Audit framework

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The Linux audit framework provides a CAPP-compliant (Controlled Access Protection Profiles) auditing system that reliably collects information about any security-relevant (or non-security-relevant) event on a system. It can help you track actions performed on a system.


Linux audit helps make your system more secure by providing you with a means to analyze what is happening on your system in great detail. It does not, however, provide additional security itself—it does not protect your system from code malfunctions or any kind of exploits. Instead, Audit is useful for tracking these issues and helps you take additional security measures, to prevent them.

The audit framework works by listening to the event reported by the kernel and logging them to a log file.

/!\ as of linux 3.11, the audit framework is not yet compatible with the namespace implementation, if you use namespaces, consider deactivate the audit framework. It may also affects the performance of the system.


Installation

audit is available in the community repository, it can be installed with:

# pacman -S audit
# systemctl start auditd.service
# systemctl enable auditd.service

Audit framework is composed of the auditd daemon, responsible for writing the audit messages that were generated through the audit kernel interface and triggered by application and system activity.

This daemon can be controled by several commands and files:

  • auditctl : to control the behavior of the daemon on the fly, adding rules
 etc.
  • /etc/audit/audit.rules : contains the rules and various parameters of the
 auditd daemon
  • aureport : generate report of the activity on a system
  • ausearch : search for various events

j auditspd : the daemon which can be used to relay event notifications to

 other applications instead of writing them to disk in the audit log
  • autrace : this command can be used to trace a process, in a similar way as
 strace.
  • /etc/audit/auditd.conf : configuration file related to the logging.

Adding rules

Before adding rules, you must know that the audit framework can be very verbose and that each rules must be carrefully tested before being effectively deployed. Indeed, just one rule can flood all your log within a few minutes and break the relevancy of your logs.


Audit files and directories access

The most basic use of the audit framework is to log the access to the files you want. To do this, you must use a watch (-w) to a file or a directory The most basic rule to set up is to track accesses to the passwd file :

# auditctl -w /etc/passwd -p rwxa

You can track access to a folder with :

# auditctl -w /etc/security/

The first rule keep track of every read (r), write (w), execution (x), attribute change (a) to the file /etc/passwd. The second keep track to any access in the /etc/security/ folder.

You can list all active rules with :

# auditctl -l

Once you validate the rule, you can add it to the /etc/audit/audit.rules :

-w /etc/audit/audit.rules -p rwxa
-w /etc/security/


Audit syscalls

The audit framework allow you to audit the syscalls performed.

A security related rule is to track the chmod syscall, to detect file ownership changes :

auditctl -a entry,always -S chmod

For a list of all syscalls : man syscalls

A lot of rules and posibilities are available, see man auditctl and man audit.rules

Search the logs

using pid

you can search events related to a particular pid using :

# ausearch -p 1

This command will show you all the events logged according to your rules related to the PID1 (i.e. systemd).

using keys

You can use the -k option in your rules to be able to find related events easily :

# auditctl -w /etc/passwd -p rwxa -k KEY_pwd

A lot of other options are available, see man ausearch

# ausearch -k KEY_pwd

A similar search with a key set up will give you

Look for anormalies

The aureport tool can be used to quicly report any anormal event performaed on the system, it include network interface used in promiscous mode, process or thread crashing or exiting with ENOMEM error etc.

The easiest way to use aureprt is :

# aureport -n

You can generate custom report, see man aureport

Which files or syscalls are worth-auditing ?

Keep in mind that each audit rule added will generate logs, so you must be ready to treat this amount of information. Basically, each security-related event/file must be monitored, like ids, ips, anti-rootkits etc. On the other side, it's totally useless to track every write syscall, the smallest compilation will fill your logs with this event.

More complex set of rules can be set up, performing auditing on a very fine-grained base. If you want to do so, the man pages of auditctl is worth-reading.