User space support for event auditing is installed as part
of the base FreeBSD operating system. Kernel support is available
in the GENERIC kernel by default,
and auditd(8) can be enabled
by adding the following line to
/etc/rc.conf:
auditd_enable="YES"
Then, start the audit daemon:
#service auditd start
Users who prefer to compile a custom kernel must include the following line in their custom kernel configuration file:
options AUDIT
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. Selection expressions are evaluated from left to right, and two expressions are combined by appending one onto the other.
Table 17.1, “Default Audit Event Classes” summarizes the default audit event classes:
| Class Name | Description | Action |
|---|---|---|
| all | all | Match all event classes. |
| aa | authentication and authorization | |
| 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) and pathconf(2). |
| fc | file create | Audit events where a file is created as a result. |
| fd | file delete | Audit events where file deletion occurs. |
| fm | file attribute modify | Audit events where file attribute modification occurs, such as by chown(8), chflags(1), and flock(2). |
| fr | file read | Audit events in which data is read or files are opened for reading. |
| fw | file write | Audit events in which data is written or files are written or modified. |
| io | ioctl | Audit use of the ioctl
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. |
| 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 event class may be 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. Table 17.2, “Prefixes for Audit Event Classes” summarizes the available prefixes:
| Prefix | Action |
|---|---|
| + | 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. |
If no prefix is present, both successful and failed instances of the event will be audited.
The following example selection string selects both successful and failed login/logout events, but only successful execution events:
lo,+ex
The following configuration files for security event
auditing are found in
/etc/security:
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, and
maximum audit trail size.
audit_event: textual names and
descriptions of system audit events and a list of which
classes each event is in.
audit_user: user-specific audit
requirements to be combined with the global defaults at
login.
audit_warn: a customizable shell
script used by auditd(8) 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.
Audit configuration files should be edited and maintained carefully, as errors in configuration may result in improper logging of events.
In most cases, administrators will only need to modify
audit_control and
audit_user. The first file controls
system-wide audit properties and policies and the second file
may be used to fine-tune auditing by user.
A number of defaults for the audit subsystem are
specified in audit_control:
dir:/var/audit dist:off flags:lo,aa minfree:5 naflags:lo,aa policy:cnt,argv filesz:2M expire-after:10M
The dir entry 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.
If the dist field is set to
on or yes, hard links
will be created to all trail files in
/var/audit/dist.
The flags field sets the system-wide
default preselection mask for attributable events. In the
example above, successful and failed login/logout events as
well as authentication and authorization are audited for all
users.
The minfree entry defines the minimum
percentage of free space for the file system where the audit
trail is stored.
The naflags entry specifies audit
classes to be audited for non-attributed events, such as the
login/logout process and authentication and
authorization.
The policy entry specifies a
comma-separated list of policy flags controlling various
aspects of audit behavior. The cnt
indicates that the system should continue running despite an
auditing failure (this flag is highly recommended). The
other flag, argv, causes command line
arguments to the execve(2) system call to be audited as
part of command execution.
The filesz entry specifies the maximum
size for an audit trail before automatically terminating and
rotating the trail file. A value of 0
disables automatic log rotation. If the requested file size
is below the minimum of 512k, it will be ignored and a log
message will be generated.
The expire-after field specifies when
audit log files will expire and be removed.
The administrator can specify further audit requirements
for specific users in audit_user.
Each line configures auditing for a user via two fields:
the alwaysaudit field specifies a set of
events that should always be audited for the user, and the
neveraudit field specifies a set of
events that should never be audited for the user.
The following example entries audit login/logout events
and successful command execution for root and file creation and
successful command execution for www. If used with the
default audit_control, the
lo entry for root is redundant, and
login/logout events will also be audited for www.
root:lo,+ex:no www:fc,+ex:no
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