|Red Hat Enterprise Linux 3: Reference Guide|
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Each PAM configuration file contains a group of directives formatted as follows:
<module interface> <control flag> <module name> <module arguments>
Each of these elements are explained in the subsequent sections.
There are four types of PAM module interfaces which correlate to different aspects of the authorization process:
auth — This module interface authenticates the 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.
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.
Module interface directives can be stacked, or placed upon one another, so that multiple modules are used together for one purpose. For this reason, the order in which the modules are listed is very important to the authentication process.
Stacking makes it very easy for an administrator to require specific conditions to exist before allowing the user to authenticate. For example, rlogin normally uses five stacked auth modules, as seen in its PAM configuration file:
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
Before someone is allowed to use rlogin, PAM verifies that the /etc/nologin file does not exist, that they are not trying to log in remotely as a root user over a network connection, and that any environmental variables can be loaded. Then, if a successful rhosts authentication is performed, the connection is allowed. If the rhosts authentication fails, then standard password authentication is performed.
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.
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 docs 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).
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 applications is linked the appropriate version of libpam, which can locate the correct version of the module.
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 will report an error to the /var/log/messages file.