.\" Copyright (c) 2006 Robert N. M. Watson
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.Dd April 28, 2019
.Dt AUDITPIPE 4
.Os
.Sh NAME
.Nm auditpipe
.Nd "pseudo-device for live audit event tracking"
.Sh SYNOPSIS
.Cd "options AUDIT"
.Sh DESCRIPTION
While audit trail files
generated with
.Xr audit 4
and maintained by
.Xr auditd 8
provide a reliable long-term store for audit log information, current log
files are owned by the audit daemon until terminated making them somewhat
unwieldy for live monitoring applications such as host-based intrusion
detection.
For example, the log may be cycled and new records written to a new file
without notice to applications that may be accessing the file.
.Pp
The audit facility provides an audit pipe facility for applications requiring
direct access to live BSM audit data for the purposes of real-time
monitoring.
Audit pipes are available via a clonable special device,
.Pa /dev/auditpipe ,
subject to the permissions on the device node, and provide a
.Qq tee
of the audit event stream.
As the device is clonable, more than one instance of the device may be opened
at a time; each device instance will provide independent access to all
records.
.Pp
The audit pipe device provides discrete BSM audit records; if the read buffer
passed by the application is too small to hold the next record in the
sequence, it will be dropped.
Unlike audit data written to the audit trail, the reliability of record
delivery is not guaranteed.
In particular, when an audit pipe queue fills, records will be dropped.
Audit pipe devices are blocking by default, but support non-blocking I/O,
asynchronous I/O using
.Dv SIGIO ,
and polled operation via
.Xr select 2
and
.Xr poll 2 .
.Pp
Applications may choose to track the global audit trail, or configure local
preselection parameters independent of the global audit trail parameters.
.Ss Audit Pipe Queue Ioctls
The following ioctls retrieve and set various audit pipe record queue
properties:
.Bl -tag -width ".Dv AUDITPIPE_GET_MAXAUDITDATA"
.It Dv AUDITPIPE_GET_QLEN
Query the current number of records available for reading on the pipe.
.It Dv AUDITPIPE_GET_QLIMIT
Retrieve the current maximum number of records that may be queued for reading
on the pipe.
.It Dv AUDITPIPE_SET_QLIMIT
Set the current maximum number of records that may be queued for reading on
the pipe.
The new limit must fall between the queue limit minimum and queue limit
maximum queryable using the following two ioctls.
.It Dv AUDITPIPE_GET_QLIMIT_MIN
Query the lowest possible maximum number of records that may be queued for
reading on the pipe.
.It Dv AUDITPIPE_GET_QLIMIT_MAX
Query the highest possible maximum number of records that may be queued for
reading on the pipe.
.It Dv AUDITPIPE_FLUSH
Flush all outstanding records on the audit pipe; useful after setting initial
preselection properties to delete records queued during the configuration
process which may not match the interests of the user process.
.It Dv AUDITPIPE_GET_MAXAUDITDATA
Query the maximum size of an audit record, which is a useful minimum size for
a user space buffer intended to hold audit records read from the audit pipe.
.El
.Ss Audit Pipe Preselection Mode Ioctls
By default, the audit pipe facility configures pipes to present records
matched by the system-wide audit trail, configured by
.Xr auditd 8 .
However, the preselection mechanism for audit pipes can be configured using
alternative criteria, including pipe-local flags and naflags settings, as
well as auid-specific selection masks.
This allows applications to track events not captured in the global audit
trail, as well as limit records presented to those of specific interest to
the application.
.Pp
The following ioctls configure the preselection mode on an audit pipe:
.Bl -tag -width ".Dv AUDITPIPE_GET_PRESELECT_MODE"
.It Dv AUDITPIPE_GET_PRESELECT_MODE
Return the current preselect mode on the audit pipe.
The ioctl argument should be of type
.Vt int .
.It Dv AUDITPIPE_SET_PRESELECT_MODE
Set the current preselection mode on the audit pipe.
The ioctl argument should be of type
.Vt int .
.El
.Pp
Possible preselection mode values are:
.Bl -tag -width ".Dv AUDITPIPE_PRESELECT_MODE_TRAIL"
.It Dv AUDITPIPE_PRESELECT_MODE_TRAIL
Use the global audit trail preselection parameters to select records for the
audit pipe.
.It Dv AUDITPIPE_PRESELECT_MODE_LOCAL
Use local audit pipe preselection; this model is similar to the global audit
trail configuration model, consisting of global flags and naflags parameters,
as well as a set of per-auid masks.
