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2. Terms and conventions

2.1. Definitions

The terminology surrounding PAM is rather confused. Neither Samar and Lai's original paper nor the XSSO specification made any attempt at formally defining terms for the various actors and entities involved in PAM, and the terms that they do use (but do not define) are sometimes misleading and ambiguous. The first attempt at establishing a consistent and unambiguous terminology was a whitepaper written by Andrew G. Morgan (author of Linux-PAM) in 1999. While Morgan's choice of terminology was a huge leap forward, it is in this author's opinion by no means perfect. What follows is an attempt, heavily inspired by Morgan, to define precise and unambiguous terms for all actors and entities involved in PAM.

account: The set of credentials the applicant is requesting from the arbitrator.
applicant: The user or entity requesting authentication.
arbitrator: The user or entity who has the privileges necessary to verify the applicant's credentials and the authority to grant or deny the request.
chain: A sequence of modules that will be invoked in response to a PAM request. The chain includes information about the order in which to invoke the modules, what arguments to pass to them, and how to interpret the results.
client: The application responsible for initiating an authentication request on behalf of the applicant and for obtaining the necessary authentication information from him.
facility: One of the four basic groups of functionality provided by PAM: authentication, account management, session management and authentication token update.
module: A collection of one or more related functions implementing a particular authentication facility, gathered into a single (normally dynamically loadable) binary file and identified by a single name.
policy: The complete set of configuration statements describing how to handle PAM requests for a particular service. A policy normally consists of four chains, one for each facility, though some services do not use all four facilities.
server: The application acting on behalf of the arbitrator to converse with the client, retrieve authentication information, verify the applicant's credentials and grant or deny requests.
service: A class of servers providing similar or related functionality and requiring similar authentication. PAM policies are defined on a per-service basis, so all servers that claim the same service name will be subject to the same policy.
session: The context within which service is rendered to the applicant by the server. One of PAM's four facilities, session management, is concerned exclusively with setting up and tearing down this context.
token: A chunk of information associated with the account, such as a password or passphrase, which the applicant must provide to prove his identity.
transaction: A sequence of requests from the same applicant to the same instance of the same server, beginning with authentication and session set-up and ending with session tear-down.

2.2. Usage examples

This section aims to illustrate the meanings of some of the terms defined above by way of a handful of simple examples.

2.2.1. Client and server are one

This simple example shows alice su(1)'ing to root.

% whoami
alice
% ls -l `which su`
-r-sr-xr-x  1 root  wheel  10744 Dec  6 19:06 /usr/bin/su
% su -
Password: xi3kiune
# whoami
root

The applicant is alice.
The account is root.
The su(1) process is both client and server.
The authentication token is xi3kiune.
The arbitrator is root, which is why su(1) is setuid root.

2.2.2. Client and server are separate

The example below shows eve try to initiate an ssh(1) connection to login.example.com, ask to log in as bob, and succeed. Bob should have chosen a better password!

% whoami
eve
% ssh bob@login.example.com
bob@login.example.com's password: god
Last login: Thu Oct 11 09:52:57 2001 from 192.168.0.1
Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994
    The Regents of the University of California.  All rights reserved.
FreeBSD 4.4-STABLE (LOGIN) #4: Tue Nov 27 18:10:34 PST 2001

Welcome to FreeBSD!
%

The applicant is eve.
The client is Eve's ssh(1) process.
The server is the sshd(8) process on login.example.com
The account is bob.
The authentication token is god.
Although this is not shown in this example, the arbitrator is root.

2.2.3. Sample policy

The following is FreeBSD's default policy for sshd:

sshd   auth        required    pam_nologin.so  no_warn
sshd    auth        required    pam_unix.so no_warn try_first_pass
sshd    account     required    pam_login_access.so
sshd    account     required    pam_unix.so
sshd    session     required    pam_lastlog.so  no_fail
sshd    password    required    pam_permit.so

This policy applies to the sshd service (which is not necessarily restricted to the sshd(8) server.)
auth, account, session and password are facilities.
pam_nologin.so, pam_unix.so, pam_login_access.so, pam_lastlog.so and pam_permit.so are modules. It is clear from this example that pam_unix.so provides at least two facilities (authentication and account management.)