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To control access to a directory and all of the files in it, AFS associates an access control list (ACL) with it, rather than the mode bits that the UNIX file system (UFS) associates with individual files or directories. AFS ACLs provide more refined access control because there are seven access permissions rather than UFS's three, and there is room for approximately 20 user or group entries on an ACL, rather than just the three UFS entries (owner, group, and other).
This chapter explains how to perform the following tasks by
using the indicated commands:
| Examine access control list | fs listacl |
| Edit ACL's normal permissions section | fs setacl |
| Edit ACL's negative permissions section | fs setacl with -negative flag |
| Replace an ACL | fs setacl with -clear flag |
| Copy an ACL | fs copyacl |
| Remove obsolete AFS UIDs | fs cleanacl |
This section describes the main differences between the AFS and UFS file protection systems, discusses the implications of directory-level protections, and describes the seven access permissions.
The UFS mode bits data protection system and the AFS ACL system differ in the following ways:
UFS associates a set of nine mode bits with each file element, three (rwx) for each of the element's owner, owning group, and all other users. A similar set of mode bits on the file's directory applies to the file only in an oblique way.
An AFS ACL instead protects all files in a directory in the same way. If a certain file is more sensitive than others, store it in a directory with a more restrictive ACL.
Defining access at the directory level has important consequences:
In general, then, it is best to assign fairly liberal access permissions to high-level directories (including user home directories). In particular, it often makes sense to grant at least the l permission to the system:anyuser or system:authuser group on high-level directories. For further discussion, see Using Groups on ACLs.
Mode bits are the only file-protection system in UFS. AFS allows you to set the UNIX mode bits on a file in addition to the ACL on its directory, but it interprets them differently. See How AFS Interprets the UNIX Mode Bits.
UFS defines three access permissions in the form of mode bits: r (read), w (write), and x (execute). AFS defines seven permissions, which makes access control more precise. For detailed descriptions, see The AFS ACL Permissions.
a (administer)
d (delete)
i (insert)
k (lock)
l (lookup)
r (read)
w (write)
UFS controls access for one user and two groups by providing a set of mode bits for each: the user who owns the file or directory, a single defined group, and everyone who has an account on the system.
AFS, in contrast, allows you to place many entries (individual users or groups) on an ACL, granting a different set of access permissions to each one. The number of possible entries is about 20, and depends on how much space each entry occupies in the memory allocated for the ACL itself.
AFS defines two system groups, system:anyuser and system:authuser, which represent all users and all authenticated users, respectively; for further discussion, see Using Groups on ACLs. In addition, users can define their own groups in the Protection Database, consisting of individual users or machine IP addresses. Users who have the a permission on an ACL can create entries for the system groups as well as groups defined by themselves or other users. For information on defining groups, see Administering the Protection Database.
When a user requests access to a file or directory, the File Server sums together all of the permissions that the relevant ACL extends to the user and to groups to which the user belongs. Placing group entries on ACLs therefore can control access for many more users than the ACL can accommodate as individual entries.
Functionally, the seven standard ACL permissions fall into two groups: one that applies to the directory itself and one that applies to the files it contains.
The four permissions in this group are meaningful with respect to the directory itself. For example, the i (insert) permission does not control addition of data to a file, but rather creation of a new file or subdirectory.
This permission enables a user to issue the following commands:
This permission does not enable a user to read the contents of a file in the directory, to issue the ls -l command on a file in the directory, or to issue the fs listacl command with the filename as the -path argument. Those operations require the r (read) permission which is described in The Three File Permissions.
Similarly, this permission does not enable a user to issue the ls, ls -l, ls -ld, or fs listacl commands against a subdirectory of the directory. Those operations require the l permission on the ACL of the subdirectory itself.
The three permissions in this group are meaningful with respect to files in a directory, rather than the directory itself or its subdirectories.
AFS provides eight additional permissions that do not have a defined meaning, denoted by the uppercase letters A, B, C, D, E, F, G, and H.
You can write application programs that assign a meaning to one or more of the permissions, and then place them on ACLs to control file access by those programs. For example, you can modify a print program to recognize and interpret the permissions, and then place them on directories that house files that the program accesses. Use the fs listacl and fs setacl commands to display and set the auxiliary permissions on ACLs just like the standard seven.