These parameters are configured using further ioctls.
.El
.Pp
After changing the audit pipe preselection mode, records selected under
earlier preselection configuration may still be in the audit pipe queue.
The application may flush the current record queue after changing the
configuration to remove possibly undesired records.
.Ss Audit Pipe Local Preselection Mode Ioctls
The following ioctls configure the preselection parameters used when an audit
pipe is configured for the
.Dv AUDITPIPE_PRESELECT_MODE_LOCAL
preselection mode.
.Bl -tag -width ".Dv AUDITPIPE_GET_PRESELECT_NAFLAGS"
.It Dv AUDITPIPE_GET_PRESELECT_FLAGS
Retrieve the current default preselection flags for attributable events on
the pipe.
These flags correspond to the
.Va flags
field in
.Xr audit_control 5 .
The ioctl argument should be of type
.Vt au_mask_t .
.It Dv AUDITPIPE_SET_PRESELECT_FLAGS
Set the current default preselection flags for attributable events on the
pipe.
These flags correspond to the
.Va flags
field in
.Xr audit_control 5 .
The ioctl argument should be of type
.Vt au_mask_t .
.It Dv AUDITPIPE_GET_PRESELECT_NAFLAGS
Retrieve the current default preselection flags for non-attributable events
on the pipe.
These flags correspond to the
.Va naflags
field in
.Xr audit_control 5 .
The ioctl argument should be of type
.Vt au_mask_t .
.It Dv AUDITPIPE_SET_PRESELECT_NAFLAGS
Set the current default preselection flags for non-attributable events on the
pipe.
These flags correspond to the
.Va naflags
field in
.Xr audit_control 5 .
The ioctl argument should be of type
.Vt au_mask_t .
.It Dv AUDITPIPE_GET_PRESELECT_AUID
Query the current preselection masks for a specific auid on the pipe.
The ioctl argument should be of type
.Vt "struct auditpipe_ioctl_preselect" .
The auid to query is specified via the
.Va ap_auid
field of type
.Vt au_id_t ;
the mask will be returned via
.Va ap_mask
of type
.Vt au_mask_t .
.It Dv AUDITPIPE_SET_PRESELECT_AUID
Set the current preselection masks for a specific auid on the pipe.
Arguments are identical to
.Dv AUDITPIPE_GET_PRESELECT_AUID ,
except that the caller should properly initialize the
.Va ap_mask
field to hold the desired preselection mask.
.It Dv AUDITPIPE_DELETE_PRESELECT_AUID
Delete the current preselection mask for a specific auid on the pipe.
Once called, events associated with the specified auid will use the default
flags mask.
The ioctl argument should be of type
.Vt au_id_t .
.It Dv AUDITPIPE_FLUSH_PRESELECT_AUID
Delete all auid specific preselection specifications.
.El
.Sh EXAMPLES
The
.Xr praudit 1
utility
may be directly executed on
.Pa /dev/auditpipe
to review the default audit trail.
.Sh SEE ALSO
.Xr poll 2 ,
.Xr select 2 ,
.Xr audit 4 ,
.Xr dtaudit 4 ,
.Xr audit_control 5 ,
.Xr audit 8 ,
.Xr auditd 8
.Sh HISTORY
The OpenBSM implementation was created by McAfee Research, the security
division of McAfee Inc., under contract to Apple Computer Inc.\& in 2004.
It was subsequently adopted by the TrustedBSD Project as the foundation for
the OpenBSM distribution.
.Pp
Support for kernel audit first appeared in
.Fx 6.2 .
.Sh AUTHORS
The audit pipe facility was designed and implemented by
.An Robert Watson Aq Mt rwatson@FreeBSD.org .
.Pp
The Basic Security Module (BSM) interface to audit records and audit event
stream format were defined by Sun Microsystems.
.Sh BUGS
See the
.Xr audit 4
manual page for information on audit-related bugs and limitations.
.Pp
The configurable preselection mechanism mirrors the selection model present
for the global audit trail.
It might be desirable to provide a more flexible selection model.
.Pp
The per-pipe audit event queue is fifo, with drops occurring if either the
user thread provides in sufficient for the record on the queue head, or on
enqueue if there is insufficient room.
It might be desirable to support partial reads of records, which would be
more compatible with buffered I/O as implemented in system libraries, and to
allow applications to select which records are dropped, possibly in the style
of preselection.