Extended acl in linux
POSIX ACLs (access control lists) can be used as an expansion of the traditional permission concept for file system objects. With ACLs, permissions can be defined more flexibly than with the traditional permission concept.
The term POSIX ACL suggests that this is a true POSIX ( portable operating system interface ) standard. The respective draft standards POSIX 1003.1e and POSIX 1003.2c have been withdrawn for several reasons. Nevertheless, ACLs (as found on many systems belonging to the Unix family) are based on these drafts and the implementation of file system ACLs (as described in this chapter) follows these two standards.
10.1 Traditional File Permissions #
Find detailed information about the traditional file permissions in the GNU Coreutils Info page, Node File permissions ( info coreutils «File permissions» ). More advanced features are the setuid, setgid, and sticky bit.
10.1.1 The setuid Bit #
In certain situations, the access permissions may be too restrictive. Therefore, Linux has additional settings that enable the temporary change of the current user and group identity for a specific action. For example, the passwd program normally requires root permissions to access /etc/passwd . This file contains some important information, like the home directories of users and user and group IDs. Thus, a normal user would not be able to change passwd , because it would be too dangerous to grant all users direct access to this file. A possible solution to this problem is the setuid mechanism. setuid (set user ID) is a special file attribute that instructs the system to execute programs marked accordingly under a specific user ID. Consider the passwd command:
-rwsr-xr-x 1 root shadow 80036 2004-10-02 11:08 /usr/bin/passwd
You can see the s that denotes that the setuid bit is set for the user permission. By means of the setuid bit, all users starting the passwd command execute it as root .
10.1.2 The setgid Bit #
The setuid bit applies to users. However, there is also an equivalent property for groups: the setgid bit. A program for which this bit was set runs under the group ID under which it was saved, no matter which user starts it. Therefore, in a directory with the setgid bit, all newly created files and subdirectories are assigned to the group to which the directory belongs. Consider the following example directory:
drwxrws--- 2 tux archive 48 Nov 19 17:12 backup
You can see the s that denotes that the setgid bit is set for the group permission. The owner of the directory and members of the group archive may access this directory. Users that are not members of this group are “ mapped ” to the respective group. The effective group ID of all written files will be archive . For example, a backup program that runs with the group ID archive can access this directory even without root privileges.
10.1.3 The Sticky Bit #
There is also the sticky bit . It makes a difference whether it belongs to an executable program or a directory. If it belongs to a program, a file marked in this way is loaded to RAM to avoid needing to get it from the hard disk each time it is used. This attribute is used rarely, because modern hard disks are fast enough. If this bit is assigned to a directory, it prevents users from deleting each other’s files. Typical examples include the /tmp and /var/tmp directories:
drwxrwxrwt 2 root root 1160 2002-11-19 17:15 /tmp
10.2 Advantages of ACLs #
Traditionally, three permission sets are defined for each file object on a Linux system. These sets include the read ( r ), write ( w ), and execute ( x ) permissions for each of three types of users—the file owner, the group, and other users. In addition to that, it is possible to set the set user id , the set group id , and the sticky bit. This lean concept is fully adequate for most practical cases. However, for more complex scenarios or advanced applications, system administrators formerly needed to use several workarounds to circumvent the limitations of the traditional permission concept.
ACLs can be used as an extension of the traditional file permission concept. They allow the assignment of permissions to individual users or groups even if these do not correspond to the original owner or the owning group. Access control lists are a feature of the Linux kernel and are currently supported by ReiserFS, Ext2, Ext3, JFS, and XFS. Using ACLs, complex scenarios can be realized without implementing complex permission models on the application level.
The advantages of ACLs are evident if you want to replace a Windows server with a Linux server. Some connected workstations may continue to run under Windows even after the migration. The Linux system offers file and print services to the Windows clients with Samba. With Samba supporting access control lists, user permissions can be configured both on the Linux server and in Windows with a graphical user interface (only Windows NT and later). With winbindd , part of the Samba suite, it is even possible to assign permissions to users only existing in the Windows domain without any account on the Linux server.
10.3 Definitions #
The conventional POSIX permission concept uses three classes of users for assigning permissions in the file system: the owner, the owning group, and other users. Three permission bits can be set for each user class, giving permission to read ( r ), write ( w ), and execute ( x ).
The user and group access permissions for all kinds of file system objects (files and directories) are determined by means of ACLs.
Default ACLs can only be applied to directories. They determine the permissions a file system object inherits from its parent directory when it is created.
Each ACL consists of a set of ACL entries. An ACL entry contains a type, a qualifier for the user or group to which the entry refers, and a set of permissions. For some entry types, the qualifier for the group or users is undefined.
10.4 Handling ACLs #
Table 10.1, “ACL Entry Types” summarizes the six possible types of ACL entries, each defining permissions for a user or a group of users. The owner entry defines the permissions of the user owning the file or directory. The owning group entry defines the permissions of the file’s owning group. The superuser can change the owner or owning group with chown or chgrp , in which case the owner and owning group entries refer to the new owner and owning group. Each named user entry defines the permissions of the user specified in the entry’s qualifier field. Each named group entry defines the permissions of the group specified in the entry’s qualifier field. Only the named user and named group entries have a qualifier field that is not empty. The other entry defines the permissions of all other users.
The mask entry further limits the permissions granted by named user, named group, and owning group entries by defining which of the permissions in those entries are effective and which are masked. If permissions exist in one of the mentioned entries and in the mask, they are effective. Permissions contained only in the mask or only in the actual entry are not effective—meaning the permissions are not granted. All permissions defined in the owner and owning group entries are always effective. The example in Table 10.2, “Masking Access Permissions” demonstrates this mechanism.
There are two basic classes of ACLs: A minimum ACL contains only the entries for the types owner, owning group, and other, which correspond to the conventional permission bits for files and directories. An extended ACL goes beyond this. It must contain a mask entry and may contain several entries of the named user and named group types.