You can combine the seven permissions in any way in an ACL entry, but certain combinations are more useful than others. Four of the more common combinations have corresponding shorthand forms. When using the fs setacl command to define ACL entries, you can provide either one or more of the individual letters that represent the permissions, or one of the following shorthand forms:
ACLs enable you both to grant and to deny access to a directory and the files in it. To grant access, use the fs setacl command to create an ACL entry that associates a set of permissions with a user or group, as described in Setting ACL Entries. When you use the fs listacl command to display an ACL (as described in Displaying ACLs), such entries appear underneath the following header, which uses the term rights to refer to permissions:
Normal rights
There are two ways to deny access:
Negative rights
When determining what type of access to grant to a user, the File Server first compiles a set of permissions by examining all of the entries in the Normal rights section of the ACL. It then subtracts any permissions associated with the user (or with groups to which the user belongs) on the Negative rights section of the ACL. Therefore, negative permissions always cancel out normal permissions.
Using negative permissions reverses the usual semantics of the fs setacl command, introducing the potential for confusion. In particular, combining the none shorthand and the -negative flag constitutes a double negative: by removing an entry from the Negative rights section of the ACL, you enable a user once again to obtain permissions via entries in the Normal rights section. Combining the all shorthand with the -negative flag explicitly denies all permissions.
Note also that it is pointless to create an entry in the Negative rights section if an entry in the Normal rights section grants the denied permissions to the system:anyuser group. In this case, users can obtain the permissions simply by using the unlog command to discard their tokens. When they do so, the File Server recognizes them as the anonymous user, who belongs to the system:anyuser group but does not match the entries on the Negative rights section of the ACL.
As previously mentioned, placing a group entry on an ACL enables you to control access for many users at once. You can grant a new user access to many files and directories simply by adding the user to a group that appears on the relevant ACLs. You can also create groups of machines, in which case any user logged on to the machine obtains the access that is granted to the group. On directories where they have the a permission on the ACL, users can define their own groups and can create ACL entries for any groups, not just groups that they create or own themselves. For instructions on creating groups of users or machines, and a discussion of the most effective ways to use different types of groups, see Administering the Protection Database.
AFS also defines the following two system groups, which can be very useful on ACLs because they potentially represent a large group of people. For more information about these groups, see The System Groups.
Note that creating an ACL entry for this group is the only way to extend access to AFS users from foreign cells, unless you create local authentication accounts for them.
It is particularly useful to grant the l (lookup) permission to the system:anyuser group on the ACL of most directories in the file system, especially at the upper levels. This permission enables users only to learn the names of files and subdirectories in a directory, but without it they cannot traverse their way through the directories in the path to a target file.
A slightly more restrictive alternative is to grant the l permission to the system:authuser group. If that is still not restrictive enough, you can grant the l to specific users or groups, which cannot exceed about 20 in number on a given ACL.
Another reason to grant certain permissions to the system:anyuser group is to enable the correct operation of processes that provide services such as printing and mail delivery. For example, in addition to the l permission, a print process possibly needs the r (read) permission in order to access the contents of files, and a mail delivery process possibly requires the i (insert) permission to deliver new pieces of mail.
The ACL on the root directory of every newly created volume grants all permissions to the system:administrators group. You can remove this entry if you wish, but members of the system:administrators group always implicitly have the a (administer), and by default also the l, permission on every directory's ACL. The a permission enables them to grant themselves other permissions explicitly when necessary. To learn about changing this default set of permissions, see Administering the system:administrators Group.
To display the ACL associated with a file, directory or symbolic link, issue the fs listacl command. The output for a symbolic link displays the ACL that applies to its target file or directory, rather than the ACL on the directory that houses the symbolic link.
Note for AFS/DFS Migration Toolkit users: If the machine on which you issue the fs listacl command is configured to access a DCE cell's DFS filespace via the AFS/DFS Migration Toolkit, you can use the command to display the ACL on DFS files and directories. To display a DFS directory's Initial Container and Initial Object ACL instead of the regular one, include the fs listacl command's -id or -if flag. For instructions, see the IBM AFS/DFS Migration Toolkit Administration Guide and Reference. The fs command interpreter ignores the -id and -if flags if you include them when displaying an AFS ACL.
% fs listacl [<dir/file path>+]
where
The following error message indicates that you do not have the permissions needed to display an ACL. To specify a directory name as the dir/file path argument, you must have the l (lookup) permission on the ACL. To specify a filename, you must also have the r (read) permission on its directory's ACL.
fs: You don't have the required access permissions on 'dir/file path'
Members of the system:administrators group and the directory's owner (as reported by the ls -ld command) implicitly have the a (administer) permission on every directory's ACL, and can use the fs setacl command to grant themselves the required permissions; for instructions, see Setting ACL Entries.
