Costs of increased processing time
Costs of analysis of audit data
Costs of storage of audit data
The cost of increased processing time is the least significant of the three costs of auditing. The first reason is that auditing generally does not occur during computational-intensive tasks--image processing, complex calculations, and so forth. The other reason that processing cost is usually insignificant is that single-user workstations have plenty of extra CPU cycles.
The cost of analysis is roughly proportional to the amount of audit data collected. The cost of analysis includes the time it takes to merge and review audit records, and the time it takes to archive them and keep them in a safe place.
The fewer records you generate, the less time it takes to analyze them, so upcoming sections describe how you can reduce the amount of data collected, while still providing enough coverage to achieve your site's security goals.
Number of users
Number of machines
Amount of use
Degree of security required
Because the factors vary from one situation to the next, no formula can determine in advance the amount of disk space to set aside for audit data storage.
Full auditing (with the all flag) can fill up a disk in no time. Even a simple task like compiling a program of modest size (for example, 5 files, 5000 lines total) in less than a minute could generate thousands of audit records, occupying many megabytes of disk space. Therefore, it is very important to use the preselection features to reduce the volume of records generated. For example, omitting the fr class instead of all classes can reduce the audit volume by more than two-thirds. Efficient audit file management is also important after the audit records are created, to reduce the amount of storage required.
The following sections give some ideas on how to reduce the costs of storage by auditing selectively to reduce the amount of audit data collected, while still meeting your site's security needs. Also discussed are how to set up audit file storage and archiving procedures to reduce storage requirements.
Before configuring auditing, understand the audit flags and the types of events they flag. Develop a philosophy of auditing for your organization that is based on the amount of security your site requires, and the types of users you administer.
Unless the process audit preselection mask is modified dynamically, the audit characteristics in place when a user logs in are inherited by all processes during the login session, and, unless the databases are modified, the process preselection mask applies in all subsequent login sessions.
Dynamic controls refer to controls put in place by the administrator while processes are running. These persist only while the affected processes (and any of their children) exist, but will not continue in effect at the next login. Dynamic controls apply to one machine at a time, since the audit command only applies to the current machine where you are logged in. However, if you make dynamic changes on one machine, you should make them on all machines at the same time.
Each process has two sets of one-bit flags for audit classes. One set controls whether the process is audited when an event in the class is requested successfully; the other set, when an event is requested but fails (for any reason). It is common for processes to be more heavily audited for failures than for successes, since this can be used to detect attempts at browsing and other types of attempts at violating system security.
In addition to supplying the per-user audit control information in the static databases, you may dynamically adjust the state of auditing while a user's processes are active on a single machine.
To change the audit flags for a specific user to a supplied value, use the auditconfig command with the -setpmask, -setsmask, or -setumask options. The command changes the process audit flags for one process, one audit session ID, or one audit user ID respectively. See the auditconfig(1M) man page and "The auditconfig Command".