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<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Chapter 3. Overview</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="Linux-PAM_SAG.html" title="The Linux-PAM System Administrators' Guide"><link rel="up" href="Linux-PAM_SAG.html" title="The Linux-PAM System Administrators' Guide"><link rel="prev" href="sag-text-conventions.html" title="Chapter 2. Some comments on the text"><link rel="next" href="sag-configuration.html" title="Chapter 4. The Linux-PAM configuration file"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 3. Overview</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="sag-text-conventions.html">Prev</a> </td><th width="60%" align="center"> </th><td width="20%" align="right"> <a accesskey="n" href="sag-configuration.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h1 class="title"><a name="sag-overview"></a>Chapter 3. Overview</h1></div></div></div><p>
For the uninitiated, we begin by considering an example. We take an
application that grants some service to users;
<span class="command"><strong>login</strong></span> is one such program.
<span class="command"><strong>Login</strong></span> does two things, it first establishes that
the requesting user is whom they claim to be and second provides
them with the requested service: in the case of
<span class="command"><strong>login</strong></span> the service is a command shell
(bash, tcsh, zsh, etc.) running with the identity of the user.
</p><p>
Traditionally, the former step is achieved by the
<span class="command"><strong>login</strong></span> application prompting the user for a
password and then verifying that it agrees with that located on
the system; hence verifying that as far as the system is concerned
the user is who they claim to be. This is the task that is delegated
to <span class="emphasis"><em>Linux-PAM</em></span>.
</p><p>
From the perspective of the application programmer (in this case
the person that wrote the <span class="command"><strong>login</strong></span> application),
<span class="emphasis"><em>Linux-PAM</em></span> takes care of this
authentication task -- verifying the identity of the user.
</p><p>
The flexibility of <span class="emphasis"><em>Linux-PAM</em></span> is
that <span class="emphasis"><em>you</em></span>, the system administrator, have
the freedom to stipulate which authentication scheme is to be
used. You have the freedom to set the scheme for any/all
PAM-aware applications on your Linux system. That is, you can
authenticate from anything as naive as
<span class="emphasis"><em>simple trust</em></span> (<span class="command"><strong>pam_permit</strong></span>)
to something as paranoid as a combination of a retinal scan, a
voice print and a one-time password!
</p><p>
To illustrate the flexibility you face, consider the following
situation: a system administrator (parent) wishes to improve the
mathematical ability of her users (children). She can configure
their favorite ``Shoot 'em up game'' (PAM-aware of course) to
authenticate them with a request for the product of a couple of
random numbers less than 12. It is clear that if the game is any
good they will soon learn their
<span class="emphasis"><em>multiplication tables</em></span>. As they mature, the
authentication can be upgraded to include (long) division!
</p><p>
<span class="emphasis"><em>Linux-PAM</em></span> deals with four
separate types of (management) task. These are:
<span class="emphasis"><em>authentication management</em></span>;
<span class="emphasis"><em>account management</em></span>;
<span class="emphasis"><em>session management</em></span>; and
<span class="emphasis"><em>password management</em></span>.
The association of the preferred management scheme with the behavior
of an application is made with entries in the relevant
<span class="emphasis"><em>Linux-PAM</em></span> configuration file.
The management functions are performed by <span class="emphasis"><em>modules</em></span>
specified in the configuration file. The syntax for this
file is discussed in the section
<a class="link" href="sag-configuration.html" title="Chapter 4. The Linux-PAM configuration file">below</a>.
</p><p>
Here is a figure that describes the overall organization of
<span class="emphasis"><em>Linux-PAM</em></span>:
</p><pre class="programlisting">
+----------------+
| application: X |
+----------------+ / +----------+ +================+
| authentication-[---->--\--] Linux- |--<--| PAM config file|
| + [----<--/--] PAM | |================|
|[conversation()][--+ \ | | | X auth .. a.so |
+----------------+ | / +-n--n-----+ | X auth .. b.so |
| | | __| | | _____/
| service user | A | | |____,-----'
| | | V A
+----------------+ +------|-----|---------+ -----+------+
+---u-----u----+ | | |
| auth.... |--[ a ]--[ b ]--[ c ]
+--------------+
| acct.... |--[ b ]--[ d ]
+--------------+
| password |--[ b ]--[ c ]
+--------------+
| session |--[ e ]--[ c ]
+--------------+
</pre><p>
By way of explanation, the left of the figure represents the
application; application X. Such an application interfaces with the
<span class="emphasis"><em>Linux-PAM</em></span> library and knows none of
the specifics of its configured authentication method. The
<span class="emphasis"><em>Linux-PAM</em></span> library (in the center)
consults the contents of the PAM configuration file and loads the
modules that are appropriate for application-X. These modules fall
into one of four management groups (lower-center) and are stacked in
the order they appear in the configuration file. These modules, when
called by <span class="emphasis"><em>Linux-PAM</em></span>, perform the
various authentication tasks for the application. Textual information,
required from/or offered to the user, can be exchanged through the
use of the application-supplied <span class="emphasis"><em>conversation</em></span>
function.
</p><p>
If a program is going to use PAM, then it has to have PAM
functions explicitly coded into the program. If you have
access to the source code you can add the appropriate PAM
functions. If you do not have access to the source code, and
the binary does not have the PAM functions included, then
it is not possible to use PAM.
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