The output for each file or directory specified as dir/file path begins with the following header to identify it:
Access list for dir/file path is
The Normal rights header appears on the next line, followed by lines that each pair a user or group name and a set of permissions. The permissions appear as the single letters defined in The AFS ACL Permissions, and always in the order rlidwka. If there are any negative permissions, the Negative rights header appears next, followed by pairs of negative permissions.
The following example displays the ACL on user terry's home directory in the ABC Corporation cell:
% fs la /afs/abc.com/usr/terry
Access list for /afs/abc.com/usr/terry is
Normal permissions:
system:authuser rl
pat rlw
terry rlidwka
Negative permissions:
terry:other-dept rl
jones rl
where pat, terry, and jones are individual users, system:authuser is a system group, and terry:other-dept is a group that terry owns. The list of normal permissions grants all permissions to terry, the r (read), l (lookup), and w (write) permissions to pat, and the r and l permissions to the members of the system:authuser group.
The list of negative permissions denies the r and l permissions to jones and the members of the terry:other-dept group. These entries effectively prevent them from accessing terry's home directory in any way, because they cancel out the r and l permissions extended to the system:authuser group, which is the only entry on the Normal rights section of the ACL that possibly applies to them.
To add, remove, or edit ACL entries, use the fs setacl command. By default, the command manipulates entries on the normal permissions section of the ACL. To manipulate entries on the negative permissions section, include the -negative flag.
You must have the a (administer) permission on an ACL to edit it. The owner of a directory (as reported by the ls -ld) command and members of the system:administrators group always implicitly have it on every ACL. By default, members of the system:administrators group also implicitly have the l (lookup) permission.
Note for AFS/DFS Migration Toolkit users: If the machine on which you issue the fs setacl command is configured to access a DCE cell's DFS filespace via the AFS/DFS Migration Toolkit, you can use the command to set the ACL on DFS files and directories. To set a DFS directory's Initial Container and Initial Object ACL instead of the regular one, include the fs setacl command's -id or -if flag. For instructions, see the IBM AFS/DFS Migration Toolkit Administration Guide and Reference. The fs command interpreter ignores the -id and -if flags if you include them when setting an AFS ACL.
% fs listacl [<dir/file path>]
% fs setacl -dir <directory>+ -acl <access list entries>+
where
Specify the read/write path to each directory, to avoid the failure that results when you attempt to change a read-only volume. By convention, you indicate the read/write path by placing a period before the cell name at the pathname's second level (for example, /afs/.abc.com). For further discussion of the concept of read/write and read-only paths through the filespace, see The Rules of Mount Point Traversal.
You can also use the following notation on its own or as part of a pathname:
fs: 'filename': Not a directory
If you specify only one directory or file name, you can omit the -dir and -acl switches.
To define the permissions, provide either:
For a more detailed description of the permissions and shorthand notations, see The AFS ACL Permissions.
On a single command line, you can combine user and group entries. You can also use individual letters in some pairs and the shorthand notations in other pairs, but cannot combine letters and shorthand notation within a single pair.
Either of the following examples grants user pat the r (read) and l (lookup) permissions on the ACL of the notes subdirectory in the issuer's home directory. They illustrate how it is possible to omit the -dir and -acl switches when you name only one directory.
% fs sa ~/notes pat rl % fs sa ~/notes pat read
The following example edits the ACL for the current working directory. It removes the entry for the system:anyuser group, and adds two entries: one grants all permissions except a (administer) to the members of the terry:colleagues group and the other grants the r (read) and l (lookup) permissions to the system:authuser group. The command appears on two lines here only for legibility.
% fs sa -dir . -acl system:anyuser none terry:colleagues write \
system:authuser rl
% fs listacl [<dir/file path>]
% fs setacl -dir <directory>+ -acl <access list entries>+ -negative
where
The following example denies user pat the w (write) and d (delete) permissions for the project subdirectory of the current working directory.
% fs sa project pat wd -neg
It is sometimes simplest to clear an ACL completely before defining new permissions on it, for instance if the mix of normal and negative permissions makes it difficult to understand how their interaction affects a user's access to the directory. To clear an ACL completely while you define new entries, include the -clear flag on the fs setacl command. When you include this flag, you can create entries on either the normal permissions or the negative permissions section of the ACL, but not on both at once.
