System Administration Guide: Security Services

Audit Trail

The audit trail is created by the audit daemon (see the auditd(1M) man page). The audit daemon starts on each machine when the machine is brought up. After the auditd daemon starts at boot time, it is responsible for collecting the audit trail data and writing the audit records into audit files, which are also called audit log files. For a description of the file format, see the audit.log(4) man page.

Even though you can physically locate audit directories within other file systems that are not dedicated to auditing, do not do so except for directories of last resort. Directories of last resort are directories where audit files are written only when there is no other suitable directory available.

There is one other scenario where locating audit directories outside of dedicated audit file systems could be acceptable. You might do so in a software development environment where auditing is optional, and where it is more important to make full use of disk space than to keep an audit trail. Putting audit directories within other file systems is not acceptable in a security-conscious production environment.

You should also consider the following:

A host should have at least one local audit directory, which it can use as a directory of last resort if the host is unable to communicate with the audit server.
Mount audit directories with the read-write (rw) option. When you mount audit directories remotely (using NFS software), also use the intr and noac options.
List the audit file systems on the audit server where they reside. The export list should include all machines in the configuration.

More About the Audit Files

Each audit file is a self-contained collection of records. The file's name identifies the time span during which the records were generated and the machine that generated them.

Audit File Naming

Audit files that are complete have names of the following form:

start-time.finish-time.machine

where start-time is the time that the first audit record in the audit file was generated, finish-time is the time that the last record was written to the file, and machine is the name of the machine that generated the file. For an example of these names, see Example of a Closed Audit File Name.

An audit log file that is still active has a name of the following form:

start-time.not_terminated.machine

How Audit File Names Are Used

The file name time stamps are used by the auditreduce command to locate files that contain records for the specific time range that has been requested. These time stamps are important because there can be a month's accumulation or more of audit files online, and searching them all for records that were generated in the last 24 hours would be unacceptably expensive.

Time-Stamp Format and Interpretation

The start-time and end-time are timestamps with one-second resolution. They are specified in Greenwich Mean Time (GMT). The format is four digits for the year, followed by two digits for each month, day, hour, minute, and second, as follows:

YYYYMMDDHHMMSS

The timestamps are in GMT to ensure that they will sort in proper order even across a daylight savings time boundary. Because they are in GMT, the date and hour must be translated to the current time zone to be meaningful. Beware of this point whenever you manipulate these files with standard file commands rather than with the auditreduce command.

Example of a File Name for a Still-Active File

The format of a file name of a still-active file is as follows:

YYYYMMDDHHMMSS.not_terminated.machine

Here is an example:

19990327225243.not_terminated.dopey

The audit log files are named by the beginning date. So, in the example above the audit file was created in 1999, on March 27, at 10:52:43 p.m, GMT. The not_terminated in the file name means either that the file is still active or that the auditd daemon was unexpectedly interrupted. The name dopey at the end is the host name of the machine whose audit data is being collected.

Example of a Closed Audit File Name

The format of the name of a closed audit log file is as follows:

YYYYMMDDHHMMSS.YYYYMMDDHHMMSS.hostname

Here is an example:

19990320005243.19900327225351.dopey

In this example, the audit log file was created in 1999, on March 20, at 12:52:43 a.m., GMT. The file was closed March 27, at 10:53:51 p.m., GMT. The name dopey at the end is the host name of the machine whose audit data was collected.

Whenever auditd is unexpectedly interrupted, the audit file that is open at the time retains the not_terminated file name designation. Also, when a machine is writing to a remotely mounted audit file and the file server crashes or becomes inaccessible, the not_terminated designation remains in the current file's name. The audit daemon opens a new audit file and keeps the old audit file name intact.