Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
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  administra-
       tors' 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 privi-
       lege 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 appli-
       cations 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  man-
       ual  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 appli-
       cations (services) and the pluggable authentication modules (PAMs) that
       perform the actual authentication tasks.

       PAM separates the tasks of authentication into four independent manage-
       ment 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  ser-

       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  authenti-
       cation  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  authen-
       tication  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  pass-

       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 direc-
       tory. 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  acti-
       vates 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 identi-
       cal 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 file-
       name 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  appli-
       cation:  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 associ-
       ated 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 sat-
       isfy 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 argu-
       ments 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 develop-
       ers, and application developers.

PAM 0.56                          1997 Feb 9                            PAM(8)


Want to link to this manual page? Use this URL:

home | help