PAM(7) Linux-PAM Manual PAM(7)
NAME
PAM, pam - Pluggable Authentication Modules for Linux
DESCRIPTION
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 and preferably, the configuration can be
set by individual configuration files located in a pam.d directory. The
presence of this directory will cause Linux-PAM to ignore /etc/pam.conf.
Vendor-supplied PAM configuration files might be installed in the system
directory /usr/lib/pam.d/ or a configurable vendor specific directory
instead of the machine configuration directory /etc/pam.d/. If no
machine configuration file is found, the vendor-supplied file is used.
All files in /etc/pam.d/ override files with the same name in other
directories.
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
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
password.
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.
FILES
/etc/pam.conf
the configuration file
/etc/pam.d
the Linux-PAM configuration directory. Generally, if this directory
is present, the /etc/pam.conf file is ignored.
/usr/lib/pam.d
the Linux-PAM vendor configuration directory. Files in /etc/pam.d
override files with the same name in this directory.
ERRORS
Typically errors generated by the Linux-PAM system of libraries, will be
written to syslog(3).
CONFORMING TO
DCE-RFC 86.0, October 1995. Contains additional features, but remains
backwardly compatible with this RFC.
SEE ALSO
pam(3), pam_authenticate(3), pam_sm_setcred(3), pam_strerror(3), PAM(8)
Linux-PAM 06/29/2025 PAM(7)
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