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Medium/Average (~15 mins)
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Become familiar with the auditing capabilities of Red Hat® Enterprise Linux® (RHEL)
The Linux Audit System provides a way to track security-relevant information on your system. Based on preconfigured rules, Audit generates log entries to record as much information as possible about the events happening on your system. This information is crucial for mission-critical environments to determine any violations of security policy and what actions the violator performed. Audit does not provide additional security to your system; rather, it can be used to discover violations of security policies used on your system. These violations can be prevented by additional security measures such as SELinux.
The lab is performed on a system installed with Red Hat Enterprise Linux 8 and may require modification to work in other lab environments.
All of the exercises in this lab are run on the audit.example.com host, either as the root or auditlab user. In each section, you are instructed which user to use. The username is reflected in the command prompt as shown here:
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The root user prompt on audit.example.com:
[root@audit ~]#
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The auditlab user prompt on audit.example.com:
[auditlab@audit ~]$
To access the audit.example.com, Red Hat recommends that you open the terminal and remotely connect using SSH into the audit.example.com host from the bastion workstation system as shown. If done correctly, you do not need to enter a password.
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Use SSH to remotely log in to the bastion workstation system (replacing
GUIDwith your lab-provided GUID and using r3dh4t1! as the password):[localhost ~]$ ssh [email protected]
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Use SSH to remotely log in to audit.example.com as root:
[lab-user@workstation-GUID ~]$ ssh [email protected]
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Repeat the first SSH command to log in to your bastion workstation system in a new terminal, then use SSH to log in to audit.example.com as
auditlabto avoid exiting your root session on audit.example.com:[lab-user@workstation-GUID ~]$ ssh [email protected]
There are two main audit components in Red Hat Enterprise Linux: the audit daemon and the kernel itself. In this section, you configure both.
When the audit daemon is started during the boot process, it reads its
configuration information from the file /etc/audit/auditd.conf.
The configuration options are explained in the auditd.conf(5)
man page. Three of the more interesting options are the flush, freq, and log_format options:
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flushdetermines the method by which audit events are flushed to disk. -
freqcontrols how frequently the flush takes place. -
log_formatoption controls the on-disk audit log format.
In this section, you verify that flush is set to INCREMENTAL_ASYNC
(for asynchronous flushing for performance), freq is set to 50 (to flush the log
every 50 records), and log_format is set to ENRICHED (to resolve some
information for improved archival value).
The /etc/audit/auditd.conf file
can be modified using any text editor. In this section, you use the sed command to edit the file.
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If you are not already there, log in to the workstation bastion host as lab-user from your desktop system (replacing
GUIDwith your lab-provided GUID and, if needed, using r3dh4t1! as the password):[localhost ~]$ ssh [email protected]
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Log in to the audit.example.com host as root:
[lab-user@workstation-GUID ~]$ ssh [email protected] -
As root, edit the
auditd.conffile:[root@audit ~]# sed -e 's/^flush.*/flush = INCREMENTAL_ASYNC/' -i /etc/audit/auditd.conf [root@audit ~]# sed -e 's/^freq.*/freq = 50/' -i /etc/audit/auditd.conf [root@audit ~]# sed -e 's/^log_format.*/log_format = ENRICHED/' -i /etc/audit/auditd.conf
After the configuration file is updated, you must signal the audit daemon to reload its configuration.
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As root, force
auditdto reload its configuration:[root@audit ~]# service auditd reload
Expect to see it return an acknowledgement similar to:
Reconfiguring: [ OK ]
The audit daemon can dump a small report about its configuration and some facts about its current state. This can help you diagnose problems encountered by the audit daemon.
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As root, determine the audit daemon’s state:
[root@audit ~]# service auditd state
Expect to see it return an acknowledgement similar to the following, with a list of values including the current time, process priority, configuration options, and disk free space:
Getting auditd internal state: [ OK ]
Usually the audit daemon enables the kernel’s audit subsystem.
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As root, get the kernel’s state to verify that the audit system is enabled:
[root@audit ~]# auditctl -s
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1in the first line of the output of theauditctlcommand indicates that it is enabled. Thepidline provides the PID of the audit daemon. The rest of the output is the kernel status, not including rules.
