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<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE article PUBLIC "-//FreeBSD//DTD DocBook XML V5.0-Based Extension//EN"
"http://www.FreeBSD.org/XML/share/xml/freebsd50.dtd">
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<article xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
<info><title>Pluggable Authentication Modules</title>
<abstract>
<para>This article describes the underlying principles and
mechanisms of the Pluggable Authentication Modules (PAM)
library, and explains how to configure PAM, how to integrate
PAM into applications, and how to write PAM modules.</para>
</abstract>
<copyright>
<year>2001</year>
<year>2002</year>
<year>2003</year>
<holder>Networks Associates Technology, Inc.</holder>
</copyright>
<authorgroup>
<author><personname><firstname>Dag-Erling</firstname><surname>Sm&oslash;rgrav</surname></personname><contrib>Contributed by </contrib></author>
</authorgroup>
<legalnotice xml:id="pam-legalnotice">
<para>This article was written for the FreeBSD Project by
ThinkSec AS and Network Associates Laboratories, the Security
Research Division of Network Associates, Inc. under
DARPA/SPAWAR contract N66001-01-C-8035 (<quote>CBOSS</quote>),
as part of the DARPA CHATS research program.</para>
</legalnotice>
<legalnotice xml:id="trademarks" role="trademarks">
&tm-attrib.freebsd;
&tm-attrib.linux;
&tm-attrib.opengroup;
&tm-attrib.sun;
&tm-attrib.general;
</legalnotice>
<releaseinfo>$FreeBSD$</releaseinfo>
</info>
<section xml:id="pam-intro">
<title xml:id="pam-intro.title">Introduction</title>
<para>The Pluggable Authentication Modules (PAM) library is a
generalized API for authentication-related services which allows
a system administrator to add new authentication methods simply
by installing new PAM modules, and to modify authentication
policies by editing configuration files.</para>
<para>PAM was defined and developed in 1995 by Vipin Samar and
Charlie Lai of Sun Microsystems, and has not changed much since.
In 1997, the Open Group published the X/Open Single Sign-on
(XSSO) preliminary specification, which standardized the PAM API
and added extensions for single (or rather integrated) sign-on.
At the time of this writing, this specification has not yet been
adopted as a standard.</para>
<para>Although this article focuses primarily on FreeBSD 5.x,
which uses OpenPAM, it should be equally applicable to FreeBSD
4.x, which uses Linux-PAM, and other operating systems such as
Linux and &solaris;.</para>
</section>
<section xml:id="pam-terms">
<title xml:id="pam-terms.title">Terms and conventions</title>
<section xml:id="pam-definitions">
<title xml:id="pam-definitions.title">Definitions</title>
<para>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.</para>
<glosslist>
<glossentry>
<glossterm>account</glossterm>
<glossdef>
<para>The set of credentials the applicant is requesting
from the arbitrator.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>applicant</glossterm>
<glossdef>
<para>The user or entity requesting authentication.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>arbitrator</glossterm>
<glossdef>
<para>The user or entity who has the privileges necessary
to verify the applicant's credentials and the authority
to grant or deny the request.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>chain</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>client</glossterm>
<glossdef>
<para>The application responsible for initiating an
authentication request on behalf of the applicant and
for obtaining the necessary authentication information
from him.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>facility</glossterm>
<glossdef>
<para>One of the four basic groups of functionality
provided by PAM: authentication, account management,
session management and authentication token
update.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>module</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>policy</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>server</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>service</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>session</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>token</glossterm>
<glossdef>
<para>A chunk of information associated with the account,
such as a password or passphrase, which the applicant
must provide to prove his identity.</para>
</glossdef>
</glossentry>
<glossentry>
<glossterm>transaction</glossterm>
<glossdef>
<para>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.</para>
</glossdef>
</glossentry>
</glosslist>
</section>
<section xml:id="pam-usage-examples">
<title xml:id="pam-usage-examples.title">Usage examples</title>
<para>This section aims to illustrate the meanings of some of
the terms defined above by way of a handful of simple
examples.</para>
<section>
<title>Client and server are one</title>
<para>This simple example shows <literal>alice</literal>
&man.su.1;'ing to <literal>root</literal>.</para>
<screen>&prompt.user; <userinput>whoami</userinput>
alice
&prompt.user; <userinput>ls -l `which su`</userinput>
-r-sr-xr-x 1 root wheel 10744 Dec 6 19:06 /usr/bin/su
&prompt.user; <userinput>su -</userinput>
