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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  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)


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