PAM, pam - Pluggable Authentication Modules for Linux
This manual is intended to offer a quick introduction to Linux-PAM. For
more information the reader is directed to the Linux-PAM system
Linux-PAM is a system of libraries that handle the authentication tasks
of applications (services) on the system. The library provides a stable
general interface (Application Programming Interface - API) that
privilege granting programs (such as login(1) and su(1)) defer to to
perform standard authentication tasks.
The principal feature of the PAM approach is that the nature of the
authentication is dynamically configurable. In other words, the system
administrator is free to choose how individual service-providing
applications will authenticate users. This dynamic configuration is set
by the contents of the single Linux-PAM configuration file
/etc/pam.conf. Alternatively, the configuration can be set by
individual configuration files located in the /etc/pam.d/ directory.
The presence of this directory will cause Linux-PAM to
From the point of view of the system administrator, for whom this
manual is provided, it is not of primary importance to understand the
internal behavior of the Linux-PAM library. The important point to
recognize is that the configuration file(s) define the connection
between applications (services) and the pluggable authentication
modules (PAMs) that perform the actual authentication tasks.
Linux-PAM separates the tasks of authentication into four independent
management groups: account management; authentication management;
password management; and session management. (We highlight the
abbreviations used for these groups in the configuration file.)
Simply put, these groups take care of different aspects of a typical
user's request for a restricted service:
account - provide account verification types of service: has the user's
password expired?; is this user permitted access to the requested
authentication - authenticate a user and set up user credentials.
Typically this is via some challenge-response request that the user
must satisfy: if you are who you claim to be please enter your
password. Not all authentications are of this type, there exist
hardware based authentication schemes (such as the use of smart-cards
and biometric devices), with suitable modules, these may be substituted
seamlessly for more standard approaches to authentication - such is the
flexibility of Linux-PAM.
password - this group's responsibility is the task of updating
authentication mechanisms. Typically, such services are strongly
coupled to those of the auth group. Some authentication mechanisms lend
themselves well to being updated with such a function. Standard UN*X
password-based access is the obvious example: please enter a
session - this group of tasks cover things that should be done prior to
a service being given and after it is withdrawn. Such tasks include the
maintenance of audit trails and the mounting of the user's home
directory. The session management group is important as it provides
both an opening and closing hook for modules to affect the services
available to a user.
the configuration file
the Linux-PAM configuration directory. Generally, if this directory
is present, the /etc/pam.conf file is ignored.
Typically errors generated by the Linux-PAM system of libraries, will
be written to syslog(3).
DCE-RFC 86.0, October 1995. Contains additional features, but remains
backwardly compatible with this RFC.
pam(3), pam_authenticate(3), pam_sm_setcred(3), pam_strerror(3), PAM(7)