16.3. PAM Configuration File Format

16.3.1. Module Interface

There are four types of PAM module interfaces which correlate to different aspects of the authorization process:

• auth — This module interface authenticates use. For example, it asks for and verifies the validity of a password. Modules with this interface can also set credentials, such as group memberships or Kerberos tickets.

• account — This module interface verifies that access is allowed. For example, it may check if a user account is expired or is allowed to log in at a particular time of day.

• password — This module interface sets and verifies passwords.

• session — This module interface configures and manages user sessions. Modules with this interface can also perform additional tasks that are needed to allow access, like mounting a user's home directory and making the user's mailbox available.

	Note
	An individual module can provide any or all module interfaces. For instance, pam_unix.so provides all four module interfaces.

In a PAM configuration file, the module interface is the first field defined. For example, a typical line in a configuration may look like this:

auth      required  pam_unix.so

This instructs PAM to use the pam_unix.so module's auth interface.

auth       required     pam_nologin.so
auth       required     pam_securetty.so
auth       required     pam_env.so
auth       sufficient   pam_rhosts_auth.so
auth       required     pam_stack.so service=system-auth

16.3.2. Control Flag

All PAM modules generate a success or failure result when called. Control flags tell PAM what do with the result. Since modules can be stacked in a particular order, control flags decide how important the success or failure of a particular module is to the overall goal of authenticating the user to the service.

There are four predefined control flags:

• required — The module result must be successful for authentication to continue. If a required module result fails, the user is not notified until results on all modules referencing that interface are completed.

• requisite — The module result must be successful for authentication to continue. However, if a requisite module result fails, the user is notified immediately with a message reflecting the first failed required or requisite module.

• sufficient — The module result is ignored if it fails. However, if a sufficient flagged module result is successful and no required flagged modules above it have failed, then no other results are required and the user is authenticated to the service.

• optional — The module result is ignored. A module flagged as optional only becomes necessary for successful authentication when there are no other modules referencing the interface.

	Important
	The order in which required modules are called is not critical. The sufficient and requisite control flags cause order to become important.

A newer control flag syntax which allows for more precise control is now available for PAM. Please see the PAM documentation located in the /usr/share/doc/pam-<version-number>/ directory for information on this new syntax (where <version-number> is the version number for PAM).

16.3.3. Module Name

The module name provides PAM the name of the pluggable module containing the specified module interface. Under older versions of Red Hat Enterprise Linux, the full path to the module was provided within the PAM configuration file, such as /lib/security/pam_stack.so. However, since the advent of multilib systems, which store 64-bit PAM modules within the /lib64/security/ directory, the directory name is omitted because the application is linked to the appropriate version of libpam, which can locate the correct version of the module.

16.3.4. Module Arguments

PAM uses arguments to pass information to a pluggable module during authentication for some modules.

For example, the pam_userdb.so module uses secrets stored in a Berkeley DB file to authenticate the user. Berkeley DB is an open source database system embedded in many applications. The module takes a db argument so that Berkeley DB knows which database to use for the requested service.

A typical pam_userdb.so line within a PAM configuration file looks like this:

auth      required  pam_userdb.so db=<path-to-file>

In the previous example, replace <path-to-file> with the full path to the Berkeley DB database file.

Invalid arguments are ignored and do not otherwise affect the success or failure of the PAM module. However, most modules report errors to the /var/log/messages file.