Linux-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 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; pass-
word 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 - establish the user is who they claim to be. 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
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 passwordbased
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.
The configuration file(s) [Toc] [Back]
When a Linux-PAM aware privilege granting application is started, it
activates its attachment to the PAM-API. This activation performs a
number of tasks, the most important being the reading of the configuration
file(s): /etc/pam.conf. Alternatively, this may be the contents
of the /etc/pam.d/ directory.
These files list the PAMs that will do the authentication tasks
required by this service, and the appropriate behavior of the PAM-API
in the event that individual PAMs fail.
The syntax of the /etc/pam.conf configuration file is as follows. The
file is made up of a list of rules, each rule is typically placed on a
single line, but may be extended with an escaped end of line: `\<LF>'.
Comments are preceded with `#' marks and extend to the next end of
The format of each rule is a space separated collection of tokens, the
first three being case-insensitive:
service type control module-path module-arguments
The syntax of files contained in the /etc/pam.d/ directory, are identical
except for the absence of any service field. In this case, the ser-
vice is the name of the file in the /etc/pam.d/ directory. This filename
must be in lower case.
An important feature of Linux-PAM, is that a number of rules may be
stacked to combine the services of a number of PAMs for a given authentication
The service is typically the familiar name of the corresponding application:
login and su are good examples. The service-name, other, is
reserved for giving default rules. Only lines that mention the current
service (or in the absence of such, the other entries) will be associated
with the given service-application.
The type is the management group that the rule corresponds to. It is
used to specify which of the management groups the subsequent module is
to be associated with. Valid entries are: account; auth; password; and
session. The meaning of each of these tokens was explained above.
The third field, control, indicates the behavior of the PAM-API should
the module fail to succeed in its authentication task. Valid control
values are: requisite - failure of such a PAM results in the immediate
termination of the authentication process; required - failure of such a
PAM will ultimately lead to the PAM-API returning failure but only
after the remaining stacked modules (for this service and type) have
been invoked; sufficient - success of such a module is enough to satisfy
the authentication requirements of the stack of modules (if a
prior required module has failed the success of this one is ignored);
optional - the success or failure of this module is only important if
it is the only module in the stack associated with this service+type.
module-path - this is the full filename of the PAM to be used by the
module-arguments - these are a space separated list of tokens that can
be used to modify the specific behavior of the given PAM. Such arguments
will be documented for each individual module.
/etc/pam.conf - the configuration file
/etc/pam.d/ - the Linux-PAM configuration directory. If this directory
is present, the /etc/pam.conf file is ignored.
/usr/lib/libpam.so.X - the dynamic library
/usr/lib/security/*.so - the PAMs
Note, to conform to the Linux File-system standard, the libraries and
modules in your system may be located in /lib and /lib/security respectively.
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, currently under consideration by the DCERFC
The three Linux-PAM Guides, for System administrators, module develop-
ers, and application developers.
Linux-PAM 0.56 1997 Feb 9 PAM(7)
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