Access Control Lists

Access control list (ACL) provides an additional, more flexible permission mechanism for file systems. It is designed to assist with UNIX file permissions. ACL allows you to give permissions for any user or group to any disk resource.

Installation

The acl package is a dependency of systemd, it should already be installed.

Enable ACL

To enable ACL, the filesystem must be mounted with the acl option. You can use fstab entries to make it permanent on your system.

There is a possibility that the acl option is already active as one of the default mount options on the filesystem. Btrfs and Ext2/3/4 filesystems are affected by this. Use the following command to check ext2/3/4 formatted partitions for the option: 

# tune2fs -l /dev/sdXY | grep "Default mount options:"
Default mount options:    user_xattr acl

Also check that the default mount options are not overridden, in such case you will see noacl in /proc/mounts in the relevant line.

You can set the default mount options of a filesystem using the tune2fs -o option partition command, for example: 

# tune2fs -o acl /dev/sdXY

Using the default mount options instead of an entry in /etc/fstab is very useful for external drives, such partition will be mounted with acl option also on other Linux machines. There is no need to edit /etc/fstab on every machine.

Note:
  • acl is specified as a default mount option when creating an ext2/3/4 filesystem. This is configured in /etc/mke2fs.conf.
  • The default mount options are not listed in /proc/mounts.

Usage

Set ACL

The ACL can be modified using the setfacl command.

Tip:
  • You can list file/directory permission changes without modifying the permissions (i.e. dry-run) by appending the --test flag.
  • To apply operations to all files and directories recursively, append the -R/--recursive argument.

To set permissions for a user (user is either the user name or ID): 

# setfacl -m "u:user:permissions" <file/dir>

To set permissions for a group (group is either the group name or ID): 

# setfacl -m "g:group:permissions" <file/dir>

To set permissions for others: 

# setfacl -m "other:permissions" <file/dir>

To allow all newly created files or directories to inherit entries from the parent directory (this will not affect files which will be moved into the directory): 

# setfacl -dm "entry" <dir>

To remove a specific entry: 

# setfacl -x "entry" <file/dir>

To remove the default entries: 

# setfacl -k <file/dir>

To remove all entries (entries of the owner, group and others are retained): 

# setfacl -b <file/dir>
Note: By default, setfacl automatically recalculates the ACL mask entry unless the -n (--no-mask) option is used. The mask entry defines the maximum effective permissions for all named users, named groups, and the owning group (but not the file owner or others). When recalculated, the mask is set to the union (bitwise OR) of all permissions from these entries — ensuring that each retains its requested permissions.

The mask then acts as a limiting filter: each affected entry's effective permissions are the intersection (bitwise AND) of its own permissions and the mask. For example, if user:bob is granted rwx and the mask is r-x, Bob’s effective permissions will be limited to r-x. This behavior ensures consistent access semantics, especially when sharing files across systems that do or do not support ACLs.

The example below helps clarify two distinct steps in how the ACL mask works in setfacl, especially in the context of the --no-mask and default behavior:

  1. Recalculation of the mask (bitwise OR / "union")
  2. Applying the mask (bitwise AND / limiting)
user:bob: rw-
group:dev: r--
group:: r-x
→ mask: rwx (union)

Effective rights:
bob: rw-
dev: r--
group:: r-x

The 2003 USENIX document POSIX Access Control Lists on Linux has more information.

Show ACL

To show permissions, use: 

# getfacl <file/dir>

Examples

Set all permissions for user johnny to file named abc: 

# setfacl -m "u:johnny:rwx" abc

Check permissions: 

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
user:johnny:rwx
group::r--
mask::rwx
other::r--

Change permissions for user johnny: 

# setfacl -m "u:johnny:r-x" abc

Check permissions: 

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
user:johnny:r-x
group::r--
mask::r-x
other::r--

Remove all ACL entries: 

# setfacl -b abc

Check permissions: 

# getfacl abc
# file: abc
# owner: someone
# group: someone
user::rw-
group::r--
other::r--

Output of ls command

You will notice that there is an ACL for a given file because it will exhibit a + (plus sign) after its Unix permissions in the output of ls -l. 

$ ls -l /dev/audio
crw-rw----+ 1 root audio 14, 4 nov.   9 12:49 /dev/audio

$ getfacl /dev/audio
getfacl: Removing leading '/' from absolute path names
# file: dev/audio
# owner: root
# group: audio
user::rw-
user:solstice:rw-
group::rw-
mask::rw-
other::---

Execution permissions for private files

The following technique describes how a process like a web server can be granted access to files that reside in a user's home directory, without compromising security by giving the whole world access.

In the following we assume that the web server runs as the user http and grant it access to geoffrey's home directory /home/geoffrey.

The first step is granting execution permissions for the user http: 

# setfacl -m "u:http:--x" /home/geoffrey
Note: Execution permissions to a directory are necessary for a process to list the directory's content.

Since the user http is now able to access files in /home/geoffrey, others no longer need access: 

# chmod o-rx /home/geoffrey

Use getfacl to verify the changes: 

$ getfacl /home/geoffrey
getfacl: Removing leading '/' from absolute path names
# file: home/geoffrey
# owner: geoffrey
# group: geoffrey
user::rwx
user:http:--x
group::r-x
mask::r-x
other::---

As the above output shows, other's no longer have any permissions, but the user http is still able to access the files, thus security might be considered increased.

If you need to give write access for the user http on specific directories and/or files, run: 

# setfacl -dm "u:http:rwx" /home/geoffrey/project1/cache

See also

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