PAM(3) BSD Library Functions Manual PAM(3)
-- Pluggable Authentication Modules Library
Pluggable Authentication Module Library (libpam, -lpam)
The Pluggable Authentication Modules (PAM) library abstracts a number of common authentication-related operations and provides a framework
for dynamically loaded modules that implement these operations in various ways.
In PAM parlance, the application that uses PAM to authenticate a user is the server, and is identified for configuration purposes by a ser-
vice name, which is often (but not necessarily) the program name.
The user requesting authentication is called the applicant, while the user (usually, root) charged with verifying his identity and granting
him the requested credentials is called the arbitrator.
The sequence of operations the server goes through to authenticate a user and perform whatever task he requested is a PAM transaction; the
context within which the server performs the requested task is called a session.
The functionality embodied by PAM is divided into six primitives grouped into four facilities: authentication, account management, session
management and password management.
The PAM library expects the application to provide a conversation callback which it can use to communicate with the user. Some modules may
use specialized conversation functions to communicate with special hardware such as cryptographic dongles or biometric devices. See
pam_conv(3) for details.
Initialization and Cleanup
The pam_start() function initializes the PAM library and returns a handle which must be provided in all subsequent function calls. The
transaction state is contained entirely within the structure identified by this handle, so it is possible to conduct multiple transactions in
The pam_end() function releases all resources associated with the specified context, and can be called at any time to terminate a PAM trans-
The pam_set_item() and pam_get_item() functions set and retrieve a number of predefined items, including the service name, the names of the
requesting and target users, the conversation function, and prompts.
The pam_set_data() and pam_get_data() functions manage named chunks of free-form data, generally used by modules to store state from one
invocation to another.
There are two authentication primitives: pam_authenticate() and pam_setcred(). The former authenticates the user, while the latter manages
The pam_acct_mgmt() function enforces policies such as password expiry, account expiry, time-of-day restrictions, and so forth.
The pam_open_session() and pam_close_session() functions handle session setup and teardown.
The pam_chauthtok() function allows the server to change the user's password, either at the user's request or because the password has
The pam_putenv(), pam_getenv() and pam_getenvlist() functions manage a private environment list in which modules can set environment vari-
ables they want the server to export during the session.
The pam_strerror() function returns a pointer to a string describing the specified PAM error code.
The following return codes are defined by <security/pam_constants.h>:
[PAM_ABORT] General failure.
[PAM_ACCT_EXPIRED] User account has expired.
Authentication information is unavailable.
Authentication token aging disabled.
[PAM_AUTHTOK_ERR] Authentication token failure.
Password has expired.
Authentication token lock busy.
Failed to recover old authentication token.
[PAM_AUTH_ERR] Authentication error.
[PAM_BUF_ERR] Memory buffer error.
[PAM_CONV_ERR] Conversation failure.
[PAM_CRED_ERR] Failed to set user credentials.
[PAM_CRED_EXPIRED] User credentials have expired.
[PAM_CRED_UNAVAIL] Failed to retrieve user credentials.
Unknown authentication domain.
[PAM_IGNORE] Ignore this module.
[PAM_MAXTRIES] Maximum number of tries exceeded.
Unknown module type.
New authentication token required.
Module data not found.
[PAM_OPEN_ERR] Failed to load module.
[PAM_PERM_DENIED] Permission denied.
[PAM_SERVICE_ERR] Error in service module.
[PAM_SESSION_ERR] Session failure.
[PAM_SYMBOL_ERR] Invalid symbol.
[PAM_SYSTEM_ERR] System error.
[PAM_TRY_AGAIN] Try again.
[PAM_USER_UNKNOWN] Unknown user.
X/Open Single Sign-On Service (XSSO) - Pluggable Authentication Modules, June 1997.
The OpenPAM library and this manual page were developed for the FreeBSD Project by ThinkSec AS and Network Associates Laboratories, the Secu-
rity Research Division of Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035 (``CBOSS''), as part of the DARPA CHATS
The OpenPAM library is maintained by Dag-Erling Smorgrav <firstname.lastname@example.org>.
September 12, 2014 BSD