/to/daemon
supports TCP Wrapper, check the documentation, or run: $ ldd /path/to/daemon | grep libwrap.so
If this command returns any output, then the daemon probably supports TCP Wrapper.
An alternative to TCP Wrapper support is packet filtering using iptables. Note
that iptables works at the network level, while TCP Wrapper works at the application
level. This means that iptables filtering is more efficient and more resistant to flaws in
the software being protected, but TCP Wrapper provides support for logging, banners, and
other application-level tricks which iptables cannot provide.How TCP Wrapper Protects Services
TCP Wrapper provides access control for the system's network
services using two configuration files. When a connection is attempted: The file
/etc/hosts.allow is searched for a rule matching the connection. If one is found, the
connection is allowed. Otherwise, the file /etc/hosts.deny is searched for a rule
matching the connection. If one is found, the connection is rejected. If no matching
rules are found in either file, then the connection is allowed. By default, TCP Wrapper
does not block access to any services.
In the simplest case, each rule in /etc/hosts.allow and /etc/hosts.deny takes the form:
daemon : client
where daemon is the
name of the server process for which the connection is destined, and client is the
partial or full hostname or IP address of the client. It is valid for daemon and client
to contain one item, a comma-separated list of items, or a special keyword like ALL,
which matches any service or client. (See the hosts access(5) manpage for a list of
other keywords.)
Note: Partial hostnames start at the root domain and are delimited by
the . character. So the client machine host03.dev.example.com, with IP address 10.7.2.3,
could be matched by any of the specifications:
.example.com
.dev.example.com
10.7.2.Reject All Connections From Other Hosts if Appropriate
Restrict all connections to non-public services to localhost
only. Suppose pubsrv1 and pubsrv2 are the names of daemons which must be accessed
remotely. Configure TCP Wrapper as follows.
Edit /etc/hosts.allow. Add the following lines:
pubsrv1 ,pubsrv2 : ALL
ALL: localhost
Edit /etc/hosts.deny. Add the following line:
ALL: ALL
These rules deny connections to all TCP Wrapper enabled services from any
host other than localhost, but allow connections from anywhere to the services which
must be publicly accessible. (If no public services exist, the first line in
/etc/hosts.allow may be omitted.)Allow Connections Only From Hosts in This Domain if Appropriate
For each daemon, domainsrv , which only needs to be contacted
from inside the local domain, example.com , configure TCP Wrapper to deny remote
connections.
Edit /etc/hosts.allow. Add the following line:
domainsrv : .example.com
Edit /etc/hosts.deny. Add the following line:
domainsrv : ALL
There are many possible
examples of services which need to communicate only within the local domain. If a
machine is a local compute server, it may be necessary for users to connect via SSH from
their desktop workstations, but not from outside the domain. In that case, you should
protect the daemon sshd using this method. As another example, RPC-based services such
as NFS might be enabled within the domain only, in which case the daemon portmap should
be protected. Note: This example protects only the service domainsrv . No filtering is
done on other services unless a line is entered into /etc/hosts.deny which refers to
those services by name, or which restricts the special service ALL.Monitor Syslog for Relevant Connections and Failures
Ensure that the following line exists in /etc/syslog.conf.
(This is the default, so it is likely to be correct if the configuration has not been
modified):
authpriv.* /var/log/secure
Configure logwatch or other log monitoring tools
to periodically summarize failed connections reported by TCP Wrapper at the facility
authpriv.info.
By default, TCP Wrapper audits all rejected connections at the facility
authpriv, level info. In the log file, TCP Wrapper rejections will contain the
substring:
daemon [pid ]: refused connect from ipaddr
These lines can be used to detect
malicious scans, and to debug failures resulting from an incorrect TCP Wrapper
configuration.
If appropriate, it is possible to change the syslog facility and level
used by a given TCP Wrapper rule by adding the severity option to each desired
configuration line in /etc/hosts.deny:
daemon : client : severity facility .level
By default, successful connections are not logged by TCP Wrapper. See Section 2.6 for
more information about system auditing.Further Resources
For more information about TCP Wrapper, see the tcpd(8) and
hosts access(5) manpages and the documentation directory /usr/share/doc/tcp
wrappers-version.
Some information may be available from the Tools section of the
author's website, http://www.porcupine.org, and from the RHEL4 Reference Guide [6].Iptables and Ip6tables
A host-based firewall called Netfilter is included as part of the
Linux kernel distributed with the system. It is activated by default. This firewall is
controlled by the program iptables, and the entire capability is frequently referred to by
this name. An analogous program called ip6tables handles filtering for IPv6.
Unlike TCP
Wrappers, which depends on the network server program to support and respect the rules
written, Netfilter filtering occurs at the kernel level, before a program can even process
the data from the network packet. As such, any program on the system is affected by the
rules written.
This section provides basic information about strengthening the iptables
and ip6tables configurations included with the system. For more complete information that
may allow the construction of a sophisticated ruleset tailored to your environment, please
consult the references at the end of this section.Inspect and Activate Default Rules
View the currently-enforced iptables rules by running the
command: # iptables -nL --line-numbers
The command is analogous for the ip6tables program.
If the firewall does not appear to be active (i.e., no rules appear), activate
it and ensure that it starts at boot by issuing the following commands (and analogously
for ip6tables): # service iptables restart
# chkconfig iptables on
The default iptables rules are:
Chain INPUT (policy ACCEPT)
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy ACCEPT)
num target prot opt source destination
1 RH-Firewall-1-INPUT all -- 0.0.0.0/0 0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
num target prot opt source destination
Chain RH-Firewall-1-INPUT (2 references)
num target prot opt source destination
1 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
2 ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0 icmp type 255
3 ACCEPT esp -- 0.0.0.0/0 0.0.0.0/0
4 ACCEPT ah -- 0.0.0.0/0 0.0.0.0/0
5 ACCEPT udp -- 0.0.0.0/0 224.0.0.251 udp dpt:5353
6 ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:631
7 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:631
8 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
9 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
10 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited
The ip6tables default rules are similar, with
its rules 2 and 10 reflecting protocol naming and addressing differences. Instead of
rule 8, however, ip6tables includes two rules that accept all incoming udp and tcp
packets with a particular destination port range. This is because the current Netfilter
implementation for IPv6 lacks reliable connection-tracking functionality.Verify ip6tables is enabledThe ip6tables service should be enabled.CCE-4167-3chkconfig ip6tables onVerify iptables is enabledThe iptables service should be enabled.CCE-4189-7chkconfig iptables onUnderstand the Default Ruleset
Understanding and creating firewall rules can be a challenging
activity, filled with corner cases and difficult-todebug problems. Because of this,
administrators should develop a thorough understanding of the default ruleset before
carefully modifying it.
The default ruleset is divided into four sections, each of which
is called a chain: INPUT, FORWARD, OUTPUT, and RH-Firewall-1-INPUT. INPUT, OUTPUT, and
FORWARD are built-in chains. The INPUT chain is activated on packets destined for
(i.e., addressed to) the system. The OUTPUT chain is activated on packets which are
originating from the system. The FORWARD chain is activated for packets that the
system will process and send through another interface, if so configured. The
RH-Firewall-1-INPUT chain is a custom (or user-defined) chain, which is used by the
INPUT and FORWARD chains.
A packet starts at the first rule in the appropriate chain and
proceeds until it matches a rule. If a match occurs, then control will jump to the
specified target. The default ruleset uses the built-in targets ACCEPT and REJECT, and
also the user-defined target/chain RH-Firewall-1-INPUT. Jumping to the target ACCEPT
means to allow the packet through, while REJECT means to drop the packet and send an
error message to the sending host. A related target called DROP means to drop the packet
on the floor without even sending an error message.
The default policy for all of the
built-in chains (shown after their names in the rule output above) is set to ACCEPT.
This means that if no rules in the chain match the packets, they are allowed through.
Because no rules at all are written for the OUTPUT chain, this means that iptables does
not stop any packets originating from the system. The INPUT and FORWARD chains jump to
the user-defined target RH-Firewall-1-INPUT for all packets.
RH-Firewall-1-INPUT tries
to match, in order, the following rules for both iptables and ip6tables: Rule 1
appears to accept all packets. However, this appears true only because the rules are not
presented in verbose mode. Executing the command # iptables -vnL --line-numbers
reveals
that this rule applies only to the loopback (lo) interface (see column in), while all
other rules apply to all interfaces. Thus, packets not coming from the loopback
interface do not match and proceed to the next rule. Rule 2 explicitly allows all icmp
packet types; iptables uses the code 255 to mean all icmp types. Rule 3 explicitly
allows all esp packets; these are packets which contain IPsec ESP headers.Rule 4
explicitly allows all ah packets; these are packets which contain an IPsec
authentication header SPI. Rule 5 allows inbound communication on udp port 5353
(mDNS), which the avahi daemon uses. Rules 6 and 7 allows inbound communication on
both tcp and udp port 631, which the cups daemon uses. Rule 8, in the iptables rules,
allows inbound packets that are part of a session initiated by the system. In ip6tables,
rules 8 and 9 allow any inbound packets with a destination port address between 32768
and 61000. Rule 9 (10, for ip6tables) allows inbound connections in tcp port 22, which
is the SSH protocol. Rule 10 (11, for ip6tables) rejects all other packets and sends
an error message to the sender. Because this is the last rule and matches any packet, it
effectively prevents any packet from reaching the chain's default ACCEPT target.
Preventing the acceptance of any packet that is not explicitly allowed is proper design
for a firewall.Strengthen the Default Ruleset
The default rules can be strengthened. The system scripts that
activate the firewall rules expect them to be defined in the configuration files
iptables and ip6tables in the directory /etc/sysconfig. Many of the lines in these files
are similar to the command line arguments that would be provided to the programs
/sbin/iptables or /sbin/ip6tables – but some are quite different.
The following recommendations describe how to strengthen the default
ruleset configuration file. An alternative to editing this configuration file is to
create a shell script that makes calls to the iptables program to load in rules, and
then invokes service iptables save to write those loaded rules to
/etc/sysconfig/iptables.
The following alterations can be made directly to
/etc/sysconfig/iptables and /etc/sysconfig/ip6tables. Instructions apply to both unless
otherwise noted. Language and address conventions for regular iptables are used
throughout this section; configuration for ip6tables will be either analogous or
explicitly covered.The program
system-config-securitylevel allows additional services to penetrate the default firewall
rules and automatically adjusts /etc/ sysconfig/ iptables . This program is only useful
if the default ruleset meets your security requirements. Otherwise, this program should
not be used to make changes to the firewall configuration because it re-writes the saved
configuration file. Change the Default Policies
Change the default policy to DROP (from ACCEPT) for the INPUT
and FORWARD built-in chains:
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
Changing
the default policy in this way implements proper design for a firewall, i.e. any
packets which are not explicitly permitted should not be accepted.Change the default policy to DROP (from ACCEPT) for the INPUT built-in chainChange the default policy to DROP (from ACCEPT) for the INPUT built-in chain.(1) via chkconfigChange the default policy to DROP (from ACCEPT) for the FORWARD built-in chainChange the default policy to DROP (from ACCEPT) for the FORWARD built-in chain.(1) via chkconfigRestrict ICMP Message Types
In /etc/sysconfig/iptables, the accepted ICMP messages types
can be restricted. To accept only ICMP echo reply, destination unreachable, and time
exceeded messages, remove the line:
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
and insert the lines:
-A RH-Firewall-1-INPUT -p icmp --icmp-type echo-reply -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type destination-unreachable -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type time-exceeded -j ACCEPT
To allow the system to respond to pings, also insert the following line:
-A RH-Firewall-1-INPUT -p icmp --icmp-type echo-request -j ACCEPT
Ping responses can also be limited to certain
networks or hosts by using the -s option in the previous rule.
Because IPv6 depends so
heavily on ICMPv6, it is preferable to deny the ICMPv6 packets you know you don't need
(e.g. ping requests) in /etc/sysconfig/ip6tables, while letting everything else
through:
-A RH-Firewall-1-INPUT -p icmpv6 --icmpv6-type echo-request -j DROP
If you
are going to statically configure the machine's address, it should ignore Router
Advertisements which could add another IPv6 address to the interface or alter
important network settings:
-A RH-Firewall-1-INPUT -p icmpv6 --icmpv6-type router-advertisement -j DROP
Restricting other ICMPv6 message types in
/etc/sysconfig/ip6tables is not recommended because the operation of IPv6 depends
heavily on ICMPv6. Thus, more care must be taken when blocking ICMPv6 types.Remove IPsec Rules
If the system will not process IPsec traffic, then remove the
following rules:
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPTLog and Drop Packets with Suspicious Source Addresses
Packets with non-routable source addresses should be
rejected, as they may indicate spoofing. Because the modified policy will reject
non-matching packets, you only need to add these rules if you are interested in also
logging these spoofing or suspicious attempts before they are dropped. If you do
choose to log various suspicious traffic, add identical rules with a target of DROP
after each LOG.
To log and then drop these IPv4 packets, insert the following rules in
/etc/sysconfig/iptables (excepting any that are intentionally used):
-A INPUT -i eth0 -s 10.0.0.0/8 -j LOG --log-prefix "IP DROP SPOOF A: "
-A INPUT -i eth0 -s 172.16.0.0/12 -j LOG --log-prefix "IP DROP SPOOF B: "
-A INPUT -i eth0 -s 192.168.0.0/16 -j LOG --log-prefix "IP DROP SPOOF C: "
-A INPUT -i eth0 -s 224.0.0.0/4 -j LOG --log-prefix "IP DROP MULTICAST D: "
-A INPUT -i eth0 -s 240.0.0.0/5 -j LOG --log-prefix "IP DROP SPOOF E: "
-A INPUT -i eth0 -d 127.0.0.0/8 -j LOG --log-prefix "IP DROP LOOPBACK: "
Similarly, you might wish to log packets containing some IPv6
reserved addresses if they are not expected on your network:
-A INPUT -i eth0 -s ::1 -j LOG --log-prefix "IPv6 DROP LOOPBACK: "
-A INPUT -s 2002:E000::/20 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:7F00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:0000::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:FF00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:0A00::/24 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:AC10::/28 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
-A INPUT -s 2002:C0A8::/32 -j LOG --log-prefix "IPv6 6to4 TRAFFIC: "
If you are not expecting to see site-local multicast or auto-tunneled traffic, you
can log those:
-A INPUT -s FF05::/16 -j LOG --log-prefix "IPv6 SITE-LOCAL MULTICAST: "
-A INPUT -s ::0.0.0.0/96 -j LOG --log-prefix "IPv4 COMPATIBLE IPv6 ADDR: "
If you wish to block multicasts to all
link-local nodes (e.g. if you are not using router autoconfiguration and do not plan
to have any services that multicast to the entire local network), you can block the
link-local all-nodes multicast address (before accepting incoming ICMPv6):
-A INPUT -d FF02::1 -j LOG --log-prefix "Link-local All-Nodes Multicast: "
However, if you're
going to allow IPv4 compatible IPv6 addresses (of the form ::0.0.0.0/96), you should
then consider logging the non-routable IPv4-compatible addresses:
-A INPUT -s ::0.0.0.0/104 -j LOG --log-prefix "IP NON-ROUTABLE ADDR: "
-A INPUT -s ::127.0.0.0/104 -j LOG --log-prefix "IP DROP LOOPBACK: "
-A INPUT -s ::224.0.0.0.0/100 -j LOG --log-prefix "IP DROP MULTICAST D: "
-A INPUT -s ::255.0.0.0/104 -j LOG --log-prefix "IP BROADCAST: "
If you are not expecting to see any IPv4 (or IPv4-compatible) traffic
on your network, consider logging it before it gets dropped:
-A INPUT -s ::FFFF:0.0.0.0/96 -j LOG --log-prefix "IPv4 MAPPED IPv6 ADDR: "
-A INPUT -s 2002::/16 -j LOG --log-prefix "IPv6 6to4 ADDR: "
The following rule will log all traffic
originating from a site-local address, which is deprecated address space:
-A INPUT -s FEC0::/10 -j LOG --log-prefix "SITE-LOCAL ADDRESS TRAFFIC: "Log and Drop All Other Packets
To log before dropping all packets that are not explicitly
accepted by previous rules, change the final lines from
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
to
-A RH-Firewall-1-INPUT -j LOG
-A RH-Firewall-1-INPUT -j DROP
COMMIT
The rule to log all dropped packets must be used
with care. Chatty but otherwise non-malicious network protocols (e.g. NetBIOS) may
result in voluminous logs; insertion of earlier rules to explicitly drop their packets
without logging may be appropriate.Further Strengthening
Further strengthening, particularly as a result of
customization to a particular environment, is possible for the iptables rules. Consider
the following options, though their practicality depends on the network environment and
usage scenario: Restrict outgoing traffic. As shown above, the OUTPUT chain's default
policy can be changed to DROP, and rules can be written to specifically allow only
certain types of outbound traffic. Such a policy could prevent casual usage of insecure
protocols such as ftp and telnet, or even disrupt spyware. However, it would still not
prevent a sophisticated user or program from using a proxy to circumvent the intended
effects, and many client programs even try to automatically tunnel through port 80 to
avoid such restrictions.SYN flood protection. SYN flood protection can be provided by
iptables, but might run into limiting issues for servers. For example, the iplimit match
can be used to limit simultaneous connections from a given host or class. Similarly, the
recent match allows the firewall to deny additional connections from any host within a
given period of time (e.g. more than 3 –state NEW connections on port 22 within a minute
to prevent dictionary login attacks).
A more precise option for DoS protection is using
TCP SYN cookies. (See Section 2.5.1.2 for more information.)Further Resources
More complex, restrictive, and powerful rulesets can be
created, but this requires careful customization that relies on knowledge of the
particular environment. The following resources provide more detailed information: The iptables(8) man page The Netfilter Project's documentation at http://www.netfilter.orgThe Red Hat Enterprise Linux Reference GuideSecure Sockets Layer Support
The Secure Sockets Layer (SSL) protocol provides encrypted and
authenticated network communications, and many network services include support for it.
Using SSL is recommended, especially to avoid any plaintext transmission of sensitive
data, even over a local network. The SSL implementation included with the system is called
OpenSSL. Recent implementations of SSL may also be referred to as Transport Layer Security
(TLS).
SSL uses public key cryptography to provide authentication and encryption. Public
key cryptography involves two keys, one called the public key and the other called the
private key. These keys are mathematically related such that data encrypted with one key
can only be decrypted by the other, and vice versa. As their names suggest, public keys
can be distributed to anyone while a private key must remain known only to its owner.
SSL uses certificates, which are files that hold cryptographic data: a public key, and a
signature of that public key. In SSL authentication, a server presents a client with its
certificate as a means of demonstrating that it is who it claims it is. If everything goes
correctly, the client can verify the server's certificate by determining that the
signature inside the certificate could only have been generated by a third party whom the
client trusts. This third party is called a Certificate Authority (CA). Each client system
should also have certificates from trusted CAs, and the client uses these CA certificates
to verify the authenticity of the server's certificate. After authenticating a server
using its certificate and a CA certificate, SSL provides encryption by using the server
certificate to securely negotiate a shared secret key.
If your server must communicate
using SSL with systems that might not be able to securely accept a new CA certificate
prior to any SSL communication, then paying an established CA (whose certificates your
clients already have) to sign your server certificates is recommended. The steps for doing
this vary by vendor. Once the signed certificates have been obtained, configuration of the
services is the same whether they were purchased from a vendor or signed by your own CA.
For setting up an internal network and encrypting local traffic, creating your own CA to
sign SSL certificates can be appropriate. The major steps in this process are: Create a CA to sign certificates Create SSL certificates for servers using that CAEnableclient support by distributing the CA's certificateCreate a CA to Sign Certificates
The following instructions apply to OpenSSL since it is
included with the system, but creating a CA is possible with any standards-compliant SSL
toolkit. The security of certificates depends on the security of the CA that signed
them, so performing these steps on a secure machine is critical. The system used as a CA
should be physically secure and not connected to any network. It should receive any
certificate signing requests (CSRs) via removable media and output certificates onto
removable media.
The script /etc/pki/tls/misc/CA is included to assist in the process of
setting up a CA. This script uses many settings in /etc/pki/tls/openssl.cnf. The
settings in this file can be changed to suit your needs and allow easier selection of
default settings, particularly in the [req distinguished name] section.
To create the CA: # cd /etc/pki/tls/misc
# ./CA -newca When prompted, press enter to create a new CA key with the default name cakey.pem.When prompted, enter a password that will protect the private key, then enter the same password
again to verify it.At the prompts, fill out as much of the CA information as is relevant for your site. You must specify
a common name, or generation of the CA certificate will fail. Next, you will be prompted for the password, so that the script can re-open the private key in order
to write the certificate.
This step performs the following actions:
creates the directory
/etc/pki/CA (by default), which contains files necessary for the operation of a
certificate authority. These are:serial, which contains the current serial number for certificates signed by the CAindex.txt, which is a text database file that contains information about certificates signedcrl, which is a directory for holding revoked certificatesprivate, a directory which stores the CA's private keycreates a public-private key pair for the CA in the file /etc/pki/CA/private/cakey.pem. The
private key must be kept private in order to ensure the security of the certificates the CA will later sign.signs the public key (using the corresponding private key, in a process called self-signing) to create the CA
certificate, which is then stored in /etc/pki/CA/cacert.pem.
When the CA later signs a server certificate using its private
key, it means that it is vouching for the authenticity of that server. A client can then
use the CA's certificate (which contains its public key) to verify the authenticity of
the server certificate. To accomplish this, it is necessary to distribute the CA
certificate to any clients as covered in Section 2.5.6.3.Create SSL Certificates for Servers
Creating an SSL certificate for a server involves the following steps: A public-private key pair for the server must be generated.A certificate signing request (CSR) must be created from the key pair.The CSR must be signed by a
certificate authority (CA) to create the server certificate. If a CA has been set up as
described in Section 2.5.6.1, it can sign the CSR.The server certificate and keys must be installed on the server.
Instructions on how to generate and sign SSL certificates are provided for the following
common services:Mail server, in Section 3.11.4.6.Dovecot, in Section 3.17.2.2. Apache, in Section 3.16.4.1.Enable Client Support
The system ships with certificates from well-known commercial
CAs. If your server certificates were signed by one of these established CAs, then this
step is not necessary since the clients should include the CA certificate already.
If your servers use certificates signed by your own CA, some user applications will warn
that the server's certificate cannot be verified because the CA is not recognized. Other
applications may simply fail to accept the certificate and refuse to operate, or
continue operating without ever having properly verified the server certificate.
To avoid this warning, and properly authenticate the servers, your CA certificate must be
exported to every application on every client system that will be connecting to an
SSL-enabled server.Adding a Trusted CA for Firefox
Firefox needs to have a certificate from the CA that signed
the web server's certificate, so that it can authenticate the web server.
To import a new CA certificate into Firefox 1.5:Launch Firefox and choose Preferences from the Edit menu. Click the Advanced button.Select the Security pane.Click the View Certificates button.Click the Authorities tab. Click the Import button at the bottom of the screen.Navigate to the CA certificate and import it.Adding a Trusted CA for Thunderbird
Thunderbird needs to have a certificate from the CA that
signed the mail server's certificates, so that it can authenticate the mail server(s).
To import a new CA certificate into Thunderbird 2: Launch Thunderbird and choose Account Settings from the Edit menu.Click the Advanced button.Select the Certificates tabClick the View Certificates button.Select the Authorities tab.Click the Import button at the bottom of the screen.Navigate to the CA certificate and import it. Determine whether the CA should
be used to identify web sites, e-mail users, and software developers and trust it for
each accordingly.Adding a Trusted CA for Evolution
The Evolution e-mail client needs to have a certificate from
the CA that signed the mail server's certificates, so that it can authenticate the
mail server(s).
To import a new CA certificate into Evolution: Launch Evolution and choose Preferences from the Edit menu.Select Certificates from the icon list on the left.Select the Authorities tab.Click the Import button.Navigate to the CA certificate and import it.Further ResourcesThe OpenSSL Project home page at http://www.openssl.orgThe openssl(1) man pageJeremy Mates's how-to: http://sial.org/howto/opensslUncommon Network Protocols
The system includes support for several network protocols which are not commonly used. Although security vul-
nerabilities in kernel networking code are not frequently discovered, the consequences can be dramatic. Ensuring
uncommon network protocols are disabled reduces the system’s risk to attacks targeted at its implementation of
those protocols.Disable Support for DCCP
To prevent the DCCP kernel module from being loaded, add the following line to /etc/modprobe.conf:install dccp /bin/true
The Datagram Congestion Control Protocol (DCCP) is a relatively new transport layer protocol, designed to
support streaming media and telephony.Disable Support for DCCPSupport for DCCP should be disabled.(1) via /etc/modprobe.confDisable Support for SCTP
To prevent the SCTP kernel module from being loaded, add the following line to /etc/modprobe.conf:install sctp /bin/true
The Stream Control Transmission Protocol (SCTP) is a transport layer protocol, designed to support the idea
of message-oriented communication, with several streams of messages within one connection.Disable Support for SCTPSupport for SCTP should be disabled.(1) via /etc/modprobe.confDisable Support for RDS
To prevent the RDS kernel module from being loaded, add the following line to /etc/modprobe.conf:install rds /bin/true
The Reliable Datagram Sockets (RDS) protocol is a transport layer protocol designed to provide reliable high-
bandwidth, low-latency communications between nodes in a cluster.Disable Support for RDSSupport for RDS should be disabled.(1) via /etc/modprobe.confDisable Support for TIPC
To prevent the TIPC kernel module from being loaded, add the following line to /etc/modprobe.conf:install rds /bin/true
The Transparent Inter-Process Communication (TIPC) protocol is designed to provide communications between
nodes in a cluster..Disable Support for TIPCSupport for TIPC should be disabled.(1) via /etc/modprobe.confLogging and Auditing
Successful local or network attacks on systems do not necessarily
leave clear evidence of what happened. It is necessary to build a configuration in advance
that collects this evidence, both in order to determine that something anomalous has
occurred, and in order to respond appropriately. In addition, a well-configured logging and
audit infrastructure will show evidence of any misconfiguration which might leave the system
vulnerable to attack.
Logging and auditing take different approaches to collecting data. A
logging infrastructure provides a framework for individual programs running on the system to
report whatever events are considered interesting: the sshd program may report each
successful or failed login attempt, while the sendmail program may report each time it sends
an e-mail on behalf of a local or remote user. An auditing infrastructure, on the other
hand, reports each instance of certain low-level events, such as entry to the setuid system
call, regardless of which program caused the event to occur.
Auditing has the advantage of
being more comprehensive, but the disadvantage of reporting a large amount of information,
most of which is uninteresting. Logging (particularly using a standard framework like
syslog) has the advantage of being compatible with a wide variety of client applications,
and of reporting only information considered important by each application, but the
disadvantage that the information reported is not consistent between applications.
A robust
infrastructure will perform both logging and auditing, and will use configurable automated
methods of summarizing the reported data, so that system administrators can remove or
compress reports of events known to be uninteresting in favor of alert monitoring for events
known to be interesting.
This section discusses how to configure logging, log monitoring,
and auditing, using tools included with RHEL5. It is recommended that syslog be used for
logging, with logwatch providing summarization, and that auditd be used for auditing, with
aureport providing summarization.Configure Syslog
Syslog has been the default Unix logging mechanism for many years. This section
discusses how to configure syslog for best effect, and how to use tools provided with the
system to maintain and monitor your logs.Configure RsyslogThe rsyslog service should be enabled.CCE-3679-8chkconfig rsyslog onEnsure All Important Messages are CapturedEdit the file /etc/syslog.conf. Add or correct whichever of the
following lines are appropriate for your environment:
auth,info.* /var/log/messages
kern.* /var/log/kern.log
daemon.* /var/log/daemon.log
syslog.* /var/log/syslog
lpr,news,uucp,local0,local1,local2,local3,local4,local5,local6.* /var/log/unused.log
When a message is sent to syslog for logging, it is sent with a facility name (such as
mail, auth, or local2), and a priority (such as debug, notice, or emerg). Each line of
syslog's configuration file is a directive which specifies a set of facility/priority
pairs, and then gives a filename or host to which log messages of matching types should
be sent. In order for a message to match a type, the facility must match, and the
priority must be the priority named in the rule or any higher priority. (See
syslog.conf(5) for an ordered list of priorities.)
Older versions of syslog mandated a
very restrictive format for the syslog.conf file. However, the version of syslog shipped
with RHEL5 allows any sort of whitespace (spaces or tabs, not just tabs) to separate the
selection criteria from the message disposition, and allows the use of facility.* as a
wildcard matching a given facility at any priority.
The default RHEL5 syslog
configuration stores the facilities authpriv, cron, and mail in named logs. This guide
describes the implementation of the following configuration, but any configuration which
stores the important facilities and is usable by the administrators will suffice:Store each of the facilities kern, daemon, and syslog in its own log, so that it will be
easy to access information about messages from those facilities. Restrict the
information stored in /var/log/messages to only the facilities auth and user, and store
all messages from those facilities. Messages can easily become cluttered otherwise. Store information about all facilities which should not be in use at this site in a file
called /var/log/ unused.log. If any messages are logged to this file at some future
point, this may be an indication that an unknown service is running, and should be
investigated. In addition, if news and uucp are not in use at this site, remove the
directive from the default syslog.conf which stores those facilities.
Making use of the
local facilities is also recommended. Specific configuration is beyond the scope of this
guide, but applications such as SSH can easily be configured to log to a local facility
which is not being used for anything else. If this is done, reconfigure /etc/syslog.conf
to store this facility in an appropriate named log or in /var/log/messages, rather than
in /var/log/unused.log.Confirm Existence and Permissions of System Log Files
For each log file LOGFILE referenced in /etc/syslog.conf, run
the commands: # touch LOGFILE
# chown root:root LOGFILE
# chmod 0600 LOGFILE
Syslog will
refuse to log to a file which does not exist. All messages intended for that file will
be silently discarded, so it is important to verify that all log files exist. Some logs
may contain sensitive information, so it is better to restrict permissions so that only
administrative users can read or write logfiles.User who owns log filesSpecify user owner of all logfiles specified in /etc/syslog.conf.Specify user owner of all logfiles specified in /etc/syslog.confrootrootgroup who owns log filesSpecify group owner of all logfiles specified in /etc/syslog.conf.Specify group owner of all logfiles specified in /etc/syslog.confrootrootFile permissions on logfilesSpecify file permissions of all logfiles specified in /etc/syslog.conf.Specify permissions of all logfiles specified in /etc/syslog.conf110000000100000000110000000111000000Confirm user that owns System Log FilesAll syslog log files should be owned by root.CCE-4366-1(1) via chownConfirm group that owns System Log FilesAll syslog log files should be group owned by root.CCE-3701-0(1) via chownConfirm Permissions of System Log FilesFile permissions for all syslog log files should be set correctly.CCE-4233-3(1) via chmodSend Logs to a Remote Loghost
Edit /etc/syslog.conf. Add or correct the line:
*.* @loghost.example.com
where loghost.example.com is the name of your central log server.
If system logs are to be useful in detecting malicious activities, it is necessary to
send logs to a remote server. An intruder who has compromised the root account on a
machine may delete the log entries which indicate that the system was attacked before
they are seen by an administrator.
However, it is recommended that logs be stored on the
local host in addition to being sent to the loghost, because syslog uses the UDP
protocol to send messages over a network. UDP does not guarantee reliable delivery, and
moderately busy sites will lose log messages occasionally, especially in periods of high
traffic which may be the result of an attack. In addition, remote syslog messages are
not authenticated in any way, so it is easy for an attacker to introduce spurious
messages to the central log server. Also, some problems cause loss of network
connectivity, which will prevent the sending of messages to the central server. For all
of these reasons, it is better to store log messages both centrally and on each host, so
that they can be correlated if necessary.Send Logs to a Remote LoghostSyslog logs should be sent to a remote loghostCCE-4260-6(1) via /etc/syslog.confEnable syslogd to Accept Remote Messages on Loghosts Only
Is this machine the central log server for your organization?
If so, edit the file /etc/sysconfig/syslog. Add or correct the following line:
SYSLOGD_OPTIONS="-m 0 -r -s example.com "
where example.com is the name of your domain.
If the machine is not a log server, edit /etc/sysconfig/syslog, and instead add or
correct the line:
SYSLOGD_OPTIONS="-m 0"
By default, RHEL5's syslog does not listen over
the network for log messages. The -r flag enables syslogd to listen over a network, and
should be used only if necessary. The -s example.com flag strips the domain name
example.com from each sending machine's hostname before logging messages from that host,
to reduce the amount of redundant information placed in log files. See the syslogd(8)
man page for further information.Disable syslogd from Accepting Remote Messages on Loghosts OnlySyslogd should reject remote messagesCCE-3382-9(1) via /etc/sysconfig/syslogEnsure All Logs are Rotated by logrotate
Edit the file /etc/logrotate.d/syslog. Find the first line,
which should look like this (wrapped for clarity):
/var/log/messages /var/log/secure /var/log/maillog /var/log/spooler \
/var/log/boot.log /var/log/cron {
Edit this line so
that it contains a one-space-separated listing of each log file referenced in
/etc/syslog.conf.
All logs in use on a system must be rotated regularly, or the log
files will consume disk space over time, eventually interfering with system operation.
The file /etc/logrotate.d/syslog is the configuration file used by the logrotate program
to maintain all log files written by syslog. By default, it rotates logs weekly and
stores four archival copies of each log. These settings can be modified by editing
/etc/logrotate.conf, but the defaults are sufficient for purposes of this guide.