The Linux kernel’s audit subsystem can be configured with the auditctl
command. By using auditctl the administrator can add audit event filtering
rules as well as tune the audit subsystem in the kernel. The configuration
parameters are explained in the
auditctl(8) man page.
A number of preconfigured audit filter rules are provided with Red Hat
Enterprise Linux. You can find them in /usr/share/doc/audit-*. These filter
rules can be enabled by copying them to the system’s audit filter rule
directory, regenerating the filter configuration, and loading the resulting
filter rule configuration into the kernel.
In this section, you enable some basic audit filters designed to help administrators meet the U.S. Department of Defense Security Technical Implementation Guide (STIG) for Red Hat Enterprise Linux.
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While logged in to the audit.example.com system as root, enable a number of pre-defined audit filters:
[root@audit ~]# cat /usr/share/doc/audit/rules/README-rules [root@audit ~]# rm /etc/audit/rules.d/* [root@audit ~]# cp /usr/share/doc/audit/rules/10-base-config.rules /etc/audit/rules.d [root@audit ~]# cp /usr/share/doc/audit/rules/30-stig.rules /etc/audit/rules.d [root@audit ~]# cp /usr/share/doc/audit/rules/31-privileged.rules /etc/audit/rules.d [root@audit ~]# cp /usr/share/doc/audit/rules/99-finalize.rules /etc/audit/rules.d [root@audit ~]# augenrules --load
The
augenrulestool combines all of the*.rulesfiles located in/etc/audit/rules.dinto the/etc/audit/audit.rulesfile and loads them using theauditctlcommand. You can remove or rename any of these files and rerun theaugenrules --loadcommand to reconfigure your system. -
Now that rules are loaded, working as root, have the kernel dump the currently loaded rules so that you can inspect what is loaded:
[root@audit ~]# auditctl -l
Expect to see many audit rules output from the kernel.
Custom audit filters can be loaded into the kernel using the auditctl
command. The various filter options are explained in the
auditctl(8) man page.
Custom audit filters can be made persistent by creating a new file in the
/etc/audit/rules.d directory with the .rules file extension. While not
required, the following naming convention is suggested:
<priority>-<name>.rules
Where the <priority> value falls into these categories:
10: Kernel and `auditctl` configuration 20: Rules that could match general rules but we want a different match 30: Main rules 40: Optional rules 50: Server specific rules 70: System local rules 90: Finalize (immutable)
The preconfigured filter rules provide a useful example for how to structure
your custom audit filter rule files. The basic syntax is that each line is
a series of arguments passed to the auditctl command; lines starting with a
# are treated as comments and ignored.
In this section, you create an audit filter that captures audit
events created by the /usr/bin/ping program. You also configure the
system to tag all of those events with the rhkey key, using the -k
option, to make the search through the audit log easier. The -a always,exit is
a common way to add audit filter rules; it adds a filter rule to be executed at
syscall exit time. (See the
auditctl(8) man page for
more detail.)
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While logged into the audit.example.com system as root, add a custom audit filter for the
/usr/bin/pingapplication:[root@audit ~]# auditctl -a always,exit -F exe=/usr/bin/ping -k rhkey
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As root, add a new rule file to
/etc/audit/rules.dand reload your configuration to make your custom filter rule persistent:[root@audit ~]# echo "-a always,exit -S all -F exe=/usr/bin/ping -F key=rhkey" > /etc/audit/rules.d/70-rhkey_lab.rules [root@audit ~]# augenrules --load
In addition to modifying custom filter rules, you can adjust the base configuration of the audit subsystem in the Linux kernel using
auditctl. -
As root, increase the audit backlog buffer to
8192entries:[root@audit ~]# auditctl -b 8192
This setting is confirmed by output similar to the status command.
