All configuration files for security audit are found in /etc/security. The following files must be present before the audit
daemon is started:
audit_class - Contains the definitions of the audit
classes.
audit_control - Controls aspects of the audit subsystem,
such as default audit classes, minimum disk space to leave on the audit log volume,
maximum audit trail size, etc.
audit_event - Textual names and descriptions of system audit
events, as well as a list of which classes each event in.
audit_user - User-specific audit requirements, which are
combined with the global defaults at login.
audit_warn - A customizable shell script used by auditd to generate warning messages in exceptional situations,
such as when space for audit records is running low or when the audit trail file has been
rotated.
Warning: Audit configuration files should be edited and maintained carefully,
as errors in configuration may result in improper logging of events.
Selection expressions are used in a number of places in the audit configuration to
determine which events should be audited. Expressions contain a list of event classes to
match, each with a prefix indicating whether matching records should be accepted or
ignored, and optionally to indicate if the entry is intended to match successful or
failed operations. Selection expressions are evaluated from left to right, and two
expressions are combined by appending one onto the other.
The following list contains the default audit event classes present in audit_class:
all - all - Match all event classes.
ad - administrative - Administrative actions performed on the
system as a whole.
ap - application - Application defined action.
cl - file
close - Audit calls to the close system
call.
ex - exec - Audit program execution. Auditing of command line
arguments and environmental variables is controlled via audit_control(5) using
the argv and envv parameters to the policy setting.
fa - file
attribute access - Audit the access of object attributes such as stat(1), pathconf(2) and
similar events.
fc - file
create - Audit events where a file is created as a result.
fm - file
attribute modify - Audit events where file attribute modification occurs, such
as chown(8), chflags(1), flock(2), etc.
fr - file
read - Audit events in which data is read, files are opened for reading,
etc.
fw - file
write - Audit events in which data is written, files are written or modified,
etc.
io - ioctl - Audit use of the ioctl(2) system
call.
ip - ipc - Audit various forms of Inter-Process Communication,
including POSIX pipes and System V IPC operations.
lo - login_logout - Audit login(1) and logout(1) events
occurring on the system.
na - non
attributable - Audit non-attributable events.
no - invalid
class - Match no audit events.
nt - network - Audit events related to network actions, such as connect(2) and accept(2).
ot - other - Audit miscellaneous events.
pc - process - Audit process operations, such as exec(3) and exit(3).
These audit event classes may be customized by modifying the audit_class and audit_event configuration
files.
Each audit class in the list is combined with a prefix indicating whether
successful/failed operations are matched, and whether the entry is adding or removing
matching for the class and type.
(none) Audit both successful and failed instances of the event.
+ Audit successful events in this class.
- Audit failed events in this class.
^ Audit neither successful nor failed events in this
class.
^+ Do not audit successful events in this class.
^- Do not audit failed events in this class.
The following example selection string selects both successful and failed login/logout
events, but only successful execution events:
In most cases, administrators will need to modify only two files when configuring the
audit system: audit_control and audit_user. The first controls system-wide audit properties and
policies; the second may be used to fine-tune auditing by user.
The dir option is used to set one or more directories
where audit logs will be stored. If more than one directory entry appears, they will be
used in order as they fill. It is common to configure audit so that audit logs are stored
on a dedicated file system, in order to prevent interference between the audit subsystem
and other subsystems if the file system fills.
The flags field sets the system-wide default preselection
mask for attributable events. In the example above, successful and failed login and
logout events are audited for all users.
The minfree option defines the minimum percentage of free
space for the file system where the audit trail is stored. When this threshold is
exceeded, a warning will be generated. The above example sets the minimum free space to
twenty percent.
The naflags option specifies audit classes to be audited
for non-attributed events, such as the login process and system daemons.
The policy option specifies a comma-separated list of
policy flags controlling various aspects of audit behavior. The default cnt flag indicates that the system should continue running despite
an auditing failure (this flag is highly recommended). Another commonly used flag is argv, which causes command line arguments to the execve(2) system call
to be audited as part of command execution.
The filesz option specifies the maximum size in bytes to
allow an audit trail file to grow to before automatically terminating and rotating the
trail file. The default, 0, disables automatic log rotation. If the requested file size
is non-zero and below the minimum 512k, it will be ignored and a log message will be
generated.
The audit_user file permits the administrator to specify
further audit requirements for specific users. Each line configures auditing for a user
via two fields: the first is the alwaysaudit field, which
specifies a set of events that should always be audited for the user, and the second is
the neveraudit field, which specifies a set of events that
should never be audited for the user.
The following example audit_user file audits login/logout
events and successful command execution for the root user, and
audits file creation and successful command execution for the www user. If used with the example audit_control file above, the lo entry for
root is redundant, and login/logout events will also be audited
for the www user.