Note
that logrotate is run nightly by the cron job /etc/cron.daily/logrotate. If particularly
active logs need to be rotated more often than once a day, some other mechanism must be
used.Ensure All Logs are Rotated by logrotateThe logrotate (syslog rotater) service should be enabled.CCE-4182-2(1) via cronMonitor Suspicious Log Messages using Logwatch
The system includes an extensible program called Logwatch for
reporting on unusual items in syslog. Logwatch is valuable because it provides a parser
for the syslog entry format and a number of signatures for types of lines which are
considered to be mundane or noteworthy. Logwatch has a number of downsides: the
signatures can be inaccurate and are not always categorized consistently, and you must
be able to program in Perl in order to customize the signature database. However, it is
recommended that all Linux sites which do not have time to deploy a third-party log
monitoring application run Logwatch in its default configuration. This provides some
useful information about system activity in exchange for very little administrator
effort.
This guide recommends that Logwatch be run only on the central logserver, if
your site has one, in order to focus administrator attention by sending all daily logs
in a single e-mail.Monitor Suspicious Log Messages using LogwatchThe logwatch service should be enabledCCE-4323-2(1) via cronConfigure Logwatch on the Central Log Server
Is this machine the central log server? If so, edit the file
/etc/logwatch/conf/logwatch.conf. Add or correct the following lines: HostLimit = no
SplitHosts = yes
MultiEmail = no
Service = -zz-disk_space
Ensure that logwatch.pl is run nightly from cron. (This is the default): # cd /etc/cron.daily
# ln -s /usr/share/logwatch/scripts/logwatch.pl 0logwatch
On a central logserver, you want
Logwatch to summarize all syslog entries, including those which did not originate on
the logserver itself. The HostLimit setting tells Logwatch to report on all hosts, not
just the one on which it is running.
If SplitHosts is set, Logwatch will separate
entries by hostname. This makes the report longer but significantly more usable. If it
is not set, then Logwatch will not report which host generated a given log entry, and
that information is almost always necessary. If MultiEmail is set, then each host's
information will be sent in a separate e-mail message. This is a matter of preference.
The Service directive -zz-disk space tells Logwatch not to run the zz-disk space
report, which reports on free disk space. Since all log monitoring is being done on
the central logserver, the disk space listing will always be that of the logserver,
regardless of which host is being monitored. This is confusing, so disable that
service. Note that this does mean that Logwatch will not monitor disk usage
information. Many workarounds are possible, such as running df on each host daily via
cron and sending the output to syslog so that it will be reported to the logserver.Disable Logwatch on Clients if a Logserver Exists
Does your site have a central logserver which has been
configured to report on logs received from all systems? If so: # rm /etc/cron.daily/0logwatch
If no logserver exists, it will be necessary for each
machine to run Logwatch individually. Using a central logserver provides the security
and reliability benefits discussed earlier, and also makes monitoring logs easier and
less time-intensive for administrators.System Accounting with auditd
The audit service is the current Linux recommendation for
kernel-level auditing. By default, the service audits about SELinux AVC denials and
certain types of security-relevant events such as system logins, account modifications,
and authentication events performed by programs such as sudo.
Under its default
configuration, auditd has modest disk space requirements, and should not noticeably impact
system performance. The audit service, in its default configuration, is strongly
recommended for all sites, regardless of whether they are running SELinux.
DoD or federal networks often have substantial auditing requirements and auditd can be
configured to meet these requirements.
Typical DoD requirements include:Ensure Auditing is Configured to Collect Certain System Events
Information on the Use of Print Command (unsuccessful and successful)Startup and Shutdown Events (unsuccessful and successful)Ensure the auditing software can record the following for each audit event:
Date and time of the eventUserid that initiated the eventType of eventSuccess or failure of the eventFor I&A events, the origin of the request (e.g., terminal ID)For events that introduce an object into a user’s address space, and for object deletion events, the
name of the object, and in MLS systems, the objects security level.Ensure files are backed up no less than weekly onto a different system than the system being audited or
backup media.Ensure old logs are closed out and new audit logs are started dailyEnsure the configuration is immutable. With the -e 2 setting a reboot will be required to change any audit
rules.Ensure that the audit data files have permissions of 640, or more restrictive.Enable the auditd Service
Ensure that the auditd service is enabled (this is the default): # chkconfig auditd on
By default, auditd logs only SELinux denials, which are
helpful for debugging SELinux and discovering intrusion attempts, and certain types of
security events, such as modifications to user accounts (useradd, passwd, etc), login
events, and calls to sudo.
Data is stored in /var/log/audit/audit.log. By default,
auditd rotates 4 logs by size (5MB), retaining a maximum of 20MB of data in total, and
refuses to write entries when the disk is too full. This minimizes the risk of audit
data filling its partition and impacting other services. However, it is possible to lose
audit data if the system is busy.Enable the auditd ServiceThe auditd service should be enabled.CCE-4292-9(1) via chkconfigConfigure auditd Data RetentionDetermine STOREMB , the amount of audit data (in megabytes) which should be retained in each log
file. Edit the file /etc/audit/auditd.conf. Add or modify the following line:
max_log_file = STOREMBUse a dedicated partition (or logical volume) for log files. It is straightforward to create such a partition
or logical volume during system installation time. The partition should be larger than the maximum
space which auditd will ever use, which is the maximum size of each log file (max log file) multiplied
by the number of log files (num logs). Ensure the partition is mounted on /var/log/audit.If your site requires that the machine be disabled when auditing cannot be performed, configure auditd
to halt the system when disk space for auditing runs low. Edit /etc/audit/auditd.conf, and add or
correct the following lines:
space_left_action = email
action_mail_acct = root
admin_space_left_action = halt
The default action to take when the logs reach their maximum size is to rotate the log files, discarding the
oldest one. If it is more important to retain all possible auditing information, even if that opens the possibility
of running out of space and taking the action defined by admin space left action, add or correct the line:
max_log_file_action = keep_logs
By default, auditd retains 4 log files of size 5Mb apiece. For a busy system or a system which is thoroughly
auditing system activity, this is likely to be insufficient.
The log file size needed will depend heavily on what types of events are being audited. First configure auditing
to log all the events of interest. Then monitor the log size manually for awhile to determine what file size will
allow you to keep the required data for the correct time period.
Using a dedicated partition for /var/log/audit prevents the auditd logs from disrupting system functionality if
they fill, and, more importantly, prevents other activity in /var from filling the partition and stopping the audit
trail. (The audit logs are size-limited and therefore unlikely to grow without bound unless configured to do so.)
Some machines may have requirements that no actions occur which cannot be audited. If this is the case, then
auditd can be configured to halt the machine if it runs out of space.
Note: Since older logs are rotated, configuring auditd this way does not prevent older logs from being rotated
away before they can be viewed.
If your system is configured to halt when logging cannot be performed, make sure this can never
happen under normal circumstances! Ensure that /var/ log/ audit is on its own partition, and
that this partition is larger than the maximum amount of data auditd will retain normally.Enable Auditing for Processes Which Start Prior to the Audit Daemon
To ensure that all processes can be audited, even those which start prior to the audit daemon, add the
argument audit=1 to the kernel line in /etc/grub.conf, in the manner below:
kernel /vmlinuz-version ro vga=ext root=/dev/VolGroup00/LogVol00 rhgb quiet audit=1
Each process on the system carries an ”auditable” flag which indicates whether its activities can be audited.
Although auditd takes care of enabling this for all processes which launch after it does, adding the kernel
argument ensures that it is set for every process during boot.
Enable Auditing for Processes Which Start Prior to the Audit Daemon
To ensure that all processes can be audited, even those which start prior to the audit daemon, add the argument audit=1
to the kernel line in /etc/grub.conf, in the manner below:kernel /vmlinuz-version ro vga=ext root=/dev/VolGroup00/LogVol00 rhgb quiet audit=1(1) via /etc/grub.conf add audit=1 to kernel lineConfigure auditd Rules for Comprehensive Auditing
The auditd program can perform comprehensive monitoring of system activity. This section describes rec-
ommended configuration settings for comprehensive auditing, but a full description of the auditing system’s
capabilities is beyond the scope of this guide. The mailing list linux-audit@redhat.com may be a good source
of further information.
The audit subsystem supports extensive collection of events, including:Tracing of arbitrary system calls (identified by name or number) on entry or exit.Filtering by PID, UID, call success, system call argument (with some limitations), etc.Monitoring of specific files for modifications to the file’s contents or metadata.
Auditing rules are controlled in the file /etc/audit/audit.rules. Add rules to it to meet the auditing re-
quirements for your organization. Each line in /etc/audit/audit.rules represents a series of arguments that
can be passed to auditctl and can be individually tested as such. See documentation in /usr/share/doc/
audit-version and in the related man pages for more details.
Recommended audit rules are provided in /usr/share/doc/audit-version /stig.rules. In order to activate
those rules:# cp /usr/share/doc/audit-version /stig.rules /etc/audit/audit.rules
and then edit /etc/audit/audit.rules and comment out the lines containing arch= which are not appropriate
for your system’s architecture. Then review and understand the following rules, ensuring rules are activated as
needed for the appropriate architecture.
After reviewing all the rules, reading the following sections, and editing as needed, activate the new rules:# service auditd restartRecords Events that Modify Date and Time Information
Add the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your
system:
-a always,exit -F arch=ARCH -S adjtimex -S settimeofday -S stime -k time-change
-a always,exit -F arch=ARCH -S clock_settime -k time-change
-w /etc/localtime -p wa -k time-change
Records Events that Modify Date and Time InformationAudit rules about time(1) via /etc/audit/audit.rulesRecord Events that Modify User/Group Information
Add the following to /etc/audit/audit.rules, in order to capture events that modify account changes:
-w /etc/group -p wa -k identity
-w /etc/passwd -p wa -k identity
-w /etc/gshadow -p wa -k identity
-w /etc/shadow -p wa -k identity
-w /etc/security/opasswd -p wa -k identity
Record Events that Modify User/Group InformationAudit rules about User/Group Information(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesRecord Events that Modify the System’s Network Environment
Add the following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your
system:
-a exit,always -F arch=ARCH -S sethostname -S setdomainname -k system-locale
-w /etc/issue -p wa -k system-locale
-w /etc/issue.net -p wa -k system-locale
-w /etc/hosts -p wa -k system-locale
-w /etc/sysconfig/network -p wa -k system-localeRecord Events that Modify the System’s Network EnvironmentAudit rules about the System’s Network Environment(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesRecord Events that Modify the System’s Mandatory Access Controls
Add the following to /etc/audit/audit.rules:
-w /etc/selinux/ -p wa -k MAC-policy
Record Events that Modify the System’s Mandatory Access ControlsAudit rules about the System’s Mandatory Access Controls(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Logon and Logout Events
At a minimum the audit system should collect login info for all users and root. Add the following to
/etc/audit/audit.rules:
-w /var/log/faillog -p wa -k logins
-w /var/log/lastlog -p wa -k logins
Ensure auditd Collects Logon and Logout EventsAudit rules about the Logon and Logout Events(1) via /etc/audit/audit.rulesEnsure auditd Collects Process and Session Initiation Information
At a minimum the audit system should collect process information for all users and root. Add the following
to /etc/audit/audit.rules:
-w /var/run/utmp -p wa -k session
-w /var/log/btmp -p wa -k session
-w /var/log/wtmp -p wa -k session
Ensure auditd Collects Process and Session Initiation InformationAudit rules about the Process and Session Initiation Information(1) via /etc/audit/audit.rulesEnsure auditd Collects Discretionary Access Control Permission Modification Events
At a minimum the audit system should collect file permission changes for all users and root. Add the
following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system:
-a always,exit -F arch=ARCH -S chmod -S fchmod -S fchmodat -F auid>=500 \
-F auid!=4294967295 -k perm_mod
-a always,exit -F arch=ARCH -S chown -S fchown -S fchownat -S lchown -F auid>=500 \
-F auid!=4294967295 -k perm_mod
-a always,exit -F arch=ARCH -S setxattr -S lsetxattr -S fsetxattr -S removexattr -S \
lremovexattr -S fremovexattr -F auid>=500 -F auid!=4294967295 -k perm_mod
Ensure auditd Collects Discretionary Access Control Permission Modification EventsAudit rules about the Discretionary Access Control Permission Modification Events(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Unauthorized Access Attempts to Files (unsuccessful)
At a minimum the audit system should collect unauthorized file accesses for all users and root. Add the
following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system:
-a always,exit -F arch=ARCH -S creat -S open -S openat -S truncate -S ftruncate \
-F exit=-EACCES -F auid>=500 -F auid!=4294967295 -k access
-a always,exit -F arch=ARCH -S creat -S open -S openat -S truncate -S ftruncate \
-F exit=-EPERM -F auid>=500 -F auid!=4294967295 -k access
Ensure auditd Collects Unauthorized Access Attempts to Files (unsuccessful)Audit rules about the Unauthorized Access Attempts to Files (unsuccessful)(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Information on the Use of Privileged Commands
At a minimum the audit system should collect the execution of privileged commands for all users and root.
Add the following to /etc/audit/audit.rules:
-a always,exit -F path=/bin/ping -F perm=x -F auid>=500 -F auid!=4294967295 \
-k privileged
Ensure auditd Collects Information on the Use of Privileged CommandsAudit rules about the Information on the Use of Privileged Commands(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Information on Exporting to Media (successful)
At a minimum the audit system should collect media exportation events for all users and root. Add the
following to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system:
-a always,exit -F arch=ARCH -S mount -F auid>=500 -F auid!=4294967295 -k export
Ensure auditd Collects Information on Exporting to Media (successful)Audit rules about the Information on Exporting to Media (successful)(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Files Deletion Events by User (successful and unsuccessful)
At a minimum the audit system should collect file deletion events for all users and root. Add the following
to /etc/audit/audit.rules, setting ARCH to either b32 or b64 as appropriate for your system:
-a always,exit -F arch=ARCH -S unlink -S unlinkat -S rename -S renameat -F auid>=500 \
-F auid!=4294967295 -k delete
Ensure auditd Collects Files Deletion Events by User (successful and unsuccessful)Audit rules about the Files Deletion Events by User (successful and unsuccessful)(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects System Administrator Actions
At a minimum the audit system should collect administrator actions for all users and root. Add the following
to /etc/audit/audit.rules:
-w /etc/sudoers -p wa -k actionsEnsure auditd Collects System Administrator ActionsAudit rules about the System Administrator Actions(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesEnsure auditd Collects Information on Kernel Module Loading and Unloading
Add the following to /etc/audit/audit.rules in order to capture kernel module loading and unloading
events:
-w /sbin/insmod -p x -k modules
-w /sbin/rmmod -p x -k modules
-w /sbin/modprobe -p x -k modules
-a always,exit -S init_module -S delete_module -k modulesEnsure auditd Collects Information on Kernel Module Loading and UnloadingAudit rules about the Information on Kernel Module Loading and Unloading(1) via /etc/audit/audit.rulesMake the auditd Configuration Immutable
Add the following to /etc/audit/audit.rules in order to make the configuration immutable:
-e 2
With this setting, a reboot will be required to change any audit rules.
Make the auditd Configuration ImmutableForce a reboot to change audit rules(1) via /etc/audit/audit.rulescp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesSummarize and Review Audit Logs using aureport
Familiarize yourself with the aureport(8) man page, then design a short series of audit reporting commands
suitable for exploring the audit logs on a daily (or more frequent) basis. These commands can be added as a cron
job by placing an appropriately named file in /etc/cron.daily. See the next section for information on how to
ensure that the audit system collects all events needed.
For example, to generate a daily report of every user to login to the machine, the following command could be
run from cron:# aureport -l -i -ts yesterday -te today
To review all audited activity for unusual behavior, a good place to start is to see a summary of which audit
rules have been triggering:aureport --key --summary
If access violations stand out, review them with:# ausearch --key access --raw | aureport --file --summary
To review what executables are doing:# ausearch --key access --raw | aureport -x --summary
If access violations have been occurring on a particular file (such as /etc/shadow) and you want to determine
which user is doing this:# ausearch --key access --file /etc/shadow --raw | aureport --user --summary -i
Check for anomalous activity (such as device changing to promiscuous mode, processes ending abnormally, login
failure limits being reached) using:# aureport --anomaly
The foundation to audit analysis is using keys to classify the events. Information about using ausearch to find
an SELinux problem can be found in Section 2.4.6.
ServicesDisable All Unneeded Services at Boot Time
The best protection against vulnerable software is running less
software. This section describes how to review the software which Red Hat Enterprise Linux
installs on a system and disable software which is not needed. It then enumerates the
software packages installed on a default RHEL5 system and provides guidance about which ones
can be safely disabled.Determine which Services are Enabled at Boot
Run the command: # chkconfig --list | grep :on
The first column
of this output is the name of a service which is currently enabled at boot. Review each
listed service to determine whether it can be disabled.
If it is appropriate to disable
some service srvname , do so using the command: # chkconfig srvname off
Use the guidance below for information about unfamiliar services.Guidance on Default Services
The table in this section contains a list of all services which
are enabled at boot by a default RHEL5 installation. For each service, one of the
following recommendations is made: Enable: The service provides a significant capability
with limited risk exposure. Leave the service enabled. Configure: The service either is
required for most systems to function properly or provides an important security function.
It should be left enabled by most environments. However, it must be configured securely on
all machines, and different options may be needed for workstations than for servers. See
the referenced section for recommended configuration of this service.Disable if
possible: The service opens the system to some risk, but may be required by some
environments. See the appropriate section of the guide, and disable the service if at all
possible.Servers only: The service provides some function to other machines over the
network. If that function is needed in the target environment, the service should remain
enabled only on a small number of dedicated servers, and should be disabled on all other
machines on the network. Service nameActionReferenceacpidEnable3.3.15.2anacronDisable if possible3.4apmdDisable if possible3.3.15.1atdConfigure3.4auditdConfigure2.6.2autofsDisable if possible2.2.2.3avahi-daemonDisable if possible3.7bluetoothDisable if possible3.3.14cpuspeedEnable3.3.15.3 crondConfigure3.4cupsDisable if possible3.8firstbootDisable if possible3.3.1gpmDisable if possible3.3.2haldaemonDisable if possible3.3.13.2hiddDisable if possible3.3.14.2hplipDisable if possible3.8.4.1ip6tablesConfigure2.5.5iptablesConfigure2.5.5irqbalanceEnable3.3.3isdnDisable if possible3.3.4kdumpDisable if possible3.3.5kudzuDisable if possible3.3.6 mcstransDisable if possible2.4.3.2 (SELinux) mdmonitorDisable if possible3.3.7 messagebusDisable if possible3.3.13.1microcodeDisable if possible3.3.8netfsDisable if possible3.13 (NFS)networkEnable3.3.9nfslockDisable if possible3.13 (NFS)pcscdDisable if possible3.3.10portmapDisable if possible3.13 (NFS) readahead_earlyDisable if possible3.3.12readahead_laterDisable if possible3.3.12restorecondEnable2.4.3.3 (SELinux)rhnsdDisable if possible2.1.2.2 rpcgssdDisable if possible3.13 (NFS) rpcidmapdDisable if possible3.13 (NFS) sendmailConfigure3.11setroubleshootDisable if possible2.4.3.1 (SELinux)smartdEnable3.3.11 sshdServers only3.5syslogConfigure2.6.1xfsDisable if possible3.6 (X11) yum-updatesdDisable if possible2.1.2.3.2Guidance for Unfamiliar Services
If the system is running any services which have not been
covered, determine what these services do, and disable them if they are not needed or if
they pose a high risk.
If a service srvname is unknown, try running: $ rpm -qf /etc/init.d/srvname
to discover which RPM package installed the service. Then, run: $ rpm -qi rpmname
for a brief description of what that RPM does.Obsolete Services
This section discusses a number of network-visible services which
have historically caused problems for system security, and for which disabling or severely
limiting the service has been the best available guidance for some time. As a result of this
consensus, these services are not installed as part of RHEL5 by default.
Organizations which
are running these services should prioritize switching to more secure services which provide
the needed functionality. If it is absolutely necessary to run one of these services for
legacy reasons, care should be taken to restrict the service as much as possible, for
instance by configuring host firewall software (see Section 2.5.5) to restrict access to the
vulnerable service to only those remote hosts which have a known need to use it.Inetd and Xinetd
Is there an operational need to run the deprecated inetd or
xinetd software packages? If not, ensure that they are removed from the system: # yum erase inetd xinetd
Beginning with Red Hat Enterprise Linux 5, the xinetd service is no
longer installed by default. This change represents increased awareness that the dedicated
network listener model does not improve security or reliability of services, and that
restriction of network listeners is better handled using a granular model such as SELinux
than using xinetd's limited security options.Disable InetdThe inetd service should be disabled.CCE-4234-1(1) via chkconfigDisable XinetdThe xinetd service should be disabled.CCE-4252-3(1) via chkconfigUninstall InetdThe inetd package should be uninstalled.CCE-4023-8(1) via yum# yum erase inetdUninstall XinetdThe xinetd package should be uninstalled.CCE-4164-0(1) via yum# yum erase xinetdTelnet
Is there a mission-critical reason for users to access the system
via the insecure telnet protocol, rather than the more secure SSH protocol? If not, ensure
that the telnet server is removed from the system: # yum erase telnet-server
The telnet
protocol uses unencrypted network communication, which means that data from the login
session, including passwords and all other information transmitted during the session, can
be stolen by eavesdroppers on the network, and also that outsiders can easily hijack the
session to gain authenticated access to the telnet server. Organizations which use telnet
should be actively working to migrate to a more secure protocol.
See Section 3.5 for information about the SSH service.Remove Telnet Clients
In order to prevent users from casually attempting to use a telnet server, and thus exposing their credentials
over the network, remove the telnet package, which contains a telnet client program:# yum erase telnet
If Kerberos is not used, remove the krb5-workstation package, which also includes a telnet client:# yum erase krb5-workstationRemove the telnet client command from the SystemThe telnet package should be uninstalled.(1) via yum# yum erase telnetRemove the kerberos telnet client from the SystemThe krb5-workstation package should be uninstalled.(1) via yum# yum erase rsh-serverUninstall Telnet serverThe telnet-server package should be uninstalled.CCE-4330-7(1) via yum# yum erase telnet-serverDisable telnet servicetelnet service should be disabled.CCE-3390-2(1) via chkconfigRlogin, Rsh, and RcpThe Berkeley r-commands are legacy services which allow cleartext remote access and have an insecure trust model.Remove the Rsh Server Commands from the System
Is there a mission-critical reason for users to access the
system via the insecure rlogin, rsh, or rcp commands rather than the more secure ssh and
scp? If not, ensure that the rsh server is removed from the system: # yum erase rsh-server
SSH was designed to be a drop-in replacement for the r-commands, which suffer
from the same hijacking and eavesdropping problems as telnet. There is unlikely to be a
case in which these commands cannot be replaced with SSH.Remove the Rsh Server Commands from the SystemThe rsh-server package should be uninstalled.CCE-4308-3(1) via yum# yum erase rsh-serverdisable rcpThe rcp service should be disabled.CCE-3974-3(1) via chkconfig# chkconfig rcp offdisable rshThe rsh service should be disabled.CCE-4141-8(1) via chkconfig# chkconfig rsh offdisable rloginThe rlogin service should be disabled.CCE-3537-8(1) via chkconfig# chkconfig rlogin offRemove .rhosts Support from PAM Configuration Files
Check that pam_rhosts authentication is not used by any PAM
services. Run the command: # grep -l pam_rhosts /etc/pam.d/*
This command should return no output.
The RHEL5 default is not to rely on .rhosts or /etc/hosts.equiv for any
PAM-based services, so, on an uncustomized system, this command should return no output.
If any files do use pam_rhosts, modify them to make use of a more secure authentication
method instead. For more information about PAM, see Section 2.3.3.Remove .rhosts Support from PAM Configuration FilesCheck that pam_rhosts authentication is not used by any PAM services.Remove the Rsh Client Commands from the System
In order to prevent users from casually attempting to make use of an rsh server and thus exposing their
credentials over the network, remove the rsh package, which contains client programs for many of r-commands
described above:# yum erase rsh
Users should be trained to use the SSH client, and never attempt to connect to an rsh or telnet server. The
krb5-workstation package also contains r-command client programs and should be removed as described in
Section 3.2.2.1, if Kerberos is not in use.
Remove the Rsh Client Commands from the SystemThe rsh package, which contains client programs for many of r-commands should be uninstalled.(1) via yum# yum erase rshNIS
The NIS client service ypbind is not activated by default. In the
event that it was activated at some point, disable it by executing the command: # chkconfig ypbind off
The NIS server package is not installed by default. In the event that
it was installed at some point, remove it from the system by executing the command: # yum erase ypserv
The Network Information Service (NIS), also known as 'Yellow Pages' (YP), and
its successor NIS+ have been made obsolete by Kerberos, LDAP, and other modern centralized
authentication services. NIS should not be used because it suffers from security problems
inherent in its design, such as inadequate protection of important authentication
information.Uninstall NISThe ypserv package should be uninstalled.CCE-4348-9(1) via yum# yum erase ypservDisable NISThe ypbind service should be disabled.CCE-3705-1(1) via chkconfig# chkconfig ypbind offTFTP Server
Is there an operational need to run the deprecated TFTP server
software? If not, ensure that it is removed from the system: # yum erase tftp-server
TFTP is a lightweight version of the FTP protocol which has traditionally been used to
configure networking equipment. However, TFTP provides little security, and modern
versions of networking operating systems frequently support configuration via SSH or
other more secure protocols. A TFTP server should be run only if no more secure method of
supporting existing equipment can be found.Uninstall TFTP ServerThe tftp-server package should be uninstalled.CCE-3916-4(1) via yum# yum erase tftp-serverDisable TFTP ServerThe tftp service should be disabled.CCE-4273-9(1) via chkconfig# chkconfig tftp offBaseServices
This section addresses the base services that are configured to
start up on boot in a RHEL5 default installation. Some of these services listen on the
network and should be treated with particular discretion. The other services are local
system utilities that may or may not be extraneous. Each of these services should be
disabled if not required.Installation Helper Service (firstboot)
Firstboot is a daemon specific to the Red Hat installation
process. It handles 'one-time' configuration following successful installation of the
operating system. As such, there is no reason for this service to remain enabled.
Disable firstboot by issuing the command: # chkconfig firstboot offInstallation Helper Service (firstboot)The firstboot service should be disabled.CCE-3412-4(1) via chkconfig# chkconfig firstboot offConsole Mouse Service (gpm)
GPM is the service that controls the text console mouse pointer.
(The X Windows mouse pointer is unaffected by this service.)
If mouse functionality in the console is not required, disable this service: # chkconfig gpm off
Although it is
preferable to run as few services as possible, the console mouse pointer can be useful for
preventing administrator mistakes in runlevel 3 by enabling copy-and-paste operations.Console Mouse Service (gpm)The gpm service should be disabled.CCE-4229-1(1) via chkconfig# chkconfig gpm offInterrupt Distribution on Multiprocessor Systems (irqbalance)
The goal of the irqbalance service is to optimize the balance
between power savings and performance through distribution of hardware interrupts across
multiple processors.
In a server environment with multiple processors, this provides a
useful service and should be left enabled. If a machine has only one processor, the
service may be disabled: # chkconfig irqbalance offInterrupt Distribution on Multiprocessor Systems (irqbalance)The irqbalance service should be disabled.CCE-4123-6(1) via chkconfig# chkconfig irqbalance offISDN Support (isdn)
The ISDN service facilitates Internet connectivity in the
presence of an ISDN modem. If an ISDN modem is not being used, disable this service: # chkconfig isdn offISDN Support (isdn)The isdn service should be disabled.CCE-4286-1(1) via chkconfig# chkconfig isdn offKdump Kernel Crash Analyzer (kdump)
Kdump is a new kernel crash dump analyzer. It uses kexec to boot
a secondary kernel ('capture' kernel) following a system crash. The kernel dump from the
system crash is loaded into the capture kernel for analysis.
Unless the system is used for kernel development or testing, disable the service: # chkconfig kdump offKdump Kernel Crash Analyzer (kdump)The kdump service should be disabled.CCE-3425-6(1) via chkconfig# chkconfig kdump offKudzu Hardware Probing Utility (kudzu)
Is there a mission-critical reason for console users to add new
hardware to the system? If not: # chkconfig kudzu off
Kudzu, Red Hat's hardware detection
program, represents an unnecessary security risk as it allows unprivileged users to
perform hardware configuration without authorization. Unless this specific functionality
is required, Kudzu should be disabled.Kudzu Hardware Probing Utility (kudzu)The kudzu service should be disabled.CCE-4211-9(1) via chkconfig# chkconfig kudzu offSoftware RAID Monitor (mdmonitor)
The mdmonitor service is used for monitoring a software RAID
(hardware RAID setups do not use this service). This service is extraneous unless software
RAID is in use (which is not common).
If software RAID monitoring is not required, disable this service: # chkconfig mdmonitor offSoftware RAID Monitor (mdmonitor)The mdmonitor service should be disabled.CCE-3854-7(1) via chkconfig# chkconfig mdmonitor offIA32 Microcode Utility(microcodectl)
microcode ctl is a microcode utility for use with Intel IA32
processors (Pentium Pro, PII, Celeron, PIII, Xeon, Pentium 4, etc)
If the system is not running an Intel IA32 processor, disable this service: # chkconfig microcode ctl offIA32 Microcode Utility(microcodectl)The microcode_ctl service should be disabled.CCE-4356-2(1) via chkconfig# chkconfig microcode ctl offNetwork Service (network)
The network service allows associated network interfaces to
access the network. This section contains general guidance for controlling the operation
of the service. For kernel parameters which affect networking, see SectionDisable All Networking if Not Needed
If the system is a standalone machine with no need for network
access or even communication over the loopback device, then disable this service: # chkconfig network offDisable All Networking if Not Needed)The network service should be disabled.CCE-4369-5(1) via chkconfig# chkconfig network offDisable All External Network Interfaces if Not Needed
If the system does not require network communications but still
needs to use the loopback interface, remove all files of the form ifcfg-interface except
for ifcfg-lo from /etc/sysconfig/network-scripts: # rm /etc/sysconfig/network-scripts/ifcfg-interfaceDisable All External Network Interfaces if Not NeededAll files of the form ifcfg-interface except for ifcfg-lo in /etc/sysconfig/network-scripts should be removedvia /etc/sysconfig/network-scripts# rm /etc/sysconfig/network-scripts/ifcfg-interfaceDisable Zeroconf Networking
Zeroconf networking allows the system to assign itself an IP
address and engage in IP communication without a statically-assigned address or even a
DHCP server. Automatic address assignment via Zeroconf (or DHCP) is not recommended.
To disable Zeroconf automatic route assignment in the 169.245.0.0 subnet, add or correct
the following line in /etc/sysconfig/network:
NOZEROCONF=yes
Zeroconf addresses are in
the network 169.254.0.0. The networking scripts add entries to the system's routing
table for these addresses. Zeroconf address assignment commonly occurs when the system
is configured to use DHCP but fails to receive an address assignment from the DHCP
server.Disable Zeroconf NetworkingDisable Zeroconf automatic route assignment in the 169.245.0.0 subnet.CCE-4369-5(1) via /etc/sysconfig/networkSmart Card Support (pcscd)
The pcscd service provides support for Smart Cards and Smart Card
Readers. If Smart Cards are not in use on the system, disable this service: # chkconfig pcscd offSmart Card Support (pcscd)The pcscd service should be disabled.CCE-4100-4(1) via chkconfig# chkconfig pcscd offSMART Disk Monitoring Support (smartd)
SMART (Self-Monitoring, Analysis, and Reporting Technology) is a
feature of hard drives that allows them to detect symptoms of disk failure and relay an
appropriate warning. This technology is considered to bring relatively low security risk,
and can be useful.
Leave this service running if the system's hard drives are
SMART-capable. Otherwise, disable it: # chkconfig smartd offSMART Disk Monitoring Support (smartd)The smartd service should be disabled.CCE-3455-3(1) via chkconfig# chkconfig smartd offBoot Caching (readahead early/readahead later)
The following services provide one-time caching of files
belonging to some boot services, with the goal of allowing the system to boot faster.
It is recommended that this service be disabled on most machines: # chkconfig readahead_early off
# chkconfig readahead_later off
The readahead services do not substantially increase a
system's risk exposure, but they also do not provide great benefit. Unless the system is
running a specialized application for which the file caching substantially improves system
boot time, this guide recommends disabling the services.Boot Caching (readahead early/readahead later)The readahead_early service should be disabled.CCE-4421-4(1) via chkconfig# chkconfig readahead early offBoot Caching (readahead early/readahead later)The readahead_later service should be disabled.CCE-4302-6(1) via chkconfig# chkconfig readahead later offApplication Support Services
The following services are software projects of freedesktop.org
that are meant to provide system integration through a series of common APIs for
applications. They are heavily integrated into the X Windows environment. If the system is
not using X Windows, these services can typically be disabled.D-Bus IPC Service (messagebus)
D-Bus is an IPC mechanism that provides a common channel for
inter-process communication.
If no services which require D-Bus are in use, disable this service: # chkconfig messagebus off
A number of default services make use of D-Bus,
including X Windows (Section 3.6), Bluetooth (Section 3.3.14) and Avahi (Section 3.7).
This guide recommends that D-Bus and all its dependencies be disabled unless there is a
mission-critical need for them.
