The Protection Database stores information about AFS users, client machines, and groups which the File Server process uses to determine whether clients are authorized to access AFS data.

To obtain authenticated access to an AFS cell, a user must have an entry in the cell's Protection Database. The first time that a user requests access to the data stored on a file server machine, the File Server on that machine contacts the Protection Server to request the user's current protection subgroup (CPS), which lists all the groups to which the user belongs. The File Server scans the access control list (ACL) of the directory that houses the data, looking for groups on the CPS. It grants access in accordance with the permissions that the ACL extends to those groups or to the user individually. (The File Server stores the CPS and uses it as long as the user has the same tokens. When a user's group membership changes, he or she must reauthenticate for the File Server to recognize the change.)

Only administrators who belong to the cell's system:administrators group can create user entries (the group is itself defined in the Protection Database, as discussed in The System Groups). Members of the system:administrators group can also create machine entries, which can then be used to control access based on the machine from which the access request originates. After creating a machine entry, add it to a Protection Database group and place the group on ACLs (a machine cannot appear on ACLs directly). A machine entry can represent a single machine or multiple machines with consecutive IP addresses as specified by a wildcard notation. For instructions, see Creating User and Machine Entries. Because all replicas of a volume share the same ACL (the one on the volume's root directory mount point), machine entries enable you to replicate the volume that houses a program's binary file while still complying with a machine-based license agreement as required by the program's manufacturer. See Creating User and Machine Entries.

A group entry is a list of user entries, machine entries, or both (groups cannot belong to other groups). Putting a group on an ACL is a convenient way to extend or deny access to a set of users without listing them on the ACL individually. Similarly, adding users to a group automatically grants them access to all files and directories for which the associated ACL lists that group. Both administrators and regular users can create groups.