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PAM(8)                         Linux-PAM Manual                         PAM(8)

       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
       administrators' guide.

       Linux-PAM Is a system of libraries that handle the authentication tasks
       of applications (services) on the system.  The library provides a  sta-
       ble  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 appli-
       cations 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  man-
       ual  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  mod-
       ules (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 abbrevia-
       tions 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

       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 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 configura-
       tion  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 Linux-PAM, is that a number  of  rules  may  be
       stacked to combine the services of a number of PAMs for a given authen-
       tication task.

       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.  There  are  two
       types  of  syntax  for  this control field: the simple one has a single
       simple keyword; the more complicated one  involves  a  square-bracketed
       selection of value=action pairs.

       For  the simple (historical) syntax 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 ulti-
       mately lead to the PAM-API returning failure but only after the remain-
       ing stacked modules (for this service and type) have been invoked; suf-
       ficient - success of such a module is enough to satisfy the authentica-
       tion  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.

       For the more complicated syntax valid control values have the following


       Where  valueN  corresponds to the return code from the function invoked
       in the module for which the line is defined. It is selected from one of
       these: success; open_err; symbol_err; service_err; system_err; buf_err;
       perm_denied;     auth_err;     cred_insufficient;     authinfo_unavail;
       user_unknown;  maxtries;  new_authtok_reqd;  acct_expired; session_err;
       cred_unavail; cred_expired; cred_err; no_module_data;  conv_err;  auth-
       tok_err; authtok_recover_err; authtok_lock_busy; authtok_disable_aging;
       try_again; ignore; abort;  authtok_expired;  module_unknown;  bad_item;
       and  default.   The  last  of these, default, implies 'all valueN's not
       mentioned explicitly. Note, the full list of PAM errors is available in
       /usr/include/security/_pam_types.h  .  The  actionN can be: an unsigned
       integer, J, signifying an action of 'jump over the next  J  modules  in
       the stack'; or take one of the following forms:
       ignore  - when used with a stack of modules, the module's return status
       will not contribute to the return code the application obtains;
       bad - this action indicates that the return code should be  thought  of
       as indicative of the module failing. If this module is the first in the
       stack to fail, its status value will be used  for  that  of  the  whole
       die  - equivalent to bad with the side effect of terminating the module
       stack and PAM immediately returning to the application.
       ok - this tells PAM that the  administrator  thinks  this  return  code
       should contribute directly to the return code of the full stack of mod-
       ules. In other words, if the former state of the stack would lead to  a
       return  of  PAM_SUCCESS,  the  module's  return code will override this
       value. Note, if the former state of the stack holds some value that  is
       indicative  of  a  modules failure, this 'ok' value will not be used to
       override that value.
       done - equivalent to ok with the side effect of terminating the  module
       stack and PAM immediately returning to the application.
       reset  -  clear  all  memory of the state of the module stack and start
       again with the next stacked module.

       module-path - this is either the full filename of the PAM to be used by
       the application (it begins with a '/'), or a relative pathname from the
       default module location: /lib/security/.

       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 Linux-PAM configuration directory. Generally, if this
       directory is present, the /etc/pam.conf file is ignored.
       /lib/libpam.so.X - the dynamic library
       /lib/security/*.so - the PAMs

       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, but remains  backwardly  compatible  with
       this RFC.

       None known.

       The  three Linux-PAM Guides, for system administrators, module develop-
       ers, and application developers.

Linux-PAM 0.74                    2001 Jan 20                           PAM(8)