Password: <userinput>xi3kiune</userinput>
&prompt.root; whoami
root
</screen>
<itemizedlist>
<listitem>
<para>The applicant is <literal>alice</literal>.</para>
</listitem>
<listitem>
<para>The account is <literal>root</literal>.</para>
</listitem>
<listitem>
<para>The &man.su.1; process is both client and
server.</para>
</listitem>
<listitem>
<para>The authentication token is
<literal>xi3kiune</literal>.</para>
</listitem>
<listitem>
<para>The arbitrator is <literal>root</literal>, which is
why &man.su.1; is setuid <literal>root</literal>.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>Client and server are separate</title>
<para>The example below shows <literal>eve</literal> try to
initiate an &man.ssh.1; connection to
<literal>login.example.com</literal>, ask to log in as
<literal>bob</literal>, and succeed. Bob should have chosen
a better password!</para>
<screen>&prompt.user; <userinput>whoami</userinput>
eve
&prompt.user; <userinput>ssh bob@login.example.com</userinput>
bob@login.example.com's password: <userinput>god</userinput>
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!
&prompt.user;</screen>
<itemizedlist>
<listitem>
<para>The applicant is <literal>eve</literal>.</para>
</listitem>
<listitem>
<para>The client is Eve's &man.ssh.1; process.</para>
</listitem>
<listitem>
<para>The server is the &man.sshd.8; process on
<literal>login.example.com</literal></para>
</listitem>
<listitem>
<para>The account is <literal>bob</literal>.</para>
</listitem>
<listitem>
<para>The authentication token is
<literal>god</literal>.</para>
</listitem>
<listitem>
<para>Although this is not shown in this example, the
arbitrator is <literal>root</literal>.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>Sample policy</title>
<para>The following is FreeBSD's default policy for
<literal>sshd</literal>:</para>
<programlisting>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</programlisting>
<itemizedlist>
<listitem>
<para>This policy applies to the <literal>sshd</literal>
service (which is not necessarily restricted to the
&man.sshd.8; server.)</para>
</listitem>
<listitem>
<para><literal>auth</literal>, <literal>account</literal>,
<literal>session</literal> and
<literal>password</literal> are facilities.</para>
</listitem>
<listitem>
<para><filename>pam_nologin.so</filename>,
<filename>pam_unix.so</filename>,
<filename>pam_login_access.so</filename>,
<filename>pam_lastlog.so</filename> and
<filename>pam_permit.so</filename> are modules. It is
clear from this example that
<filename>pam_unix.so</filename> provides at least two
facilities (authentication and account
management.)</para>
</listitem>
</itemizedlist>
</section>
</section>
<!--
<section id="pam-conventions">
<title id="pam-conventions.title">Conventions</title>
<para>This section has not yet been written.</para>
</section>
-->
</section>
<section xml:id="pam-essentials">
<title xml:id="pam-essentials.title">PAM Essentials</title>
<section xml:id="pam-facilities-primitives">
<title xml:id="pam-facilities-primitives.title">Facilities and
primitives</title>
<para>The PAM API offers six different authentication primitives
grouped in four facilities, which are described below.</para>
<variablelist>
<varlistentry>
<term><literal>auth</literal></term>
<listitem>
<para><emphasis>Authentication.</emphasis> This facility
concerns itself with authenticating the applicant and
establishing the account credentials. It provides two
primitives:</para>
<itemizedlist>
<listitem>
<para>&man.pam.authenticate.3; authenticates the
applicant, usually by requesting an authentication
token and comparing it with a value stored in a
database or obtained from an authentication
server.</para>
</listitem>
<listitem>
<para>&man.pam.setcred.3; establishes account
credentials such as user ID, group membership and
resource limits.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>account</literal></term>
<listitem>
<para><emphasis>Account management.</emphasis> This
facility handles non-authentication-related issues of
account availability, such as access restrictions based
on the time of day or the server's work load. It
provides a single primitive:</para>
<itemizedlist>
<listitem>
<para>&man.pam.acct.mgmt.3; verifies that the
requested account is available.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>session</literal></term>
<listitem>
<para><emphasis>Session management.</emphasis> This
facility handles tasks associated with session set-up
and tear-down, such as login accounting. It provides
two primitives:</para>
<itemizedlist>
<listitem>
<para>&man.pam.open.session.3; performs tasks
associated with session set-up: add an entry in the
<filename>utmp</filename> and
<filename>wtmp</filename> databases, start an SSH
agent, etc.</para>
</listitem>
<listitem>
<para>&man.pam.close.session.3; performs tasks
associated with session tear-down: add an entry in
the <filename>utmp</filename> and
<filename>wtmp</filename> databases, stop the SSH
agent, etc.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>password</literal></term>
<listitem>
<para><emphasis>Password management.</emphasis> This
facility is used to change the authentication token
