Create Flexible Permissions Hierarchies with POSIX ACLs






Create Flexible Permissions Hierarchies with POSIX ACLs

When Unix mode-based permissions just aren't enough, use an ACL.

Most of the time, the traditional Unix file permissions system fits the bill just fine. But in a highly collaborative environment with multiple people needing access to files, this scheme can become unwieldy. Access control lists, otherwise known as ACLs (pronounced to rhyme with "hackles"), are a relatively new feature of open source Unix operating systems, but they have been available in their commercial counterparts for some time. While ACLs do not inherently add "more security" to a system, they do reduce the complexity of managing permissions. ACLs provide new ways to apply file and directory permissions without resorting to the creation of unnecessary groups.

ACLs are stored as extended attributes within the filesystem metadata. As the name implies, they allow you to define lists that either grant or deny access to a given file or directory based on the criteria you provide. However, ACLs do not abandon the traditional permissions system completely. ACLs can be specified for both users and groups and are still separated into the realms of read, write, and execute access. In addition, a control list may be defined for any user or group that does not correspond to any of the other user or group ACLs, much like the "other" mode bits of a file.

Access control lists also have what is called an ACL mask, which acts as a permission mask for all ACLs that specifically mention a user and a group. This is similar to a umask, but not quite the same. For instance, if you set the ACL mask to r--, any ACLs that pertain to a specific user or group and are looser in permissions (e.g., rw-) will effectively become r--. Directories also may contain a default ACL, which specifies the initial ACLs of files and subdirectories created within them.

Enabling ACLs

Most filesystems in common use today under Linux (Ext2/3, ReiserFS, JFS, and XFS) are capable of supporting ACLs. If you're using Linux, make sure one of the following kernel configuration options is set, corresponding to the type of filesystem you're using:

CONFIG_EXT2_FS_POSIX_ACL=y
CONFIG_EXT3_FS_POSIX_ACL=y
CONFIG_REISERFS_FS_POSIX_ACL=y
CONFIG_JFS_POSIX_ACL=y
CONFIG_FS_POSIX_ACL=y
CONFIG_XFS_POSIX_ACL=y

To enable ACLs in FreeBSD, mount any filesystems you want to use them on with the acls mount option:

# mount -o acls -u /usr
# mount
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1e on /tmp (ufs, local, soft-updates)
/dev/ad0s1f on /usr (ufs, local, soft-updates, acls)
/dev/ad0s1d on /var (ufs, local, soft-updates)

The -u option updates the mount, which lets you change the mount options for a currently mounted filesystem. If you want to undo this, you can disable ACLs by using the noacls option instead. To enable ACLs automatically at boot for a filesystem, modify the filesystem's /etc/fstab entry to look like this:

/dev/ad0s1f             /usr            ufs     rw,acls         2       2

Managing ACLs

Once they've been enabled, ACLs can be set, modified, and removed using the setfacl command. To create or modify an ACL, use the -m option, followed by an ACL specification and a filename or list of filenames. You can delete an ACL by using the -x option and specifying an ACL or list of ACLs.

There are three general forms of an ACL: one for users, another for groups, and one for others. Let's look at them here:

# User ACL
u:[user]:<mode>
# Group ACL
g:[group]:<mode>
# Other ACL
o:<mode>

Notice that in user and group ACLs, the actual user and group names that the ACL applies to are optional. If these are omitted, it means that the ACL will apply to the base ACL, which is derived from the file's mode bits. Thus, if you modify these, the mode bits will be modified, and vice versa.

See for yourself by creating a file and then modifying its base ACL:

$ touch myfile
$ ls -l myfile
-rw-rw-r--    1 andrew   andrew          0 Oct 13 15:57 myfile
$ setfacl -m u::---,g::---,o:--- myfile
$ ls -l myfile
----------    1 andrew   andrew          0 Oct 13 15:57 myfile

From this example, you can also see that multiple ACLs can be listed by separating them with commas.

You can also specify ACLs for an arbitrary number of groups or users:

$ touch foo
$ setfacl -m u:jlope:rwx,g:wine:rwx,o:--- foo
$ getfacl foo
# file: foo
# owner: andrew
# group: andrew
user::rw-
user:jlope:rwx
group::---
group:wine:rwx
mask::rwx
other::---

Now if you changed the mask to r--, the ACLs for jlope and wine would effectively become r-- as well:

$ setfacl -m m:r-- foo
$ getfacl foo
# file: foo
# owner: andrew
# group: andrew
user::rw-
user:jlope:rwx                  #effective:r--
group::---
group:wine:rwx                  #effective:r--
mask::r--
other::---

As mentioned earlier, a directory can have a default ACL that will automatically be applied to files that are created within that directory. To designate an ACL as the default, prefix it with a d::

$ mkdir mydir
$ setfacl -m d:u:jlope:rwx mydir
$ getfacl mydir
# file: mydir
# owner: andrew
# group: andrew
user::rwx
group::---
other::---
default:user::rwx
default:user:jlope:rwx
default:group::---
default:mask::rwx
default:other::---

$ touch mydir/bar
$ getfacl mydir/bar
# file: mydir/bar
# owner: andrew
# group: andrew
user::rw-
user:jlope:rwx                  #effective:rw-
group::---
mask::rw-
other::---

As you may have noticed from the previous examples, you can list ACLs by using the getfacl command. This command is pretty straightforward and has only a few options. The most useful is the -R option, which allows you to list ACLs recursively and works very much like ls -R.



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