PAM

Debian Packages

libpam-runtime
libpam0g
libpam-doc

References

PAM web site: http://www.kernel.org/pub/linux/libs/pam/
System Administrator's Guide: http://www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/Linux-PAM_SAG.html
Local documentation 1: /usr/share/doc/libpam-doc/Linux-PAM_SAG.txt.gz
Local documentation 2: each PAM has its own man page (e.g. man pam_unix)
Wikipedia: http://en.wikipedia.org/wiki/Pluggable_Authentication_Modules

Glossary

PAM: Pluggable Authentication Modules. The name of the library/framework. Can also be used to refer to the modules themselves ("the PAM" (singular), or "the PAMs" (plural))
Conversation function: A callback function that clients must supply PAM with; PAM modules can use the conversation function to acquire information from the client.
Authentication token: Typically a password.

Overview: What is PAM

PAM is a framework that applications (services) can use to authenticate users and perform other authentication-related tasks. PAM decouples applications (services) from the concrete method of authentication that the application (service) should use.

A rough overview of how an authentication session might look when PAM is involved:

the user provides his credentials (usually a user name and a password) to some piece of application software
the application uses the API of the PAM system library to delegate checking whether the credentials are valid
the application identifies itself to PAM with a "service name"
PAM maps the service name to a configuration file in /etc/pam.d; for instance, a service named "imap" requires /etc/pam.d/imap to be present
PAM evaluates the configuration file to determine which authentication mechanism it should use to perform the check
the result of the check is reported back to the application

PAM service types

The PAM library provides different service types to its clients, each type deals with a separate type of management task. Some modules (e.g. pam_unix) can be used for all service types, while others provide only one or two service types (e.g. pam_env).

auth: This service type provides authentication management. A module that provides this service type provides two aspects of authenticating the user: Firstly, it establishes that the user is who they claim to be, by instructing the application to prompt the user for a password or other means of identification. Secondly, the module can grant group membership or other privileges through its credential granting properties.

account: This service type provides non-authentication based account management. A module that provides this service type is typically used to restrict/permit access to a service based on the time of day, currently available system resources (maximum number of users) or perhaps the location of the applicant user -- 'root' login only on the console.

password: This service type provides password management. A module that provides this service type is required for updating the authentication token (e.g. a password) associated with the user. Typically, there is one module for each 'challenge/response' based authentication (auth) type.

session: This service type provides session management. A module that provides this service type is associated with doing things that need to be done for the user before/after they can be given service. Such things include the logging of information concerning the opening/closing of some data exchange with a user, mounting directories, etc.

The configuration file

Overview

A service's PAM configuration file lists the modules that will do the authentication tasks required by the service, and the appropriate behavior of the PAM-API in the event that individual modules fail.

The filename of a PAM configuration file must be in lower case. It represents the name of a service, usually a familiar application name such as login, sshd, samba or imap. A configuration file with the service name other is reserved for providing defaults.

Each line in the configuration file is a rule. A number of rules may be stacked to combine the services of a number of PAMs for a given authentication task. The format of each rule is this:

<service-type> <control> <module-path> <module-arguments>

`service-type` field

The service-type field refers to the service type (see chapter above) that the rule corresponds to. Possible values are:

account
auth
password
session

`control` field

The control field 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 - which is not further explored on this page - involves a square-bracketed selection of value=action pairs. Possible keywords for control are:

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
requisite : like required, however, in the case that such a module returns a failure, control is directly returned to the application. The return value is that associated with the first required or requisite module to fail. Note, this flag can be used to protect against the possibility of a user getting the opportunity to enter a password over an unsafe medium. It is conceivable that such behavior might inform an attacker of valid accounts on a system. This possibility should be weighed against the not insignificant concerns of exposing a sensitive password in a hostile environment.
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). A failure of this module is not deemed as fatal to satisfying the application that this type has succeeded. If the module succeeds the PAM framework returns success to the application immediately without trying any other modules.
optional : the success or failure of this module is only important if it is the only module in the stack associated with this type.
include : include all lines of given type from the configuration file specified as an argument to this control.
substack : include all lines of given type from the configuration file specified as an argument to this control. This differs from include in that evaluation of the done and die actions in a substack does not cause skipping the rest of the complete module stack, but only of the substack. Jumps in a substack also can not make evaluation jump out of it, and the whole substack is counted as one module when the jump is done in a parent stack. The reset action will reset the state of a module stack to the state it was in as of beginning of the substack evaluation.