associated with an account, either because it has
expired or because the user wishes to change it. It
provides a single primitive:</para>
<itemizedlist>
<listitem>
<para>&man.pam.chauthtok.3; changes the authentication
token, optionally verifying that it is sufficiently
hard to guess, has not been used previously,
etc.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="pam-modules">
<title xml:id="pam-modules.title">Modules</title>
<para>Modules are a very central concept in PAM; after all,
they are the <quote>M</quote> in <quote>PAM</quote>. A PAM
module is a self-contained piece of program code that
implements the primitives in one or more facilities for one
particular mechanism; possible mechanisms for the
authentication facility, for instance, include the &unix;
password database, NIS, LDAP and Radius.</para>
<section xml:id="pam-module-naming">
<title xml:id="pam-module-naming.title">Module Naming</title>
<para>FreeBSD implements each mechanism in a single module,
named
<literal>pam_<replaceable>mechanism</replaceable>.so</literal>
(for instance, <literal>pam_unix.so</literal> for the &unix;
mechanism.) Other implementations sometimes have separate
modules for separate facilities, and include the facility
name as well as the mechanism name in the module name. To
name one example, &solaris; has a
<literal>pam_dial_auth.so.1</literal> module which is
commonly used to authenticate dialup users.</para>
</section>
<section xml:id="pam-module-versioning">
<title xml:id="pam-module-versioning.title">Module Versioning</title>
<para>FreeBSD's original PAM implementation, based on
Linux-PAM, did not use version numbers for PAM modules.
This would commonly cause problems with legacy applications,
which might be linked against older versions of the system
libraries, as there was no way to load a matching version of
the required modules.</para>
<para>OpenPAM, on the other hand, looks for modules that have
the same version number as the PAM library (currently 2),
and only falls back to an unversioned module if no versioned
module could be loaded. Thus legacy modules can be provided
for legacy applications, while allowing new (or newly built)
applications to take advantage of the most recent
modules.</para>
<para>Although &solaris; PAM modules commonly have a version
number, they are not truly versioned, because the number is a
part of the module name and must be included in the
configuration.</para>
</section>
</section>
<section xml:id="pam-chains-policies">
<title xml:id="pam-chains-policies.title">Chains and
policies</title>
<para>When a server initiates a PAM transaction, the PAM library
tries to load a policy for the service specified in the
&man.pam.start.3; call. The policy specifies how
authentication requests should be processed, and is defined in
a configuration file. This is the other central concept in
PAM: the possibility for the admin to tune the system security
policy (in the wider sense of the word) simply by editing a
text file.</para>
<para>A policy consists of four chains, one for each of the four
PAM facilities. Each chain is a sequence of configuration
statements, each specifying a module to invoke, some
(optional) parameters to pass to the module, and a control
flag that describes how to interpret the return code from the
module.</para>
<para>Understanding the control flags is essential to
understanding PAM configuration files. There are four
different control flags:</para>
<variablelist>
<varlistentry>
<term><literal>binding</literal></term>
<listitem>
<para>If the module succeeds and no earlier module in the
chain has failed, the chain is immediately terminated
and the request is granted. If the module fails, the
rest of the chain is executed, but the request is
ultimately denied.</para>
<para>This control flag was introduced by Sun in &solaris; 9
(&sunos; 5.9), and is also supported by OpenPAM.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>required</literal></term>
<listitem>
<para>If the module succeeds, the rest of the chain is
executed, and the request is granted unless some other
module fails. If the module fails, the rest of the
chain is also executed, but the request is ultimately
denied.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>requisite</literal></term>
<listitem>
<para>If the module succeeds, the rest of the chain is
executed, and the request is granted unless some other
module fails. If the module fails, the chain is
immediately terminated and the request is denied.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>sufficient</literal></term>
<listitem>
<para>If the module succeeds and no earlier module in the
chain has failed, the chain is immediately terminated
and the request is granted. If the module fails, the
module is ignored and the rest of the chain is
executed.</para>
<para>As the semantics of this flag may be somewhat
confusing, especially when it is used for the last
module in a chain, it is recommended that the
<literal>binding</literal> control flag be used instead
if the implementation supports it.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>optional</literal></term>
<listitem>
<para>The module is executed, but its result is ignored.
