Working with Access Control Lists

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© IKO, Fotolia

© IKO, Fotolia

Article from Issue 91/2008
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The ancient Linux permission system is often insufficient for complex production environments. Access Control Lists offer a flexible alternative.

Alice appreciates the convenience of a PC-based electronic calendar, but to maintain her privacy, she has set strict permissions for her calendar file: She can add new appointments herself, but other members of her workgroup have read-only access. Others outside of her workgroup are not even allowed to look.

This configuration is fine at first, but one day Bob from another department agrees to collaborate with Alice. To allow this to happen, she has to give him access to her calendar data.

In this scenario, it is clear that the legacy Linux permission system has outlived its usefulness. To allow Bob to read the file with her calendar data, Alice can ask the administrator to move the new colleague into her own group, but this would allow Bob to view all the other documents produced by Alice's team. Another approach would be to temporarily set up a completely new user group with both Alice's and Bob's accounts as members. In this simple scenario, a temporary group might be an acceptable solution, but in a real-world enterprise environment, group management becomes far more complicated, and the habit of creating temporary groups on the fly can lead to too many groups with no good way of tracking them.

Access Control Lists, or ACLs for short, promise a solution. They add flexible access control to the legacy Unix permissions system, letting users add permissions for any group or users. Alice doesn't even need to talk to the administrator; she can simply put Bob on the list of authorized users and even specify default permissions for all new files.

Access Control Lists have been around for a while, and they are gradually becoming part of daily life in many production environments; however, the ACL security structure is still unfamiliar to many Linux users. In this article, I show you how to get started with ACLs in Linux.

Rotating Disks

If you plan to use ACLs, your filesystem must support extended attributes. Of the current crop of filesystems, Ext2, Ext3, Ext4, ReiserFS, JFS, and XFS all have ACL support. JFS and XFS support extended attributes by default; for all others, you need to stipulate the acl mount option to enable ACLs:

mount -o remount,acl,defaults mount_point

Most current distributions set these parameters by default in /etc/fstab:

/dev/hda1 / ext3 acl,user_xattr 1 1

For internal disks, you need not change anything, and ACLs also work over NFS as of NFSv3, assuming the server has a filesystem and operating system that support ACLs.

Kernel Issues

Besides the filesystem, the kernel also must support ACLs – after all, it is the kernel that finally grants or refuses access to a file. All current kernels in the 2.6 series have ACL support, and patches exist for the legacy 2.4.x kernel. The major distributions typically enable extended attributes for all of the filesystems mentioned previously, allowing users to start assigning permissions from scratch. To be sure, just enter the following command:

grep "XATTR\|POSIX_ACL" /boot/config-$(uname -r)

It should show two entries with =y if ACLs are supported. Ext2, for example, would show:

CONFIG_EXT2_FS_XATTR=y
CONFIG_EXT2_FS_POSIX_ACL=y

Otherwise, you have no alternative but to install a new kernel.

Band of Two

Besides filesystem and Linux kernel support, you also need a package with applications that display the ACLs for each file and modify them as needed. Most distributions include a package called acl for this purpose. Two of the programs it includes are particularly useful:

  • getfacl displays the ACL for a file, and
  • setfacl sets or changes the permissions for a file

Both tools rely on the libattr and libacl libraries, which many distros install by default.

POSIX and ACLs

You might stumble across the term POSIX (Portable Operating System Interface) ACLs on the Internet and in documentation. Although various drafts appeared at the end of the last century (POSIX 1003.1e, commonly referred to as POSIX.1e, and 1003.2c), for several reasons, the drafts were never approved. Most ACL implementations are still oriented on these drafts. To underline the close connection, many authors use the term POSIX ACLs [1].

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