17.4. Working with Audit Trails

Since audit trails are stored in the BSM binary format, several built-in tools are available to modify or convert these trails to text. To convert trail files to a simple text format, use praudit. To reduce the audit trail file for analysis, archiving, or printing purposes, use auditreduce. This utility supports a variety of selection parameters, including event type, event class, user, date or time of the event, and the file path or object acted on.

For example, to dump the entire contents of a specified audit log in plain text:

# praudit /var/audit/AUDITFILE

Where AUDITFILE is the audit log to dump.

Audit trails consist of a series of audit records made up of tokens, which praudit prints sequentially, one per line. Each token is of a specific type, such as header (an audit record header) or path (a file path from a name lookup). The following is an example of an execve event:

header,133,10,execve(2),0,Mon Sep 25 15:58:03 2006, + 384 msec
exec arg,finger,doug

This audit represents a successful execve call, in which the command finger doug has been run. The exec arg token contains the processed command line presented by the shell to the kernel. The path token holds the path to the executable as looked up by the kernel. The attribute token describes the binary and includes the file mode. The subject token stores the audit user ID, effective user ID and group ID, real user ID and group ID, process ID, session ID, port ID, and login address. Notice that the audit user ID and real user ID differ as the user robert switched to the root account before running this command, but it is audited using the original authenticated user. The return token indicates the successful execution and the trailer concludes the record.

XML output format is also supported and can be selected by including -x.

Since audit logs may be very large, a subset of records can be selected using auditreduce. This example selects all audit records produced for the user trhodes stored in AUDITFILE:

# auditreduce -u trhodes /var/audit/AUDITFILE | praudit

Members of the audit group have permission to read audit trails in /var/audit. By default, this group is empty, so only the root user can read audit trails. Users may be added to the audit group in order to delegate audit review rights. As the ability to track audit log contents provides significant insight into the behavior of users and processes, it is recommended that the delegation of audit review rights be performed with caution.

17.4.1. Live Monitoring Using Audit Pipes

Audit pipes are cloning pseudo-devices which allow applications to tap the live audit record stream. This is primarily of interest to authors of intrusion detection and system monitoring applications. However, the audit pipe device is a convenient way for the administrator to allow live monitoring without running into problems with audit trail file ownership or log rotation interrupting the event stream. To track the live audit event stream:

# praudit /dev/auditpipe

By default, audit pipe device nodes are accessible only to the root user. To make them accessible to the members of the audit group, add a devfs rule to /etc/devfs.rules:

add path 'auditpipe*' mode 0440 group audit

See devfs.rules(5) for more information on configuring the devfs file system.


It is easy to produce audit event feedback cycles, in which the viewing of each audit event results in the generation of more audit events. For example, if all network I/O is audited, and praudit is run from an SSH session, a continuous stream of audit events will be generated at a high rate, as each event being printed will generate another event. For this reason, it is advisable to run praudit on an audit pipe device from sessions without fine-grained I/O auditing.

17.4.2. Rotating and Compressing Audit Trail Files

Audit trails are written to by the kernel and managed by the audit daemon, auditd(8). Administrators should not attempt to use newsyslog.conf(5) or other tools to directly rotate audit logs. Instead, audit should be used to shut down auditing, reconfigure the audit system, and perform log rotation. The following command causes the audit daemon to create a new audit log and signal the kernel to switch to using the new log. The old log will be terminated and renamed, at which point it may then be manipulated by the administrator:

# audit -n

If auditd(8) is not currently running, this command will fail and an error message will be produced.

Adding the following line to /etc/crontab will schedule this rotation every twelve hours:

0     */12       *       *       *       root    /usr/sbin/audit -n

The change will take effect once /etc/crontab is saved.

Automatic rotation of the audit trail file based on file size is possible using filesz in audit_control as described in Section, “The audit_control File”.

As audit trail files can become very large, it is often desirable to compress or otherwise archive trails once they have been closed by the audit daemon. The audit_warn script can be used to perform customized operations for a variety of audit-related events, including the clean termination of audit trails when they are rotated. For example, the following may be added to /etc/security/audit_warn to compress audit trails on close:

# Compress audit trail files on close.
if [ "$1" = closefile ]; then
        gzip -9 $2

Other archiving activities might include copying trail files to a centralized server, deleting old trail files, or reducing the audit trail to remove unneeded records. This script will be run only when audit trail files are cleanly terminated. It will not be run on trails left unterminated following an improper shutdown.

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