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PAM(8)                            PAM Manual                            PAM(8)

       PAM - Pluggable Authentication Modules


       This manual is intended to offer a quick introduction to PAM.  For more
       information the reader is directed to the Linux-PAM system
       administrators' guide.

       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 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 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 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.

       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 -  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 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.

The configuration file(s)
       When a 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 service is the name of the file in the /etc/pam.d/ directory. This
       filename must be in lower case.

       An important feature of 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 PAM configuration directory. If this directory is
       present, the /etc/pam.conf file is ignored.
       /usr/lib/ - the dynamic library
       /usr/lib/pam_*.so - the PAMs

       Typically errors generated by the PAM system of libraries, will be
       written to syslog(3).

       DCE-RFC 86.0, October 1995.
       Contains additional features, currently under consideration by the DCE-
       RFC committee.

       None known.

       The three Linux-PAM Guides, for System administrators,  module
       developers,  and application developers.

PAM 0.56                          1997 Feb 9                            PAM(8)


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