Stricter configuration of D-Bus is possible and
documented in the man page dbus-daemon(1). D-Bus maintains two separate configuration
files, located in /etc/dbus-1/, one for system-specific configuration and the other for
session-specific configuration.D-Bus IPC Service (messagebus)The messagebus service should be disabled.CCE-3822-4(1) via chkconfig# chkconfig messagebus offHAL Daemon (haldaemon)
The haldaemon service provides a dynamic way of managing device
interfaces. It automates device configuration and provides an API for making devices
accessible to applications through the D-Bus interface.HAL Daemon (haldaemon)The haldaemon service should be disabled.CCE-4364-6(1) via chkconfig# chkconfig haldaemon offDisable HAL Daemon if Possible
HAL provides valuable attack surfaces to attackers as an
intermediary to privileged operations and should be disabled unless necessary: # chkconfig haldaemon offConfigure HAL Daemon if Necessary
HAL provides a limited user the ability to mount system
devices. This is primarily used by X utilities such as gnome-volume-manager to perform
automounting of removable media.
HAL configuration is currently
only possible through a series of fdi files located in
/usr/share/hal/fdi/
Note: The HAL future road map includes a
mandatory framework for managing administrative privileges called
PolicyKit.
To prevent users from accessing devices through HAL,
create the
file
/etc/hal/fdi/policy/99-policy-all-drives.fdi
with the contents:
<?xml version="1.0"
encoding="UTF-8"?><deviceinfo
version="0.2"><device><match key="info.capabilities"
contains="volume"><merge key="volume.ignore"
type="bool">true</merge></match></device></deviceinfo>
The
above code matches any device labeled with the volume capability (any device capable
of being mounted will be labeled this way) and sets the corresponding volume.ignore
key to true, indicating that the volume should be ignored. This both makes the volume
invisible to the UI, and denies mount attempts by unprivileged users.
Bluetooth Support
Bluetooth provides a way to transfer information between devices
such as mobile phones, laptops, PCs, printers, digital cameras, and video game consoles
over a short-range wireless link. Any wireless communication presents a serious security
risk to sensitive or classified systems. Section 2.5.2 contains information on the related
topic of wireless networking.
Removal of hardware is the only way to ensure that the
Bluetooth wireless capability remains disabled. If it is completely impractical to remove
the Bluetooth hardware module, and site policy still allows the device to enter sensitive
spaces, every effort to disable the capability via software should be made. In general,
acquisition policy should include provisions to prevent the purchase of equipment that
will be used in sensitive spaces and includes Bluetooth capabilities.Bluetooth Host Controller Interface Daemon (bluetooth)
The bluetooth service enables the system to use Bluetooth
devices. If the system requires no Bluetooth devices, disable this service:# chkconfig bluetooth offBluetooth Host Controller Interface Daemon (bluetooth)The bluetooth service should be disabled.CCE-4355-4(1) via chkconfig# chkconfig bluetooth offBluetooth Input Devices (hidd)
The hidd service provides support for Bluetooth input devices.
If the system has no Bluetooth input devices (e.g. keyboard or mouse), disable this
service: # chkconfig hidd offBluetooth Input Devices (hidd)The hidd service should be disabled.CCE-4377-8(1) via chkconfig# chkconfig hidd offDisable Bluetooth Kernel Modules
The kernel's module loading system can be configured to prevent
loading of the Bluetooth module.
Add the following to /etc/modprobe.conf to prevent the
loading of the Bluetooth module:
alias net-pf-31 off
The unexpected name, net-pf-31, is
a result of how the kernel requests modules for network protocol families; it is an
alias for the bluetooth module.Disable Bluetooth Kernel ModulesPrevent loading of the Bluetooth module.(1) via /etc/modprobe.confPower Management Support
The following services provide an interface to power management
functions. These functions include monitoring battery power, system hibernate/suspend, CPU
throttling, and various power-save utilities.Advanced Power Management Subsystem (apmd)
The apmd service provides last generation power management
support.
If the system is capable of ACPI support, or if power management is not
necessary, disable this service: # chkconfig apmd off
APM is being replaced by ACPI and
should be considered deprecated. As such, it can be disabled if ACPI is supported by
your hardware and kernel. If the file /proc/acpi/info exists and contains ACPI version
information, then APM can safely be disabled without loss of functionality.Advanced Power Management Subsystem (apmd)The apmd service should be disabled.CCE-4289-5(1) via chkconfig# chkconfig apmd offAdvanced Configuration and Power Interface (acpid)
The acpid service provides next generation power management
support.
Unless power management features are not necessary, leave this service enabled.Advanced Configuration and Power Interface (acpid)The acpid service should be disabled.CCE-4298-6(1) via chkconfigCPU Throttling (cpuspeed)
The cpuspeed service uses hardware support to throttle the CPU
when the system is idle. Unless CPU power optimization is unnecessary, leave this
service enabled.CPU Throttling (cpuspeed)The cpuspeed service should be disabled.CCE-4051-9(1) via chkconfigCron and At Daemons
The cron and at services are used to allow commands to be executed
at a later time. The cron service is required by almost all systems to perform necessary
maintenance tasks, while at may or may not be required on a given system. Both daemons
should be configured defensively.Enable cron DaemonThe crond service should be enabled.CCE-4324-0(1) via chkconfigDisable anacron if Possible
Is this a machine which is designed to run all the time, such as
a server or a workstation which is left on at night? If so: # yum erase anacron
The
anacron subsystem is designed to provide cron functionality for machines which may be shut
down during the normal times that system cron jobs run, frequently in the middle of the
night. Laptops and workstations which are shut down at night should keep anacron enabled,
so that standard system cron jobs will run when the machine boots.
However, on machines
which do not need this additional functionality, anacron represents another piece of
privileged software which could contain vulnerabilities. Therefore, it should be removed
when possible to reduce system risk.Disable anacron if PossibleThe anacron service should be disabled.CCE-4406-5(1) via chkconfigUninstall anacron if PossibleThe anacron package should be uninstalled.CCE-4428-9(1) via yum# yum erase anacronRestrict Permissions on Files Used by cronRestrict the permissions on the primary system crontab file: # chown root:root /etc/crontab
# chmod 600 /etc/crontabIf anacron has not been removed,
restrict the permissions on its primary configuration file: # chown root:root /etc/anacrontab
# chmod 600 /etc/anacrontab Restrict the permission on all system
crontab directories: # cd /etc
# chown -R root:root cron.hourly cron.daily cron.weekly cron.monthly cron.d
# chmod -R go-rwx cron.hourly cron.daily cron.weekly cron.monthly cron.d Restrict the permissions on the spool directory for user crontab files: # chown root:root /var/spool/cron
# chmod -R go-rwx /var/spool/cron
Cron and anacron make use of a
number of configuration files and directories. The system crontabs need only be edited by
root, and user crontabs are edited using the setuid root crontab command. If unprivileged
users can modify system cron configuration files, they may be able to gain elevated
privileges, so all unnecessary access to these files should be disabled.group owner of /etc/crontabSpecify group owner of /etc/crontab.Specify group owner of /etc/crontabrootrootuser owner of /etc/crontabSpecify user owner of /etc/crontab.Specify user owner of /etc/crontabrootrootpermissions on /etc/crontab fileSpecify file permissions on /etc/crontab.Specify permissions of /etc/crontab110100100110100100100000000110000000111000000group owner of /etc/anacrontabSpecify group owner of /etc/ancrontab.Specify group owner of /etc/anacrontabrootrootuser owner of /etc/anacrontabSpecify user owner of /etc/anacrontab.Specify user owner of /etc/anacrontabrootrootpermissions on /etc/anacrontab fileSpecify file permissions on /etc/crontab.Specify permissions of /etc/anacrontab110100100110100100100000000110000000111000000group owner of cron.hourly cron.daily cron.weekly cron.monthly cron.dSpecify group owner of /etc/cron.* files and directories.Specify group owner of /etc/cron.* files and directoriesrootrootuser owner of cron.hourly cron.daily cron.weekly cron.monthly cron.dSpecify user owner of /etc/cron.* files and directories.Specify user owner of /etc/cron.* files and directoriesrootrootpermissions on cron.hourly cron.daily cron.weekly cron.monthly cron.dSpecify file and directory permissions on /etc/cron.*.Specify permissions of /etc/cron.* files and directories111101101111101101100000000110000000111000000group owner of /var/spool/cronSpecify group owner of /var/spool/cron.Specify group owner of /var/spool/cronrootrootuser owner of /var/spool/cronSpecify user owner of /var/spool/cron.rootrootpermissions on /var/spool/cron fileSpecify file permissions on /var/spool/cron.Specify file permissions of /var/spool/cron111000000100000000110000000111000000Set group owner on /etc/crontabThe /etc/crontab file should be owned by the appropriate group.CCE-3626-9(1) via chownSet user owner on /etc/crontabThe /etc/crontab file should be owned by the appropriate user.CCE-3851-3(1) via chownSet Permissions on /etc/crontabRestrict Permissions on Files Used by cronFile permissions for /etc/crontab should be set correctly.CCE-4388-5(1) via chmodSet group owner on /etc/anacrontabThe /etc/anacrontab file should be owned by the appropriate group.CCE-3604-6(1) via chownSet user owner on /etc/anacrontabThe /etc/anacrontab file should be owned by the appropriate user.CCE-4379-4(1) via chownSet Permissions on /etc/anacrontabFile permissions for /etc/anacrontab should be set correctly.CCE-4304-2(1) via chmodSet group owner on /etc/cron.hourlyThe /etc/cron.hourly file should be owned by the appropriate group.CCE-4054-3(1) via chownSet group owner on /etc/cron.dailyThe /etc/cron.daily file should be owned by the appropriate group.CCE-3481-9(1) via chownSet group owner on /etc/cron.weeklyThe /etc/cron.weekly file should be owned by the appropriate group.CCE-4331-5(1) via chownSet group owner on /etc/cron.monthlyThe /etc/cron.monthly file should be owned by the appropriate group.CCE-4322-4(1) via chownSet group owner on /etc/cron.dThe /etc/cron.d file should be owned by the appropriate group.CCE-4212-7(1) via chownSet user owner on /etc/cron.hourlyThe /etc/cron.hourly file should be owned by the appropriate user.CCE-3983-4(1) via chownSet user owner on /etc/cron.dailyThe /etc/cron.daily file should be owned by the appropriate user.CCE-4022-0(1) via chownSet user owner on /etc/cron.weeklyThe /etc/cron.weekly file should be owned by the appropriate user.CCE-3833-1(1) via chownSet user owner on /etc/cron.monthlyThe /etc/cron.monthly file should be owned by the appropriate user.CCE-4441-2(1) via chownSet user owner on /etc/cron.dThe /etc/cron.d file should be owned by the appropriate user.CCE-4380-2(1) via chownSet permissions on /etc/cron.hourlyFile permissions for /etc/cron.hourly should be set correctly.CCE-4106-1(1) via chmodSet permissions on /etc/cron.dailyFile permissions for /etc/cron.daily should be set correctly.CCE-4450-3(1) via chmodSet permissions on /etc/cron.weeklyFile permissions for /etc/cron.weekly should be set correctly.CCE-4203-6(1) via chmodSet permissions on /etc/cron.monthlyFile permissions for /etc/cron.monthly should be set correctly.CCE-4251-5(1) via chmodSet permissions on /etc/cron.dFile permissions for /etc/cron.d should be set correctly.CCE-4250-7(1) via chmodRestrict group owner on /var/spool/cron directoryThe /var/spool/cron directory should be owned by the appropriate group.(1) via chownRestrict user owner on /var/spool/cron directoryThe /var/spool/cron directory should be owned by the appropriate user.(1) via chownRestrict Permissions on /var/spool/cron directoryDirectory permissions for /var/spool/cron should be set correctly.(1) via chmodDisable at if PossibleUnless the at daemon is required, disable it with the following command:# chkconfig atd off
Many of the periodic or delayed execution features of the at daemon can be provided through the cron daemon
instead.
Disable at DaemonThe atd service should be disabled.(1) via chkconfiguninstall at DaemonThe at package should be removed.(1) via chkconfigRestrict at and cron to Authorized UsersRemove the cron.deny file: # rm /etc/cron.denyEdit /etc/cron.allow, adding one line for each user allowed to use the crontab command to
create cron jobs. Remove the at.deny file: # rm /etc/at.deny Edit /etc/at.allow, adding one line for each user allowed to use the at command to create at jobs.
The
/etc/cron.allow and /etc/at.allow files contain lists of users who are allowed to use cron
and at to delay execution of processes. If these files exist and if the corresponding
files /etc/cron.deny and /etc/at.deny do not exist, then only users listed in the relevant
allow files can run the crontab and at commands to submit jobs to be run at scheduled
intervals.
On many systems, only the system administrator needs the ability to schedule
jobs. Note that even if a given user is not listed in cron.allow, cron jobs can still be
run as that user. The cron.allow file controls only administrative access to the crontab
command for scheduling and modifying cron jobs.Remove /etc/cron.deny/etc/cron.deny file should not exist.rm /etc/cron.denyRemove /etc/at.deny/etc/at.deny file should not exist.rm /etc/at.denySSH Server
The SSH protocol is recommended for remote login and remote file
transfer. SSH provides confidentiality and integrity for data exchanged between two systems,
as well as server authentication, through the use of public key cryptography. The
implementation included with the system is called OpenSSH, and more detailed documentation
is available from its website, http://www.openssh.org. Its server program is called sshd and
provided by the RPM package openssh-server.Disable OpenSSH Server if Possible
Unless the system needs to provide the remote login and file
transfer capabilities of SSH, disable and remove the OpenSSH server and its configuration.Disable and Remove OpenSSH Software
Disable and remove openssh-server with the commands: # chkconfig sshd off
# yum erase openssh-server
Users of the system will still be able to
use the SSH client program /usr/bin/ssh to access SSH servers on other systems.Disable OpenSSH SoftwareThe sshd service should be disabled.CCE-4268-9(1) via chkconfig# chkconfig sshd offRemove OpenSSH SoftwareSSH should be uninstalledCCE-4272-1(1) via yum# yum erase openssh-serverRemove SSH Server iptables Firewall Exception
Edit the files /etc/sysconfig/iptables and
/etc/sysconfig/ip6tables (if IPv6 is in use). In each file, locate and delete the line:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
By default, inbound connections to SSH's port are allowed. If the SSH server is not
being used, this exception should be removed from the firewall configuration. See
Section 2.5.5 for more information about Iptables.Remove SSH Server iptables Firewall ExceptionInbound connections to the ssh port should be deniedCCE-4295-2(1) /etc/sysconfig/iptablesRemove SSH Server ip6tables Firewall ExceptionInbound connections to the ssh port should be denied(1) /etc/sysconfig/ip6tablesConfigure OpenSSH Server if Necessary
If the system needs to act as an SSH server, then certain changes
should be made to the OpenSSH daemon configuration file /etc/ssh/sshd config. The
following recommendations can be applied to this file. See the sshd config(5) man page for
more detailed information.Ensure Only Protocol 2 Connections Allowed
Only SSH protocol version 2 connections should be permitted.
Version 1 of the protocol contains security vulnerabilities. The default setting shipped
in the configuration file is correct, but it is important enough to check.
Verify that the following line appears:
Protocol 2Ensure Only Protocol 2 Connections AllowedSSH version 1 protocol support should be disabled.CCE-4325-7(1) via /etc/ssh/sshd_configLimit Users SSH Access'
By default, the SSH configuration allows any user to access the
system. In order to allow all users to login via SSH but deny only a few users, add or
correct the following line:
DenyUsers USER1 USER2
Alternatively, if it is appropriate to allow only a few users access to the system via
SSH, add or correct the following line:
AllowUsers USER1 USER2Set Idle Timeout Interval for User Logins
SSH allows administrators to set an idle timeout interval.
After this interval has passed, the idle user will be automatically logged out.
Find and edit the following lines in /etc/ssh/sshd config as follows:
ClientAliveInterval interval
ClientAliveCountMax 0
The timeout interval is given in seconds.
To have a timeout of 5 minutes, set interval to 300.
If a shorter timeout has already been set for
the login shell, as in Section 2.3.5.5, that value will preempt any SSH setting made
here. Keep in mind that some processes may stop SSH from correctly detecting that the
user is idle.SSH session Idle timeSpecify duration of allowed idle time.Specify duration of allowed idle time (in seconds) for SSH session300300600SSH session ClientAliveCountMaxSets the number of client alive messages which may be sent without sshd receiving any messages back from the client.Specify the number of clients alive messages which may be sent without sshd receiving any messages back from the client303Set Idle Timeout Interval for User LoginsThe SSH idle timout interval should be set to an appropriate
valueCCE-3845-5(1) via /etc/ssh/sshd_configSet ClientAliveCountMax for User LoginsThe ClientAliveCountMax should be set to an appropriate value(1) via /etc/ssh/sshd_configDisable .rhosts Files
SSH can emulate the behavior of the obsolete rsh command in
allowing users to enable insecure access to their accounts via .rhosts files.
To ensure that this behavior is disabled, add or correct the following line:
IgnoreRhosts yesDisable .rhosts FilesEmulation of the rsh command through the ssh server should be disabledCCE-4475-0(1) via /etc/ssh/sshd_configDisable Host-Based Authentication
SSH's cryptographic host-based authentication is slightly more
secure than .rhosts authentication, since hosts are cryptographically authenticated.
However, it is not recommended that hosts unilaterally trust one another, even within an
organization.
To disable host-based authentication, add or correct the following line:
HostbasedAuthentication noDisable Host-Based AuthenticationSSH host-based authentication should be disabledCCE-4370-3(1) via /etc/ssh/sshd_configDisable root Login via SSH
The root user should never be allowed to login directly over a
network, as this both reduces auditable information about who ran privileged commands on
the system and allows direct attack attempts on root's password.
To disable root login via SSH, add or correct the following line:
PermitRootLogin noDisable root Login via SSHRoot login via SSH should be disabledCCE-4387-7(1) via /etc/ssh/sshd_configDisable Empty Passwords
To explicitly disallow remote login from accounts with empty
passwords, add or correct the following line:
PermitEmptyPasswords no
Measures should also be taken to disable accounts with empty passwords system-wide,
as described in Section 2.3.1.5.Disable Empty PasswordsRemote connections from accounts with empty passwords should be disabledCCE-3660-8(1) via /etc/ssh/sshd_configEnable a Warning Banner
Section 2.3.7 contains information on how to create an
appropriate warning banner.
To enable a warning banner, add or correct the following line:
Banner /etc/issueEnable a Warning BannerSSH warning banner should be enabledCCE-4431-3(1) via /etc/ssh/sshd_configDo Not Allow Users to Set Environment Options
To prevent users from being able to present environment options to the SSH daemon and potentially bypass
some access restrictions, add or correct the following line:
PermitUserEnvironment no
Do Not Allow Users to Set Environment OptionsPermitUserEnvironment should be disabled(1) via /etc/ssh/sshd_configUse Only Approved Ciphers
Limit the ciphers to those which are FIPS-approved and only use ciphers in counter (CTR) mode. The
following line demonstrates use of FIPS-approved ciphers in CTR mode:
Ciphers aes128-ctr,aes192-ctr,aes256-ctr
The man page sshd_config(5) contains a list of the ciphers supported for the current release of the SSH daemon.Use Only Approved CiphersUse only FIPS approved ciphers not in CBC mode(1) via /etc/ssh/sshd_configStrengthen Firewall Configuration if Possible
If the SSH server must only accept connections from the local
network, then strengthen the default firewall rule for the SSH service.
Determine an
appropriate network block, netwk, and network mask, mask, representing the machines on
your network which must be allowed to access this SSH server.
Edit the files
/etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in use). In each file,
locate the line:
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
and replace it with:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 22 -j ACCEPT
If your site uses IPv6, and you are editing ip6tables, use the line:
-A RH-Firewall-1-INPUT -s ipv6netwk::/ipv6mask -m tcp -p tcp --dport 22 -j ACCEPT
instead because Netfilter does not yet reliably support stateful filtering for
IPv6.
See Section 2.5.5 for more information about Iptables configuration.X Window SystemThe X Window System implementation included with the system is called X.org.Disable X Windows if Possible
Unless there is a mission-critical reason for the machine to run
a GUI login screen, prevent X from starting automatically at boot. There is usually no
reason to run X Windows on a dedicated server machine, since administrators can login via
SSH or on the text console.Disable X Windows at System Boot
Edit the file /etc/inittab, and correct the line
id:5:initdefault: to:
id:3:initdefault:
This action changes the default boot runlevel of
the system from 5 to 3. These two runlevels should be identical except that runlevel 5
starts X on boot, while runlevel 3 does not.default boot levelSpecify whether to start in single user mode, text UI or graphical UI.Specify whether to start in single user mode, text UI or graphical UI5531Disable X Windows at System BootX Windows should be disabled at system bootCCE-4462-8(1) via /etc/inittabRemove X Windows from the System if Possible
Remove the X11 RPMs from the system: # yum groupremove "X Window System"
As long as X.org remains installed on the system, users can still run X
Windows by typing startx at the shell prompt. This may run X Windows using configuration
settings which are less secure than the system defaults. Therefore, if the machine is a
dedicated server which does not need to provide graphical logins at all, it is safest to
remove the X.org software entirely.
The command given here will remove over 100
packages. It should safely and effectively remove X from machines which do not need it.Remove X Windows from the System if PossibleX Windows should be removedCCE-4422-2(1) via yum# yum groupremove "X Window System"Lock Down X Windows startx Configuration if Necessary
If X is not to be started at boot time but the software must
remain installed, users will be able to run X manually using the startx command. In some
cases, this runs X with a configuration which is less safe than the default. Follow
these instructions to mitigate risk from this configuration.Disable X Font Server
Disable the xfs helper service: # chkconfig xfs off
The
system's X.org requires the X Font Server service (xfs) to function. The xfs service
will be started automatically if X.org is activated via startx. Therefore, it is safe
to prevent xfs from starting at boot when X is disabled, even if users are allowed to
run X manually.Disable X Window System Listening
To prevent X.org from listening for remote connections,
create the file /etc/X11/xinit/xserverrc and fill it with the following line:
exec X :0 -nolisten tcp $@
One of X.org's features is the ability to provide remote graphical
display. This feature should be disabled unless it is required. If the system uses
runlevel 5, which is the default, the GDM display manager starts X safely, with remote
listening disabled. However, if X is started from the command line with the startx
command, then the server will listen for new connections on X's default port, 6000.
See the xinit(1), startx(1), and Xserver(1) man pages for more information.Disable X Window System ListeningDisable the ability to provide remote graphical displayCCE-4074-1(1) via /etc/X11/xinit/xserverrcecho "exec X :0 -nolisten tcp $@" > /etc/X11/xinit/xserverrcConfigure X Windows if Necessary
If there is a mission-critical reason for this machine to run a
GUI, improve the security of the default X configuration by following the guidance in this
section.Create Warning Banners for GUI Login Users
Edit the file /etc/gdm/custom.conf. Locate the [greeter]
section, and correct that section to contain the lines:
[greeter]
InfoMsgFile=/etc/issue
See Section 2.3.7 for an explanation of banner file use. This setting will cause the
system greeting banner to be displayed in a box prior to GUI login. If the default
banner font is inappropriate, it can be changed by specifying the InfoMsgFont directive
as well, for instance:
InfoMsgFont=Sans 12Create Warning Banners for GUI Login UsersEnable warning banner for GUI loginCCE-3717-6(1) via /etc/gdm/custom.confAvahi Server
The Avahi daemon implements the DNS Service Discovery and Multicast
DNS protocols, which provide service and host discovery on a network. It allows a system to
automatically identify resources on the network, such as printers or web servers. This
capability is also known as mDNSresponder and is a major part of Zeroconf networking. By
default, it is enabled.Disable Avahi Server if Possible
Because the Avahi daemon service keeps an open network port, it
is subject to network attacks. Disabling it is particularly important to reduce the
system's vulnerability to such attacks.Disable Avahi Server Software
Issue the command: # chkconfig avahi-daemon offDisable Avahi Server SoftwareThe avahi-daemon service should be disabled.CCE-4365-3(1) via chkconfig# chkconfig avahi-daemon offRemove Avahi Server iptables Firewall Exception
Edit the files /etc/sysconfig/iptables and
/etc/sysconfig/ip6tables (if IPv6 is in use). In each file, locate and delete the line:
-A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT
By default, inbound
connections to Avahi's port are allowed. If the Avahi server is not being used, this
exception should be removed from the firewall configuration. See Section 2.5.5 for more
information about the Iptables firewall.Configure Avahi if Necessary
If your system requires the Avahi daemon, its configuration can
be restricted to improve security. The Avahi daemon configuration file is
/etc/avahi/avahi-daemon.conf. The following security recommendations should be applied to
this file. See the avahi-daemon.conf(5) man page or documentation at http://www.avahi.org
for more detailed information about the configuration options.Serve Only via Required Protocol
The default setting in the configuration file allows Avahi to
use both IPv4 and IPv6 sockets.
If you are using only IPv4, edit
/etc/avahi/avahi-daemon.conf and ensure the following line exists in the [server]
section:
use-ipv6=no
Similarly, if you are using only IPv6, disable IPv4 sockets with the line:
use-ipv4=noServe Only via Required ProtocolThe Avahi daemon should be configured not to serve via Ipv6CCE-4136-8(1) via /etc/avahi/avahi-daemon.confServe Only via Required ProtocolThe Avahi daemon should be configured not to serve via Ipv4CCE-4409-9(1) via /etc/avahi/avahi-daemon.confCheck Responses TTL Field '
Avahi can be set to ignore IP packets unless their TTL field is
255. To make Avahi ignore packets unless the TTL field is 255, edit
/etc/avahi/avahi-daemon.conf and ensure the following line appears in the [server]
section:
check-response-ttl=yes
This helps to ensure that only mDNS responses from the
local network are processed, because the TTL field in a packet is decremented from its
initial value of 255 whenever it is routed from one network to another. Although a
properly-configured router or firewall should not allow mDNS packets into the local
network at all, this option provides another check to ensure they are not trusted.Check Responses' TTL FieldAvahi should be configured to reject packets with a TTL field not equal to 255CCE-4426-3(1) via /etc/avahi/avahi-daemon.confPrevent Other Programs from Using Avahis Port '
Avahi can stop other mDNS stacks from running on the host by
preventing other processes from binding to port 5353.
To prevent other mDNS stacks from
running, edit /etc/avahi/avahi-daemon.conf and ensure the following line appears in the
[server] section:
disallow-other-stacks=yes
This is designed to help ensure that only
Avahi is responsible for mDNS traffic coming from that port on the system.Prevent Other Programs from Using Avahi's PortAvahi should be configured to not allow other stacks from binding to port 5353CCE-4193-9(1) via /etc/avahi/avahi-daemon.confDisable Publishing if Possible
The default setting in the configuration file allows the
avahi-daemon to send information about the local host, such as its address records and
the services it offers, to the local network.
To stop sending this information but still
allow Avahi to query the network for services, ensure the configuration file includes
the following line in the [publish] section:
disable-publishing=yes
This line may be
particularly useful if Avahi is needed for printer discovery, but not to advertise
services. This configuration is highly recommended for client systems that should not
advertise their services (or existence).Disable Publishing if PossibleAvahi publishing of local information should be disabledCCE-4444-6(1) via /etc/avahi/avahi-daemon.confRestrict Published Information
If it is necessary to publish some information to the network,
it should not be joined by any extraneous information, or by information supplied by a
non-trusted source on the system. Prevent user applications from using Avahi to publish
services by adding or correcting the following line in the [publish] section:
disable-user-service-publishing=yes
Implement as many of the following lines as
possible, to restrict the information published by Avahi:
publish-addresses=no
publish-hinfo=no
publish-workstation=no
publish-domain=no
Inspect the files in the
directory /etc/avahi/services/. Unless there is an operational need to publish
information about each of these services, delete the corresponding file.
These options
should be used even if publishing is disabled entirely via disable-publishing, since
that option prevents publishing attempts from succeeding, while these options prevent
the attempts from being made in the first place. Using both approaches is recommended
for completeness.Restrict disable-user-service-publishingAvahi publishing of local information by user applications should be disabledCCE-4352-1(1) via /etc/avahi/avahi-daemon.confRestrict publish-addressesAvahi publishing of hardware information should be disabledCCE-4433-9(1) via /etc/avahi/avahi-daemon.confRestrict publish-hinfoAvahi publishing of workstation name should be disabledCCE-4451-1(1) via /etc/avahi/avahi-daemon.confRestrict publish-workstationAvahi publishing of IP addresses should be disabledCCE-4341-4(1) via /etc/avahi/avahi-daemon.confRestrict publish-domainAvahi publishing of domain name should be disabledCCE-4358-8(1) via /etc/avahi/avahi-daemon.confPrint Support
The Common Unix Printing System (CUPS) service provides both local
and network printing support. A system running the CUPS service can accept print jobs from
other systems, process them, and send them to the appropriate printer. It also provides an
interface for remote administration through a web browser. The CUPS service is installed and
activated by default. The project homepage and more detailed documentation are available at
http://www.cups.org.
The HP Linux Imaging and Printing service (HPLIP) is a separate package
that provides support for some of the additional features that HP printers provide that CUPS
may not necessarily support. It relies upon the CUPS service.Disable the CUPS Service if Possible
Do you need the ability to print from this machine or to allow
others to print to it? If not: # chkconfig cups offDisable the CUPS Service if PossibleThe cups service should be disabled.CCE-4112-9(1) via chkconfig# chkconfig cups offDisable Firewall Access to Printing Service if Possible
Does this system need to operate as a network print server? If
not, edit the files /etc/sysconfig/iptables and /etc/sysconfig/ip6tables (if IPv6 is in
use). In each file, locate and delete the lines:
-A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp --dport 631 -j ACCEPT
By
default, inbound connections to the Internet Printing Protocol port are allowed. If the
print server does not need to be accessed, either because the machine is not running the
print service at all or because the machine is not providing a remote network printer to
other machines, this exception should be removed from the firewall configuration. See
Section 2.5.5 for more information about the Iptables firewall.accept udp over IPv4Open firewall to allow udp over IPv4.Enable/Disable UDP over IPv4disabledenableddisabledenabled|disabledenableddisabledaccept udp over IPv6Open firewall to allow udp over IPv6.Enable/Disable UDP over IPv6disabledenableddisabledenabled|disabledenableddisabledDisable Firewall Access to Printing Service over IPv4 if PossibleFirewall access to printing service should be disabledCCE-3649-1(1) via /etc/sysconfig/iptablesDisable Firewall Access to Printing Service over IPv6 if PossibleFirewall access to printing service should be disabled(1) via /etc/sysconfig/ip6tablesConfigure the CUPS Service if Necessary
CUPS provides the ability to easily share local printers with
other machines over the network. It does this by allowing machines to share lists of
available printers. Additionally, each machine that runs the CUPS service can potentially
act as a print server. Whenever possible, the printer sharing and print server
capabilities of CUPS should be limited or disabled. The following recommendations should
demonstrate how to do just that.Limit Printer BrowsingBy default, CUPS listens on the network for printer list broadcasts on UDP port 631. This functionality is called printer browsing.Disable Printer Browsing Entirely if Possible
To disable printer browsing entirely, edit the CUPS
configuration file, located at /etc/cups/cupsd.conf:
Browsing Off
BrowseAllow none
The
CUPS print service can be configured to broadcast a list of available printers to the
network. Other machines on the network, also running the CUPS print service, can be
configured to listen to these broadcasts and add and configure these printers for
immediate use. By disabling this browsing capability, the machine will no longer
generate or receive such broadcasts.Disable Printer Browsing Entirely if PossibleRemote print browsing should be disabledCCE-4420-6(1) via /etc/cups/cupsd.confDeny CUPS ability to listen for Incoming printer informationCUPS should be denied the ability to listen for Incoming printer informationCCE-4407-3(1) via /etc/cups/cupsd.confLimit Printer Browsing to a Particular Subnet if Necessary
It is possible to disable outgoing printer list broadcasts
without affecting incoming broadcasts from other machines. To do so, open the CUPS
configuration file, located at /etc/cups/cupsd.conf. Look for the line that begins
with BrowseAddress and remove it. The line will look like the following:
BrowseAddress @LOCAL
If the intent is not to block printer sharing, but to limit it to a particular
set of machines, you can limit the UDP printer broadcasts to trusted network
addresses.
BrowseAddress ip-address :631
Likewise, to ignore incoming UDP printer list
broadcasts, or to limit the set of machines to listen to, use the BrowseAllow and
BrowseDeny directives.
BrowseDeny all
BrowseAllow ip-address
This combination will
deny incoming broadcasts from any machine except those that are explicitly allowed
with BrowseAllow.
By default, when printer sharing is enabled, CUPS will broadcast to
every network that its host machine is connected to through all available network
interfaces on port 631. It will also listen to incoming broadcasts from other machines
on the network. Either list one BrowseAddress line for each client machine and one
BrowseAllow line for each print server or use one of the supported shorthand notations
that the CUPS service recognizes. Please see the cupsd.conf(5) man page or the
documentation provided at http://www.cups.org for more information on other ways to
format these directives.Disable Print Server Capabilities if Possible
To prevent remote users from potentially
connecting to and using locally configured printers, disable the CUPS print server
sharing capabilities. To do so, limit how the server will listen for print jobs by
removing the more generic port directive from /etc/cups/cupsd.conf:
Port 631
and replacing it with the Listen directive:
Listen localhost:631
This will prevent remote
users from printing to locally configured printers while still allowing local users on
the machine to print normally.
By default, locally configured printers will not be
shared over the network, but if this functionality has somehow been enabled, these
recommendations will disable it again. Be sure to disable outgoing printer list
broadcasts, or remote users will still be able to see the locally configured printers,
even if they cannot actually print to them. To limit print serving to a particular set
of users, use the Policy directive.Disabling the print server capabilities in this manner will
also disable the Web Administration interface. Limit Access to the Web Administration Interface
By default, access to the CUPS web administration interface is
limited to the local machine. It is recommended that this not be changed, especially
since the authentication mechanisms that CUPS provides are limited in their
effectiveness. If it is absolutely necessary to allow remote users to administer locally
installed printers, be sure to limit that access as much as possible by taking advantage
of the Location and Policy directive blocks.
