MAC(4) FreeBSD Kernel Interfaces Manual MAC(4)


macMandatory Access Control


options MAC



The Mandatory Access Control, or MAC, framework allows administrators to finely control system security by providing for a loadable security policy architecture. It is important to note that due to its nature, MAC security policies may only restrict access relative to one another and the base system policy; they cannot override traditional UNIX security provisions such as file permissions and superuser checks.

Currently, the following MAC policy modules are shipped with FreeBSD:

Name Description Labeling Load time
mac_biba(4) Biba integrity policy yes boot only
mac_bsdextended(4) File system firewall no any time
mac_ifoff(4) Interface silencing no any time
mac_lomac(4) Low-Watermark MAC policy yes boot only
mac_mls(4) Confidentiality policy yes boot only
mac_none(4) Sample no-op policy no any time
mac_partition(4) Process partition policy yes any time
mac_portacl(4) Port bind(2) access control no any time
mac_seeotheruids(4) See-other-UIDs policy no any time
mac_test(4) MAC testing policy no any time

MAC Labels

Each system subject (processes, sockets, etc.) and each system object (file system objects, sockets, etc.) can carry with it a MAC label. MAC labels contain data in an arbitrary format taken into consideration in making access control decisions for a given operation. Most MAC labels on system subjects and objects can be modified directly or indirectly by the system administrator. The format for a given policy's label may vary depending on the type of object or subject being labeled. More information on the format for MAC labels can be found in the maclabel(7) man page.

MAC Support for UFS2 File Systems

By default, file system enforcement of labeled MAC policies relies on a single file system label (see MAC Labels) in order to make access control decisions for all the files in a particular file system. With some policies, this configuration may not allow administrators to take full advantage of features. In order to enable support for labeling files on an individual basis for a particular file system, the “multilabel” flag must be enabled on the file system. To set the “multilabel” flag, drop to single-user mode and unmount the file system, then execute the following command:

tunefs -l enable filesystem

where filesystem is either the mount point (in fstab(5)) or the special file (in /dev) corresponding to the file system on which to enable multilabel support.

Policy Enforcement

Policy enforcement is divided into the following areas of the system:
File System
File system mounts, modifying directories, modifying files, etc.
Loading, unloading, and retrieving statistics on loaded kernel modules
Network interfaces, bpf(4), packet delivery and transmission, interface configuration ( ioctl(2), ifconfig(8))
Creation of and operation on pipe(2) objects
Debugging (e.g. ktrace(2)), process visibility ( ps(1)), process execution ( execve(2)), signalling ( kill(2))
Creation of and operation on socket(2) objects
Kernel environment ( kenv(1)), system accounting ( acct(2)), reboot(2), settimeofday(2), swapon(2), sysctl(3), nfsd(8)-related operations
mmap(2)-ed files

Setting MAC Labels

From the command line, each type of system object has its own means for setting and modifying its MAC policy label.
Subject/Object Utility
File system object setfmac(8), setfsmac(8)
Network interface ifconfig(8)
TTY (by login class) login.conf(5)
User (by login class) login.conf(5)

Additionally, the su(1) and setpmac(8) utilities can be used to run a command with a different process label than the shell's current label.

Programming With MAC

MAC security enforcement itself is transparent to application programs, with the exception that some programs may need to be aware of additional errno(2) returns from various system calls.

The interface for retrieving, handling, and setting policy labels is documented in the mac(3) man page.


The mac implementation first appeared in FreeBSD 5.0 and was developed by the TrustedBSD Project.


This software was contributed to the FreeBSD Project by Network Associates Labs, the Security Research Division of Network Associates Inc. under DARPA/SPAWAR contract N66001-01-C-8035 (“CBOSS”), as part of the DARPA CHATS research program.


See mac(9) concerning appropriateness for production use. The TrustedBSD MAC Framework is considered experimental in FreeBSD.

While the MAC Framework design is intended to support the containment of the root user, not all attack channels are currently protected by entry point checks. As such, MAC Framework policies should not be relied on, in isolation, to protect against a malicious privileged user.

October 30, 2007 FreeBSD