13.9. Access Control Lists

13.9. Access Control Lists
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Contributed by Tom Rhodes.

Access Control Lists (ACLs) extend the standard UNIX® permission model in a POSIX®.1e compatible way. This permits an administrator to take advantage of a more fine-grained permissions model.

The FreeBSD GENERIC kernel provides ACL support for UFS file systems. Users who prefer to compile a custom kernel must include the following option in their custom kernel configuration file:

options UFS_ACL

If this option is not compiled in, a warning message will be displayed when attempting to mount a file system with ACL support. ACLs rely on extended attributes which are natively supported in UFS2.

This chapter describes how to enable ACL support and provides some usage examples.

13.9.1. Enabling ACL Support

ACLs are enabled by the mount-time administrative flag, acls, which may be added to /etc/fstab. The mount-time flag can also be automatically set in a persistent manner using tunefs(8) to modify a superblock ACLs flag in the file system header. In general, it is preferred to use the superblock flag for several reasons:

The superblock flag cannot be changed by a remount using mount -u as it requires a complete umount and fresh mount. This means that ACLs cannot be enabled on the root file system after boot. It also means that ACL support on a file system cannot be changed while the system is in use.
Setting the superblock flag causes the file system to always be mounted with ACLs enabled, even if there is not an fstab entry or if the devices re-order. This prevents accidental mounting of the file system without ACL support.

Note:

It is desirable to discourage accidental mounting without ACLs enabled because nasty things can happen if ACLs are enabled, then disabled, then re-enabled without flushing the extended attributes. In general, once ACLs are enabled on a file system, they should not be disabled, as the resulting file protections may not be compatible with those intended by the users of the system, and re-enabling ACLs may re-attach the previous ACLs to files that have since had their permissions changed, resulting in unpredictable behavior.

File systems with ACLs enabled will show a plus (+) sign in their permission settings:

drwx------  2 robert  robert  512 Dec 27 11:54 private
drwxrwx---+ 2 robert  robert  512 Dec 23 10:57 directory1
drwxrwx---+ 2 robert  robert  512 Dec 22 10:20 directory2
drwxrwx---+ 2 robert  robert  512 Dec 27 11:57 directory3
drwxr-xr-x  2 robert  robert  512 Nov 10 11:54 public_html

In this example, directory1, directory2, and directory3 are all taking advantage of ACLs, whereas public_html is not.

13.9.2. Using ACLs

File system ACLs can be viewed using getfacl. For instance, to view the ACL settings on test:

% getfacl test
	#file:test
	#owner:1001
	#group:1001
	user::rw-
	group::r--
	other::r--

To change the ACL settings on this file, use setfacl. To remove all of the currently defined ACLs from a file or file system, include -k. However, the preferred method is to use -b as it leaves the basic fields required for ACLs to work.

% setfacl -k test

To modify the default ACL entries, use -m:

% setfacl -m u:trhodes:rwx,group:web:r--,o::--- test

In this example, there were no pre-defined entries, as they were removed by the previous command. This command restores the default options and assigns the options listed. If a user or group is added which does not exist on the system, an Invalid argument error will be displayed.

Refer to getfacl(1) and setfacl(1) for more information about the options available for these commands.

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