For example, to enable
remote access for ip-address for user username, modify each of the Location and Policy
directive blocks as follows:
<Location />
AuthType Basic
Require user username
Order allow,deny
Allow localhost
Allow ip-address
</Location>
As with the
BrowseAllow directive, use one Allow directive for each machine that needs access or use
one of the available CUPS directive definition shortcuts to enable access from a class
of machines at once. The Require user directive can take a list of individual users, a
group of users (prefixed with @), or the shorthand valid-user.
Host-based authentication has known limitations,
especially since IP addresses are easy to spoof. Requiring users to authenticate
themselves can alleviate this problem, but it cannot eliminate it. Do not use the root
account to manage and administer printers. Create a separate account for this purpose
and limit access to valid users with Require valid-user or Require user printeradmin .
Take Further Security Measures When Appropriate
Whenever possible, limit outside networks' access to port 631.
Consider using CUPS directives that limit the number of incoming clients, such as
MaxClients or MaxClientsPerHost. Additionally, there are a series of Policy and Location
directives intended to limit which users can perform different printing tasks. When used
together, these may help to mitigate the possibility of a denial of service attack. See
cupsd.conf(5) for a full list of possible directives.The HP Linux Imaging and Printing (HPLIP) Toolkit
The HPLIP package is an HP printing support utility that is
installed and enabled in a default installation. The HPLIP package is comprised of two
separate components. The first is the main HPLIP service and the second is a smaller
subcomponent called HPIJS. HPLIP is a feature-oriented network service that provides
higher level printing support (such as bi-directional I/O, scanning, photo card, and
toolbox functionality). HPIJS is a lower level basic printing driver that provides basic
support for non-PostScript HP printers.Disable HPLIP Service if Possible
Since the HPIJS driver will still function without the added
HPLIP service, HPLIP should be disabled unless the specific higher level functions that
HPLIP provides are needed by a non-PostScript HP printer on the system. # chkconfig hplip off
Note: If installing the HPLIP package from scratch, it should be noted that
HPIJS can be installed directly without HPLIP. Please see the FAQ at the HPLIP web site
at http://hplip.sourceforge.net/faqs.html for more information on how to do this.Disable HPLIP Service if PossibleThe hplip service should be disabled.CCE-4425-5(1) via chkconfigDHCP
The Dynamic Host Configuration Protocol (DHCP) allows systems to
request and obtain an IP address and many other parameters from a server.
In general, sites
use DHCP either to allow a large pool of mobile or unknown machines to share a limited
number of IP addresses, or to standardize installations by avoiding static, individual IP
address configuration on hosts. It is recommended that sites avoid DHCP as much as possible.
Since DHCP authentication is not well-supported, DHCP clients are open to attacks from rogue
DHCP servers. Such servers can give clients incorrect information (e.g. malicious DNS server
addresses) which could lead to their compromise.
If a machine must act as a DHCP client or
server, configure it defensively using the guidance in this section. This guide recommends
configuring networking on clients by manually editing the appropriate files under
/etc/sysconfig. It is also possible to use the graphical front-end programs
system-config-network and system-config-network-tui, but these programs rewrite
configuration files from scratch based on their defaults – destroying any manual changes –
and should therefore be used with caution.Disable DHCP Client if Possible
For each interface IFACE on the system (e.g. eth0), edit
/etc/sysconfig/network-scripts/ifcfg-IFACE and make the following changes: Correct the BOOTPROTO line to read:
BOOTPROTO=static
Add or correct the following lines,
substituting the appropriate values based on your site's addressing scheme:
NETMASK=255.255.255.0
IPADDR=192.168.1.2
GATEWAY=192.168.1.1
DHCP is the default network
configuration method provided by the system installer, so it may be enabled on many
systems.DHCP BOOTPROTOIf BOOTPROTO is not "static", then the only other item that must be set is the DEVICE item; all the rest will be determined by the boot protocol. No "dummy" entries need to be created.Choose DHCP BOOTPROTOstaticbootpdhcpstaticbootpdhcpstaticDisable DHCP Client if PossibleThe dhcp client service should be disabled for each interface.CCE-4191-3(1) via /etc/sysconfig/network-scripts/ifcfg-eth*Configure DHCP Client if necessary
If DHCP must be used, then certain configuration changes can
minimize the amount of information it receives and applies from the network, and thus the
amount of incorrect information a rogue DHCP server could successfully distribute.
For more information on configuring dhclient, see the dhclient(8) and dhclient.conf(5)
man pages.Minimize the DHCP-Configured Options
Create the file /etc/dhclient.conf, and add an appropriate
setting for each of the ten configuration settings which can be obtained via DHCP. For
each setting, setting , do one of the following: If the setting should not be
configured remotely by the DHCP server, select an appropriate static value, and add the
line:
supersede setting value ; If the setting should be configured remotely by the
DHCP server, add the lines:
request setting ;
require setting ;
For example, suppose the
DHCP server should provide only the IP address itself and the subnet mask. Then the
entire file should look like:
supersede domain-name "example.com ";
supersede domain-name-servers 192.168.1.2 ;
supersede nis-domain "";
supersede nis-servers "";
supersede ntp-servers "ntp.example.com ";
supersede routers 192.168.1.1 ;
supersede time-offset -18000 ;
request subnet-mask;
require subnet-mask;
By default, the DHCP
client program, dhclient, requests and applies ten configuration options (in addition to
the IP address) from the DHCP server: subnet-mask, broadcast-address, time-offset,
routers, domain-name, domain-name-servers, host-name, nis-domain, nis-servers, and
ntp-servers.
Many of the options requested and applied by dhclient may be the same for
every system on a network. It is recommended that almost all configuration options be
assigned statically, and only options which must vary on a host-by-host basis be
assigned via DHCP. This limits the damage which can be done by a rogue DHCP server.
If
appropriate for your site, it is also possible to supersede the host-name directive in
/etc/dhclient.conf, establishing a static hostname for the machine. However, dhclient
does not use the host name option provided by the DHCP server (instead using the value
provided by a reverse DNS lookup).
Note: In this example, the options nis-servers and
nis-domain are set to empty strings, on the assumption that the deprecated NIS protocol
is not in use. (See Section 3.2.4.) It is necessary to supersede settings for unused
services so that they cannot be set by a hostile DHCP server. If an option is set to an
empty string, dhclient will typically not attempt to configure the service.Disable DHCP Server if possible
If the dhcp package has been installed on a machine which does
not need to operate as a DHCP server, disable the daemon: # chkconfig dhcpd off
If possible, remove the software as well: # yum erase dhcp
The DHCP server dhcpd is not
installed or activated by default. If the software was installed and activated, but the
system does not need to act as a DHCP server, it should be disabled and removed. Unmanaged
DHCP servers will provide faulty information to clients, interfering with the operation of
a legitimate site DHCP server if there is one, or causing misconfigured machines to
exhibit unpredictable behavior if there is not.Disable DHCP Server if possibleThe dhcpd service should be disabled.CCE-4336-4(1) via chkconfig# chkconfig dhcpd offUninstall DHCP Server if possibleThe dhcp package should be uninstalled.CCE-4464-4(1) via yum# yum erase dhcpConfigure the DHCP Server if necessary
If the system must act as a DHCP server, the configuration
information it serves should be minimized. Also, support for other protocols and
DNS-updating schemes should be explicitly disabled unless needed.
The configuration file
for dhcpd is called /etc/dhcpd.conf. The file begins with a number of global configuration
options. The remainder of the file is divided into sections, one for each block of
addresses offered by dhcpd, each of which contains configuration options specific to that
address block.Do Not Use Dynamic DNS
To prevent the DHCP server from receiving DNS information from
clients, edit /etc/dhcpd.conf, and add or correct the following global option:
ddns-update-style none;
The Dynamic DNS protocol is used to remotely update the data
served by a DNS server. DHCP servers can use Dynamic DNS to publish information about
their clients. This setup carries security risks, and its use is not recommended.
If Dynamic DNS must be used despite the risks it poses, it is critical that Dynamic DNS
transactions be protected using TSIG or some other cryptographic authentication
mechanism. See Section 3.14 for more information about DNS servers, including further
information about TSIG and Dynamic DNS. Also see dhcpd.conf(5) for more information
about protecting the DHCP server from passing along malicious DNS data from its clients.
Note: The ddns-update-style option controls only whether the DHCP server will attempt to
act as a Dynamic DNS client. As long as the DNS server itself is correctly configured to
reject DDNS attempts, an incorrect ddns-update-style setting on the client is harmless
(but should be fixed as a best practice).Do Not Use Dynamic DNSThe dynamic DNS feature of the DHCP server should be disabledCCE-4257-2(1) via /etc/dhcpd.confDeny Decline Messages
Edit /etc/dhcpd.conf and add or correct the following global
option to prevent the DHCP server from responding the DHCPDECLINE messages, if possible:
deny declines;
The DHCPDECLINE message can be sent by a DHCP client to indicate that it
does not consider the lease offered by the server to be valid. By issuing many
DHCPDECLINE messages, a malicious client can exhaust the DHCP server's pool of IP
addresses, causing the DHCP server to forget old address allocations.Deny Decline MessagesDHCPDECLINE messages should be denied by the DHCP serverCCE-4403-2(1) via /etc/dhcpd.confDeny BOOTP Queries
Unless your network needs to support older BOOTP clients,
disable support for the bootp protocol by adding or correcting the global option:
deny bootp;
The bootp option tells dhcpd to respond to BOOTP queries. If support for this
simpler protocol is not needed, it should be disabled to remove attack vectors against
the DHCP server.Deny BOOTP QueriesBOOTP queries should be denied by the DHCP serverCCE-4345-5(1) via /etc/dhcpd.confMinimize Served Information
Edit /etc/dhcpd.conf. Examine each address range section within
the file, and ensure that the following options are not defined unless there is an
operational need to provide this information via DHCP:
option domain-name
option domain-name-servers
option nis-domain
option nis-servers
option ntp-servers
option routers
option time-offset
Because the configuration information provided by the DHCP
server could be maliciously provided to clients by a rogue DHCP server, the amount of
information provided via DHCP should be minimized. Remove these definitions from the
DHCP server configuration to ensure that legitimate clients do not unnecessarily rely on
DHCP for this information.
Note: By default, the RHEL5 client installation uses DHCP to
request much of the above information from the DHCP server. In particular, domain-name,
domain-name-servers, and routers are configured via DHCP. These settings are typically
necessary for proper network functionality, but are also usually static across machines
at a given site. See Section 3.9.2.1 for a description of how to configure static site
information within the DHCP client configuration.DHCP should not send domain-nameDomain name server information should not be sent by the DHCP server.CCE-3724-2(1) via /etc/dhcpd.confDHCP should not send domain-name-serversDefault routers should not be sent by the DHCP server.CCE-4243-2(1) via /etc/dhcpd.confDHCP should not send nis-domainDomain name should not be sent by the DHCP server.CCE-4389-3(1) via /etc/dhcpd.confDHCP should not send nis-serversNIS domain should not be sent by the DHCP server.CCE-3913-1(1) via /etc/dhcpd.confDHCP should not send ntp-serversNIS servers should not be sent by the DHCP server.CCE-4169-9(1) via /etc/dhcpd.confDHCP should not send routersTime offset should not be sent by the DHCP server.CCE-4318-2(1) via /etc/dhcpd.confDHCP should not send time-offsetNTP servers should not be sent by the DHCP server.CCE-4319-0(1) via /etc/dhcpd.confConfigure Logging
Ensure that the following line exists in /etc/syslog.conf:
daemon.* /var/log/daemon.log
Configure logwatch or other log monitoring tools to
summarize error conditions reported by the dhcpd process.
By default, dhcpd logs notices
to the daemon facility. Sending all daemon messages to a dedicated log file is part of
the syslog configuration outlined in Section 2.6.1.1.Configure DHCP Loggingdhcpd logging should be enabled.CCE-3733-3(1) via /etc/syslog.confFurther Resources* The man pages dhcpd.conf(5) and dhcpd(8) * ISC web page http://isc.org/products/DHCPNetwork Time Protocol
The Network Time Protocol is used to manage the system clock over a
network. Computer clocks are not very accurate, so time will drift unpredictably on
unmanaged systems. Central time protocols can be used both to ensure that time is consistent
among a network of machines, and that their time is consistent with the outside world.
Local time synchronization is recommended for all networks. If every machine on your network
reliably reports the same time as every other machine, then it is much easier to correlate
log messages in case of an attack. In addition, a number of cryptographic protocols (such as
Kerberos) use timestamps to prevent certain types of attacks. If your network does not have
synchronized time, these protocols may be unreliable or even unusable.
Depending on the specifics of the network, global time accuracy may be just as important as
local synchronization, or not very important at all. If your network is connected to the
Internet, it is recommended that you make use of a public timeserver, since globally
accurate timestamps may be necessary if you need to investigate or respond to an attack
which originated outside of your network.
Whether or not you use an outside timeserver, configure
the network to have a small number of machines operating as NTP servers, and the remainder
obtaining time information from those internal servers.Select NTP Software
The Network Time Protocol (RFC 1305) is designed to synchronize
time with a very high degree of accuracy even on an unreliable network. NTP is therefore a
complex protocol. The Simple Network Time Protocol (RFC 4330) implements a subset of NTP
which is intended to be good enough to meet the time requirements of most networks.
The primary implementation of NTP comes from ntp.org, and is shipped with RHEL5 as the ntp
RPM. An alternative is OpenNTPD, which is an implementation of SNTP, and which can be
obtained as source code from http://www.openntpd.org. OpenNTPD may be simpler to configure
than the reference NTP implementation, at the cost of the need to install and maintain
third-party software.
This guide does not recommend the use of a particular NTP/SNTP
software package, but does recommend that some NTP software be selected and installed on
all machines. The remainder of this section describes how to securely configure NTP
clients and servers, and discusses both the reference NTP implementation and OpenNTPD.Configure Reference NTP if AppropriateThe ntp RPM implements the reference NTP server.Configure an NTP Client
There are a number of options for configuring clients to work with the reference NTP server. It is possible to run
ntpd as a service (i.e., continuously) on each host, configuring clients so that the ntp protocol ignores all network
access. This still introduces an additional network listener on client machines, and is therefore not recommended.
This guide instead recommends running ntpd periodically via cron. It is also possible to run ntpdate via cron
with the -u option, but it is being obsoleted in favor of ntpd.
Alternately, even if the server is running the reference NTP implementation, it is possible for clients to access it
using SNTP. See Section 3.10.3.2 for information about configuring SNTP clients.Set Up Client NTP Configuration File
A valid configuration file for the client system’s ntpd must exist at /etc/ntp.conf. Ensure that /etc/ntp.conf
contains the following line, where ntp-server is the hostname or IP address of the site NTP server:
server ntp-server
Note: The ntpd software also includes authentication and encryption support which allows for clients to verify the
identity of the server, and thus guarantee the integrity of time data with high probability. See ntpd documentation
at http://www.ntp.org for more details on implementing this recommended feature.
Run ntpdate using Cron
Create a file /etc/cron.d/ntpd containing the following crontab:
15 * * * * root /usr/sbin/ntpd -q -u ntp:ntp
The -q option instructs ntpd to exit just after setting the clock, and the -u option instructs it to run as the
specified user.
Note: When setting the clock for the first time, execute the above command with the -g option, as ntpd
will refuse to set the clock if it is significantly different from the source.
This crontab will execute ntpd to synchronize the time to the NTP server at 15 minutes past every hour. (It is
possible to choose a different minute, or to vary the minute between machines in order to avoid heavy traffic to
the NTP server.) Hourly synchronization should be sufficiently frequent that clock drift will not be noticeable.Configure an NTP Server
The site’s NTP server contacts a central NTP server, probably either one provided by your ISP or a public time
server, to obtain accurate time data. The server then allows other machines on your network to request the time
data.
The NTP server configuration file is located at /etc/ntp.conf.Enable the NTP Daemon
If this machine is an NTP server, ensure that ntpd is enabled
at boot time: # chkconfig ntpd onEnable the NTP DaemonThe ntpd service should be enabled.CCE-4376-0(1) via chkconfig# chkconfig ntpd onDeny All Access to ntpd by Default
Edit the file /etc/ntp.conf. Prepend or correct the following
line:
restrict default ignore
Since ntpd is a complex software package which listens
for network connections and runs as root, it must be protected from network access by
unauthorized machines. This setting uses ntpd's internal authorization to deny all
access to any machine, server or client, which is not specifically authorized by other
policy settings.Deny All Access to ntpd by DefaultNetwork access to ntpd should be deniedCCE-4134-3(1) via /etc/ntp.confSpecify a Remote NTP Server for Time Data
Find the IP address, server-ip , of an appropriate remote NTP
server. Edit the file /etc/ntp.conf, and add or correct the following lines:
restrict server-ip mask 255.255.255.255 nomodify notrap noquery
server server-ip
If your site
does not require time data to be accurate, but merely to be synchronized among local
machines, this step can be omitted, and the NTP server will default to providing time
data from the local clock. However, it is a good idea to periodically synchronize the
clock to some source of accurate time, even if it is not appropriate to do so
automatically.
The previous step disabled all remote access to this NTP server's state
data. This NTP server must contact a remote server to obtain accurate data, so NTP's
configuration must allow that remote data to be used to modify the system clock. The
restrict line changes the default access permissions for that remote server. The
server line specifies the remote server as the preferred NTP server for time data. If
you intend to synchronize to more than one server, specify restrict and server lines
for each server.
Note: It would be possible to specify a hostname, rather than an IP
address, for the server field. However, the restrict setting applies only to network
blocks of IP addresses, so it is considered more maintainable to use the IP address in
both fields.Specify a Remote NTP Server for Time DataA remote NTP Server for time synchronization should be specifiedCCE-4385-1(1) via /etc/ntp.confAllow Legitimate NTP Clients to Access the Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which will synchronize to this
server. Edit /etc/ntp.conf and add the line:
restrict netwk mask mask nomodify notrap
Edit /etc/sysconfig/iptables. Add the following line, ensuring that it appears before
the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport 123 -j ACCEPT
If the clients are
spread across more than one netblock, separate restrict and ACCEPT lines should be
added for each netblock.
The iptables configuration is needed because the default
iptables configuration does not allow inbound access to any services. See Section
2.5.5 for more information about iptables.
Note: The reference NTP implementation will
refuse to serve time data to clients until enough time has elapsed that the server
host's time can be assumed to have settled to an accurate value. While testing, wait
ten minutes after starting ntpd before attempting to synchronize clients.Configure OpenNTPD if Appropriate
OpenNTPD is an implementation of the SNTP protocol which is
provided as a simple alternative to the reference NTP server. Advantages of OpenNTPD
include simplicity of configuration, built-in privilege separation and chroot jailing of
the NTP protocol code, and a small codebase which lacks many of the management and other
protocol features used by the reference NTP server. This simplicity comes at the cost of
degraded time accuracy, but SNTP is probably accurate enough for most sites with typical
monitoring requirements.Obtain NTP Software
If your site intends to use the OpenNTPD implementation, it is
necessary to compile and install the software. (If your site intends to use the
reference NTP implementation, no installation is necessary.) Obtain the software by
downloading an appropriate source version, openntpd-version .tar.gz, from
http://www.openntpd.org/portable.html. Unpack the source code: $ tar xzf openntpd-version .tar.gzConfigure and compile the source. (By default, the code will
be compiled for installation into /usr/ local): $ cd openntpd-version
$ ./configure --with-privsep-user=ntp
$ make As root, install the resulting program into
/usr/local: # make install
The configuration option --with-privsep-user=ntp tells
OpenNTPD to use the existing system account ntp for the non-root portion of its
operation.Obtain NTP SoftwareOpenNTPD should be installedCCE-4032-9(1) via openntpd packageConfigure an SNTP Client
OpenNTPD runs only in daemon mode — there is no command line
suitable to be run from cron. However, this is considered reasonably safe for client use
because the daemon does not listen on any network ports by default, and because OpenNTPD
is a small codebase with no remote management interface or other complex features.
However, it is possible to run a time-stepping program, such as rdate(1), from cron
instead of configuring the daemon as outlined in this section.Enable the NTP DaemonEdit the file /etc/rc.local. Add or correct the following line: /usr/local/sbin/ntpd -sEnable the NTP DaemonThe ntp daemon should be enabledCCE-4424-8(1) via /etc/rc.localConfigure the Client NTP Daemon to Use the Local Server
Edit the file /usr/local/etc/ntpd.conf. Add or correct the
following line:
server local-server.example.com
where local-server.example.com is the
hostname of the site's local NTP or SNTP server.Configure the Client NTP Daemon to Use the Local ServerThe ntp daemon synchronization server should be set appropriatelyCCE-3487-6(1) via /usr/local/etc/ntpd.confConfigure an SNTP ServerThe SNTP server obtains time data from a remote server, and then listens on a network interface for time queries from local machines.Enable the NTP Daemon
Edit the file /etc/rc.local. Add or correct the following
line:
/usr/local/sbin/ntpd -s
Since OpenNTPD is third-party software, it does not have
a standard startup script, so the daemon is started at boot using the local facility.Listen for Client Connections
Edit the file /usr/local/etc/ntpd.conf. Add or correct the
following line:
listen on ipaddr
where ipaddr is the primary IP address of this server.
By default, ntpd does not listen for any connections over a network. Listening
must be actively enabled on NTP servers so that clients may obtain time data.Allow Legitimate NTP Clients to Access the Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which will synchronize to this
server.
Edit /etc/sysconfig/iptables. Add the following line, ensuring that it appears
before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport 123 -j ACCEPT
The iptables configuration is needed because the default iptables configuration does
not allow inbound access to any services. See Section 2.5.5 for more information about
iptables.Specify a Remote NTP Server for Time Data
Find the hostname, server-host , of an appropriate remote NTP
server. Edit the file /usr/local/etc/ ntpd.conf, and add or correct the following
line:
server server-host
This setting configures ntpd to obtain time data from the
remote host. To use multiple time servers, add one line for each server.Mail Transfer Agent
Mail servers are used to send and receive mail over a network on
behalf of site users. Mail is a very common service, and MTAs are frequent targets of
network attack. Ensure that machines are not running MTAs unnecessarily, and configure
needed MTAs as defensively as possible.Mail Transfer AgentThe sendmail service should be disabled.CCE-4416-4(1) via chkconfigSelect Mail Server Software and Configuration
Select one of the following options for configuring e-mail on the
machine: If this machine does not need to operate as a mail server, follow the
instructions in Section 3.11.2 to run sendmail in submission-only mode.If the machine
must operate as a mail server, read the strategies for MTA configuration in Section 3.11.3
for information about configuration options. Then apply both the MTA-independent operating
system configuration guidance in Section 3.11.4, and the specific guidance for your MTA:
If the Sendmail MTA is preferred, see Section 3.11.5. If the Postfix MTA is preferred, see Section 3.11.6. If another MTA is preferred, use that MTA's documentation to
implement the ideas in Section 3.11.3.
It is recommended that very few machines at any
site be configured to receive mail over a network. However, it may be necessary for most
machines at a given site to send e-mail, for instance so that cron jobs can report output
to an administrator. Sendmail supports a submission-only mode in which mail can be sent
from the machine to a central site MTA, but the machine cannot receive mail over a
network.
If a Mail Transfer Agent (MTA) is needed, the system default is Sendmail.
Postfix, a popular alternative written with security in mind, is also available. Postfix
can be more effectively contained by SELinux as its modular design has resulted in
separate processes performing specific actions. More information on these MTAs is
available from their respective websites, http://www.sendmail.org and
http://www.postfix.org.Hildebrandt, R., and Koetter, P. The Book of Postfix. No Starch Press, 2005Configure SMTP For Mail Client
This guide discusses the use of Sendmail for submission-only
e-mail configuration. It is also possible to use Postfix.Hunt, C. Sendmail Cookbook. O’Reilly and Associates, 2003Disable the Listening Sendmail Daemon
Edit the file /etc/sysconfig/sendmail. Add or modify the line:
DAEMON=no
The MTA performs two functions: listening over a network for incoming SMTP
e-mail requests, and sending mail from the local machine. Since outbound mail may be
delayed due to network outages or other problems, the outbound MTA runs in a queue-only
mode, in which it periodically attempts to resend any delayed mail. Setting DAEMON=no
tells sendmail to execute only the queue runner on this machine, and never to receive
SMTP mail requests.Disable the Listening Sendmail DaemonThe listening sendmail daemon should be disabled.CCE-4293-7(1) via /etc/sysconfig/sendmailConfigure Mail Submission if Appropriate
If it is appropriate to configure mail submission with a
central MTA, edit /etc/mail/submit.cf. Locate the line beginning with D{MTAHost}, and
modify it to read:
D{MTAHost}mailserver
where mailserver is the hostname of the server
to which this machine should forward its outgoing mail.
This suggestion is provided as a
simple way to migrate away from a configuration in which each machine at a site runs its
own MTA, to a configuration in which client machines do not run listening daemons. If
this modification is made to /etc/mail/submit.cf, then, when a local process on a
machine attempts to send mail, the message will be forwarded to the machine mailserver
for processing.
Modifying /etc/mail/submit.cf directly is only appropriate if your site
does not perform any other mailserver customization on clients. If other customization
is done, use your usual Sendmail change procedure to define the MTA host.
Note: In
addition to making this change on the client, it may also be necessary to reconfigure
the MTA on mailserver so that it will relay mail on behalf of this host.Strategies for MTA Security
This section discusses several types of MTA configuration which
should be performed in order to protect against attacks involving the mail system. Though
configuration syntax will differ depending on which MTA is in use (see Section 3.11.5 for
Sendmail configuration syntax and Section 3.11.6 for Postfix), these strategies are
generally advisable for any MTA, including ones not covered by this guide.Use Resource Limits to Mitigate Denial of Service
It is often desirable to constrain an attacker's ability to
consume a mail server's resources simply by sending otherwise valid mail at a high rate,
whether maliciously or accidentally. Relevant resource limits include con106 CHAPTER 3.
SERVICES straints on: the number of MTA daemons which may run at one time, the rate at
which incoming messages may be received, the size and complexity of each message, or the
amount of mail queue space which must remain free in order for mail to be delivered.
That last parameter deserves additional explanation. Most MTAs require queue space for
temporary files in order to process existing messages in their queues. Therefore, if the
queue filesystem is allowed to fill completely in a denial of service, the MTA will not
be able to clear its own queue even when the malicious traffic has stopped. This will
delay recovery from an attack.Configure SMTP Greeting Banner
When remote mail senders connect to the MTA on port 25, they
are greeted by an initial banner as part of the SMTP dialogue. This banner is necessary,
but it frequently gives away too much information, including the MTA software which is
in use, and sometimes also its version number. Remote mail senders do not need this
information in order to send mail, so the banner should be changed to reveal only the
hostname (which is already known and may be useful) and the word ESMTP, to indicate that
the modern SMTP protocol variant is supported.Control Mail Relaying
The sending of Unsolicited Bulk E-mail, referred to variously
as UBE, UCE, or spam, is a major problem on the Internet today. The security
implications of spam are that it operates as a Denial of Service attack on legitimate
e-mail use. Strategies for fighting spam receipt at your site are complex and quickly
evolving, and thus far beyond the scope of this guide. The problem of relaying
unauthorized e-mail, however, can and should be addressed by any network-connected site.
Most MTAs perform two functions: to accept mail from remote sites on behalf of local
users, and to allow local users to send mail to remote sites. The former function is
relatively easy — mail whose recipient address is local can be assumed to be destined
for a local user. The latter function is more complex. Since it is typically considered
neither secure nor desirable for users to log in to the MTA host itself to send mail,
the MTA must be able to remotely accept mail addressed to anyone from the user's
workstation. If the MTA is running very old software or is configured poorly, it can be
possible for attackers to take advantage of this feature, using your MTA to relay their
spam from one remote site to another. This is undesirable for many reasons, not least
that your site will quickly be blacklisted as a spam source, leaving you unable to send
legitimate e-mail to your correspondents.
The simplest solution described in this guide
is to configure the MTA to relay mail only from the local site's address range, and some
variant on this is the default for most modern MTAs.
That solution may be insufficient
for sites whose users need to send mail from remote machines, for instance while
travelling, as well as for sites where mail submission must be accepted from network
ranges which are not considered secure, either because authorized machines are unmanaged
or because it is possible to connect unauthorized machines to the network.
If remote or
mobile hosts are authorized to relay, or if local clients exist in insecure netblocks,
the SMTP AUTH protocol should be used to require mail senders to authenticate before
submitting messages. For better protection and to allow support for a wide range of
authentication mechanisms without sending passwords over a network in clear text, SMTP
AUTH transactions should be encrypted using SSL.
Another approach is to require mail to
be submitted on port 587, the designated Message Submission Port. Use of a separate port
allows the mail relay function to be entirely separated from the mail delivery function.
This may become a best practice in the future, but description of how to configure the
Message Submission Port is currently beyond the scope of this guide. See RFC 2476 for
information about this configuration.Configure Operating System to Protect Mail Server
The guidance in this section is appropriate for any host which is
operating as a site MTA, whether the mail server runs using Sendmail, Postfix, or some
other software.Use Separate Hosts for External and Internal Mail if Possible
The mail server is a frequent target of network attack from the
outside. However, since all site users receive mail, the mail server must be open to
some connection from each inside users. It is strongly recommended that these functions
be separated, by having an externally visible mail server which processes all incoming
and outgoing mail, then forwards internal mail to a separate machine from which users
can access it.Protect the MTA Host from User Access
The mail server contains privileged data belonging to all users
and performs a vital network function. Preventing users from logging into this server is
a precaution against privilege escalation or denial of service attacks which might
compromise the mail service. Take steps to ensure that only system administrators are
allowed shell access to the MTA host.Restrict Remote Access to the Mail Spool
If users directly connect to this machine to receive mail,
ensure that there is a single, well-secured mechanism for access to the directory
/var/spool/mail (the directory /var/mail is a symlink to this).
Allowing unrestricted
access to /var/spool/mail can be dangerous, since this directory contains sensitive
information belonging to all users. Protocols such as NFS, which have an insecure
authorization mechanism by default, should be considered insufficient for these
purposes. See Section 3.17 for details on secure configuration of POP3 or IMAP, which
are the preferred ways to provide user access to mail.Configure iptables to Allow Access to the Mail Server
Edit /etc/sysconfig/iptables. Add the following line, ensuring
that it appears before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT
The default
Iptables configuration does not allow inbound access to the SMTP service. This
modification allows that access, while keeping other ports on the server in their
default protected state. See Section 2.5.5 for more information about Iptables.Verify System Logging and Log Permissions for Mail
Edit the file /etc/syslog.conf. Add or correct the following
line if necessary (this is the default):
mail.* -/var/log/maillog
Run the following commands to ensure correct permissions on the mail log: # chown root:root /var/log/maillog
# chmod 600 /var/log/maillog
The mail server logs contain a record of
every e-mail which is sent or received on the system, which is considered sensitive
information by most sites. It is necessary that these logs be collected for purposes of
debugging and statistics, but their contents should be protected from unauthorized
access.Configure SSL Certificates for Use with SMTP AUTH
If SMTP AUTH is to be used (see Section 3.11.3.3 for a
description of possible anti-relaying mechanisms), the use of SSL to protect credentials
in transit is strongly recommended. There are also configurations for which it may be
desirable to encrypt all mail in transit from one MTA to another, though such
configurations are beyond the scope of this guide. In either event, the steps for
creating and installing an SSL certificate are independent of the MTA in use, and are
described here.Create an SSL Certificate
Note: This step must be performed on your CA system, not on
the MTA host itself. If you will have a commercial CA sign certificates, then this
step should be performed on a separate, physically secure system devoted to that
purpose.
Change into the CA certificate directory: # cd /etc/pki/tls/certs
Generate a key pair for the mail server: # openssl genrsa -out mailserverkey.pem 2048
Next,
generate a certificate signing request (CSR) for the CA to sign, making sure to supply
your mail server's fully qualified domain name as the Common Name: # openssl req -new -key mailserverkey.pem -out mailserver.csr
Next, the mail server CSR must be signed to
create the mail server certificate. You can either send the CSR to an established CA
or sign it with your CA.