Remember to create an entry that grants appropriate permissions to the directory's owner. The owner implicitly has the a (administer) permission required to replace a deleted entry, but the effects of a missing ACL entry (particularly the lack of the lookup permission) can be so confusing that it becomes difficult for the owner to realize that the missing entry is causing the problems.
% fs listacl [<dir/file path>]
% fs setacl -dir <directory>+ -acl <access list entries>+ -clear \
[-negative]
where
The fs copyacl command copies a source directory's ACL to one or more destination directories. It does not affect the source ACL at all, but changes each destination ACL as follows:
Note for AFS/DFS Migration Toolkit users: If the machine is configured to enable AFS users to access a DCE cell's DFS filespace via the AFS/DFS Migration Toolkit, then you can use the fs copyacl command to copy ACLs between DFS files and directories also. The command includes -id and -if flags for altering a DFS directory's Initial Container and Initial Object ACLs as well as its regular ACL; see the IBM AFS/DFS Migration Toolkit Administration Guide and Reference. You cannot copy ACLs between AFS and DFS directories, because they use different ACL formats. The fs command interpreter ignores the -id and -if flags if you include them when copying AFS ACLs.
% fs listacl [<dir/file path>]
% fs copyacl -fromdir <source directory> -todir <destination directory>+ \
[-clear]
where
Specify the read/write path to each directory, to avoid the failure that results when you attempt to change a read-only volume. By convention, you indicate the read/write path by placing a period before the cell name at the pathname's second level (for example, /afs/.abc.com). For further discussion of the concept of read/write and read-only paths through the filespace, see The Rules of Mount Point Traversal.
The following example copies the ACL from the current working directory's notes subdirectory to the plans subdirectory. The issuer does not include the -clear flag, so the entry for user pat remains on the plans directory's ACL although there is no corresponding entry on the notes directory's ACL.
% fs la notes plans
Access list for notes is
Normal permissions:
terry rlidwka
smith rl
jones rl
Access list for plans is
Normal permissions:
terry rlidwk
pat rlidwk
% fs copyacl notes plans
% fs la notes plans
Access list for notes is
Normal permissions:
terry rlidwka
smith rl
jones rl
Access list for plans is
Normal permissions:
terry rlidwka
pat rlidwk
smith rl
jones rl
When you remove a user or group entry from the Protection Database, the fs listacl command displays the user's AFS UID (or group's AFS GID) in ACL entries, rather than the name. In the following example, user terry has an ACL entry for the group terry:friends (AFS GID -567) on her home directory in the ABC Corporation cell, and then removes the group from the Protection Database.
% fs listacl /afs/abc.com/usr/terry
Access list for /afs/abc.com/usr/terry is
Normal permissions:
terry:friends rlik
system:anyuser l
terry rlidwka
% pts delete terry:friends
% fs listacl /afs/abc.com/usr/terry
Access list for /afs/abc.com/usr/terry is
Normal permissions:
-567 rlik
system:anyuser l
terry rlidwka
Leaving AFS IDs on ACLs serves no function, because the ID no longer corresponds to an active user or group. Furthermore, if the ID is ever assigned to a new user or group, then the new possessor of the ID gains access that the owner of the directory actually intended for the previous possessor. (Reusing AFS IDs is not recommended precisely for this reason.)
To remove obsolete AFS UIDs from ACLs, use the fs cleanacl command.
% fs listacl [<dir/file path>]
% fs cleanacl [<dir/file path>+]
where
Specify the read/write path to each directory, to avoid the failure that results when you attempt to change a read-only volume. By convention, you indicate the read/write path by placing a period before the cell name at the pathname's second level (for example, /afs/.abc.com). For further discussion of the concept of read/write and read-only paths through the filespace, see The Rules of Mount Point Traversal.
You can also use the following notation on its own or as part of a pathname:
fs: 'filename': Not a directory
If there are obsolete AFS IDs on a directory, the command interpreter displays its cleaned ACL under the following header.
Access list for directory is now
If a directory's ACL has no obsolete AFS IDs on it, the following message appears for each.
Access list for directory is fine.
Although AFS uses ACLs to protect file data rather than the mode bits that UFS uses, it does not ignore the mode bits entirely. When you issue the chmod command on an AFS file or directory, AFS changes the bits appropriately. To change a file's mode bits, you must have the AFS w (write) permission on the ACL of the file's directory. To change a directory's mode bits, you must have the d (delete), i (insert), and l (lookup) permissions on its ACL.
AFS also uses the UNIX mode bits as follows:
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