If you want to make the configuration change persistent, you can create a new file in
/etc/audit/rules.dwith the configuration and reload the audit rules. -
As root, make the backlog changes persistent:
[root@audit ~]# echo "-b 8192" > /etc/audit/rules.d/15-rhkey_kernel.rules [root@audit ~]# augenrules --load
The kernel also has two kernel boot command-line options that affect the audit system:
audit and audit_backlog_limit. The audit configuration option takes either a 1 or 0, which
means enabled or disabled, respectively. If you plan to use the audit system, you should boot
with audit enabled. As the system is booting, it generates events. By default the kernel
has room to hold 64 events. But systemd logs an event for every service started and stopped, and
the kernel logs events as it gets configured. This can easily overrun the 64 reserved event spots.
To hold a lot of events until auditd can start reading them, you increase the backlog.
In this section, you modify /etc/default/grub to add audit-related configuration to the
kernel’s boot prompt. Then you regenerate the boot menu so that it takes effect.
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As root, back up the current
/etc/default/grubfile and edit it to set theauditandaudit_backlog_limitoptions:[root@audit ~]# cp /etc/default/grub /etc/default/grub.bak [root@audit ~]# sed -e '/GRUB_CMDLINE_LINUX/s/\"/ audit=1 audit_backlog_limit=8192\"/2' -i /etc/default/grub
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Verify that the
audit=1 and audit_backlog_limit=8192options are present:[root@audit ~]# grep GRUB_CMDLINE_LINUX /etc/default/grub
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As root, regenerate the grub boot menu:
[root@audit ~]# grub2-mkconfig -o `find /boot -name grub.cfg`
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The menu is in different places based on whether you have a BIOS- or UEFI-based machine. The
findcommand locates the file for you.
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The exercises below show how to search through the audit logs and generate summary audit reports. It is important to note that this section requires that the system is configured as described earlier in this lab.
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Open another terminal shell on the workstation bastion host, and from there log in to the audit.example.com system as the auditlab user:
[lab-user@workstation-GUID ~]$ ssh [email protected]
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Run the following commands to generate some interesting events in the audit log:
[auditlab@audit ~]$ vi /etc/shadow (Type :q! to exit vi)
[auditlab@audit ~]$ ping -c 1 127.0.0.1
[auditlab@audit ~]$ vi ~/project_tps_report.txt (Type: i to go into insert mode) (Type: This is my TPS report) (Press *Esc*) (Type :wq! to save the file and exit vi)
[auditlab@audit ~]$ chmod 0664 ~/project_tps_report.txt
While the audit logs are plain text files, and normal Linux text searching tools
(e.g. grep) can be used to search the audit logs, the audit userspace tools
include ausearch—, which was designed to search and interpret the audit logs.
The ausearch tool can take a number of command-line parameters, which are described in the
ausearch(8) man page.
The --start option specifies at what point in the audit logs to start searching,
--start today indicates that only events from today should be considered. The
-m option indicates that you are interested in audit events with the given
record type.
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While logged into the audit.example.com system as root, examine the login events on the test system:
[root@audit ~]# ausearch --start today -m USER_LOGIN
Expect to see one event shown with SSHD for the current session that is hosting this search command.
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As root, list all of the service start and stop events:
[root@audit ~]# ausearch --start this-month -m SERVICE_START -m SERVICE_STOP
Multiple record types can be specified; the results include events that contain either record type.
Expect the results to show an event for each service run or stopped in that time.
The
-ioption instructsausearchto interpret the results, translating some fields into a more human-readable form. The-koption searches on the key assigned to an audit rule. -
As root, display all of the events from today matching the access key:
[root@audit ~]# ausearch --start today -i -k access
This command lists any events triggered by the pre-defined rules with the
accesskey, and the-iinterpretation option makes theproctitlefield readable in the output. -
As root, display today’s events from the auditlab user that match the
perm_modkey:[root@audit ~]# ausearch --start today -i -k perm_mod --uid auditlab
The
--uidoption searches for events that match the given UID.Expect this command to list the event generated by the example above in the [Audit Events Generation] section.
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As root, display all of today’s accesses of the project_tps_report.txt file:
[root@audit ~]# ausearch --start today -i -f project_tps_report.txt
The
-foption searches for events that match the given file name.Expect the command to list the creation and permission modification events from the [Audit Events Generation] section.
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As root, view all of the events from today matching the
rhkeykey, to search for audit events generated by your custom filter rule:[root@audit ~]# ausearch --start today -i -k rhkey
Expect this to list the event from the
pingcommand in the [Audit Events Generation] section.