To sign mailserver.csr using your CA: # openssl ca -in mailserver.csr -out mailservercert.pem
This step creates a private key,
mailserverkey.pem, and a public certificate, mailservercert.pem. The mail server will
use these to prove its identity by demonstrating that it has a certificate which has
been signed by a CA. Mail clients at your site should be willing to send their mail
only to a server they can authenticate.Install the SSL Certificate
Create the PKI directory for mail certificates, if it does
not already exist: # mkdir /etc/pki/tls/mail
# chown root:root /etc/pki/tls/mail
# chmod 755 /etc/pki/tls/mail
Using removable media or some other secure transmission
format, install the files generated in the previous step onto the mail server: /etc/pki/tls/mail/serverkey.pem: the private key mailserverkey.pem/etc/pki/tls/mail/servercert.pem: the certificate file mailservercert.pem
Verify the ownership and permissions of these files: # chown root:root /etc/pki/tls/mail/serverkey.pem
# chown root:root /etc/pki/tls/mail/servercert.pem
# chmod 600 /etc/pki/tls/mail/serverkey.pem
# chmod 644 /etc/pki/tls/mail/servercert.pem
Verify that the CA's public certificate file has been installed as
/etc/pki/tls/CA/cacert.pem, and has the correct permissions: # chown root:root /etc/pki/tls/CA/cacert.pem
# chmod 644 /etc/pki/tls/CA/cacert.pemConfigure Sendmail Server if Necessary
When sendmail is configured to act as a server for incoming mail,
it listens on port 25 for connections, and responds to those connections using the
configuration in /etc/mail/sendmail.cf. This file has a somewhat opaque format, and
modifying it directly is generally not recommended. Instead, the following procedure
should be used to modify the sendmail configuration: Install the sendmail-cf RPM, which
is required in order to compile a new configuration file: # yum install sendmail-cfEdit the M4 source file /etc/mail/sendmail.mc as directed by the configuration step you
are applying. Inside the directory /etc/mail/, use make to build the configuration
according to the Makefile provided by Sendmail: # cd /etc/mail
# make sendmail.cfLimit Denial of Service Attacks
Edit /etc/mail/sendmail.mc, and add or correct the following
options:
define(`confMAX_DAEMON_CHILDREN',`40')dnl
define(`confCONNECTION_RATE_THROTTLE', `3 ')dnl
define(`confMIN_FREE_BLOCKS',`20971520')dnl
define(`confMAX_HEADERS_LENGTH',`51200')dnl
define(`confMAX_MESSAGE_SIZE',`10485760')dnl
define(`confMAX_RCPTS_PER_MESSAGE',`100')dnl
Note: The values given here are examples, and may need to be modified for any
particular site, especially one with high e-mail volume.
These configuration options
serve to make it more difficult for attackers to consume resources on the MTA host. (See
Section 3.11.3.1 for details on why this is done.) The MAX DAEMON CHILDREN option limits
the number of sendmail processes which may be deployed to handle incoming connections at
any one time, while CONNECTION RATE THROTTLE limits the number of connections per second
which each listener may receive. The MIN FREE BLOCKS option stops e-mail receipt when
the queue filesystem is close to full. The MAX HEADERS LENGTH (bytes), MAX MESSAGE SIZE
(bytes), and MAX RCPTS PER MESSAGE (distinct recipients) options place bounds on the
legal sizes of messages received via SMTP.Configure SMTP Greeting Banner
Edit /etc/mail/sendmail.mc, and add or correct the following
line, substituting an appropriate greeting string for $j :
define(`confSMTP_LOGIN_MSG', `$j ')dnl
and recompile sendmail's configuration.
The default greeting banner discloses
that the listening mail process is Sendmail rather than some other MTA, and also
provides the version number. See Section 2.3.7 for more about warning banners, and
Section 3.11.3.2 for strategies regarding SMTP greeting banners in particular.
The Sendmail variable $j contains the hostname of the mail server, which may be an
appropriate greeting string for most environments.Control Mail Relaying
This guide will discuss two mechanisms for controlling mail
relaying in Sendmail. The /etc/mail/relay-domains file contains a list of hostnames that
are allowed to relay mail. Follow the guidance in Section 3.11.5.3.1 to configure
relaying for trusted machines.
If there are machines which must be allowed to relay
mail, but which cannot be trusted to relay unconditionally, configure SMTP AUTH with TLS
support using the guidance in Sections 3.11.5.3.2 and following.Configure Trusted Networks and HostsIf all machines which share a common domain or subdomain
name may relay, then edit /etc/mail/ relay-domains, adding a line for each domain or
subdomain, e.g.:
example.com
trusted-subnet.school.edu
... If the machines which are
allowed to relay must be specified on a per-host basis, then edit /etc/mail/
relay-domains, adding a line for each such host:
host1.example.com
host5.subnet.example.com
smtp.trusted-subnet.school.edu
Then edit /etc/mail/sendmail.mc, add or correct the line:
FEATURE(`relay_hosts_only')dnl
and recompile sendmail's configuration.
The file /etc/mail/relay-domains must contain only
the set of machines for which this MTA should unconditionally relay mail. This
configures both inbound and outbound relaying, that is, hosts mentioned in
relay-domains may send mail through the MTA, and the MTA will also accept inbound mail
addressed to such hosts. This is a trust relationship — if spammers gain access to
these machines, your site will effectively become an open relay. It is recommended
that only machines which are managed by you or by another trusted organization be
placed in relay-domains, and that users of all other machines be required to use SMTP
AUTH to send mail.
Note: The relay-domains file must be configured to contain either a
list of domains (in which case every host in each of those domains will be allowed to
relay) or a list of hosts (in which case each individual relaying host must be listed
and the sendmail.cf must be reconfigured to interpret the relay-domains file in the
desired way).Require SMTP AUTH Before Relaying from Untrusted Clients
By default, Sendmail uses the Cyrus-SASL library to provide
authentication.
To enable the use of SASL authentication for relaying, edit
/etc/mail/sendmail.mc and add or correct the following settings:
TRUST_AUTH_MECH(`LOGIN PLAIN')
define(`confAUTH_MECHANISMS', `LOGIN PLAIN')
and recompile sendmail.cf.
Then edit /usr/lib/sasl2/Sendmail.conf and add or correct the following lines:
pwcheck_method: saslauthd
Enable the saslauthd daemon: # chkconfig saslauthd on
The AUTH MECHANISMS configuration option tells sendmail to allow the
specified authentication mechanisms to be used during the SMTP dialogue. The two
listed mechanisms use SASL to test a password provided by the user. Since these
mechanisms transmit plaintext passwords, they should be protected using TLS as
described in the next section.
The TRUST AUTH MECH command tells sendmail that senders
who successfully authenticate using the specified mechanism may relay mail through
this MTA even if their addresses are not in relay-domains.
The file
/usr/lib/sasl/Sendmail.conf is the Cyrus-SASL configuration file for Sendmail. The
pwcheck method directive tells SASL how to find passwords. The simplest method,
described here, is to run a separate authentication daemon, saslauthd, which is able
to communicate with the system authentication service. On Red Hat, saslauthd uses PAM
by default, which should work in most cases. If you have a centralized authentication
system which does not work via PAM, look at the saslauthd(8) manpage to determine how
to configure saslauthd for your environment.Require TLS for SMTP AUTH
Edit /etc/mail/sendmail.mc, add or correct the following
lines:
define(`confAUTH_OPTIONS', `A p')dnl
define(`confCACERT_PATH', `/etc/pki/tls/CA')dnl
define(`confCACERT', `/etc/pki/tls/CA/cacert.pem')dnl
define(`confSERVER_CERT', `/etc/pki/tls/mail/servercert.pem')dnl
define(`confSERVER_KEY', `/etc/pki/tls/mail/serverkey.pem')dnl
and recompile sendmail.cf.
These options, combined with the previous settings, tell Sendmail to
protect all SMTP AUTH transactions using TLS. The first four options describe the
location of the necessary TLS certificate and key files.
The AUTH OPTIONS parameter
configures the SMTP AUTH dialogue. The A option is enabled by default, and simply says
that authentication is allowed if an appropriate mechanism can be found. The p option
tells Sendmail to protect against passive attacks. The PLAIN and LOGIN authentication
mechanisms, recommended by this guide for compatibility with PAM, send passwords in
the clear. (Cleartext password transmissions are vulnerable to passive attack.)
Therefore, if p is set, the SMTP daemon will not make the AUTH command available until
after the client has used the STARTTLS command to encrypt the session. If other
authentication mechanisms were enabled which did not send passwords in the clear, then
TLS would not necessarily be required.Configure Postfix if Necessary
Postfix stores its configuration files in the directory
/etc/postfix by default. The primary configuration file is /etc/postfix/main.cf. Other
files will be introduced as needed.Limit Denial of Service Attacks
Edit /etc/postfix/main.cf. Add or correct the following lines:
default_process_limit = 100
smtpd_client_connection_count_limit = 10
smtpd_client_connection_rate_limit = 30
queue_minfree = 20971520
header_size_limit = 51200
message_size_limit = 10485760
smtpd_recipient_limit = 100
Note: The values given
here are examples, and may need to be modified for any particular site. By default, the
Postfix anvil process gathers mail receipt statistics. To get information about about
what connection rates are typical at your site, look in /var/log/maillog for lines with
the daemon name postfix/anvil.
These configuration options serve to make it more
difficult for attackers to consume resources on the MTA host. (See Section 3.11.3.1 for
details on why this is done.) The default process limit parameter controls how many
smtpd processes can exist at a time, while smtpd_client_connection_count_limit controls
the number of those which can be occupied by any one remote sender, and
smtpd_client_connection_rate_limit controls the number of connections any one client
can make per minute. By default, local hosts (those in mynetworks) are exempted from
per-client rate limiting.
The queue_minfree parameter establishes a free space threshold, in order to
stop e-mail receipt before the queue filesystem is entirely full. The header_size_limit,
message_size_limit, and smtpd recipient limit parameters place bounds on the legal sizes
of messages received via SMTP.Configure SMTP Greeting Banner
Edit /etc/postfix/main.cf, and add or correct the following
line, substituting some other wording for the banner information if you prefer:
smtpd_banner = $myhostname ESMTP
The default greeting banner discloses that the
listening mail process is Postfix. See Section 2.3.7 for more about warning banners, and
Section 3.11.3.2 for strategies regarding SMTP greeting banners in particular.Control Mail Relaying
Postfix's mail relay controls are implemented with the help of
the smtpd recipient restrictions option, which controls the restrictions placed on the
SMTP dialogue once the sender and recipient envelope addresses are known.
The guidance
in Sections 3.11.6.3.1–3.11.6.3.2 should be applied to all machines. If there are
machines which must be allowed to relay mail, but which cannot be trusted to relay
unconditionally, configure SMTP AUTH with SSL support using the guidance in Sections
3.11.6.3.3 and following.Configure Trusted Networks and Hosts
Edit /etc/postfix/main.cf, and configure the contents of the
mynetworks variable in one of the following ways: If any machine in the subnet
containing the MTA may be trusted to relay messages, add or correct the line:
mynetworks_style = subnet If only the MTA host itself is trusted to relay messages,
add or correct:
mynetworks_style = host If the set of machines which can relay is
more complicated, manually specify an entry for each netblock or IP address which is
trusted to relay by setting the mynetworks variable directly:
mynetworks = 10.0.0.0/16 , 192.168.1.0/24 , 127.0.0.1
The mynetworks variable must contain only the set of
machines for which this MTA should unconditionally relay mail. This is a trust
relationship — if spammers gain access to these machines, your site will effectively
become an open relay. It is recommended that only machines which are managed by you or
by another trusted organization be placed in mynetworks, and users of all other
machines be required to use SMTP AUTH to send mail.Allow Unlimited Relaying for Trusted Networks Only
Edit /etc/postfix/main.cf, and add or correct the smtpd
recipient restrictions definition so that it contains at least:
smtpd_recipient_restrictions =
...
permit_mynetworks,
reject_unauth_destination,
...
The full contents of smtpd recipient restrictions will vary by site, since this is a
common place to put spam restrictions and other site-specific options. The permit
mynetworks option allows all mail to be relayed from the machines in mynetworks. Then,
the reject unauth destination option denies all mail whose destination address is not
local, preventing any other machines from relaying. These two options should always
appear in this order, and should usually follow one another immediately unless SMTP
AUTH is used.Require SMTP AUTH Before Relaying from Untrusted Clients
SMTP authentication allows remote clients to relay mail
safely by requiring them to authenticate before submitting mail. Postfix's SMTP AUTH
uses an authentication library called SASL, which is not part of Postfix itself. This
section describes how to configure authentication using the Cyrus-SASL implementation.
See below for a discussion of other options.
To enable the use of SASL authentication,
edit /etc/postfix/main.cf and add or correct the following settings:
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions =
...
permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destination,
...
Then edit
/usr/lib/sasl/smtpd.conf and add or correct the following line with the correct
authentication mechanism for SASL to use:
pwcheck_method: saslauthd
Enable the saslauthd daemon: # chkconfig saslauthd on
Postfix can use either the Cyrus library or
Dovecot as a source for SASL authentication. If this host is running Dovecot for some
other reason, it is recommended that Dovecot's SASL support be used instead of running
the Cyrus code as well. See http://www.postfix.org/SASL README.html for instructions
on implementing that configuration, which is not described in this guide.
In Postfix's
configuration, the directive smtpd sasl auth enable tells smtpd to allow the use of
the SMTP AUTH command during the SMTP dialogue, and to support that command by getting
authentication information from SASL. The smtpd recipient restrictions directive is
changed so that, if the client is not connecting from a trusted address, it is allowed
to attempt authentication (permit sasl authenticated) in order to relay mail.
The file
/usr/lib/sasl/smtpd.conf is the Cyrus-SASL configuration file. The pwcheck method
directive tells SASL how to find passwords. The simplest method, described above, is
to run a separate authentication daemon, saslauthd, which is able to communicate with
the system authentication system. On RHEL5, saslauthd uses PAM by default, which
should work in most cases. If you have a centralized authentication system which does
not work via PAM, look at the saslauthd(8) manpage to find out how to configure
saslauthd for your environment.Require TLS for SMTP AUTH
Edit /etc/postfix/main.cf, and add or correct the following
lines:
smtpd_tls_CApath = /etc/pki/tls/CA
smtpd_tls_CAfile = /etc/pki/tls/CA/cacert.pem
smtpd_tls_cert_file = /etc/pki/tls/mail/servercert.pem
smtpd_tls_key_file = /etc/pki/tls/mail/serverkey.pem
smtpd_tls_security_level = may
smtpd_tls_auth_only = yes
These options tell Postfix to protect all SMTP AUTH transactions using TLS. The first
four options describe the locations of the necessary TLS key files.
The smtpd_tls_security_level directive tells smtpd to allow the STARTTLS command during the SMTP
protocol exchange, but not to require it for mail senders. (Unless your site receives
mail only from other trusted sites whose sysadmins can be asked to maintain a copy of
your site certificate, you do not want to require TLS for all SMTP exchanges.)
The smtpd_tls_auth_only directive tells smtpd to require the STARTTLS command before allowing the
client to attempt to authenticate for relaying using SMTP AUTH. It may not be possible
to use this directive if you must allow relaying from non-TLS-capable client software.
If this is the case, simply omit that line.LDAP
LDAP is a popular directory service, that is, a standardized way of
looking up information from a central database. It is relatively simple to configure a RHEL5
machine to obtain authentication information from an LDAP server. If your network uses LDAP
for authentication, be sure to configure both clients and servers securely.Use OpenLDAP to Provide LDAP Service if Possible
The system's default LDAP client/server program is called
OpenLDAP. Its documentation is available at the project web page: http://www.openldap.org.Configure OpenLDAP Clients
This guide recommends configuring OpenLDAP clients by manually
editing the appropriate configuration files. RHEL5 provides an automated configuration
tool called authconfig and a graphical wrapper for authconfig called
system-config-authentication. However, these tools do not give sufficient flexibility over
configuration. The authconfig tools do not allow you to specify locations of SSL
certificate files, which is useful when trying to use SSL cleanly across several
protocols. They are also overly aggressive in placing services such as netgroups and
automounter maps under LDAP control, where it is safer to use LDAP only for services to
which it is relevant in your environment.Before configuring any machine to be an LDAP client, ensure that
a working LDAP server is present on the network. See Section 3.12.3 for instructions on
configuring an LDAP server. Configure the Appropriate LDAP Parameters for the Domain
Assume the fully qualified host name of your LDAP server is
ldap.example.com and the base DN of your domain is dc=example,dc=com (it is conventional
to use the domain name as a base DN). Edit /etc/ldap. conf, and add or correct the
following lines:
base dc=example,dc=com
uri ldap://ldap.example.com/
Then edit /etc/openldap/ldap.conf, and add or correct the following lines:
BASE dc=example,dc=com
URI ldap://ldap.example.com/
The machine whose hostname is given here must be
configured as an LDAP server, serving data identified by the base DN used here. See
Section 3.12.3 for details on configuring an LDAP server.Configure LDAP to Use TLS for All TransactionsEnsure a copy of the site's CA certificate has been placed
in the file /etc/pki/tls/CA/cacert.pem. Configure LDAP to enforce TLS use and to
trust certificates signed by the site's CA. First, edit the file /etc/ldap.conf, and add
or correct the following lines:
ssl start_tls
tls_checkpeer yes
tls_cacertdir /etc/pki/tls/CA
tls_cacertfile /etc/pki/tls/CA/cacert.pem
Then edit /etc/openldap/ldap.conf, and add or correct the following lines:
TLS_CACERTDIR /etc/pki/tls/CA
TLS_CACERT /etc/pki/tls/CA/cacert.pem
Section 2.5.6 describes the
system-wide configuration of SSL for your enterprise. It is possible to place your
certificate information under some directory other than /etc/pki/tls, but using a
consistent directory structure across all SSL services at your site is recommended. The
LDAP server must be configured with a certificate signed by the CA certificate named
here.Configure LDAP to Use TLS for All TransactionsClients require LDAP servers to provide valid certificates for SSL communications.(1) via /etc/ldap.confConfigure Authentication Services to Use OpenLDAP
Edit the file /etc/ldap.conf, and add or correct the following
lines:
pam_password md5
Edit the file /etc/nsswitch.conf, and add or correct the following lines:
passwd: files ldap
shadow: files ldap
group: files ldap
Edit the file
/etc/pam.d/system-auth-ac. Make the following changes, which will add references to LDAP
in each of the four sections of the file: Immediately before the last line in the auth
section (the one containing pam_deny.so), insert the line:
auth sufficient pam_ldap.so use_first_pass Modify the first line in the account section by adding the option
broken shadow. The line should then read:
account required pam_unix.so broken_shadow Immediately before the last line in the account section (the one containing pam
permit.so), insert the line:
account [default=bad success=ok user_unknown=ignore] pam_ldap.so Immediately before the last line in the password section (the one
containing pam_deny.so), insert the line:
password sufficient pam_ldap.so use_authtokAt the end of the file (after the last line in the session section), append the line:
session optional pam_ldap.so
The first modification tells LDAP to expect passwords in
MD5 hash format, rather than clear text.
Red Hat systems use the file /etc/nsswitch.conf
to determine the appropriate sources to search for certain kinds of data, such as
usernames, groups, hostnames, netgroups, or protocols. It is possible to manage many
other types of data using LDAP, but this guide recommends that only usernames (passwd
data), passwords (shadow data), and groups (group data) be managed using LDAP. If your
site uses netgroups, it may be appropriate to manage these via LDAP as well.
However,
data which almost never changes, such as the contents of the /etc/services file, is a
poor choice for central administration, since it introduces risk with little benefit. It
is recommended that the automounter not be used at all, so LDAP control of automounter
maps is unlikely to be appropriate.
The file /etc/pam.d/system-auth-ac is used by PAM to
control access to most authenticated services. The syntax of the PAM configuration file
is somewhat cryptic. The lines recommended here have the combined effect of using LDAP
to find authentication data for users who cannot be found in the local /etc/passwd file.
This means that, for instance, it is still possible to use a local root password. The
details of options such as broken_shadow, use_authtok, and use_first_pass may be looked
up in the man pages for the various PAM modules. Their basic effect is to attempt to
authenticate given a password against both the local /etc/shadow and the central LDAP
server, without forcing the user to type the password more than once. PAM configuration
is discussed further in Section 2.3.3.Configure OpenLDAP Server
This section contains guidance on how to configure an OpenLDAP
server to securely provide information for use in a centralized authentication service.
This is not a comprehensive guide to maintaining an OpenLDAP server, but may be helpful in
transitioning to an OpenLDAP infrastructure nonetheless.Install OpenLDAP Server RPM
Is this machine the OpenLDAP server? If so: # yum install openldap-servers
# chkconfig ldap on
The openldap-servers RPM is not installed by
default on RHEL5 machines. It is needed only by the OpenLDAP server, not by the clients
which use LDAP for authentication.Disable OpenLDAP serviceThe ldap service should be disabled.CCE-3501-4(1) via chkconfigConfigure Domain-Specific Parameters
Edit the file /etc/openldap/slapd.conf. Add or correct the
following lines:
suffix "dc=example,dc=com "
rootdn "cn=Manager,dc=example,dc=com "
where dc=example,dc=com is the same root you will use on the LDAP clients.
These are
basic LDAP configuration directives. The suffix parameter gives the root name of all
information served by this LDAP server, and should be some name related to your domain.
The rootdn parameter names LDAP's privileged user, who is allowed to read or write all
data managed by this LDAP server.Configure an LDAP Root Password
Ensure that the configuration file has reasonable permissions
before putting the hashed root password in that file: # chown root:ldap /etc/openldap/slapd.conf
# chmod 640 /etc/openldap/slapd.conf
Generate a hashed password using the slappasswd utility: # slappasswd
New password:
Re-enter new password:
This
will output a hashed password string. Edit the file /etc/openldap/slapd.conf, and add or
correct the line:
rootpw {SSHA}hashed-password-string
Be sure to select a secure
password for the LDAP root user, since this user has permission to read and write all
LDAP data, so a compromise of the LDAP root password will probably enable a full
compromise of your site. Protect configuration files containing the hashed password the
same way you would protect other files, such as /etc/shadow, which contain hashed
authentication data. In addition, be sure to use a reasonably strong hash function, such
as SHA-1, rather than an insecure scheme such as crypt.If you are using SHA-1, the hashed password string will begin with “{SHA}” or “{SSHA}”Configure the LDAP Server to Require TLS for All Transactions
Because LDAP queries and responses, particularly those
containing authentication information or other sensitive data, must be protected from
disclosure or modification while in transit over the network, this guide recommends
using SSL to protect all transactions. In order to do this, it is necessary to have a
site-wide SSL infrastructure in which a CA certificate is used to verify that other
certificates, such as that presented by the LDAP server to its clients, are authentic.
Therefore, this procedure involves using the CA system to create a certificate for the
LDAP server, then installing that certificate on the LDAP server and configuring slapd
to require its use. See Section 2.5.6 for details about the process of creating SSL
certificates for use by servers at your site.Create the Certificate for the LDAP Server
Note: This step must be performed on the CA system, not on
the LDAP server itself.
Change into the CA certificate directory: # cd /etc/pki/tls/certs
Generate a key pair for the LDAP server: # openssl genrsa -out ldapserverkey.pem 2048
Next, generate a certificate signing request (CSR) for the CA to sign: # openssl req -new -key ldapserverkey.pem -out ldapserver.csr
Sign the ldapserver.csr request: # openssl ca -in ldapserver.csr -out ldapservercert.pem
This step creates a private key, ldapserverkey.pem, and a public certificate,
ldapservercert.pem. The LDAP server will use these to prove its identity by
demonstrating that it has a certificate which has been signed by the site CA. LDAP
clients at your site should only be willing to accept authentication data from a
verified LDAP server.Install the Certificate on the LDAP Server
Create the PKI directory for LDAP certificates if it does not
already exist: # mkdir /etc/pki/tls/ldap
# chown root:root /etc/pki/tls/ldap
# chmod 755 /etc/pki/tls/ldap
Using removable media or some other secure transmission format,
install the files generated in the previous step onto the LDAP server: /etc/pki/tls/ldap/serverkey.pem: the private key ldapserverkey.pem/etc/pki/tls/ldap/servercert.pem: the certificate file ldapservercert.pem
Verify the ownership and permissions of these files: # chown root:ldap /etc/pki/tls/ldap/serverkey.pem
# chown root:ldap /etc/pki/tls/ldap/servercert.pem
# chmod 640 /etc/pki/tls/ldap/serverkey.pem
# chmod 640 /etc/pki/tls/ldap/servercert.pem
Verify that the CA's public certificate file has been installed as
/etc/pki/tls/CA/cacert.pem, and has the correct permissions: # mkdir /etc/pki/tls/CA
# chown root:root /etc/pki/tls/CA/cacert.pem
# chmod 644 /etc/pki/tls/CA/cacert.pem
As a
result of these steps, the LDAP server will have access to its own private certificate
and the key with which that certificate is encrypted, and to the public certificate
file belonging to the CA. Note that it would be possible for the key to be protected
further, so that processes running as ldap could not read it. If this were done, the
LDAP server process would need to be restarted manually whenever the server rebooted.Configure slapd to Use the Certificates
Edit the file /etc/openldap/slapd.conf. Add or correct the
following lines:
TLSCACertificateFile /etc/pki/tls/CA/cacert.pem
TLSCertificateFile /etc/pki/tls/ldap/servercert.pem
TLSCertificateKeyFile /etc/pki/tls/ldap/serverkey.pem
security simple_bind=128
The first set of lines tell slapd where to find the
appropriate SSL certificates to present to clients when they request an encrypted
transaction. The last setting tells slapd never to allow clients to present
credentials (i.e. passwords) in an unencrypted session. It is a good security
principle never to allow unencrypted passwords to traverse a network, so ensure that
LDAP mandates this.Install Account Information into the LDAP Database
There are many ways to maintain an OpenLDAP database. Methods
include: Input entries in ldif(5) format into a file /path/to/new entries , and use
slapadd to import those entries while slapd is not running: # slapadd -l /path/to/new_entries Write a script to create and modify LDAP entries by connecting to the LDAP
server normally. The Perl Net::LDAP module is appropriate for this, there is a Python
API called python-ldap, and functionality is likely available for other scripting
languages as well. Use an LDAP front-end program which provides an interface for
editing the database. If the front-end program is web-based or otherwise accessible over
a network, ensure that authentication information is protected via SSL between the
administrator's client and the program, as well as between the program and the LDAP
database.
Any of these methods or others may be appropriate for your site. This guide
does not provide a recommendation, and there will be no further discussion of the syntax
of entering LDAP data into the database.Create Top-level LDAP Structure for Domain
Create a structure for the domain itself with at least the
following attributes:
dn: dc=example,dc=com
objectClass: dcObject
objectClass: organization
dc: example
o: Organization Description
This is a placeholder for the
root of the domain's LDAP tree. Without this entry, LDAP will not be able to find any
other entries for the domain.Create LDAP Structures for Users and Groups
Create LDAP structures for people (users) and for groups with
at least the following attributes:
dn: ou=people,dc=example,dc=com
ou: people
structuralObjectClass: organizationalUnit
objectClass: organizationalUnit
dn: ou=groups,dc=example,dc=com
ou: groups
structuralObjectClass: organizationalUnit
objectClass: organizationalUnit
Posix users and groups are the two top-level items
which will be needed in order to use LDAP for authentication. These organizational
units are used to identify the two categories within LDAP.Create Unix Accounts
For each Unix user, create an LDAP entry with at least the
following attributes (others may be appropriate for your site as well), using variable
values appropriate to that user.
dn: uid=username ,ou=people,dc=example,dc=com
structuralObjectClass: inetOrgPerson
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount
cn: fullname
sn: surname
gecos: fullname
gidNumber: primary-group-id
homeDirectory: /home/username
loginShell: /path/to/shell
uid: username
uidNumber: uid
userPassword: {MD5}md5-hashed-password
If your site
implements password expiration in which passwords must be changed every N days (see
Section 2.3.1.7), then each entry should also have the attribute:
shadowMax: N
In general, the LDAP schemas for users use uid to refer to the text username, and
uidNumber for the numeric UID. This usage may be slightly confusing when compared to
the standard Unix usage.
You should not create entries for the root account or for
system accounts which are unique to individual systems, but only for user accounts
which are to be shared across machines, and which have authentication information
(such as a password) associated with them.Create Unix Groups
For each Unix group, create an LDAP entry with at least the
following attributes:
dn: cn=groupname ,ou=groups,dc=example,dc=com
cn: groupname
structuralObjectClass: posixGroup
objectClass: posixGroup
gidNumber: gid
memberUid: username1
memberUid: username2
...
memberUid: usernameN
Note that each user has a
primary group, identified by the gidNumber field in the user's account entry. That
group must be created, but it is not necessary to list the user as a memberUid of the
group. This behavior should be familiar to administrators, since it is identical to
the handling of the /etc/passwd and /etc/group files.
Do not create entries for the
root group or for system groups, but only for groups which contain human users or
which are shared across systems.Create Groups to Administer LDAP
If a group of LDAP administrators, admins , is desired, that
group must be created somewhat differently. The specification should have these
attributes:
dn: cn=admins ,ou=groups,dc=example,dc=com
cn: admins
structuralObjectClass: groupOfUniqueNames
objectClass: groupOfUniqueNames
uniqueMember: cn=Manager,dc=example,dc=com
uniqueMember: uid=admin1-username ,ou=people,dc=example,dc=com
uniqueMember: uid=admin2-username ,ou=people,dc=example,dc=com
...
uniqueMember: uid=adminN-username ,ou=people,dc=example,dc=com
LDAP cannot use Posix groups for its own internal
authentication — it needs to compare the username specified in an authenticated bind
to some internal groupOfUniqueNames. If you do not specify an LDAP administrators'
group, then all LDAP management will need to be done using the LDAP root user
(Manager). For reasons of auditing and error detection, it is recommended that LDAP
administrators have unique identities. (See Section 2.3.1.3 for similar reasoning
applied to the use of sudo for privileged system commands.)Configure slapd to Protect Authentication Information
Edit the file /etc/openldap/slapd.conf. Add or correct the
following access specifications: Protect the user's password by allowing the user
himself or the LDAP administrators to change it, allowing the anonymous user to
authenticate against it, and allowing no other access:
access to attrs=userPassword
by self write
by group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write
by anonymous auth
by * none
access to attrs=shadowLastChange
by self write
by group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write
by * readAllow anyone to read other
information, and allow the administrators to change it:
access to * by
group/groupOfUniqueNames/uniqueMember="cn=admins ,ou=groups,dc=example,dc=com " write
by * read
Access rules are applied in the order encountered, so more specific rules should
appear first. In particular, the rule restricting access to userPassword must appear
before the rule allowing access to all data. The shadowLastChange attribute is a
timestamp, and is only critical if your site implements password expiration. If your
site does not have an LDAP administrators group, the LDAP root user (called Manager in
this guide) will be able to change data without an explicit access statement.Correct Permissions on LDAP Server Files
Correct the permissions on the ldap server's files: # chown ldap:root /var/lib/ldap/*
Some manual methods of inserting information into the LDAP
database may leave these files with incorrect permissions. This will prevent slapd from
starting correctly.group owner of ldap filesSpecify group owner of /var/lib/ldap/*.Specify group owner of /var/lib/ldap/*rootrootuser owner of ldap filesSpecify user owner of /var/lib/ldap/*.Specify user owner of /var/lib/ldap/*ldapldapCorrect Permissions on LDAP Server FilesThe /var/lib/ldap/* files should be owned by the appropriate group.CCE-4484-2(1) via chownCorrect Permissions on LDAP Server FilesThe /var/lib/ldap/* files should be owned by the appropriate user.CCE-4502-1(1) via chownConfigure iptables to Allow Access to the LDAP Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which will synchronize to this
server.
Edit /etc/sysconfig/iptables. Add the following lines, ensuring that they appear
before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 389 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 636 -j ACCEPT
The default Iptables configuration does not allow inbound access to any services. These
modifications allow access to the LDAP primary (389) and encrypted-only (636) ports,
while keeping all other ports on the server in their default protected state. See
Section 2.5.5 for more information about Iptables.
Note: Even if the LDAP server
restricts connections so that only encrypted queries are allowed, it will probably be
necessary to allow traffic to the default port 389. This is true because many LDAP
clients implement encryption by connecting to the primary port and issuing the STARTTLS
command.Configure Logging for LDAPEdit the file /etc/syslog.conf. Add or correct the following line:
local4.* /var/log/ldap.log Create the log file with safe permissions: # touch /var/log/ldap.log
# chown root:root /var/log/ldap.log
# chmod 0600 /var/log/ldap.log Edit the file /etc/logrotate.d/syslog and add the pathname
/var/log/ldap.log
to the space-separated list in the first line. Edit the LDAP configuration file
/etc/openldap/slapd.conf and set a reasonable set of default log parameters, such as:
loglevel stats2
OpenLDAP sends its log data to the syslog facility local4 at priority
debug. By default, RHEL5 does not store this facility at all. The syslog configuration
suggested here will store any output logged by slapd in the file /var/log/ldap.log, and
will include that file in the standard log rotation for syslog files.
By default, LDAP's
logging is quite verbose. The loglevel parameter is a space-separated list of items to
be logged. Specifying stats2 will reduce the log output somewhat, but this level will
still produce some logging every time an LDAP query is made. (This may be appropriate,
depending on your site's auditing requirements.) In order to capture only slapd startup
messages, specify loglevel none.
See slapd.conf(5) for detailed information about the
loglevel parameter. See Section 2.6.1 for more information about syslog.NFS and RPC
The Network File System is the most popular distributed filesystem
for the Unix environment, and is very widely deployed. Unfortunately, NFS was not designed
with security in mind, and has a number of weaknesses, both in terms of the protocol itself
and because any NFS installation must expose several daemons, running on both servers and
clients, to network attack.
This section discusses the circumstances under which it is
possible to disable NFS and its dependencies, and then details steps which should be taken
to secure, as much as possible, NFS's configuration. This section is relevant to machines
operating as NFS clients, as well as to those operating as NFS servers.Disable All NFS Services if Possible
Is there a mission-critical reason for this machine to operate as
either an NFS client or an NFS server?
If not, follow all instructions in the remainder of
Section 3.13.1 to disable subsystems required by NFS.
NFS is a commonly used mechanism for
sharing data between machines in an organization. However, its use opens many potential
security holes. If NFS is not universally needed in your organization, improve the
security posture of any machine which does not require NFS by disabling it entirely.The steps in Section 3.13.1 will prevent a machine from operating
as either an NFS client or an NFS server. Only perform these steps on machines which do
not need NFS at all. Disable Services Used Only by NFS
If NFS is not needed, perform the following steps to disable
NFS client daemons: # chkconfig nfslock off
# chkconfig rpcgssd off
# chkconfig rpcidmapd off
The nfslock, rpcgssd, and rpcidmapd daemons all perform NFS client functions.
