Module name: mac_bsdextended.ko
Kernel configuration line: options MAC_BSDEXTENDED
Boot option: mac_bsdextended_load="YES"
The mac_bsdextended(4)
module enforces the file system firewall. This module's policy provides an extension to
the standard file system permissions model, permitting an administrator to create a
firewall-like ruleset to protect files, utilities, and directories in the file system
hierarchy. When access to a file system object is attempted, the list of rules is
iterated until either a matching rule is located or the end is reached. This behavior may
be changed by the use of a sysctl(8) parameter,
security.mac.bsdextended.firstmatch_enabled. Similar to other firewall modules in
FreeBSD, a file containing access control rules can be created and read by the system at
boot time using an rc.conf(5)
variable.
The rule list may be entered using a utility, ugidfw(8), that has a
syntax similar to that of ipfw(8). More tools
can be written by using the functions in the libugidfw(3)
library.
Extreme caution should be taken when working with this module; incorrect use could
block access to certain parts of the file system.
After the mac_bsdextended(4)
module has been loaded, the following command may be used to list the current rule
configuration:
# ugidfw list
0 slots, 0 rules
As expected, there are no rules defined. This means that everything is still
completely accessible. To create a rule which will block all access by users but leave
root unaffected, simply run the following command:
# ugidfw add subject not uid root new object not uid root mode n
Note: In releases prior to FreeBSD 5.3, the add parameter did not exist. In those cases the set should be used instead. See below for a command example.
This is a very bad idea as it will block all users from issuing even the most simple
commands, such as ls. A more patriotic list of rules might
be:
# ugidfw set 2 subject uid user1 object uid user2 mode n
# ugidfw set 3 subject uid user1 object gid user2 mode n
This will block any and all access, including directory listings, to user2's home directory from the
username user1.
In place of user1, the not uid user2 could be passed. This will enforce the same
access restrictions above for all users in place of just one user.
Note: The root user will be unaffected by these
changes.
This should provide a general idea of how the mac_bsdextended(4)
module may be used to help fortify a file system. For more information, see the mac_bsdextended(4) and
the ugidfw(8) manual
pages.
