Access Control Lists - ArchWiki

Jump to content

# Access Control Lists

From ArchWiki

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

- getfacl(1)
- setfacl(1)
- POSIX Access Control Lists on Linux
- How to set default file permissions for all folders/files in a directory?

Category:

- Access control

Access Control Lists
Add topic