Included in the Audit userspace tools are three utilities that can be used to
generate a number of reports from the audit log: aureport, aulast, and
aulastlog. The aureport tool can generate a number of different reports,
all of which are described in the
aureport(8) man page.
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While logged into the audit.example.com system as root, run the following commands to create several audit reports for today’s activity:
[root@audit ~]# aureport --start today --summary [root@audit ~]# aureport --start today --summary -i --file [root@audit ~]# aureport --start today --summary -i --executable [root@audit ~]# aureport --start today --summary -i --login
The
aureportandausearchtools may be used together if you want to identify who triggered a specific audit rule. The strategy is to search for the key that is associated with the audit rule and then feed the results to the kind of report you are interested in. This works only if the output fromausearchis exactly as it is in the logs. To tellausearchto leave the event unaltered, pass the--rawformatting option. -
As root, run the following command:
[root@audit ~]# ausearch --start today -k access --raw | aureport --summary -i --file
The
aulasttool generates a report similar to thelastcommand, except the information is collected from the audit log instead of the less reliableutmplogs. The aulast(8) man page provides details on how to runaulast; without any options, the output is familiar with thelastcommand.The
aulastutility can also help you find anausearchcommand to extract just the audit events for a specific login whenever you pass the--proofcommand-line option. This is helpful when investigating which programs or files a user accessed during a specific session. -
As root, examine an example of
aulastreport:[root@audit ~]# aulast [root@audit ~]# aulast --proof
Similar to
aulast,aulastlogis designed as a replacement for thelastlogcommand—the important difference being thataulastlogcollects data from the audit log. The aulastlog(8) man page provides more information, but even runningaulastlogwithout any options results in a useful report. -
As root, examine an
aulastlogreport:[root@audit ~]# aulastlog
In addition to searching through the audit logs, the ausearch tool can also
be used to transform the results into different formats. If you have already
completed the rest of this lab, you are most likely familiar with the raw
and interpreted default formats. In addition to these formats, there are also
csv and text formats, which can be selected using the --format argument.
The --format option, as well as several others that can customize the output
of ausearch, can be found in the
ausearch(8) man page.
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While logged into the audit.example.com system as root, view samples of the
csvandtextformats:[root@audit ~]# ausearch --start today --format csv [root@audit ~]# ausearch --start today --format text
The CSV output is particularly interesting as it can be imported into LibreOffice or any other spreadsheet program that accepts files in the Comma Separated Values (CSV) format.
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As root, transform today’s audit log into the CSV format suitable for use within LibreOffice:
[root@audit ~]# ausearch --start today --format csv --extra-labels --extra-obj2 > /tmp/audit.log.csv
In this section, you transfer the CSV file you just created from the audit.example.com system to the workstation bastion host system by using scp. Then you open the CSV file using LibreOffice from the workstation bastion host.
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If you are not already there, log in to the workstation bastion host as lab-user from your desktop system (replacing
GUIDwith your lab-provided GUID and using r3dh4t1! as the password):[localhost ~]$ ssh [email protected]
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Use
scpto transfer the CSV file from the audit.example.com system to the desktop of the workstation bastion host system:[lab-user@workstation-GUID ~]$ scp [email protected]:/tmp/audit.log.csv ~/Desktop/
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Return to your Lab Information web page and click CONSOLE for your workstation bastion host:
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Log in as lab-user with r3dh4t1! as the password:
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Locate the CSV file you just copied from the audit.example.com system on your workstation bastion host’s desktop:
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Double-click the audit report to view it, then click Ok on the Text Import dialog:
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Take a look at your CSV report:
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If you want to restart the lab from scratch, run the following as root on audit.example.com:
[root@audit ~]# rm /etc/audit/rules.d/* [root@audit ~]# cp /usr/share/doc/audit/rules/10-base-config.rules /etc/audit/rules.d [root@audit ~]# augenrules --load [root@audit ~]# cp /etc/default/grub.bak /etc/default/grub [root@audit ~]# grub2-mkconfig -o `find /boot -name grub.cfg`