Chapter 5. Access Control Lists
Files and directories have permission sets for the owner of the file, the group associated with the file, and all other users for the system. However, these permission sets have limitations. For example, different permissions cannot be configured for different users. Thus, Access Control Lists (ACLs) were implemented.
The Red Hat Enterprise Linux kernel provides ACL support for the ext3 file system and NFS-exported file systems. ACLs are also recognized on ext3 file systems accessed via Samba.
Along with support in the kernel, the
acl package is required to implement ACLs. It contains the utilities used to add, modify, remove, and retrieve ACL information.
mv commands copy or move any ACLs associated with files and directories.
5.1. Mounting File Systems
Before using ACLs for a file or directory, the partition for the file or directory must be mounted with ACL support. If it is a local ext3 file system, it can mounted with the following command:
mount -t ext3 -o acl device-name partition
mount -t ext3 -o acl /dev/VolGroup00/LogVol02 /work
Alternatively, if the partition is listed in the
/etc/fstab file, the entry for the partition can include the
LABEL=/work /work ext3 acl 1 2
If an ext3 file system is accessed via Samba and ACLs have been enabled for it, the ACLs are recognized because Samba has been compiled with the
--with-acl-support option. No special flags are required when accessing or mounting a Samba share.
By default, if the file system being exported by an NFS server supports ACLs and the NFS client can read ACLs, ACLs are utilized by the client system.
To disable ACLs on NFS shares when configuring the server, include the
no_acl option in the
/etc/exports file. To disable ACLs on an NFS share when mounting it on a client, mount it with the
no_acl option via the command line or the
5.2. Setting Access ACLs
There are two types of ACLs: access ACLs and default ACLs. An access ACL is the access control list for a specific file or directory. A default ACL can only be associated with a directory; if a file within the directory does not have an access ACL, it uses the rules of the default ACL for the directory. Default ACLs are optional.
ACLs can be configured:
- Per user
- Per group
- Via the effective rights mask
- For users not in the user group for the file
setfacl utility sets ACLs for files and directories. Use the
-m option to add or modify the ACL of a file or directory:
# setfacl -m rules files
Rules (rules) must be specified in the following formats. Multiple rules can be specified in the same command if they are separated by commas.
- Sets the access ACL for a user. The user name or UID may be specified. The user may be any valid user on the system.
- Sets the access ACL for a group. The group name or GID may be specified. The group may be any valid group on the system.
- Sets the effective rights mask. The mask is the union of all permissions of the owning group and all of the user and group entries.
- Sets the access ACL for users other than the ones in the group for the file.
Permissions (perms) must be a combination of the characters
x for read, write, and execute.
If a file or directory already has an ACL, and the
setfacl command is used, the additional rules are added to the existing ACL or the existing rule is modified.
Example 5.1. Give read and write permissions
For example, to give read and write permissions to user andrius:
# setfacl -m u:andrius:rw /project/somefile
To remove all the permissions for a user, group, or others, use the
-x option and do not specify any permissions:
# setfacl -x rules files
Example 5.2. Remove all permissions
For example, to remove all permissions from the user with UID 500:
# setfacl -x u:500 /project/somefile
5.3. Setting Default ACLs
To set a default ACL, add
d: before the rule and specify a directory instead of a file name.
Example 5.3. Setting default ACLs
For example, to set the default ACL for the
/share/ directory to read and execute for users not in the user group (an access ACL for an individual file can override it):
# setfacl -m d:o:rx /share
5.4. Retrieving ACLs
To determine the existing ACLs for a file or directory, use the
getfacl command. In the example below, the
getfacl is used to determine the existing ACLs for a file.
Example 5.4. Retrieving ACLs
# getfacl home/john/picture.png
The above command returns the following output:
# file: home/john/picture.png # owner: john # group: john user::rw- group::r-- other::r--
If a directory with a default ACL is specified, the default ACL is also displayed as illustrated below. For example,
getfacl home/sales/ will display similar output:
# file: home/sales/ # owner: john # group: john user::rw- user:barryg:r-- group::r-- mask::r-- other::r-- default:user::rwx default:user:john:rwx default:group::r-x default:mask::rwx default:other::r-x
5.5. Archiving File Systems With ACLs
By default, the
dump command now preserves ACLs during a backup operation. When archiving a file or file system with
tar, use the
--acls option to preserve ACLs. Similarly, when using
cp to copy files with ACLs, include the
--preserve=mode option to ensure that ACLs are copied across too. In addition, the
-a option (equivalent to
-dR --preserve=all) of
cp also preserves ACLs during a backup along with other information such as timestamps, SELinux contexts, and the like. For more information about
cp, refer to their respective
star utility is similar to the
tar utility in that it can be used to generate archives of files; however, some of its options are different. Refer to Table 5.1, “Command Line Options for
star” for a listing of more commonly used options. For all available options, refer to
man star. The
star package is required to use this utility.
Table 5.1. Command Line Options for
Creates an archive file.
Do not extract the files; use in conjunction with
Replaces files in the archive. The files are written to the end of the archive file, replacing any files with the same path and file name.
Displays the contents of the archive file.
Updates the archive file. The files are written to the end of the archive if they do not exist in the archive, or if the files are newer than the files of the same name in the archive. This option only works if the archive is a file or an unblocked tape that may backspace.
Extracts the files from the archive. If used with
Displays the most important options.
Displays the least important options.
Do not strip leading slashes from file names when extracting the files from an archive. By default, they are stripped when files are extracted.
When creating or extracting, archives or restores any ACLs associated with the files and directories.
5.6. Compatibility with Older Systems
If an ACL has been set on any file on a given file system, that file system has the
ext_attr attribute. This attribute can be seen using the following command:
# tune2fs -l filesystem-device
A file system that has acquired the
ext_attr attribute can be mounted with older kernels, but those kernels do not enforce any ACLs which have been set.
Versions of the
e2fsck utility included in version 1.22 and higher of the
e2fsprogs package (including the versions in Red Hat Enterprise Linux 2.1 and 4) can check a file system with the
ext_attr attribute. Older versions refuse to check it.
5.7. ACL References
Refer to the following man pages for more information.
man acl— Description of ACLs
man getfacl— Discusses how to get file access control lists
man setfacl— Explains how to set file access control lists
man star— Explains more about the
starutility and its many options