All of these daemons run with elevated privileges, and many listen for
network connections. If they are not needed, they should be disabled to improve system
security posture.Disable nfslockThe nfslock service should be disabled.CCE-4396-8(1) via chkconfigDisable rpcgssdThe rpcgssd service should be disabled.CCE-3535-2(1) via chkconfigDisable rpcidmapdThe rpcidmapd service should be disabled.CCE-3568-3(1) via chkconfigDisable netfs if Possible
Determine whether any network filesystems handled by netfs are
mounted on this system: # mount -t nfs,nfs4,smbfs,cifs,ncpfs
If this command returns no output, disable netfs to improve system security: # chkconfig netfs off
The netfs script
manages the boot-time mounting of several types of networked filesystems, of which NFS
and Samba (see Section 3.18) are the most common. If these filesystem types are not in
use, the script can be disabled, protecting the system somewhat against accidental or
malicious changes to /etc/fstab and against flaws in the netfs script itself.Disable netfs if PossibleThe netfs service should be disabled.CCE-4533-6(1) via chkconfigDisable RPC Portmapper if Possible
If: NFS is not needed The site does not rely on NIS for authentication information, and The machine does not run any other RPC-based service
then disable the RPC portmapper service: # chkconfig portmap off
By design, the RPC
model does not require particular services to listen on fixed ports, but instead uses a
daemon, portmap, to tell prospective clients which ports to use to contact the services
they are trying to reach. This model weakens system security by introducing another
privileged daemon which may be directly attacked, and is unnecessary because RPC was
never adopted by enough services to risk using up all the ports on a system.
Unfortunately, the portmapper is central to RPC design, so it cannot be disabled if your
site is using any RPCbased services, including NFS, NIS (see Section 3.2.4 for
information about NIS, which is not recommended), or any third-party or custom RPC-based
program. If none of these programs are in use, however, portmap should be disabled to
improve system security.
In order to get more information about whether portmap may be
disabled on a given host, query the local portmapper using the command: # rpcinfo -p
If the only services listed are portmapper and status, it is safe to disable the
portmapper. If other services are listed and your site is not running NFS or NIS,
investigate these services and disable them if possible.Disable RPC Portmapper if PossibleThe portmap service should be disabled.CCE-4550-0(1) via chkconfigConfigure All Machines which Use NFSThe steps in this section are appropriate for all machines which run NFS, whether they operate as clients or as servers.Make Each Machine a Client or a Server, not Both
If NFS must be used, it should be deployed in the simplest
configuration possible to avoid maintainability problems which may lead to unnecessary
security exposure. Due to the reliability and security problems caused by NFS, it is not
a good idea for machines which act as NFS servers to also mount filesystems via NFS. At
the least, crossed mounts (the situation in which each of two servers mounts a
filesystem from the other) should never be used.Restrict Access to the Portmapper
Edit the file /etc/hosts.deny. Add or correct the line:
portmap: ALL
Edit the file /etc/hosts.allow. Add or correct the line:
portmap: IPADDR1 , IPADDR2 , ...
where each IPADDR is the IP address of a server or client with which this
machine shares NFS filesystems. If the machine is an NFS server, it may be simpler to
use an IP netblock specification, such as 10.3.2. (this is the TCP Wrappers syntax
representing the netblock 10.3.2.0/24), or a hostname specification, such as
.subdomain.example.com. The use of hostnames is not recommended.
The /etc/hosts.allow
and /etc/hosts.deny files are used by TCP Wrappers to determine whether specified remote
hosts are allowed to access certain services. The default portmapper shipped with RHEL5
has TCP Wrappers support built in, so this specification can be used to provide some
protection against network attacks on the portmapper. (See Section 2.5.4 for more
information about TCP Wrappers.)
Note: This step protects only the portmap service
itself. It is still possible for attackers to guess the port numbers of NFS services and
attack those services directly, even if they are denied access to the portmapper.Configure NFS Services to Use Fixed Ports
Edit the file /etc/sysconfig/nfs. Add or correct the following
lines:
LOCKD_TCPPORT=lockd-port
LOCKD_UDPPORT=lockd-port
MOUNTD_PORT=mountd-port
RQUOTAD_PORT=rquotad-port
STATD_PORT=statd-port
STATD_OUTGOING_PORT=statd-outgoing-port
where each X-port is a port which is not used by any other service on your network.
Firewalling should be done at each host and at the border firewalls to protect the NFS
daemons from remote access, since NFS servers should never be accessible from outside
the organization. However, by default, the portmapper assigns each NFS service to a port
dynamically at service startup time. Dynamic ports cannot be protected by port filtering
firewalls such as iptables (Section 2.5.5).
Therefore, restrict each service to always
use a given port, so that firewalling can be done effectively. Note that, because of the
way RPC is implemented, it is not possible to disable the portmapper even if ports are
assigned statically to all RPC services.Configure lockd to Use Fixed Ports for TCPThe lockd service should be configured to use a static port for TCPCCE-4559-1(1) via /etc/sysconfig/nfsConfigure statd to Use an outgoing static portThe statd service should be configured to use an outgoing static portCCE-4015-4(1) via /etc/sysconfig/nfsConfigure statd to Use a static portThe statd service should be configured to use a static portCCE-3667-3(1) via /etc/sysconfig/nfsConfigure lockd to Use a static port for UDPThe lockd service should be configured to use a static port for UDPCCE-4310-9(1) via /etc/sysconfig/nfsConfigure mountd to Use a static portThe mountd service should be configured to use a static portCCE-4438-8(1) via /etc/sysconfig/nfsConfigure rquotad to Use Fixed PortsThe rquotad service should be configured to use a static portCCE-3579-0(1) via /etc/sysconfig/nfsConfigure NFS ClientsThe steps in this section are appropriate for machines which operate as NFS clients.Disable NFS Server Daemons# chkconfig nfs off
# chkconfig rpcsvcgssd off
There is no need
to run the NFS server daemons except on a small number of properly secured machines
designated as NFS servers. Ensure that these daemons are turned off on clients.Disable nfs serviceThe nfs service should be disabledCCE-4473-5(1) via chkconfigDisable rpcsvcgssd serviceThe rpcsvcgssd service should be disabledCCE-4491-7(1) via chkconfigMount Remote Filesystems with Restrictive Options
Edit the file /etc/fstab. For each filesystem whose type
(column 3) is nfs or nfs4, add the text ,nodev,nosuid to the list of mount options in
column 4. If appropriate, also add ,noexec.
See Section 2.2.1.2 for a description of the
effects of these options. In general, execution of files mounted via NFS should be
considered risky because of the possibility that an adversary could intercept the
request and substitute a malicious file. Allowing setuid files to be executed from
remote servers is particularly risky, both for this reason and because it requires the
clients to extend root-level trust to the NFS server.Mount Remote Filesystems with nodevThe nodev option should be enabled for all NFS mountsCCE-4368-7(1) via /etc/fstabMount Remote Filesystems with nosuidThe nosuid option should be enabled for all NFS mountsCCE-4024-6(1) via /etc/fstabMount Remote Filesystems with noexecThe noexec option should be enabled for all NFS mountsCCE-4526-0(1) via /etc/fstabConfigure NFS ServersThe steps in this section are appropriate for machines which operate as NFS servers.Configure the Exports File Restrictively
Linux's NFS implementation uses the file /etc/exports to
control what filesystems and directories may be accessed via NFS. (See the exports(5)
manpage for more information about the format of this file.)
The syntax of the exports
file is not necessarily checked fully on reload, and syntax errors can leave your NFS
configuration more open than intended. Therefore, exercise caution when modifying the
file.
The syntax of each line in /etc/exports is
/DIR ipaddr1 (opt1 ,opt2 ) ipaddr2 (opt3 )
where /DIR is a directory or filesystem to export, ipaddrN is an IP address,
netblock, hostname, domain, or netgroup to which to export, and optN is an option.Use Access Lists to Enforce Authorization Restrictions on Mounts
Edit /etc/exports. Ensure that each export line contains a
set of IP addresses or hosts which are allowed to access that export.
If no IP
addresses or hostnames are specified on an export line, then that export is available
to any remote host which requests it. All lines of the exports file should specify the
hosts (or subnets, if needed) which are allowed to access the exported directory, so
that unknown or remote hosts will be denied.Use Root-Squashing on All Exports
Edit /etc/exports. Ensure that no line contains the option no_root_squash.
If a filesystem is exported using root squashing, requests from root on
the client are considered to be unprivileged (mapped to a user such as nobody). This
provides some mild protection against remote abuse of an NFS server. Root squashing is
enabled by default, and should not be disabled.Use Root-Squashing on All ExportsRoot squashing should be enabled for all NFS sharesCCE-4544-3(1) via /etc/exportsRestrict NFS Clients to Privileged Ports
Edit /etc/exports. Ensure that no line contains the option insecure.
By default, Linux's NFS implementation requires that all client requests be
made from ports less than 1024. If your organization has control over machines
connected to its network, and if NFS requests are prohibited at the border firewall,
this offers some protection against malicious requests from unprivileged users.
Therefore, the default should not be changed.Restrict NFS Clients to Privileged PortsRestriction of NFS clients to privileged ports should be enabledCCE-4465-1(1) via /etc/exportsExport Filesystems Read-Only if Possible
Edit /etc/exports. Ensure that every line contains the option
ro and does not contain the option rw, unless there is an operational need for remote
clients to modify that filesystem.
If a filesystem is being exported so that users can
view the files in a convenient fashion, but there is no need for users to edit those
files, exporting the filesystem read-only removes an attack vector against the server.
The default filesystem export mode is ro, so do not specify rw without a good reason.Export Filesystems Read-Only if PossibleWrite access to NFS shares should be disabledCCE-4350-5(1) via /etc/exportsAllow Legitimate NFS Clients to Access the Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which must mount NFS filesystems
from this server.
Edit /etc/sysconfig/iptables. Add the following lines, ensuring that
they appear before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport 111 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 111 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 2049 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport lockd-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport lockd-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport mountd-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport mountd-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport rquotad-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport rquotad-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport statd-port -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p udp --dport statd-port -j ACCEPT
where the variable port numbers match those selected in Section 3.13.2.3
The default iptables configuration does not allow inbound access to any services. This
modification will allow the specified block of remote hosts to initiate connections to
the set of NFS daemons, while keeping all other ports on the server in their default
protected state. See Section 2.5.5 for more information about iptables.DNS ServerMost organizations have an operational need to run at least one nameserver. However, there are many common attacks involving DNS, be configured defensively.Liu, C. DNS & BIND Cookbook. O’Reilly and Associates, Oct 2002Disable DNS Server if Possible
Is there an operational need for this machine to act as a DNS
server for this site?
If not, disable the software and remove it from the system: # chkconfig named off
# yum erase bind
DNS software should be disabled on any machine which
does not need to be a nameserver. Note that the BIND DNS server software is not installed
on RHEL5 by default. The remainder of this section discusses secure configuration of
machines which must be nameservers.Disable DNS Server if PossibleThe named service should be disabled.CCE-3578-2(1) via chkconfigUninstall bind if PossibleThe bind package should be uninstalled.CCE-4219-2(1) via yumRun the BIND9 Software if DNS Service is Needed
It is highly recommended that the BIND9 software be used to
provide DNS service. BIND is the Internet standard Unix nameserver, and, while it has had
security problems in the past, it is also well-maintained and Red Hat is likely to quickly
issue updates in response to any problems discovered in the future. In addition, BIND
version 9 has new security features and more secure default settings than earlier
versions. In particular, BIND version 4 is no longer recommended for production use, and
BIND4 servers should be upgraded to a newer version as soon as possible.Isolate DNS from Other Services
This section discusses mechanisms for preventing the DNS server
from interfering with other services. This is done both to protect the remainder of the
network should a nameserver be compromised, and to make direct attacks on nameservers more
difficult.Run DNS Software on Dedicated Servers if Possible
Since DNS is a high-risk service which must frequently be made
available to the entire Internet, it is strongly recommended that no other services be
offered by machines which act as organizational DNS servers.Run DNS Software in a chroot Jail
Install the bind-chroot package: # yum install bind-chroot
Place a valid named.conf file inside the chroot jail: # cp /etc/named.conf /var/named/chroot/etc/named.conf
# chown root:root /var/named/chroot/etc/named.conf
# chmod 644 /var/named/chroot/etc/named.conf
Create and populate an appropriate zone
directory within the jail, based on the options directive. If your named.conf includes:
options {
directory "/path/to/DIRNAME ";
...
}
then copy that directory and its contents from the original zone directory: # cp -r /path/to/DIRNAME /var/named/chroot/DIRNAME
Edit the file /etc/sysconfig/named. Add or correct the line:
ROOTDIR=/var/named/chroot
Chroot jails are not foolproof. However, they serve to make it more difficult for a
compromised program to be used to attack the entire host. They do this by restricting a
program's ability to traverse the directory upward, so that files outside the jail are
not visible to the chrooted process. Since RHEL5 supports a standard mechanism for
placing BIND in a chroot jail, you should take advantage of this feature.
Note: If you
are running BIND in a chroot jail, then you should use the jailed named.conf as the
primary nameserver configuration file. That is, when this guide recommends editing
/etc/named.conf, you should instead edit /var/named/chroot/etc/named.conf.group owner of jailSpecify group owner of /var/named/chroot/etc/named.confSpecify group owner of /var/named/chroot/etc/named.confrootrootuser owner of jailSpecify user owner of /var/named/chroot/etc/named.confSpecify user owner of /var/named/chroot/etc/named.confrootrootpermisison of jailSpecify file permissions on /var/named/chroot/etc/named.confSpecify permissions of /var/named/chroot/etc/named.conf110100100100000000110100100111000000^[01]+$Run DNS Software in a chroot Jail owned by root groupThe /var/named/chroot/etc/named.conf file should be owned by the appropriate group.CCE-3985-9(1) via chownRun DNS Software in a chroot Jail owned by root userThe /var/named/chroot/etc/named.conf file should be owned by the appropriate user.CCE-4258-0(1) via chownSet permissions on chroot Jail for DNSFile permissions for /var/named/chroot/etc/named.conf should be set correctly.CCE-4487-5(1) via chmodConfigure Firewalls to Protect the DNS Server
Edit the file /etc/sysconfig/iptables. Add the following lines,
ensuring that they appear before the final LOG and DROP lines for the
RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -m state --state NEW -p udp --dport 53 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 53 -j ACCEPT
These
lines are necessary in order to allow remote machines to contact the DNS server. If this
server is only available to the local network, it may be appropriate to insert a -s flag
into this rule to allow traffic only from packets on the local network. See Section
3.5.1.2 for an example of such a modification. See Section 2.5.5 for general information
about iptables.Protect DNS Data from Tampering or AttackThis section discusses DNS configuration options which make it more difficult for attackers to gain access to private DNS data or to modify DNS data.Run Separate DNS Servers for External and Internal Queries if
Possible
Is it possible to run external and internal nameservers on
separate machines? If so, follow the configuration guidance in this section. If not, see
Section 3.14.4.2 for an alternate approach using BIND9.
On the external nameserver, edit /etc/named.conf. Add or correct the following
directives:
options {
allow-query { any; };
recursion no;
...
};
zone "example.com " IN {
...
};
On the internal nameserver, edit
/etc/named.conf. Add or correct the following directives, where SUBNET is the numerical
IP representation of your organization in the form xxx.xxx.xxx.xxx/xx:
acl internal {
SUBNET ;
localhost;
};
options {
allow-query { internal; };
...
};
zone "internal.example.com " IN {
...
};
Enterprise nameservers generally serve two
functions. One is to provide public information about the machines in a domain for the
benefit of outside users who wish to contact those machines, for instance in order to
send mail to users in the enterprise, or to visit the enterprise's external web page.
The other is to provide nameservice to client machines within the enterprise. Client
machines require both private information about enterprise machines (which may be
different from the public information served to the rest of the world) and public
information about machines outside the enterprise, which is used to send mail or visit
websites outside of the organization.
In order to provide the public nameservice
function, it is necessary to share data with untrusted machines which request it —
otherwise, the enterprise cannot be conveniently contacted by outside users. However,
internal data should be protected from disclosure, and serving irrelevant public name
queries for outside domains leaves the DNS server open to cache poisoning and other
attacks. Therefore, local network nameservice functions should not be provided to
untrusted machines.
Separate machines should be used to fill these two functions whenever possible.Use Views to Partition External and Internal Information if Necessary
If it is not possible to run external and internal nameservers
on separate physical machines, run BIND9 and simulate this feature using views.
Edit
/etc/named.conf. Add or correct the following directives (where SUBNET is the numerical
IP representation of your organization in the form xxx.xxx.xxx.xxx/xx):
acl internal {
SUBNET ;
localhost;
};
view "internal-view" {
match-clients { internal; };
zone "." IN {
type hint;
file "db.cache";
};
zone "internal.example.com " IN {
...
};
};
view "external-view" {
match-clients { any; };
recursion no;
zone "example.com " IN {
...
};
};
The view feature is provided by BIND9 as a way to allow a single nameserver to make
different sets of data available to different sets of clients. If possible, it is always
better to run external and internal nameservers on separate machines, so that even
complete compromise of the external server cannot be used to obtain internal data or
confuse internal DNS clients. However, this is not always feasible, and use of a feature
like views is preferable to leaving internal DNS data entirely unprotected.
Note: As
shown in the example, database files which are required for recursion, such as the root
hints file, must be available to any clients which are allowed to make recursive
queries. Under typical circumstances, this includes only the internal clients which are
allowed to use this server as a general-purpose nameserver.Disable Zone Transfers from the Nameserver if Possible
Is it necessary for a secondary nameserver to receive zone data
via zone transfer from the primary server? If not, follow the instructions in this
section. If so, see the next section for instructions on protecting zone transfers.
Edit /etc/named.conf. Add or correct the following directive:
options { allow-transfer { none; };
...
}
If both the primary and secondary nameserver are under your control, or
if you have only one nameserver, it may be possible to use an external configuration
management mechanism to distribute zone updates. In that case, it is not necessary to
allow zone transfers within BIND itself, so they should be disabled to avoid the
potential for abuse.Authenticate Zone Transfers if Necessary
If it is necessary for a secondary nameserver to receive zone
data via zone transfer from the primary server, follow the instructions here.
Use dnssec-keygen to create a symmetric key file in the current directory: # cd /tmp
# dnssec-keygen -a HMAC-MD5 -b 128 -n HOST dns.example.com
Kdns.example.com .+aaa +iiiii
This output is the name of a file containing the new key. Read the file to find the
base64-encoded key string: # cat Kdns.example.com.+NNN+MMMMM.key
dns.example.com IN KEY 512 3 157 base64-key-string
Edit /etc/named.conf on the primary nameserver. Add the directives:
key zone-transfer-key {
algorithm hmac-md5;
secret "base64-key-string ";
};
zone "example.com " IN {
type master;
allow-transfer { key zone-transfer-key; };
...
}
Edit /etc/named.conf on the secondary nameserver. Add the directives:
key zone-transfer-key {
algorithm hmac-md5;
secret "base64-key-string ";
};
server IP-OF-MASTER {
keys { zone-transfer-key; };
};
zone "example.com " IN {
type slave;
masters { IP-OF-MASTER ; };
...
};
The BIND transaction signature (TSIG) functionality
allows primary and secondary nameservers to use a shared secret to verify authorization
to perform zone transfers. This method is more secure than using IP-based limiting to
restrict nameserver access, since IP addresses can be easily spoofed.
However, if you
cannot configure TSIG between your servers because, for instance, the secondary
nameserver is not under your control and its administrators are unwilling to configure
TSIG, you can configure an allow-transfer directive with numerical IP addresses or ACLs
as a last resort.
Note: The purpose of the dnssec-keygen command is to create the shared
secret string base64-key-string . Once this secret has been obtained and inserted into
named.conf on the primary and secondary servers, the key files
Kdns.example.com.+NNN+MMMMM.key and Kdns.example.com.+NNN+MMMMM.private are no longer
needed, and may safely be deleted.Disable Dynamic Updates if Possible
Is there a mission-critical reason to enable the risky dynamic
update functionality? If not:
Edit /etc/named.conf. For each zone specification, correct
the following directive if necessary:
zone "example.com " IN {
allow-update { none; };
...
}
Dynamic updates allow remote servers to add, delete, or modify any entries in your
zone file. Therefore, they should be considered highly risky, and disabled unless there
is a very good reason for their use.
If dynamic updates must be allowed, IP-based ACLs
are insufficient protection, since they are easily spoofed. Instead, use TSIG keys (see
the previous section for an example), and consider using the update-policy directive to
restrict changes to only the precise type of change needed.Disable DNS Dynamic Updates if PossibleLDAP's dynamic updates feature should be disabledCCE-4399-2(1) via /etc/named.confFTPServer
FTP is a common method for allowing remote access to files. Like
telnet, the FTP protocol is unencrypted, which means that passwords and other data
transmitted during the session can be captured and that the session is vulnerable to
hijacking. Therefore, running the FTP server software is not recommended.
However, there are
some FTP server configurations which may be appropriate for some environments, particularly
those which allow only read-only anonymous access as a means of downloading data available
to the public.Disable vsftpd if Possible
Is there a mission-critical reason for the machine to act as an
FTP server? If not, disable vsftpd if it has been installed: # chkconfig vsftpd offDisable vsftpd if PossibleThe vsftpd service should be disabled.CCE-3919-8(1) via chkconfigUninstall vsftpd if PossibleThe vsftpd service should be uninstalled.CCE-3919-8(1) via yumUse vsftpd to Provide FTP Service if Necessary
If this machine must operate as an FTP server, install the vsftpd
package via the standard channels: # yum install vsftpd After RHEL 2.1, Red Hat switched
from distributing wu-ftpd with RHEL to distributing vsftpd. For security and for
consistency with future Red Hat releases, the use of vsftpd is recommended.Configure vsftpd Securely
The primary vsftpd configuration file is /etc/vsftpd.conf, if
that file exists, or /etc/vsftpd/vsftpd.conf if it does not. For the remainder of this
section, the phrase 'the configuration file' will refer to whichever of those files is
appropriate for your environment.Enable Logging of All FTP Transactions
Edit the vsftpd configuration file. Add or correct the
following configuration options:
xferlog_std_format=NO
log_ftp_protocol=YES
The
modifications above ensure that all commands sent to the ftp server are logged using the
verbose vsftpd log format. The default vsftpd log file is /var/log/vsftpd.log.
Note: If
verbose logging to vsftpd.log is done, sparse logging of downloads to /var/log/xferlog
will not also occur. However, the information about what files were downloaded is
included in the information logged to vsftpd.log.Enable Logging of All FTP TransactionsLogging of vsftpd transactions should be enabledCCE-4549-2(1) via /etc/vsftpd.confCreate Warning Banners for All FTP Users
Edit the vsftpd configuration file. Add or correct the
following configuration options:
banner_file=/etc/issue
See Section 2.3.7 for an
explanation of banner file use. This setting will cause the system greeting banner to be
used for FTP connections as well.Create Warning Banners for All FTP UsersA warning banner for all FTP users should be enabledCCE-4554-2(1) via /etc/vsftpd.confRestrict the Set of Users Allowed to Access FTP
This section describes how to disable non-anonymous
(password-based) FTP logins, or, if it is not possible to do this entirely due to legacy
applications, how to restrict insecure FTP login to only those users who have an
identified need for this access.Restrict Access to Anonymous Users if Possible
Is there a mission-critical reason for users to transfer
files to/from their own accounts using FTP, rather than using a secure protocol like
SCP/SFTP? If not:
Edit the vsftpd configuration file. Add or correct the following
configuration option:
local_enable=NO
If non-anonymous FTP logins are necessary,
follow the guidance in the remainder of this section to secure these logins as much as
possible.
The use of non-anonymous FTP logins is strongly discouraged. Since SSH
clients and servers are widely available, and since SSH provides support for a
transfer mode which resembles FTP in user interface, there is no good reason to allow
password-based FTP access. See Section 3.5 for more information about SSH.Restrict Access to Anonymous Users if PossibleLocal user login to the vsftpd service should be disabledCCE-4443-8(1) via /etc/vsftpd.confLimit Users Allowed FTP Access if Necessary
If there is a mission-critical reason for users to access
their accounts via the insecure FTP protocol, limit the set of users who are allowed
this access.
Edit the vsftpd configuration file. Add or correct the following
configuration options:
userlist_enable=YES
userlist_file=/etc/vsftp.ftpusers
userlist_deny=NO
Edit the file /etc/vsftp.ftpusers. For each user USERNAME who should
be allowed to access the system via ftp, add a line containing that user's name.
USERNAME
If anonymous access is also required, add the anonymous usernames to
/etc/vsftp.ftpusers as well:
anonymous
ftp
Historically, the file /etc/ftpusers
contained a list of users who were not allowed to access the system via ftp. It was
used to prevent system users such as the root user from logging in via the insecure
ftp protocol.
However, when the configuration option userlist_deny=NO is set, vsftpd
interprets ftpusers as the set of users who are allowed to login via ftp. Since it
should be possible for most users to access their accounts via secure protocols, it is
recommended that this setting be used, so that non-anonymous ftp access can be limited
to legacy users who have been explicitly identified.Disable FTP Uploads if Possible
Is there a mission-critical reason for users to upload files
via FTP? If not:
Edit the vsftpd configuration file. Add or correct the following
configuration options:
write_enable=NO
If FTP uploads are necessary, follow the guidance
in the remainder of this section to secure these transactions as much as possible.
Anonymous FTP can be a convenient way to make files available for universal download.
However, it is less common to have a need to allow unauthenticated users to place files
on the FTP server. If this must be done, it is necessary to ensure that files cannot be
uploaded and downloaded from the same directory.Disable FTP Uploads if PossibleFile uploads via vsftpd should be disabledCCE-4461-0(1) via /etc/vsftpd.confPlace the FTP Home Directory on its Own Partition
By default, the anonymous FTP root is the home directory of the
ftp user account. The df command can be used to verify that this directory is on its own
partition.
If there is a mission-critical reason for anonymous users to upload files,
precautions must be taken to prevent these users from filling a disk used by other
services.Configure Firewalls to Protect the FTP Server
Edit the file /etc/sysconfig/iptables. Add the following lines,
ensuring that they appear before the final LOG and DROP lines for the
RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 21 -j ACCEPT
Edit the file /etc/sysconfig/iptables-config. Ensure that the space-separated
list of modules contains the FTP connection tracking module:
IPTABLES_MODULES="ip_conntrack_ftp"
These settings configure iptables to allow
connections to an FTP server. The first line allows initial connections to the FTP
server port.
FTP is an older protocol which is not very compatible with firewalls.
During the initial FTP dialogue, the client and server negotiate an arbitrary port to be
used for data transfer. The ip conntrack ftp module is used by iptables to listen to
that dialogue and allow connections to the data ports which FTP negotiates. This allows
an FTP server to operate on a machine which is running a firewall.Web Server
The web server is responsible for providing access to content via
the HTTP protocol. Web servers represent a significant security risk because: The HTTP port is commonly probed by malicious sources Web server software is very complex, and includes a long history of vulnerabilities The HTTP protocol is unencrypted and vulnerable to passive monitoring
The system's default web server software is Apache 2 and is provided
in the RPM package httpd.Ristic, I. Apache Security. O’Reilly and Associates, Mar 2005Disable Apache if Possible
If Apache was installed and activated, but the system does not
need to act as a web server, then it should be disabled and removed from the system: # chkconfig httpd off
# yum erase httpdDisable Apache if PossibleThe httpd service should be disabled.CCE-4338-0(1) via chkconfigUninstall Apache if PossibleThe httpd package should be uninstalled.CCE-4514-6(1) via yumInstall Apache if Necessary
If the Apache web server must be run, follow these guidelines to
install it defensively. Then follow the guidelines in the remainder of Section 3.16 to
configure the web server machine and software as securely as possible.Install Apache Software Safely
Install the Apache 2 package from the standard Red Hat
distribution channel: # yum install httpd
Note: This method of installation is
recommended over installing the 'Web Server' package group during the system
installation process. The Web Server package group includes many packages which are
likely extraneous, while the command-line method installs only the required httpd
package itself.Confirm Minimal Built-in Modules
The default Apache installation minimizes the number of modules
that are compiled directly into the binary (core prefork http core mod so). This
minimizes risk by limiting the capabilities allowed by the webserver.
Query the set of compiled-in modules using the following command: $ httpd -l
If the number of compiled-in
modules is significantly larger than the aforementioned set, this guide recommends
reinstallating Apache with a reduced configuration.Secure the Apache Configuration
The Apache configuration file is /etc/httpd/conf/httpd.conf.
Apply the recommendations in the remainder of this section to this file.Restrict Information Leakage
The ServerTokens and ServerSignature directives determine how
much information the web server discloses about the configuration of the system.
ServerTokens Prod restricts information in page headers, returning only the word
'Apache.' ServerSignature Off keeps Apache from displaying the server version on error
pages. It is a good security practice to limit the information provided to clients.
Add
or correct the following directives in /etc/httpd/conf/httpd.conf so that as little
information as possible is released:
ServerTokens Prod
ServerSignature Offvalue of ServerTokensTells apache to only return Apache in the Server header, returned on every page request.Specify restrictions of of provided information in page headers for web serverProdProdvalue of ServerSignatureTells apache not to display the server version on error pages, or other pages it generates.Enable/Disable Apache displaying the server version on error pagesOffOffRestrict Information Leakageusing ServerTokensThe apache2 server's ServerTokens value should be set appropriatelyCCE-4474-3(1) via /etc/httpd/conf/httpd.confRestrict Information Leakage using ServerSignatureThe apache2 server's ServerSignature value should be set appropriatelyCCE-3756-4(1) via /etc/httpd/conf/httpd.confMinimize Loadable Modules
A default installation of Apache includes a plethora of
'dynamically shared objects' (DSO) that are loaded at run-time. Unlike the
aforementioned 'compiled-in' modules, a DSO can be disabled in the configuration file by
removing the corresponding LoadModule directive.
Note: A DSO only provides additional
functionality if associated directives are included in the Apache configuration file. It
should also be noted that removing a DSO will produce errors on Apache startup if the
configuration file contains directives that apply to that module. Refer to
http://httpd.apache.org/docs/ for details on which directives are associated with each
DSO.
Follow each DSO removal, the configuration can be tested with the following command
to check if everything still works: # service httpd configtest
The purpose of each of
the modules loaded by default will now be addressed one at a time. If none of a module's
directives are being used, remove it.Apache Core Modules
These modules comprise a basic subset of modules that are
likely needed for base Apache functionality; ensure they are not commented out in
/etc/httpd/conf/httpd.conf:
LoadModule auth_basic_module modules/mod_auth_basic.so
LoadModule authn_default_module modules/mod_authn_default.so
LoadModule authz_host_module modules/mod_authz_host.so
LoadModule authz_user_module modules/mod_authz_user.so
LoadModule authz_groupfile_module modules/mod_authz_groupfile.so
LoadModule authz_default_module modules/mod_authz_default.so
LoadModule log_config_module modules/mod_log_config.so
LoadModule logio_module modules/mod_logio.so
LoadModule setenvif_module modules/mod_setenvif.so
LoadModule mime_module modules/mod_mome.so
LoadModule autoindex_module modules/mod_autoindex.so
LoadModule negotiation_module modules/mod_negotiation.so
LoadModule dir_module modules/mod_dir.so
LoadModule alias_module modules/mod_alias.soHTTP Basic Authentication
The following modules are necessary if this web server will
provide content that will be restricted by a password.
Authentication can be performed
using local plain text password files (authn file), local DBM password files (authn
dbm) or an LDAP directory (see Section 3.16.3.2.5). The only module required by the
web server depends on your choice of authentication. Comment out the modules you don't
need from the following:
LoadModule authn_file_module modules/mod_authn_file.so
LoadModule authn_dbm_module modules/mod_authn_dbm.so
authn_alias allows for
authentication based on aliases. authn_anon allows anonymous authentication similar to
that of anonymous ftp sites. authz owner allows authorization based on file ownership.
authz dbm allows for authorization based on group membership if the web server is
using DBM authentication.
If the above functionality is unnecessary, comment out the
related module: #LoadModule authn_alias_module modules/mod_authn_alias.so
#LoadModule authn_anon_module modules/mod_authn_anon.so
#LoadModule authz_owner_module modules/mod_authz_owner.so
#LoadModule authz_dbm_module modules/mod_authz_dbm.soHTTP Digest Authentication
This module provides encrypted authentication sessions.
However, this module is rarely used and considered experimental. Alternate methods of
encrypted authentication are recommended, such as SSL (Section 3.16.4.1)
If the above
functionality is unnecessary, comment out the related module: #LoadModule auth_digest_module modules/mod_auth_digest.somod rewrite
The mod rewrite module is very powerful and can protect
against certain classes of web attacks. However, it is also very complex and has a
significant history of vulnerabilities itself.
If the above functionality is
unnecessary, comment out the related module: #LoadModule rewrite_module modules/mod_rewrite.soLDAP Support
This module provides HTTP authentication via an LDAP
directory.
If the above functionality is unnecessary, comment out the related modules:#LoadModule ldap_module modules/mod_ldap.so
#LoadModule authnz_ldap_module modules/mod_authnz_ldap.so
If LDAP is to be used, SSL encryption (Section 3.16.4.1)
should be used as well.Server Side Includes
Server Side Includes provide a method of dynamically
generating web pages through the insertion of server-side code. However, the
technology is also deprecated and introduces significant security concerns.
