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NAME    [Toc]    [Back]

       Linux-PAM - Pluggable Authentication Modules for Linux

SYNOPSIS    [Toc]    [Back]


DESCRIPTION    [Toc]    [Back]

       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, 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.

FILES    [Toc]    [Back]

       /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.

ERRORS    [Toc]    [Back]

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

CONFORMING TO    [Toc]    [Back]

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

BUGS    [Toc]    [Back]

       None known.

SEE ALSO    [Toc]    [Back]

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

Linux-PAM 0.56			  1997 Feb 9				PAM(7)
[ Back ]
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