If all modules in a chain are marked
<literal>optional</literal>, all requests will always be
granted.</para>
</listitem>
</varlistentry>
</variablelist>
<para>When a server invokes one of the six PAM primitives, PAM
retrieves the chain for the facility the primitive belongs to,
and invokes each of the modules listed in the chain, in the
order they are listed, until it reaches the end, or determines
that no further processing is necessary (either because a
<literal>binding</literal> or
<literal>sufficient</literal> module succeeded, or because a
<literal>requisite</literal> module failed.) The request is
granted if and only if at least one module was invoked, and
all non-optional modules succeeded.</para>
<para>Note that it is possible, though not very common, to have
the same module listed several times in the same chain. For
instance, a module that looks up user names and passwords in a
directory server could be invoked multiple times with
different parameters specifying different directory servers to
contact. PAM treat different occurrences of the same module
in the same chain as different, unrelated modules.</para>
</section>
<section xml:id="pam-transactions">
<title xml:id="pam-transactions.title">Transactions</title>
<para>The lifecycle of a typical PAM transaction is described
below. Note that if any of these steps fails, the server
should report a suitable error message to the client and abort
the transaction.</para>
<orderedlist>
<listitem>
<para>If necessary, the server obtains arbitrator
credentials through a mechanism independent of
PAM&mdash;most commonly by virtue of having been started
by <literal>root</literal>, or of being setuid
<literal>root</literal>.</para>
</listitem>
<listitem>
<para>The server calls &man.pam.start.3; to initialize the
PAM library and specify its service name and the target
account, and register a suitable conversation
function.</para>
</listitem>
<listitem>
<para>The server obtains various information relating to the
transaction (such as the applicant's user name and the
name of the host the client runs on) and submits it to PAM
using &man.pam.set.item.3;.</para>
</listitem>
<listitem>
<para>The server calls &man.pam.authenticate.3; to
authenticate the applicant.</para>
</listitem>
<listitem>
<para>The server calls &man.pam.acct.mgmt.3; to verify that the
requested account is available and valid. If the password
is correct but has expired, &man.pam.acct.mgmt.3; will
return <literal>PAM_NEW_AUTHTOK_REQD</literal> instead of
<literal>PAM_SUCCESS</literal>.</para>
</listitem>
<listitem>
<para>If the previous step returned
<literal>PAM_NEW_AUTHTOK_REQD</literal>, the server now
calls &man.pam.chauthtok.3; to force the client to change
the authentication token for the requested account.</para>
</listitem>
<listitem>
<para>Now that the applicant has been properly
authenticated, the server calls &man.pam.setcred.3; to
establish the credentials of the requested account. It is
able to do this because it acts on behalf of the
arbitrator, and holds the arbitrator's credentials.</para>
</listitem>
<listitem>
<para>Once the correct credentials have been established,
the server calls &man.pam.open.session.3; to set up the
session.</para>
</listitem>
<listitem>
<para>The server now performs whatever service the client
requested&mdash;for instance, provide the applicant with a
shell.</para>
</listitem>
<listitem>
<para>Once the server is done serving the client, it calls
&man.pam.close.session.3; to tear down the session.</para>
</listitem>
<listitem>
<para>Finally, the server calls &man.pam.end.3; to notify
the PAM library that it is done and that it can release
whatever resources it has allocated in the course of the
transaction.</para>
</listitem>
</orderedlist>
</section>
</section>
<section xml:id="pam-config">
<title xml:id="pam-config.title">PAM Configuration</title>
<section xml:id="pam-config-file">
<title xml:id="pam-config-file.title">PAM policy files</title>
<section xml:id="pam-config-pam.conf">