If the above functionality is unnecessary, comment out the related module: #LoadModule include_module modules/mod_include.so
If there is a critical need for Server Side
Includes, they should be enabled with the option IncludesNoExec to prevent arbitrary
code execution. Additionally, user supplied data should be encoded to prevent
cross-site scripting vulnerabilities.MIME Magic
This module provides a second layer of MIME support that in
most configurations is likely extraneous.
If the above functionality is unnecessary, comment out the related module: #LoadModule mime_magic_module modules/mod_mime_magic.soWebDAV (Distributed Authoring and Versioning)
WebDAV is an extension of the HTTP protocol that provides
distributed and collaborative access to web content. Due to a number of security
concerns with WebDAV, its use is not recommended.
If the above functionality is unnecessary, comment out the related modules: #LoadModule dav_module modules/mod_dav.so
#LoadModule dav_fs_module modules/mod_dav_fs.so
If there is a
critical need for WebDAV, extra care should be taken in its configuration. Since DAV
access allows remote clients to manipulate server files, any location on the server
that is DAV enabled should be protected by encrypted authentication.Server Activity Status
This module provides real-time access to statistics on the
internal operation of the web server. This is an unnecessary information leak and
should be disabled.
If the above functionality is unnecessary, comment out the related module: #LoadModule status_module modules/mod_status.so
If there is a critical need
for this module, ensure that access to the status page is properly restricted to a
limited set of hosts in the status handler configuration.Web Server Configuration Display
This module creates a web page illustrating the configuration
of the web server. This is an unnecessary security leak and should be disabled.
If the above functionality is unnecessary, comment out the related module: #LoadModule info_module modules/mod_info.so
If there is a critical need for this module, use the
Location directive to provide an access control list to restrict access to the
information.URL Correction on Misspelled Entries
This module attempts to find a document match by allowing one
misspelling in an otherwise failed request.
If the above functionality is unnecessary, comment out the related module: #LoadModule speling_module modules/mod_speling.so
This functionality weakens server security by making site enumeration easier.User-specific directories
The UserDir directive provides user-specific directory
translation, allowing URLs based on associated usernames.
If the above functionality is unnecessary, comment out the related module: #LoadModule userdir_module modules/mod_userdir.so
If there is a critical need for this module, include the line
UserDir disabled root (at a minimum) in the configuration file. Ideally, UserDir
should be disabled, and then enabled on a case-by-case basis for specific users that
require this functionality.
Note: A web server's users can be trivially enumerated
using this module.Proxy Support
This module provides proxying support, allowing Apache to
forward requests and serve as a gateway for other servers.
If the above functionality is unnecessary, comment out the related modules: #LoadModule proxy_module modules/mod_proxy.so
#LoadModule proxy_balancer_module modules/mod_proxy_balancer.so
#LoadModule proxy_ftp_module modules/mod_proxy_ftp.so
#LoadModule proxy_http_module modules/mod_proxy_http.so
#LoadModule proxy_connect_module modules/mod_proxy_connect.so
If proxy support is needed, load proxy and the
appropriate proxy protocol handler module (one of proxy http, proxy ftp, or proxy
connect). Additionally, make certain that a server is secure before enabling proxying,
as open proxy servers are a security risk. proxy balancer enables load balancing, but
requires that mod status be enabled. Since mod status is not recommended, proxy
balancer should be avoided as well.Cache Support
This module allows Apache to cache data, optimizing access to
frequently accessed content. However, not only is it an experimental module, but it
also introduces potential security flaws into the web server such as the possibility
of circumventing Allow and Deny directives.
If the above functionality is unnecessary, comment out the related modules: #LoadModule cache_module modules/mod_cache.so
#LoadModule disk_cache_module modules/mod_disk_cache.so
#LoadModule file_cache_module modules/mod_file_cache.so
#LoadModule mem_cache_module modules/mod_mem_cache.so
If caching is required, it should not be enabled for any limited-access content.CGI Support (and Related Modules)
This module allows HTML to interact with the CGI web
programming language.
If the above functionality is unnecessary, comment out the related modules: #LoadModule cgi_module modules/mod_cgi.so
#LoadModule env_module modules/mod_env.so
#LoadModule actions_module modules/mod_actions.so
#LoadModule suexec_module modules/mod_suexec.so
If the web server requires the use of CGI, enable
the cgi module. If extended CGI functionality is required, include the appropriate
modules. env allows for control of the environment passed to CGI scripts. actions
allows CGI events to be triggered when files of a certain type are requested. su exec
allows CGI scripts to run as a specified user/group instead of as the server's
user/group.Various Optional Components
The following modules perform very specific tasks, sometimes
providing access to just a few additional directives. If this functionality is not
required (or if you are not using these directives), comment out the associated
module: External filtering (response passed through external program prior to client delivery) #LoadModule ext_filter_module modules/mod_ext_filter.so User-specified
Cache Control and Expiration #LoadModule expires_module modules/mod_expires.soCompression Output Filter (provides content compression prior to client delivery)#LoadModule deflate_module modules/mod_deflate.so HTTP Response/Request Header Customization #LoadModule headers_module modules/mod_headers.soUser activity monitoring via cookies #LoadModule usertrack_module modules/mod_usertrack.so Dynamically configured mass virtual hosting #LoadModule vhost_alias_module modules/mod_vhost_alias.soMinimize Configuration Files Included
The Include directive directs Apache to load supplementary
configuration files from a provided path. The default configuration loads all files that
end in .conf from the /etc/httpd/conf.d directory.
To restrict excess configuration, the
following line should be commented out and replaced with Include directives that only
reference required configuration files: #Include conf.d/*.conf
If the above change was
made, ensure that the SSL encryption remains loaded by explicitly including the
corresponding configuration file: (see Section 3.16.4.1 for further details on SSL
configuration)
Include conf.d/ssl.conf
If PHP is necessary, a similar alteration must be
made: (see Section 3.16.4.4.1 for further details on PHP configuration)
Include conf.d/php.confDirectory Restrictions
The Directory tags in the web server configuration file allow
finer grained access control for a specified directory. All web directories should be
configured on a case-by-case basis, allowing access only where needed.Restrict Root Directory
The Apache root directory should always have the most
restrictive configuration enabled.
<Directory
/>
Options None
AllowOverride None
Order
allow,deny
</Directory>
Restrict Web Directory
The default configuration for the web (/var/www/html)
Directory allows directory indexing (Indexes)and the following of symbolic links
(FollowSymLinks). Neither of these is recommended.
The
/var/www/html directory hierarchy should not be viewable via the web, and symlinks
should only be followed if the owner of the symlink also owns the linked
file.
Ensure that this policy is adhered to by altering the
related section of the configuration:
<Directory
"/var/www/html">
# ...
Options SymLinksIfOwnerMatch
# ...
</Directory>
Restrict Other Critical Directories
All accessible web directories should be configured with
similar restrictive settings. The Options directive should be limited to necessary
functionality and the AllowOverride directive should be used only if needed. The Order
and Deny access control tags should be used to deny access by default, allowing access
only where necessary.Configure Authentication if ApplicableSet up a password file.
If a password file doesn't yet exist, one must be generated with the following command: # htpasswd -cs passwdfile user WARNING: This command will overwrite an existing file at this location.
Once a password file has been generated, subsequent users can be added with the
following command: # htpasswd -s passwdfile user Optionally, set up a group file (if using group authentication).
The group file is a plain text file of the following format
(each group is on its own line, followed by a colon and a list of users that belong to
that group, separated by spaces):
group : user1 user2
group2 : user3 Modify file
permissions so that Apache can read the group and passwd files: # chgrp apache passwdfile groupfile
# chmod 640 passwdfile groupfile Turn on authentication for desired directories
Add the following options inside the appropriate Directory tag: For single-user authentication:
<Directory "directory ">
# ... AuthName "Private Data"
AuthType Basic
AuthUserFile passwdfile
require user user
# ...
</Directory> For multiple-user authentication restricted by groups:
<Directory "directory ">
# ...
AuthName "Private Data"
AuthType Basic
AuthUserFile passwdfile
AuthGroupFile groupfile
require group group
# ...
</Directory> For multiple-user authentication restricted by valid user accounts:
<Directory "directory ">
# ...
AuthName "Private Data"
AuthType Basic
AuthUserFile passwdfile
require valid-user
# ...
</Directory>
The AuthName directive specifies a label for the protected content.
The AuthType directive
specifies the kind of authentication (if using Digest authentication, this line would
instead read AuthType Digest)
The AuthUserFile and AuthGroupFile directives point to the
password and group files (if using Digest authentication, these directives would instead
be AuthDigestFile and AuthDigestGroupFile.)
The require user directive restricts access
to a single user. The require group directive restricts access to multiple users in a
designated group. The short-hand require valid-user directive restricts access to any
user in the passwdfile
Note: Make sure the AuthUserFile and AuthGroupFile locations are
outside the web server document tree to prevent remote clients from having access to
restricted usernames and passwords. This guide recommends /etc/httpd/conf as a location
for these files.Basic authentication is handled in plaintext over the network.
Therefore, all login attempts are vulnerable to password sniffing. For increased
protection against passive monitoring, encrypted authentication over a secure channel
such as SSL (Section 3.16.4.1) is recommended. Limit Available Methods
Web server methods are defined in section 9 of RFC 2616
(http://www.ietf.org/rfc/rfc2616.txt). If a web server does not require the
implementation of all available methods, they should be disabled.
Note: GET and POST are
the most common methods. A majority of the others are limited to the WebDAV protocol.
<Directory /var/www/html>
# ...
# Only allow specific methods (this command is case-sensitive!)
<LimitExcept GET POST>
Order allow,deny
</LimitExcept>
# ...
</Directory>Use Appropriate Modules to Improve Apaches Security'
Among the modules available for Apache are several whose use may
improve the security of the web server installation. This section recommends and discusses
the deployment of security-relevant modules.Deploy mod ssl
Because HTTP is a plain text protocol, all traffic is
susceptible to passive monitoring. If there is a need for confidentiality, SSL should be
configured and enabled to encrypt content.
Note: mod nss is a FIPS 140-2 certified
alternative to mod ssl. The modules share a considerable amount of code and should be
nearly identical in functionality. If FIPS 140-2 validation is required, then mod nss
should be used. If it provides some feature or its greater compatibility is required,
thenmod ssl should be used.Install mod ssl
Install mod ssl: # yum install mod sslCreate an SSL Certificate
On your CA (if you are using your own) or on another
physically secure system, generate a key pair for the web server: # cd /etc/pki/tls/certs
# openssl genrsa -des3 -out httpserverkey.pem 2048
When prompted,
enter a strong, unique passphrase to protect the web server key pair.
Next, generate a Certificate Signing Request (CSR) from the key for the CA: # openssl req -new -key httpserverkey.pem -out httpserver.csr
Enter the passphrase for the web server key pair
and then fill out the fields as completely as possible (or hit return to accept
defaults); the Common Name field is especially important. It must match the
fullyqualified domain name of your server exactly (e.g. www.example.com) or the
certificate will not work. The /etc/pki/tls/openssl.conf file will determine which
other fields (e.g. Country Name, Organization Name, etc) must match between the server
request and the CA. Leave the challenge password and an optional company name blank.
Next, the web server CSR must be signed to create the web server certificate. You can
either send the CSR to an established CA or sign it with your CA.
To sign httpserver.csr using your CA: # openssl ca -in httpserver.csr -out httpservercert.pem
When prompted, enter the CA passphrase to continue and then complete the process. The
httpservercert. pem certificate needed to enable SSL on the web server is now in the
directory.
Finally, the web server key and certificate file need to be moved to the
web server. Use removable media if possible. Place the server key and certificate file
in /etc/pki/tls/http/, naming them serverkey.pem and servercert.pem, respectively.Install SSL Certificate
Add or modify the configuration file
/etc/httpd/conf.d/ssl.conf to match the following:
# establish new listening port
Listen 443
# seed appropriately
SSLRandomSeed startup file:/dev/urandom 1024
SSLRandomSeed connect file:/dev/urandom 1024
<VirtualHost site-on-certificate.com:443>
# Enable SSL
SSLEngine On
# Path to server certificate + private key
SSLCertificateFile /etc/pki/tls/http/servercert.pem
SSLCertificateKeyFile /etc/pki/tls/http/serverkey.pem
SSLProtocol All -SSLv2
# Weak ciphers and null authentication should be denied unless absolutely necessary
# (and even then, such cipher weakening should occur within a Location enclosure)
SSLCipherSuite HIGH:MEDIUM:!aNULL:+MD5
</VirtualHost>
Ensure that all
directories that house SSL content are restricted to SSL access only in
/etc/httpd/conf/ httpd.conf:
<Directory /var/www/html/secure>
# require SSL for access
SSLRequireSSL
SSLOptions +StrictRequire
# require domain to match certificate domain
SSLRequire %{HTTP HOST} eq "site-on-certificate.com"
# rather than reply with 403 error, redirect user to appropriate site
# this is OPTIONAL - uncomment to apply
# ErrorDocument 403 https://site-on-certificate.com
</Directory>Deploy mod security
mod security provides an application level firewall for Apache.
Following the installation of mod security with the base ruleset, specific configuration
advice can be found at http://www.modsecurity.org/ to design a policy that best matches
the security needs of the web applications.Install mod security
Install mod security:
# yum install mod_securityConfigure mod security Filtering
mod security supports a significant number of options, far
too many to be fully covered in this guide. However, the following list comprises a
smaller subset of suggested filters to be added to /etc/httpd/conf/ httpd.conf:
# enable mod_security
SecFilterEngine On
# enable POST filtering
SecFilterScanPost On
# Make sure that URL encoding is valid
SecFilterCheckURLEncoding On
# Accept almost all byte values
SecFilterForceByteRange 1 255
# Prevent directory traversal
SecFilter "\.\./"
# Filter on specific system specific paths
SecFilter /etc/passwd
SecFilter /bin/
# Prevent cross-site scripting
SecFilter "<[[:space:]]* script"
# Prevent SQL injection
SecFilter "delete[[:space:]]+from"
SecFilter "insert[[:space:]]+into"
SecFilter "select.+from"Use Denial-of-Service Protection Modules
Denial-of-service attacks are difficult to detect and prevent
while maintaining acceptable access to authorized users. However, there are a number of
traffic-shaping modules that attempt to address the problem. Well-known DoS protection
modules include: mod_throttle mod_bwshare mod_limitipconn mod_dosevasive It is
recommended that denial-of-service prevention be implemented for the web server.
However, this guide leaves specific configuration details to the discretion of the
reader.Configure Supplemental Modules AppropriatelyAny required functionality added to the web server via additional modules should be configured appropriately.Configure PHP Securely
PHP is a widely used and often misconfigured server-side
scripting language. It should be used with caution, but configured appropriately when
needed.
Make the following changes to /etc/php.ini:
# Do not expose PHP error messages to external users
display_errors = Off
# Enable safe mode
safe_mode = On
# Only allow access to executables in isolated directory
safe_mode_exec_dir = php-required-executables-path
# Limit external access to PHP environment
safe_mode_allowed_env_vars = PHP_
# Restrict PHP information leakage
expose_php = Off
# Log all errors
log_errors = On
# Do not register globals for input data
register_globals = Off
# Minimize allowable PHP post size
post_max_size = 1K
# Ensure PHP redirects appropriately
cgi.force_redirect = 0
# Disallow uploading unless necessary
file_uploads = Off
# Disallow treatment of file requests as fopen calls
allow_url_fopen = Off
# Enable SQL safe mode
sql.safe_mode = OnConfigure Operating System to Protect Web Server
The following configuration steps should be taken on the machine
which hosts the web server, in order to provide as safe an environment as possible for the
web server.Restrict File and Directory Access
Minimize access to critical Apache files and directories: # chmod 511 /usr/sbin/httpd
# chmod 750 /var/log/httpd/
# chmod 750 /etc/httpd/conf/
# chmod 640 /etc/httpd/conf/*
# chgrp -R apache /etc/httpd/confDirectory permissions on /etc/httpd/confSpecify directory permissions on /etc/httpd/confSpecify directory permissions of /etc/httpd/conf111101000111101000^[01]+$File permissions on /etc/httpd/conf/*Specify file permissions on /etc/httpd/conf/*Specify file permissions of /etc/httpd/conf/*110100000110100000^[01]+$File permissions on /usr/sbin/httpdSpecify file permissions on /usr/sbin/httpdSpecify file permissions of /etc/sbin/httpd101001001101001001^[01]+$group owner of /etc/httpd/conf/*Specify group owner of /etc/httpd/conf/*Specify group owner of /etc/httpd/conf/*apacheapacheFile permissions on /var/log/httpd/Specify file permissions on /var/log/httpd/Specify file permissions of /var/log/httpd/111101000111101000^[01]+$Restrict permissions on /etc/httpd/confFile permissions for /etc/httpd/conf should be set correctly.CCE-4509-6(1) via chmodRestrict permissions on /etc/httpd/conf/*File permissions for /etc/httpd/conf/* should be set correctly.CCE-4386-9(1) via chmodRestrict permissions on /usr/sbin/httpdFile permissions for /usr/sbin/httpd should be set correctly.CCE-4029-5(1) via chmodRestrict group access to /etc/httpd/conf/*The /etc/httpd/conf/* files should be owned by the appropriate group.CCE-3581-6(1) via chgrpRestrict permissions on /var/log/httpdFile permissions for /var/log/httpd should be set correctly.CCE-4574-0(1) via chmodConfigure iptables to Allow Access to the Web Server
Edit /etc/sysconfig/iptables. Add the following lines, ensuring
that they appear before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 80 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 443 -j ACCEPT
The default
Iptables configuration does not allow inbound access to the HTTP (80) and HTTPS (443)
ports used by the web server. This modification allows that access, while keeping other
ports on the server in their default protected state. See Section 2.5.5 for more
information about Iptables.Run Apache in a chroot Jail if Possible
Putting Apache in a chroot jail minimizes the damage done by a
potential break-in by isolating the web server to a small section of the filesystem.
In
order to configure Apache to run from a chroot directory, edit the Apache configuration
file, /etc/httpd/ conf/httpd.conf, and add the directive:
SecChrootDir /chroot/apache
It
is also necessary to place all files required by Apache inside the filesystem rooted at
/chroot/apache , including Apache's binaries, modules, configuration files, and served
web pages. The details of this configuration are beyond the scope of this guide.Additional Resources
Further resources should be consulted if your web server requires
more extensive configuration guidance, especially if particular applications need to be
secured.
In particular, [26] is recommended as a more comprehensive guide to securing Apache.IMAP and POP3 Server
Dovecot provides IMAP and POP3 services. It is not installed by
default. The project page at http://www.dovecot.org contains more detailed information
about Dovecot configuration.Disable Dovecot if Possible
If the system does not need to operate as an IMAP or POP3 server,
disable and remove Dovecot if it was installed: # chkconfig dovecot off
# yum erase dovecotDisable Dovecot if PossibleThe dovecot service should be disabled.CCE-3847-1(1) via chkconfigUninstall Dovecot if PossibleThe dovecot package should be uninstalled.CCE-4239-0(1) via yumConfigure Dovecot if NecessaryDovecot's main configuration file is /etc/dovecot.conf. The settings which appear, commented out, in the file are the defaults.Support Only the Necessary Protocols
Edit /etc/dovecot.conf. Add or correct the following lines,
replacing PROTOCOL with only the subset of protocols (imap, imaps, pop3, pop3s)
required:
protocols = PROTOCOL
Dovecot supports the IMAP and POP3 protocols, as well as
SSL-protected versions of those protocols. Configure the Dovecot server to support only
the protocols needed by your site.
If possible, require SSL protection for all
transactions. The SSL protocol variants listen on alternate ports (995 instead of 110
for pop3s, and 993 instead of 143 for imaps), and require SSL-aware clients. An
alternate approach is to listen on the standard port and require the client to use the
STARTTLS command before authenticating.Dovecot should not support imapsDovecot should be configured to not support the imaps protocolCCE-4384-4(1) via /etc/dovecot.confDovecot should not support pop3sDovecot should be configured to not support the pop3s protocolCCE-3887-7(1) via /etc/dovecot.confDovecot should not support pop3Dovecot should be configured to not support the pop3 protocolCCE-4530-2(1) via /etc/dovecot.confDovecot should not support imapDovecot should be configured to not support the imap protocolCCE-4547-6(1) via /etc/dovecot.confEnable SSL Support
SSL should be used to encrypt network traffic between the
Dovecot server and its clients. Users must authenticate to the Dovecot server in order
to read their mail, and passwords should never be transmitted in clear text. In
addition, protecting mail as it is downloaded is a privacy measure, and clients may use
SSL certificates to authenticate the server, preventing another system from
impersonating the server. See Section 2.5.6 for general SSL information, including the
setup of a Certificate Authority (CA).Apache 2 with SSL/TLS: Step-by-step, Part 2. Tech. rep.Create an SSL Certificate
Note: The following steps should be performed on your CA
system, and not on the Dovecot server itself. If you will have a commercial CA sign
certificates, then these steps should be performed on a separate, physically secure
system devoted to that purpose.
On your CA (if you are using your own) or on another
physically secure system, generate a key pair for the Dovecot server: # cd /etc/pki/tls/certs
# openssl genrsa -out imapserverkey.pem 2048
Next, generate a
certificate signing request (CSR) for the CA to sign, making sure to enter the
server's fully-qualified domain name when prompted for the Common Name: # openssl req -new -key imapserverkey.pem -out imapserver.csr
Next, the mail server CSR must be
signed to create the Dovecot server certificate. You can either send the CSR to an
established CA or sign it with your CA.
To sign imapserver.csr using your CA: # openssl ca -in imapserver.csr -out imapservercert.pem
This step creates a private key,
imapserverkey.pem, and a public certificate, imapservercert.pem. The Dovecot server
will use these to prove its identity by demonstrating that it has a certificate which
has been signed by a CA. POP3 or IMAP clients at your site should only be willing to
provide users' credentials to a server they can authenticate.Install the SSL Certificate
Create the PKI directory for POP and IMAP certificates if it
does not already exist: # mkdir /etc/pki/tls/imap
# chown root:root /etc/pki/tls/imap
# chmod 755 /etc/pki/tls/imap
Using removable media or some other secure transmission
format, install the files generated in the previous step onto the Dovecot server: /etc/pki/tls/imap/serverkey.pem: the private key imapserverkey.pem/etc/pki/tls/imap/servercert.pem: the certificate file imapservercert.pem
Verify thepermissions on these files: # chown root:root /etc/pki/tls/imap/serverkey.pem
# chown root:root /etc/pki/tls/imap/servercert.pem
# chmod 600 /etc/pki/tls/imap/serverkey.pem
# chmod 600 /etc/pki/tls/imap/servercert.pem
Verify that the CA's public certificate
file has been installed as /etc/pki/tls/CA/cacert.pem, and has the correct
permissions: # chown root:root /etc/pki/tls/CA/cacert.pem
# chmod 644 /etc/pki/tls/CA/cacert.pemConfigure Dovecot to Use the SSL Certificate
Edit /etc/dovecot.conf and add or correct the following lines
(ensuring they reference the appropriate files):
ssl_cert_file = /etc/pki/tls/imap/servercert.pem
ssl_key_file = /etc/pki/tls/imap/serverkey.pem
ssl_ca_file = /etc/pki/tls/CA/cacert.pem
These options tell Dovecot where to find the
TLS configuration, allowing clients to make encrypted connections.Disable Plaintext Authentication
To prevent Dovecot from attempting plaintext authentication
of clients, edit /etc/dovecot.conf and add or correct the following line:
disable_plaintext_auth = yes
The disable_plaintext_auth command disallows
login-related commands until an encrypted session has been negotiated using SSL. If
client compatibility requires you to allow connections to the pop3 or imap ports,
rather than the alternate SSL ports, you should use this command to require STARTTLS
before authentication.Disable Plaintext AuthenticationDovecot plaintext authentication of clients should be disabledCCE-4552-6(1) via /etc/dovecot.confEnable Dovecot Options to Protect Against Code Flaws
Edit /etc/dovecot.conf and add or correct the following line:
login_process_per_connection = yes
mail_drop_priv_before_exec = yes
IMAP and POP3 are
remote authenticated protocols, meaning that the server must accept remote connections
from anyone, but provide substantial services only to clients who have successfully
authenticated. To protect against security problems, Dovecot splits these functions into
separate server processes. The imap-login and/or pop3-login processes accept connections
from unauthenticated users, and only spawn imap or pop3 processes on successful
authentication.
However, the imap-login and pop3-login processes themselves may contain
vulnerabilities. Since each of these processes operates as a daemon, handling multiple
sequential client connections from different users, bugs in the code could allow
unauthenticated users to steal credential data. If the login_process_per_connection
option is enabled, then a separate imap-login or pop3-login process is created for each
new connection, protecting against this class of problems. This option has an efficiency
cost, but is strongly recommended.
If the mail_drop_priv_before_exec option is on, the
imap-login or pop3-login process will drop privileges to the user's ID after
authentication and before executing the imap or pop3 process itself. Under some very
limited circumstances, this could protect against privilege escalation by authenticated
users. However, if the mail executable option is used to run code before starting each
user's session, it is important to drop privileges to prevent the custom code from
running as root.Enable Dovecot Option mail_drop_priv_before_execThe Dovecot option to drop privileges to user before executing mail process should be enabledCCE-4371-1(1) via /etc/dovecot.confEnable Dovecot Option mail_drop_priv_before_execThe Dovecot option to spawn a new login process per connection should be enabledCCE-4410-7(1) via /etc/dovecot.confAllow IMAP Clients to Access the Server
Edit /etc/sysconfig/iptables. Add the following line, ensuring
that it appears before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -m state --state NEW -p tcp --dport 143 -j ACCEPT
The default
iptables configuration does not allow inbound access to any services. This modification
will allow remote hosts to initiate connections to the IMAP daemon, while keeping all
other ports on the server in their default protected state. See Section 2.5.5 for more
information about iptables.Samba(SMB) Microsoft Windows File Sharing Server
When properly configured, the Samba service allows Linux machines
to provide file and print sharing to Microsoft Windows machines. There are two software
packages that provide Samba support. The first, samba-client, provides a series of command
line tools that enable a client machine to access Samba shares. The second, simply labeled
samba, provides the Samba service. It is this second package that allows a Linux machine to
act as an Active Directory server, a domain controller, or as a domain member. Only the
samba-client package is installed by default.Disable Samba if Possible
If the Samba service has been enabled and will not be used, disable it: # chkconfig smb off
Even after the Samba server package has been installed, it
will remain disabled. Do not enable this service unless it is absolutely necessary to
provide Microsoft Windows file and print sharing functionality.Disable Samba if PossibleThe smb service should be disabled.CCE-4551-8(1) via chkconfigConfigure Samba if Necessary
All settings for the Samba daemon can be found in
/etc/samba/smb.conf. Settings are divided between a [global] configuration section and a
series of user created share definition sections meant to describe file or print shares on
the system. By default, Samba will operate in user mode and allow client machines to
access local home directories and printers. It is recommended that these settings be
changed or that additional limitations be set in place.Testing the Samba Configuration File
To test the configuration file for syntax errors, use the
testparm command. It will also list all settings currently in place, including defaults
that may not appear in the configuration file. # testparm -vChoosing the Appropriate security Parameter
There are two kinds of security in Samba, share-level (share)
and user-level. User-level security is further subdivided into four separate
implementations: user, domain, ads, and server. It is recommended that the share and
server security modes not be used. In share security, everyone is given the same
password for each share, preventing individual user accountability. server security mode
has been superseded by the domain and ads security modes. It may now be considered
obsolete.
The security parameter is set in the [global] section of the Samba
configuration file. It determines how the server will handle user names and passwords.
Some security modes require additional parameters, such as workgroup, realm, or password
server names. All security modes will require that each remote user have a matching
local account. One workaround to this problem is to use the winbindd daemon. Please
consult the official Samba documentation to learn more.Use user Security for Servers Not in a Domain Context
This is the default setting with a new Samba installation and
the best choice when operating outside of a domain security context. The relevant
parameters in /etc/samba/smb.conf will read as follows:
security = user
workgroup = MYGROUP
Set the value of workgroup so that it matches the value of other machines on
the network.
In user mode, authentication requests are handled locally and not passed
on to a separate authentication server. This is the desired behavior for standalone
servers and domain controllers.Use domain Security for Servers in a Domain Context
First, change the security parameter to domain.
Next, set the workgroup and netbios name parameters (if necessary):
security = domain
workgroup = WORKGROUP
netbios name = NETBIOSNAME
domain mode is used for any machine
that will act as a domain member server. It lets Samba know that the authentication
information it needs can be found on another machine. Primary and Backup Domain
Controllers host copies of this information. Samba will try to automatically determine
which machine it should authenticate against on a domain network. If this detection
fails, it may be necessary to specify the location manually.
Unlike the Microsoft
Windows implementation of the SMB standard, a Samba machine can freely change roles
within a domain without requiring that the machine be reinstalled (such roles include
primary and backup domain controllers, domain member servers, and ordinary domain
workstations). However, there are some limitations on how each machine can fulfill
each role in a mixed network.When using Samba as a Primary or Backup Domain Controller,
use security = user, not security = domain. This tells Samba that the local machine is
hosting the authentication backend. Use ads (Active Directory Service) Security For Servers in an ADS
Domain
Context The security mode ads enables a Samba machine to act
as an ADS domain member server. Since ADS requires Kerberos, be sure to set the realm
parameter appropriately and configure the local copy of Kerberos. If necessary, it is
also possible to manually set the password server parameter.
security = ads
realm = MY_REALM
password server = your.kerberos.server
Currently, it is possible to act as an
Active Directory domain member server, but not as a domain controller. Be sure to
operate in mixed mode. Native mode may not work yet in current versions of Samba.
Future support for ADS should be forthcoming in Samba 4. See the Samba project web
site at http://www.samba.org for more details.Disable Guest Access and Local Login Support
Do not allow guest users to access local file or printer
shares. In global or in each share, set the parameter guest ok to no:
[share]
guest ok = no
It is safe to disable local login support for remote Samba users. Consider changing
the add user account script to set remote user shells to /sbin/nologin.Disable Guest Access and Local Login SupportDo not allow guest users to access local file or printer shares. In global or in each share, set the parameter guest ok to no.(1) via /etc/samba/smb.conf in [share] guest ok = no Disable Root Access
Administrators should not use administrator accounts to access
Samba file and printer shares. If possible, disable the root user and the wheel
administrator group:
[share]
invalid users = root @wheel
If administrator accounts
cannot be disabled, ensure that local machine passwords and Samba service passwords do
not match.
Typically, administrator access is required when Samba must create user and
machine accounts and shares. Domain member servers and standalone servers may not need
administrator access at all. If that is the case, add the invalid users parameter to
[global] instead.Set the Allowed Authentication Negotiation LevelsBy default, Samba will attempt to negotiate with Microsoft
Windows machines to set a common communication protocol. Whenever possible, be sure to
disable LANMAN authentication, as it is far weaker than the other supported protocols.
[global]
client lanman auth = no
Newer versions of Microsoft Windows may require the use
of NTLMv2. NTLMv2 is the preferred protocol for authentication, but since older machines
do not support it, Samba has disabled it by default. If possible, reenable it.
[global]
client ntlmv2 auth = yes
For the sake of backwards compatibility, most modern Windows
machines will still allow other machines to communicate with them over weak protocols
such as LANMAN. On Samba, by enabling NTLMv2, you are also disabling LANMAN and NTLMv1.
If NTLMv1 is required, it is still possible to individually disable LANMAN.Let Domain Controllers Create Machine Trust Accounts On-the-Fly
Add or correct an add machine script entry to the [global]
section of /etc/samba/smb.conf to allow Samba to dynamically create Machine Trust
Accounts:
[global]
add machine script = /usr/sbin/useradd -n -g machines -d /dev/null -s /sbin/nologin %u
Make sure that the group machines exists. If not, add it with the
following command:
/usr/sbin/groupadd machines
When acting as a PDC, it becomes
necessary to create and store Machine Trust Accounts for each machine that joins the
domain. On a Microsoft Windows PDC, this account is created with the Server Manager
tool, but on a Samba PDC, two accounts must be created. The first is the local machine
account, and the second is the Samba account. For security purposes, it is recommended
to let Samba create these accounts on-the-fly. When Machine Trust Accounts are created
manually, there is a small window of opportunity in which a rogue machine could join the
domain in place of the new server.Restrict Access to the [IPC$] Share
Limit access to the [IPC$] share so that only machines in your
network will be able to connect to it:
[IPC$]
hosts allow = 192.168.1. 127.0.0.1
hosts deny = 0.0.0.0/0
The [IPC$] share allows users to anonymously fetch a list of shared
resources from a server. It is intended to allow users to browse the list of available
shares. It also can be used as a point of attack into a system. Disabling it completely
may break some functionality, so it is recommended that you merely limit access to it
instead.Restrict File Sharing
Only users with local user accounts will be able to log in to
Samba shares by default. Shares can be limited to particular users or network addresses.
Use the hosts allow and hosts deny directives accordingly, and consider setting the
valid users directive to a limited subset of users or to a group of users. Separate each
address, user, or user group with a space as follows:
[share] hosts allow = 192.168.1. 127.0.0.1
valid users = userone usertwo @usergroup
It is also possible to limit read and
write access to particular users with the read list and write list options, though the
permissions set by the system itself will override these settings. Set the read only
attribute for each share to ensure that global settings will not accidentally override
the individual share settings. Then, as with the valid users directive, separate each
user or group of users with a space:
[share]
read only = yes
write list = userone usertwo @usergroup
The Samba service is only required for sharing files and printers
with Microsoft Windows workstations, and even then, other options may exist. Do not use
the Samba service to share files between Unix or Linux machines.Require Server SMB Packet Signing
To make the server use packet signing, add the following to the [global] section of the Samba configuration
file:
server signing = mandatory
The Samba server should only communicate with clients who can support SMB packet signing. Packet signing
can prevent man-in-the-middle attacks which modify SMB packets in transit.
