PAM(7) Linux-PAM Manual PAM(7)
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 administrators' guide.
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 ignore /etc/pam.conf.
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
Simply put, these groups take care of different aspects of a typical user's request for a
account - provide account verification types of service: has the user's password expired?;
is this user permitted access to the requested service?
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 replacement
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
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
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)
Linux-PAM Manual 06/04/2011 PAM(7)