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 disc resource.
The required package systemd, it should already be installed.is a dependency of
To enable ACL, the filesystem must be mounted with the
acl option. You can use fstab to make it permanent on your system.
There is a possibility that the
acl option is already active as default mount option on the filesystem. Btrfs does and possibly ext filesystems do too. Use the following command to check ext* 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 option is not overridden, in such case you will see
/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.
The ACL can be modified using the setfacl command.
To add permissions for a user (
user is either the user name or ID):
# setfacl -m "u:user:permissions" <file/dir>
To add permissions for a group (
group is either the group name or ID):
# setfacl -m "g:group:permissions" <file/dir>
To allow all files or directories to inherit ACL entries from the directory it is within:
# setfacl -dm "entry" <dir>
To remove a specific entry:
# setfacl -x "entry" <file/dir>
To remove all entries:
# setfacl -b <file/dir>
To show permissions, use:
# getfacl filename
Set all permissions for user johny to file named "abc":
# setfacl -m "u:johny:rwx" abc
# getfacl abc
# file: abc # owner: someone # group: someone user::rw- user:johny:rwx group::r-- mask::rwx other::r--
Change permissions for user johny:
# setfacl -m "u:johny:r-x" abc
# getfacl abc
# file: abc # owner: someone # group: someone user::rw- user:johny:r-x group::r-- mask::r-x other::r--
Remove all extended ACL entries:
# setfacl -b abc
# 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 /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::---
Granting execution permissions for private files to a Web Server
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
webserver and grant it access to
geoffrey's home directory
The first step is granting execution permission to
webserver so it can access
# setfacl -m "u:webserver:--x" /home/geoffrey
Remember: Execution permissions to a directory are necessary for a process to list the directory's content.
webserver is now able to access files in
other no longer needs access, so it can be safely removed:
# chmod o-rx /home/geoffrey
getfacl can be used to verify the changes:
$ getfacl /home/geoffrey getfacl: Removing leading '/' from absolute path names # file: home/geoffrey # owner: geoffrey # group: geoffrey user::rwx user:webserver:--x group::r-x mask::r-x other::---
As the above output shows,
other's no longer have any permissions, but
webserver still is able to access the files, thus security might be considered increased.
- An old but still relevant (and thorough) guide to ACL