The Samba service is only required for sharing files and printers with Microsoft Windows workstations, and even
then, other options may exist. Do not use the Samba service to share files between Unix or Linux machines.
Require Client SMB Packet Signing, if using smbclient
To require samba clients running smbclient to use packet signing, add the following to the [global] section
of the Samba configuration file:
client signing = mandatory
A Samba client should only communicate with servers who can support SMB packet signing. Packet signing can
prevent man-in-the-middle attacks which modify SMB packets in transit.
Require Client SMB Packet Signing, if using smbclient
Require samba clients running smbclient to use packet signing. A Samba client should only communicate with servers who can support SMB packet signing. Packet signing can
prevent man-in-the-middle attacks which modify SMB packets in transit.CCE-4556-7(1) via /etc/samba/smb.conf in [global] client signing = mandatoryRequire Client SMB Packet Signing, if using mount.cifs
Require packet signing of clients who mount Samba shares using the mount.cifs program (e.g., those who
specify shares in /etc/fstab). To do so, ensure that signing options (either sec=krb5i or sec=ntlmv2i) are
used.
See the mount.cifs(8) man page for more information. A Samba client should only communicate with servers
who can support SMB packet signing. Packet signing can prevent man-in-the-middle attacks which modify SMB
packets in transit.
Require Client SMB Packet Signing, if using mount.cifs
Require packet signing of clients who mount Samba shares using the mount.cifs program (e.g., those who
specify shares in /etc/fstab). To do so, ensure that signing options (either sec=krb5i or sec=ntlmv2i) are
used.
See the mount.cifs(8) man page for more information. A Samba client should only communicate with servers
who can support SMB packet signing. Packet signing can prevent man-in-the-middle attacks which modify SMB
packets in transit.CCE-4556-7(1) via /etc/fstabRestrict Printer Sharing
By default, Samba utilizes the CUPS printing service to enable
printer sharing with Microsoft Windows workstations. If there are no printers on the
local machine, or if printer sharing with Microsoft Windows is not required, disable the
printer sharing capability by commenting out the following lines, found in /etc/
samba/smb.conf:
[global]
; load printers = yes
; cups options = raw
[printers]
comment = All Printers
path = /usr/spool/samba
browseable = no
guest ok = no
writable = no
printable = yes
There may be other options present, but these are the only options
enabled and uncommented by default. Removing the [printers] share should be enough for
most users.
If the Samba printer sharing capability is needed, consider disabling the
Samba network browsing capability or restricting access to a particular set of users or
network addresses. Set the valid users parameter to a small subset of users or restrict
it to a particular group of users with the shorthand @. Separate each user or group of
users with a space. For example, under the [printers] share:
[printers]
valid users = user @printerusers
The CUPS service is capable of sharing printers with other Unix and
Linux machines on the local network without the Samba service. The Samba service is only
required when a Microsoft Windows machine needs printer access on a Unix or Linux host.Configure iptables to Allow Access to the Samba Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which should operate as clients
of the Samba server.
Edit /etc/sysconfig/iptables. Add the following lines, ensuring
that they appear before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 137 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 138 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 139 -j ACCEPT
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport 445 -j ACCEPT
The default Iptables configuration does not allow inbound access to the ports used by
the Samba service. This modification allows that access, while keeping other ports on
the server in their default protected state. Since these ports are frequent targets of
network scanning attacks, restricting access to only the network segments which need to
access the Samba server is strongly recommended. See Section 2.5.5 for more information
about Iptables.Avoid the Samba Web Administration Tool (SWAT)
SWAT is a web based configuration tool provided by the Samba team
that enables both local and remote configuration management. It is not installed by
default. It is recommended that SWAT not be used, as it requires the use of a Samba
administrator account and sends that password in the clear over a network connection.
If
SWAT is absolutely required, limit access to the local machine or tunnel SWAT connections
through SSL with stunnel.Proxy Server
A proxy server is a very desirable target for a potential adversary
because much (or all) sensitive data for a given infrastructure may flow through it.
Therefore, if one is required, the machine acting as a proxy server should be dedicated to
that purpose alone and be stored in a physically secure location. The system's default proxy
server software is Squid, and provided in an RPM package of the same name.Galarneua, E. Security Considerations with Squid proxy server. Tech. rep., Apr 2003Wessels, D. Squid: The Definitive Guide. O’Reilly and Associates, Jan 2004Disable Squid if Possible
If Squid was installed and activated, but the system does not
need to act as a proxy server, then it should be disabled and removed: # chkconfig squid off
# yum erase squidDisable Squid if PossibleThe squid service should be disabled.CCE-4556-7(1) via chkconfigUninstall Squid if PossibleThe squid package should be uninstalled.CCE-4076-6(1) via yumConfigure Squid if Necessary
The Squid configuration file is /etc/squid/squid.conf. The
following recommendations can be applied to this file.
Note: If a particular tag is not
present in the configuration file, Squid falls back to the default setting (which is often
illustrated by a comment).Listen on Uncommon Port
The default listening port for the Squid service is 3128. As
such, it is frequently scanned by adversaries looking for proxy servers.
Select an
arbitrary (but uncommon) high port to use as the Squid listening port and make the
corresponding change to the configuration file:
http_port port
Run the following command
to add a new SELinux port mapping for the service: # semanage port -a -t http_cache_port_t -p tcp portVerify Default Secure Settings
Several security-enhancing settings in the Squid configuration
file are enabled by default, but appear as comments in the configuration file (as
mentioned in Section 3.19.2). In these instances, the explicit directive is not present,
which means it is implicitly enabled. If you are operating with a default configuration
file, this section can be ignored.
Ensure that the following security settings are NOT
explicitly changed from their default values:
ftp_passive on
ftp_sanitycheck on
check_hostnames on
request_header_max_size 20 KB
reply_header_max_size 20 KB
cache_effective_user squid
cache_effective_group squid
ignore_unknown_nameservers on
ftp_passive forces FTP passive connections.
ftp_sanitycheck performs additional sanity checks on FTP data connections.
check_hostnames ensures that hostnames meet RFC compliance.
request_header_max_size and reply_header_max_size place an upper limit on
HTTP header length, precautions against denial-of-service and buffer overflow
vulnerabilities.
cache_effective_user and cache_effective_group designate the EUID and
EGID of Squid following initialization (it is essential that the EUID/EGID be set to an
unprivileged sandbox account).
ignore_unknown_nameservers checks to make sure that DNS
responses come from the same IP the request was sent to.request_header_max_sizePlace an upper limit on HTTP request header length, precautions against denial-of-service and buffer overflow vulnerabilities.Specify an upper limit on HTTP request header length20kb20kb^[\d][KMGkmg]?[Bb]?$reply_header_max_sizePlace an upper limit on HTTP reply header length, precautions against denial-of-service and buffer overflow vulnerabilities.Specify an upper limit on HTTP reply header length20kb20kb^[\d][KMGkmg]?[Bb]?$cache_effective_userDesignate the EUID of Squid following initialization (it is essential that the EUID be set to an unprivileged sandbox account)..Designate the EUID of Squid following initializationsquidsquidcache_effective_groupDesignate the EGID of Squid following initialization (it is essential that the EGID be set to an unprivileged sandbox account)..Designate the EGID of Squid following initializationsquidsquidVerify ftp_passive settingThe Squid option to force FTP passive connections should be enabledCCE-4454-5(1) via /etc/squid/squid.confVerify ftp_sanitycheck settingThe Squid option to perform FTP sanity checks should be enabledCCE-4459-4(1) via /etc/squid/squid.confVerify check_hostnames sttingThe Squid option to check for RFC compliant hostnames should be enabledCCE-4503-9(1) via /etc/squid/squid.confVerify request_header_max_size settingThe Squid max request HTTP header length should be set to an appropriate valueCCE-4353-9(1) via /etc/squid/squid.confVerify reply_header_max_size settingThe Squid max reply HTTP header length should be set to an appropriate valueCCE-4419-8(1) via /etc/squid/squid.confVerify cache_effective_user settingThe Squid EUID should be set to an appropriate userCCE-3692-1(1) via /etc/squid/squid.confVerify cache_effective_group settingThe Squid GUID should be set to an appropriate groupCCE-4476-8(1) via /etc/squid/squid.confVerify ignore_unknown_nameservers settingThe Squid option to ignore unknown nameservers should be enabledCCE-3585-7(1) via /etc/squid/squid.confChange Default Insecure Settings
The default configuration settings for the following tags are
considered to be weak security and NOT recommended.
Add or modify the configuration file to include the following lines:
allow_underscore off
httpd_suppress_version_string on
forwarded_for off
log_mime_hdrs on
allow_underscore enforces RFC 1034 compliance on
hostnames by disallowing the use of underscores.
httpd_suppress_version string prevents
Squid from revealing version information in web headers and error pages.
forwarded_for
reveals proxy client IP addresses in HTTP headers and should be disabled to prevent the
leakage of internal network configuration details.
log_mime_hdrs enables logging of HTTP
response/request headers.allow_underscoreallow_underscore enforces RFC 1034 compliance on hostnames by disallowing the use of underscores.Enable/Disable enforcing RFC 1034 compliance on hostnamesoffonoffon|offonoffhttpd_suppress_versionhttpd_suppress_version string prevents Squid from revealing version information in web headers and error pages.Enable/Disable preventing squid from revealing version information in web headers and error pagesononoffon|offonoffforwarded_forforwarded_for reveals proxy client IP addresses in HTTP headers and should be disabled to prevent the leakage of internal network configuration details. Enable/Disable revealing proxy client IP addresses in HTTP headersoffonoffon|offonofflog_mime_hdrslog_mime_hdrs enables logging of HTTP response/request headers.Enable/Disable logging of HTTP response/request headersononoffon|offonoffCheck allow_underscore settingThe Squid option to allow underscores in hostnames should be disabledCCE-4344-8(1) via /etc/squid/squid.confCheck httpd_suppress_version settingThe Squid option to suppress the httpd version string should be enabledCCE-4494-1(1) via /etc/squid/squid.confCheck forwarded_for settingThe Squid option to show proxy client IP addresses in HTTP headers should be disabledCCE-4181-4(1) via /etc/squid/squid.confCheck log_mime_hdrs settingThe Squid option to log HTTP MIME headers should be enabledCCE-4577-3(1) via /etc/squid/squid.confConfigure Authentication if Applicable
Note: Authentication cannot be used in the case of transparent
proxies due to limitations of the TCP/IP protocol.
Similar to web servers, two of the
available options are Basic and Digest authentication. The other options are NTLM and
Negotiate authentication. As noted in Section 3.16.3.5, Basic authentication transmits
passwords in plain-text and is susceptible to passive monitoring. If network sniffing is
a concern, basic authentication should not be used. Negotiate is the newest and most
secure protocol. It attempts to use Kerberos authentication and falls back to NTLM if it
cannot. It should be noted that Kerberos requires a third-party Key Distribution Center
(KDC) to function properly, whereas the other methods of authentication are two-party
schemes.
Squid also offers the ability to choose a custom external authenticator.
Designating an external authenticator (also known as a 'helper' module) allows Squid to
offer pluggable third-party authentication schemes. LDAP is one example of a helper
module that exists and is in use today.
There are comments under the auth_param tag
inside /etc/squid/squid.conf that provide extensive detail on how to configure each of
these methods. If authentication is necessary, choose a method of authentication and
configure appropriately. The recommended minimum configurations illustrated for each
method are acceptable.
To force an ACL (as discussed in Section 3.19.2.5) to require
authentication, use the following directive:
acl name-of-ACL proxy_auth REQUIRED
Note:
The keyword REQUIRED can be replaced with a user or list of users to further restrict
access to a smaller subset of users.Access Control Lists (ACL)
The acl and http access tags are used in combination to allow filtering based on a series of
access control lists.
Squid has a list of default ACLs for localhost, SSL ports, and
'safe' ports. Following the definition of these ACLs, a series of http access directives
establish the following default filtering policy: Allow cachemgr access only from localhost Allow access to only ports in the 'safe' access control listLimit CONNECT method to SSL ports onlyAllow access from localhostDeny all other requests
The
default ACL policies are reasonable from a security standpoint. However, the number of
ports listed as 'safe' could be significantly trimmed depending on the needs of your
network. Out of the box, ports 21, 70, 80, 210, 280, 443, 488, 591, 777, and 1025
through 65535 are all considered safe. Some of these ports are associated with
deprecated or rarely used protocols. As such, this list could be trimmed to further
tighten filtering.
The following actions should be taken to tighten the ACL policies: There is a filter line in the configuration file that is recommended but commented out.
This line should be uncommented or added to prevent access to localhost from the proxy:
http access deny to_localhost An access list should be setup for the specific network
or networks that the proxy is intended to serve. Only this subset of IP addresses should
be allowed access.
Add these lines where the following comment appears:
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
acl your-network-acl-name src ip-range
http_access allow your-network-acl-name
Note: ip-range is of the format xxx.xxx.xxx.xxx/xxEnsure that the final http access line to appear in the document
is the following:
http_access deny all
This guarantees that all traffic not meeting an
explicit filtering rule is denied.
Further filters should be established to meet the
specific needs of a network, explicitly allowing access only where necessary.Consult
the chart below. Corresponding acl entries for unused protocols should be commented out
and thus denied. PortServiceSummaryRecommendation21ftpFile Transfer Protocol(FTP)
is a widely used file transfer protocol. ALLOW70gopherThe gopher protocol is a
deprecated search and retrieval protocol that is almost extinct, with as few as 100
gopher servers present worldwide. Support for gopher is disabled in most modern
browsers. DENY80httpA web proxy needs to allow access to HTTP traffic. ALLOW210waisThe Wide Area Information Server port is similar to gopher, serving as a text searching
system to scour indexes on remote machines. Today, it is deprecated and nearly
non-existent on the Internet. DENY280http-mgmtNo documentation of any kind could be
found on the obscure service that resides on this port. DENY443httpsSSL traffic is
likely (and recommended) for any proxy and should be allowed. ALLOW488gss-httpNo
documentation of any kind could be found on the obscure service that resides on this
port. DENY591filemakerFilemaker is a database application originally offered by Apple
in the 1980s. Although development continues and it remains in use today, it should be
disabled if your network does not require such traffic. DENY777multiling httpNo documentation of any kind could be found on
the obscure service that resides on this portDENY1025-65535unregistered ports httpunregistered
ports Random high ports are used by a variety of applications and should be allowed.ALLOWBe very careful with the order of access control tags. Access
control is handled top-down. The first rule that matches is the only rule adhered to.
The last rule on the list defines the default behavior in the case of no rule match. Restrict gss-http trafficSquid should be configured to not allow gss-http trafficCCE-4511-2(1) via /etc/squid/squid.confRestrict https trafficSquid should be configured to not allow https trafficCCE-4529-4(1) via /etc/squid/squid.confRestrict wais trafficSquid should be configured to not allow wais trafficCCE-3610-3(1) via /etc/squid/squid.confRestrict multiling http trafficSquid should be configured to not allow multiling http trafficCCE-4466-9(1) via /etc/squid/squid.confRestrict http trafficSquid should be configured to not allow http trafficCCE-4607-8(1) via /etc/squid/squid.confRestrict ftp trafficSquid should be configured to not allow ftp trafficCCE-4255-6(1) via /etc/squid/squid.confRestrict gopher trafficSquid should be configured to not allow gopher trafficCCE-4127-7(1) via /etc/squid/squid.confRestrict filemaker trafficSquid should be configured to not allow filemaker trafficCCE-4519-5(1) via /etc/squid/squid.confRestrict proxy access to localhost Squid proxy access to localhost should be deniedCCE-4413-1(1) via /etc/squid/squid.confRestrict http-mgmt trafficSquid should be configured to not allow http-mgmt trafficCCE-4373-7(1) via /etc/squid/squid.confConfigure Internet Cache Protocol (ICP) if Necessary
The ICP protocol is a cache communication protocol that allows
multiple Squid servers to communicate. The ICP protocol was designed with no security in
mind, relying on user-defined access control lists alone to determine which ICP messages
to allow.
If a Squid server is standalone, the ICP port should be disabled by adding or
correcting the following line in the configuration file:
icp_port 0
If the Squid server
is meant to speak with peers, strict ACLs should be established to only allow ICP
traffic from trusted neighbors. To accomplish this, add or correct the following lines:
icp_access allow acl-defining-trusted-neighbors
icp_access deny allConfigure iptables to Allow Access to the Proxy Server
Determine an appropriate network block, netwk , and network
mask, mask , representing the machines on your network which should operate as clients
of the proxy server.
Edit /etc/sysconfig/iptables. Add the following line, ensuring that
it appears before the final LOG and DROP lines for the RH-Firewall-1-INPUT chain:
-A RH-Firewall-1-INPUT -s netwk /mask -m state --state NEW -p tcp --dport port -j ACCEPT
For port , use either the default 3128 or the alternate port was selected in Section
3.19.2.1.
The default Iptables configuration does not allow inbound access to the Squid
proxy service. This modification allows that access, while keeping other ports on the
server in their default protected state. See Section 2.5.5 for more information about
Iptables.Forward Log Messages to Syslog Daemon
The default behavior of Squid is to record its log messages in
/var/log/squid.log. This behavior can be supplemented so that Squid also sends messages
to syslog as well. This is useful for centralizing log data, particularly in instances
where multiple Squid servers are present.
Squid provides a command line argument to
enable syslog forwarding. Modify the SQUID OPTS line in /etc/init.d/squid to include the
-s option:
SQUID_OPTS="${SQUID_OPTS:-"-D"} -s"Do Not Run as Root
Since Squid is loaded by the system's service utility, it
starts as root and then changes its effective UID to the UID specified by the cache
effective user directive. However, since it was still executed by root, the program
maintains a saved UID of root even after changing its effective UID.
To prevent this
undesired behavior, Squid must either be configured to run in a chroot environment or it
must be executed by a non-privileged user in non-daemon mode (the service utility must
not be used).Run Squid in a chroot Jail
Chrooting Squid can be a very complicated task. Documentation
for the process is vague and a great deal of trial and error may be required to
determine all the files that need to be transitioned over to the chroot environment.
Therefore, this guide recommends instead the method detailed in Section 3.19.2.9.2 to
lower privileges. If chrooting Squid is still desired, it can be enabled with the
following directive in the configuration file:
chroot chroot-path
Then, all the
necessary files used by Squid must be copied into the chroot-path directory. The
specifics of this step cannot be covered in this guide because they are highly
dependent on the external programs used in the Squid configuration.
Note: The strace
utility is a valuable resource for discovering the files needed for the chroot
environment.Modify Service Entry to Lower PrivilegesT
he following modification to /etc/init.d/squid forces the
service utility to execute Squid as the squid user instead of the root user:
# determine the name of the squid binary
[ -f /usr/sbin/squid ] && SQUID="sudo -u squid squid"
Making this change prevents Squid from writing its pid to
/var/run. This pid file is used by service to check to see if the program started
successfully. Therefore, a new location must be chosen for this pid file that the
squid user has access to, and the corresponding references in /etc/init.d/squid must
be altered to point to it.
Make the following modification to the Squid configuration file:
pid_filename /var/spool/squid/squid.pid
Edit the file /etc/init.d/squid by
changing all occurrences of /var/run/squid.pid to /var/spool/squid/ squid.pid
Also modify the following line in /etc/init.d/squid:
[ $RETVAL -eq 0 ] && touch /var/lock/subsys/squid
and add the following lines immediately after it:
rm -f /var/lock/subsys/squid
status squidSNMP Server
The Simple Network Management Protocol allows administrators to
monitor the state of network devices, including computers. Older versions of SNMP were
well-known for weak security, such as plaintext transmission of the community string (used
for authentication) and also usage of easily-guessable choices for community string.Disable SNMP Server if Possible
The system includes an SNMP daemon that allows for its remote
monitoring, though it not installed by default. If it was installed and activated, it is
important that the software be disabled and removed.
If there is not a mission-critical
need for hosts at this site to be remotely monitored by a SNMP tool, then disable and
remove SNMP as follows: # chkconfig snmpd off
# yum erase net-snmpdDisable snmpd if PossibleThe snmpd service should be disabled.CCE-3765-5(1) via chkconfigUninstall net-snmp if PossibleThe net-snmp package should be uninstalled.CCE-4404-0(1) via yumConfigure SNMP Server if Necessary
If it is necessary to run the snmpd agent on the system, some
best practices should be followed to minimize the security risk from the installation. The
multiple security models implemented by SNMP cannot be fully covered here so only the
following general configuration advice can be offered: use only SNMP version 3 security
models and enable the use of authentication and encryption for those write access to the
MIB (Management Information Base) should be allowed only if necessary all access to the
MIB should be restricted following a principle of least privilege network access should
be limited to the maximum extent possible including restricting to expected network
addresses both in the configuration files and in the system firewall rules ensure SNMP
agents send traps only to, and accept SNMP queries only from, authorized management
stations ensure that permissions on the snmpd.conf configuration file (by default, in
/etc/snmp) are 640 or more restrictive ensure that any MIB files' permissions are also
640 or more restrictiveFurther Resources
The following resources provide more detailed information about the SNMP software: The CERT SNMP Vulnerabilities FAQ at http://www.cert.org/tech
tips/snmp faq.html The Net-SNMP project web page at http://net-snmp.sourceforge.net The snmp config(5) man page the snmpd.conf(5) man pageOSCAP Scan Resultlocalhost.localdomain127.0.0.110.142.14.51::1fe80::20c:29ff:fe84:d2d400:00:00:00:00:0000:0C:29:84:D2:D400:00:00:00:00:0000:0C:29:84:D2:D4notselectednotselectednotselectednotselectednotselectednotselectednotselectedfailnotselectedCCE-4218-4# chkconfig yum-updatesd offnotselectedecho -e "/usr/bin/yum -R 120 -e 0 -d 0 -y update yum\n/usr/bin/yum -R 10 -e 0 -d 0 -y update" > /etc/cron.weekly/yum.cronpasspassnotselectedpassnotselectedCCE-4209-3yum install aidenotselectedecho -e "/usr/sbin/aide --check" > /etc/cron.daily/aide.cronnotselectednotselectedCCE-4249-9notselectedCCE-3522-0notselectedCCE-4275-4notselectedCCE-4042-8notselectedCCE-4187-1echo -e "\nblacklist usb_storage" >> /etc/modprobe.d/blacklist.confnotselectedCCE-4006-3rm /lib/modules/2.6.*/kernel/drivers/usb/storage/usb-storage.konotselectedCCE-4173-1notselectedCCE-3944-6notselectedCCE-4072-5chkconfig autofs offnotselectedCCE-4231-7notselectedecho "blacklist cramfs" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist freevxfs" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist jffs2" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist hfs" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist hfsplus" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist squashfs" >> /etc/modprobe.d/blacklist.confnotselectedecho "blacklist udf" >> /etc/modprobe.d/blacklist.confpassCCE-3918-0passCCE-3988-3passCCE-3276-3passCCE-3883-6passCCE-4210-1passCCE-4064-2passCCE-3958-6passCCE-3495-9passCCE-4130-1passCCE-3967-7passCCE-3932-1passCCE-3566-7passCCE-3399-3passCCE-3795-2failCCE-4178-0failCCE-3324-1failCCE-4223-4failCCE-3573-3passpassCCE-4220-0notselectedCCE-4225-9passCCE-4247-3passCCE-4168-1passCCE-4146-7notselectedCCE-4177-2notselectedCCE-3820-8notselectedCCE-3485-0notselectedCCE-4111-1passCCE-4256-4notselectednotselectednotselectedCCE-4044-4echo "%wheel ALL=(ALL) ALL" >> /etc/sudoersnotselectedCCE-3987-5passCCE-4238-2passpassCCE-4009-7passCCE-4154-1notselectedCCE-4180-6notselectedCCE-4092-3passCCE-4097-2notselectednotselectednotselectedCCE-4114-5notselectedCCE-3762-2notselectedCCE-3762-2notselectedCCE-3410-8notselectednotselectedCCE-4185-5# chgrp usergroup /usr/sbin/userhelpernotselectedCCE-3952-9# chmod 4710 /usr/sbin/userhelperpass/usr/sbin/authconfig --passalgo=sha512 --updatenotselectedpassCCE-3301-9passfailCCE-4090-7passCCE-3844-8passCCE-4227-5notselectedrm .netrcfailCCE-4144-2chown root /boot/grub/grub.conffailCCE-4197-0chown :root /boot/grub/grub.conffailCCE-3923-0chmod 600 /boot/grub/grub.confnotselectedCCE-3818-2notselectedCCE-4241-6notselectedCCE-4245-7notselectedCCE-3689-7notselectedCCE-3707-7notselectedCCE-3315-9notselectednotselectednotselectednotselectedCCE-3910-7yum install vlocknotselectedCCE-4060-0notselectedCCE-4188-9passCCE-3977-6notselectedpasspassCCE-3624-4notselectednotselectedCCE-3668-1notselectedCCE-4129-3notselectednotselectedCCE-4151-7notselectedCCE-4155-8notselectedCCE-3561-8notselectedCCE-4236-6notselectedCCE-4217-6notselectedCCE-3472-8notselectedCCE-4320-8notselectedCCE-4091-5notselectedCCE-4186-3notselectedCCE-3339-9notselectedCCE-3644-2notselectedCCE-4133-5notselectedCCE-4265-5notselectedCCE-4080-8notselectedCCE-3840-6notselectedCCE-3628-5notselectedCCE-4276-2notselectedCCE-4170-7notselectedCCE-3562-6notselectedCCE-3381-1notselectedCCE-3377-9notselectedCCE-4296-0notselectedCCE-4269-7notselectedCCE-4291-1notselectedCCE-4313-3notselectedCCE-4198-8notselectedCCE-3842-2notselectedCCE-4159-0notselectedCCE-4221-8notselectedCCE-4058-4notselectedCCE-4128-5notselectedCCE-4287-9notselectedCCE-3895-0notselectedCCE-4137-6failCCE-4167-3chkconfig ip6tables onfailCCE-4189-7chkconfig iptables onnotselectednotselectednotselectednotselectednotselectednotselectedfailCCE-3679-8chkconfig rsyslog onpassCCE-4366-1passCCE-3701-0failCCE-4233-3notselectedCCE-4260-6notselectedCCE-3382-9notselectedCCE-4182-2notselectedCCE-4323-2failCCE-4292-9notselectednotselectednotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectednotselectednotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectednotselectedcp /usr/share/doc/audit-version/stig.rules /etc/audit/audit.rulesnotselectedCCE-4234-1notselectedCCE-4252-3notselectedCCE-4023-8# yum erase inetdnotselectedCCE-4164-0# yum erase xinetdnotselectedCCE-4330-7# yum erase telnet-servernotselectedCCE-3390-2notselected# yum erase telnetnotselected# yum erase rsh-servernotselectedCCE-4308-3# yum erase rsh-servernotselectedCCE-3974-3# chkconfig rcp offnotselectedCCE-4141-8# chkconfig rsh offnotselectedCCE-3537-8# chkconfig rlogin offnotselectednotselected# yum erase rshnotselectedCCE-4348-9# yum erase ypservnotselectedCCE-3705-1# chkconfig ypbind offnotselectedCCE-3916-4# yum erase tftp-servernotselectedCCE-4273-9# chkconfig tftp offnotselectedCCE-3412-4# chkconfig firstboot offnotselectedCCE-4229-1# chkconfig gpm offnotselectedCCE-4123-6# chkconfig irqbalance offnotselectedCCE-4286-1# chkconfig isdn offnotselectedCCE-3425-6# chkconfig kdump offnotselectedCCE-4211-9# chkconfig kudzu offnotselectedCCE-3854-7# chkconfig mdmonitor offnotselectedCCE-4356-2# chkconfig microcode ctl offnotselectedCCE-4369-5# chkconfig network offnotselected# rm /etc/sysconfig/network-scripts/ifcfg-interfacenotselectedCCE-4369-5notselectedCCE-4100-4# chkconfig pcscd offnotselectedCCE-3455-3# chkconfig smartd offnotselectedCCE-4421-4# chkconfig readahead early offnotselectedCCE-4302-6# chkconfig readahead later offnotselectedCCE-3822-4# chkconfig messagebus offnotselectedCCE-4364-6# chkconfig haldaemon offnotselectedCCE-4355-4# chkconfig bluetooth offnotselectedCCE-4377-8# chkconfig hidd offnotselectednotselectedCCE-4289-5# chkconfig apmd offnotselectedCCE-4298-6notselectedCCE-4051-9notselectedCCE-4324-0notselectedCCE-4406-5notselectedCCE-4428-9# yum erase anacronpassCCE-3626-9passCCE-3851-3passCCE-4388-5passCCE-3604-6passCCE-4379-4passCCE-4304-2passCCE-4054-3passCCE-3481-9passCCE-4331-5passCCE-4322-4passCCE-4212-7passCCE-3983-4passCCE-4022-0passCCE-3833-1passCCE-4441-2passCCE-4380-2passCCE-4106-1passCCE-4450-3passCCE-4203-6passCCE-4251-5passCCE-4250-7passpasspassnotselectednotselectednotselectedrm /etc/cron.denynotselectedrm /etc/at.denynotselectedCCE-4268-9# chkconfig sshd offnotselectedCCE-4272-1# yum erase openssh-servernotselectedCCE-4295-2notselectednotselectedCCE-4325-7notselectedCCE-3845-5notselectednotselectedCCE-4475-0notselectedCCE-4370-3notselectedCCE-4387-7notselectedCCE-3660-8notselectedCCE-4431-3notselectednotselectednotselectedCCE-4462-8notselectedCCE-4422-2# yum groupremove "X Window System"notselectedCCE-4074-1echo "exec X :0 -nolisten tcp $@" > /etc/X11/xinit/xserverrcnotselectedCCE-3717-6notselectedCCE-4365-3# chkconfig avahi-daemon offnotselectedCCE-4136-8notselectedCCE-4409-9notselectedCCE-4426-3notselectedCCE-4193-9notselectedCCE-4444-6notselectedCCE-4352-1notselectedCCE-4433-9notselectedCCE-4451-1notselectedCCE-4341-4notselectedCCE-4358-8notselectedCCE-4112-9# chkconfig cups offnotselectedCCE-3649-1notselectednotselectedCCE-4420-6notselectedCCE-4407-3notselectedCCE-4425-5notselectedCCE-4191-3notselectedCCE-4336-4# chkconfig dhcpd offnotselectedCCE-4464-4# yum erase dhcpnotselectedCCE-4257-2notselectedCCE-4403-2notselectedCCE-4345-5notselectedCCE-3724-2notselectedCCE-4243-2notselectedCCE-4389-3notselectedCCE-3913-1notselectedCCE-4169-9notselectedCCE-4318-2notselectedCCE-4319-0notselectedCCE-3733-3notselectedCCE-4376-0# chkconfig ntpd onnotselectedCCE-4134-3notselectedCCE-4385-1notselectedCCE-4032-9notselectedCCE-4424-8notselectedCCE-3487-6notselectedCCE-4293-7notselectednotselectedCCE-3501-4notselectedCCE-4396-8notselectedCCE-3535-2notselectedCCE-3568-3notselectedCCE-4533-6notselectedCCE-4559-1notselectedCCE-4015-4notselectedCCE-3667-3notselectedCCE-4310-9notselectedCCE-4438-8notselectedCCE-3579-0notselectedCCE-4473-5notselectedCCE-4491-7notselectedCCE-4368-7notselectedCCE-4024-6notselectedCCE-4526-0notselectedCCE-4544-3notselectedCCE-4465-1notselectedCCE-4350-5notselectedCCE-3578-2notselectedCCE-4219-2notselectedCCE-3985-9notselectedCCE-4258-0notselectedCCE-4487-5notselectedCCE-4399-2notselectedCCE-3919-8notselectedCCE-3919-8notselectedCCE-4549-2notselectedCCE-4554-2notselectedCCE-4443-8notselectedCCE-4461-0notselectedCCE-4338-0notselectedCCE-4514-6notselectedCCE-4474-3notselectedCCE-3756-4notselectedCCE-4509-6notselectedCCE-4386-9notselectedCCE-4029-5notselectedCCE-3581-6notselectedCCE-4574-0notselectedCCE-3847-1notselectedCCE-4239-0notselectedCCE-4384-4notselectedCCE-3887-7notselectedCCE-4530-2notselectedCCE-4547-6notselectedCCE-4552-6notselectedCCE-4371-1notselectedCCE-4410-7notselectedCCE-4551-8notselectednotselectedCCE-4556-7notselectedCCE-4556-7notselectedCCE-4556-7notselectedCCE-4076-6notselectedCCE-4454-5notselectedCCE-4459-4notselectedCCE-4503-9notselectedCCE-4353-9notselectedCCE-4419-8notselectedCCE-3692-1notselectedCCE-4476-8notselectedCCE-3585-7notselectedCCE-4344-8notselectedCCE-4494-1notselectedCCE-4181-4notselectedCCE-4577-3notselectedCCE-4511-2notselectedCCE-4529-4notselectedCCE-3610-3notselectedCCE-4466-9notselectedCCE-4607-8notselectedCCE-4255-6notselectedCCE-4127-7notselectedCCE-4519-5notselectedCCE-4413-1notselectedCCE-4373-7notselectedCCE-3765-5notselectedCCE-4404-0notselectednotselectedCCE-4148-3notselectedCCE-4254-9notselectedCCE-4416-4notselectedCCE-4484-2notselectedCCE-4502-1notselectedCCE-4550-02.303659620.000000