`module-path` field

module-path is either the full filename of the PAM (it begins with a '/'), or a relative pathname from the default module location (/lib/security on my Debian machine)

`module-arguments` field

module-arguments are a space separated list of tokens that can be used to modify the specific behavior of the given PAM. Such arguments are documented for each individual module. To include spaces in an argument it must be surrounded with square brackets.

Include files

Configuration files may include other files. For instance, /etc/pam.d/other has the following content:

@include common-auth
@include common-account
@include common-password
@include common-session

A module overview

Module	Services provided	Description	Links
pam_access.so	all	logdaemon style login access control based on login names, host or domain names, internet addresses or network numbers	kernel.org
pam_cracklib.so	all	Checks the password against dictionary words	kernel.org
pam_debug.so	all	Debug the PAM stack	kernel.org
pam_deny.so	all	Locking-out PAM module	kernel.org
pam_echo.so	all	Print text messages	kernel.org
pam_env.so	auth, session	Set/unset environment variables	kernel.org
pam_exec.so	all	Call an external command	kernel.org
pam_faildelay.so	auth	Change the delay on failure per-application	kernel.org
pam_filter.so	all	Filter module	kernel.org
pam_ftp.so	auth	Module for anonymous access	kernel.org
pam_group.so	auth	Module to modify group access	kernel.org
pam_issue.so	auth	Add issue file to user prompt	kernel.org
pam_keyinit.so	session	Display the keyinit file	kernel.org
pam_lastlog.so	session	Display date of last login	kernel.org
pam_limits.so	session	Limit resources	kernel.org
pam_listfile.so	all	Deny or allow services based on an arbitrary file	kernel.org
pam_localuser.so	all	Require users to be listed in /etc/passwd	kernel.org
pam_loginuid.so	session	Record user's login uid to the process attribute	kernel.org
pam_mail.so	auth, account	Inform about available mail	kernel.org
pam_mkhomedir.so	session	Create users home directory	kernel.org
pam_motd.so	session	Display the motd file	kernel.org
pam_namespace.so	session	Setup a private namespace	kernel.org
pam_nologin.so	auth, acct	Prevent non-root users from login	kernel.org
pam_permit.so	all	The promiscuous module	kernel.org
pam_rhosts.so	auth	Grant access using .rhosts file	kernel.org
pam_rootok.so	auth	Gain only root access	kernel.org
pam_securetty.so	auth	Limit root login to special devices	kernel.org
pam_selinux.so	session	Set the default security context	kernel.org
pam_shells.so	auth, account	Check for valid login shell	kernel.org
pam_succeed_if.so	all	Test account characteristics	kernel.org
pam_tally.so	auth, account	Login counter (tallying) module	kernel.org
pam_time.so	account	Time controled access	kernel.org
pam_umask.so	session	Set the file mode creation mask	kernel.org
pam_unix.so	all	Traditional password authentication	kernel.org
pam_userdb.so	auth, account	Authenticate against a db database	kernel.org
pam_warn.so	all	Logs all PAM items	kernel.org
pam_wheel.so	auth, account	Only permit root access to members of group wheel	kernel.org
pam_xauth.so	session	Forward `xauth` keys between users	kernel.org
pam_ldap.so	all	Authenticate against LDAP directories, and to change their passwords in the directory	arthurdejong.org

Notes on specific module arguments

Overview

In the following sections, specific module arguments are discussed. The semantics of each argument is usually determined by quoting from the documentation of the pam_unix module. Hopefully, other modules retain these semantics.

Note: Usually PAMs have their own man page.