pam(3)

Interface Overview

The PAM library interface consists of functions which can be grouped into five categories. The names for all the authentication library functions start with pam_.

The first category contains functions for establishing and terminating an authentication activity ( pam_start(3) and pam_end(3)), functions to maintain module specific data ( pam_get_data(3) and pam_set_data(3)), functions to maintain state information ( pam_get_item(3) and pam_set_item(3)), and a function to return error status information ( pam_strerror(3)).

The second category contains functions to authenticate an individual user ( pam_authenticate(3)) and to set the credentials of the user ( pam_setcred(3)).

The third category contains functions to do account management ( pam_acct_mgmt(3)). This includes checking for password aging and access-hour restrictions.

The fourth category contains functions to perform session management ( pam_open_session(3) and pam_close_session(3)) after access to the system has been granted.

The fifth category consists of functions to change authentication tokens pam_chauthtok(3). An authentication token is the object used to verify the identity of the user. In UNIX, an authentication token is a user's password, even when using a smart card, because the PAM Framework retrieves the password from the smart card.

All the pam_*() interfaces are implemented through the library libpam. For each of the categories listed above, excluding the first category pam_start(), pam_end(), pam_[sg]et_data(), pam_[sg]et_item(), and pam_strerror()) there exists a dynamically loadable shared module that provides the appropriate service layer functionality upon demand. The functional entry points in the service layer start with the pam_sm_ prefix. The only difference between the pam_sm_*() interfaces and their corresponding pam_ interfaces is that all the pam_sm_*() interfaces require extra parameters to pass service specific options to the shared modules. Please refer to pam_sm(3) for an overview of the PAM service module APIs.

Stateful Interface

A sequence of calls sharing a common set of state information is referred to as an authentication transaction. An authentication transaction begins with a call to pam_start(). pam_start() allocates space, performs various initialization activities, and assigns a PAM authentication handle to be used for subsequent calls to the library.

After initiating an authentication transaction, applications can invoke pam_authenticate() to authenticate a particular user, and pam_acct_mgmt() to perform system entry management (the application may want to determine if the user's password has expired).

If the user has been successfully authenticated, applications call pam_setcred() to set any user credentials associated with the authentication service. Within one authentication transaction (between pam_start() and pam_end()), all calls to the PAM interface should be made with the same authentication handle returned by pam_start(). This is necessary because certain service modules may store module-specific data in the handle that is intended for use by other modules. For example, during the call to pam_authenticate(), service modules may store data in the handle that is intended for use by pam_setcred().

To perform session management, applications call pam_open_session(). For example, the system may want to store the total time for the session. The function pam_close_session() closes the current session.

When necessary, applications can call pam_get_item() and pam_set_item() to access and update specific authentication information. Such information may include the current username.

To terminate an authentication transaction, the application simply calls pam_end(), which frees previously allocated space used to store authentication information.

Application - Authentication Service Interactive Interface

The authentication service in PAM does not communicate directly with the user; instead it relies on the application to perform all such interactions. The application passes a pointer to the function, conv(), along with any associated application data pointers, through a pam_conv structure to the authentication service when it initiates an authentication transaction (via a call to pam_start()). The service will then use the function, conv(), to prompt the user for data, output error messages, and display text information. Refer to pam_start(3) for more information.

Stacking Multiple Schemes

The PAM architecture enables authentication by multiple authentication services through stacking. System entry applications, such as login(1), stack multiple service modules to authenticate users with multiple authentication services. The order in which authentication service modules are stacked is specified in the configuration file, pam.conf(4). A system administrator determines this ordering, and also determines whether the same password can be used for all authentication services.

Administrative Interface

Various authentication services are implemented by their own loadable modules whose paths are specified through the pam.conf(4) file.

User configuration

The system administrator can determine a policy by user. These are specified in the configuration files: pam.conf(4), pam_user.conf(4).