How To Manage ACL In Linux - Set Permissions Using ACL

What Is ACL In Linux?

ACL (Access Control List) is an advanced permission mechanism in Linux.

Unlike the basic and regular way of giving permissions to one user that is the owner of a file and one group that is the group owner of a file using the “chmod” command, if you have to give additional permissions to another user or another group on a file without making the user a member of the group, you will have to use ACL to do it.

With ACL, you can give permissions to two or more users and groups that are not owners of the file.

When permission is set on a file or directory using ACL, it displays a “+” sign when a list command is used.

For example, from the screen-shot below, a list command is used on a file that the permission has been set using ACL.

You can see the “+” sign, which indicates that ACL is used on the file.

Using ACL requires that the file-system where permissions will be set is mounted with ACL mount option. File-systems like the ext2, ext3, ext4, Brtfs, etc, are mounted with ACL mount option by default.

However, some file-systems are not mounted with ACL mount option by default. In this case, you will have to remount the file system with ACL mount option before ACL can be used.

How To Verify If a File-System Is Mounted With ACL Default Option

To verify if a file-system is mounted witl ACL mount option, use the command,

tune2fs -l <device> |grep "Default mount options:"

For example, to verify the file-system on /dev/sdb1, use the command,

[root@lab02 ~]# tune2fs -l /dev/sdb1 |grep "Default mount options:"

Default mount options:    user_xattr acl

Going forward, you should know that some old Linux kernel does not support ACL.

When you verify the file system and it is not mounted with ACL option, it can also be a good idea to verify if the kernel supports ACL before mounting the file system with ACL option.

How To Verify If A Linux Kernel Supports ACL

To verify if a Linux kernel is supported with ACL, use the command,

[root@lab02 statistics]# cat /boot/config* |grep -i acl

CONFIG_EXT4_FS_POSIX_ACL=y
CONFIG_XFS_POSIX_ACL=y
CONFIG_FS_POSIX_ACL=y
CONFIG_TMPFS_POSIX_ACL=y
CONFIG_NFS_V3_ACL=y
CONFIG_NFSD_V2_ACL=y
CONFIG_NFSD_V3_ACL=y
CONFIG_NFS_ACL_SUPPORT=m

The field “=y” means yes. i.e, the kernel is ACL supported but if you see “n”, then it is not ACL supported.

Having confirmed that a kernel is ACL supported, one can go ahead to mount the file system with ACL mount option if it is not mounted.

However, ACL utilities may not be installed on the system which you will have to before you can mount the file system with ACL option.

How To Install ACL Utilities In Linux

For Red Hat or CentOS systems, use the command,

[root@lab02 ~]# yum install nfs4-acl-tools acl libacl

For Debian, Ubuntu and its likes, use the command,

[root@lab02 ~]# apt-get install nfs4-acl-tools acl libacl

How To Mount A File System With The ACL Mount Option

you can set the default mount option of a file-system by using the command,

tune2fs -o acl <device>

 mount -t <file-system-type> -o acl <device> <mount-point>

For example, to set the default mount option for the partition, /dev/sdb1, use the command,

[root@lab02 ~]# tune2fs -o acl /dev/sdb1

tune2fs 1.44.6 (5-Mar-2019)

[root@lab02 ~]# mount -t ext2 -o acl /dev/sdb1 /data

However, this is will not be persistent after reboot. To make it persistent, you will need to add the option in the fstab.

To add it to the fstab, for example, /dev/sdb1, you will need to include the line below in the fstab file,

/dev/sdb1   /data    ext4    acl,defaults    1 2

You may need to remount the file system in some cases after mounting with ACL mount option.

For example, to remount /dev/sdb1, use the command,

 # mount -o remount,acl /dev/sdb1

Moving forward, there are two commands used In managing ACL, they are the getfacl and setfacl

What is getfacl In Linux?

The getfacl is the tool that is used to get an overview of an access control list on a file.

Let’s look at an example of using getfacl command on the file, january

[root@lab02 statistics]# getfacl january

# file: january
# owner: root
# group: root
user::rw-
group::r--
other::r--

From the screen-shot above, you can see that it is also possible to view the permission on a file by using the “getfacl” command, just as when you use the “ls -l” command.

The getfacl command will display the file permissions set with chmod and the file permissions set with ACL

What Is setfacl In Linux?

The setfacl is the tool that is used to set or change the access control list on a file. ACL’s in Linux are of two types. The access ACL and the default ACL.

The default ACL can only be effective on a directory.

It is advisable that you set all the permissions you want to set on a file first using the chmod command, before setting permissions using ACL.

Having set the permissions using ACL first, and going back to set the permissions using chmod may cause a disorder of permission settings on a file. Though, it can be corrected, especially when you have to, but why waste precious time and effort if you can avoid it?

Furthermore, setting permission on a directory using ACL most times requires that you use the recursive and the default option.

The current items (files and directories) will inherit the permission that is set when the recursive option is used while the new items (files and directories) that will later be added in a directory will inherit the permission that is set when the default option is used.

For example, if I am to set a read and write permission for “finance” group using ACL on the “tekneed” directory, I will run the two commands below.

setfacl -R -m g:finance:rw tekneed

setfacl -m d:g:finance:rw tekneed

How To Set Permission For User, Group And Others On A File / Directory Using ACL

To set permission for a user using ACL, you will have to specify the username or the user id as well as the lists of permission

Also, to set permission for a group, you will have to specify the group name or the group-id as well as the lists of permission

Permission can be rwx , rw, rx, wx, r, x, w, -, etc.