<title xml:id="pam-config-pam.conf.title">The
<filename>/etc/pam.conf</filename> file</title>
<para>The traditional PAM policy file is
<filename>/etc/pam.conf</filename>. This file contains all
the PAM policies for your system. Each line of the file
describes one step in a chain, as shown below:</para>
<programlisting>login auth required pam_nologin.so no_warn</programlisting>
<para>The fields are, in order: service name, facility name,
control flag, module name, and module arguments. Any
additional fields are interpreted as additional module
arguments.</para>
<para>A separate chain is constructed for each service /
facility pair, so while the order in which lines for the
same service and facility appear is significant, the order
in which the individual services and facilities are listed
is not. The examples in the original PAM paper grouped
configuration lines by facility, and the &solaris; stock
<filename>pam.conf</filename> still does that, but FreeBSD's
stock configuration groups configuration lines by service.
Either way is fine; either way makes equal sense.</para>
</section>
<section xml:id="pam-config-pam.d">
<title xml:id="pam-config-pam.d.title">The
<filename>/etc/pam.d</filename> directory</title>
<para>OpenPAM and Linux-PAM support an alternate configuration
mechanism, which is the preferred mechanism in FreeBSD. In
this scheme, each policy is contained in a separate file
bearing the name of the service it applies to. These files
are stored in <filename>/etc/pam.d/</filename>.</para>
<para>These per-service policy files have only four fields
instead of <filename>pam.conf</filename>'s five: the service
name field is omitted. Thus, instead of the sample
<filename>pam.conf</filename> line from the previous
section, one would have the following line in
<filename>/etc/pam.d/login</filename>:</para>
<programlisting>auth required pam_nologin.so no_warn</programlisting>
<para>As a consequence of this simplified syntax, it is
possible to use the same policy for multiple services by
linking each service name to a same policy file. For
instance, to use the same policy for the
<literal>su</literal> and <literal>sudo</literal> services,
one could do as follows:</para>
<screen>&prompt.root; <userinput>cd /etc/pam.d</userinput>
&prompt.root; <userinput>ln -s su sudo</userinput></screen>
<para>This works because the service name is determined from
the file name rather than specified in the policy file, so
the same file can be used for multiple differently-named
services.</para>
<para>Since each service's policy is stored in a separate
file, the <filename>pam.d</filename> mechanism also makes it
very easy to install additional policies for third-party
software packages.</para>
</section>
<section xml:id="pam-config-file-order">
<title xml:id="pam-config-file-order.title">The policy search
order</title>
<para>As we have seen above, PAM policies can be found in a
number of places. What happens if policies for the same
service exist in multiple places?</para>
<para>It is essential to understand that PAM's configuration
system is centered on chains.<!-- XXX --></para>
</section>
</section>
<section xml:id="pam-config-breakdown">
<title xml:id="pam-config-breakdown.title">Breakdown of a
configuration line</title>
<para>As explained in <xref linkend="pam-config-file"/>, each line in
<filename>/etc/pam.conf</filename> consists of four or more
fields: the service name, the facility name, the control flag,
the module name, and zero or more module arguments.</para>
<para>The service name is generally (though not always) the name
of the application the statement applies to. If you are
unsure, refer to the individual application's documentation to
determine what service name it uses.</para>
<para>Note that if you use <filename>/etc/pam.d/</filename>
instead of <filename>/etc/pam.conf</filename>, the service
name is specified by the name of the policy file, and omitted
from the actual configuration lines, which then start with the
facility name.</para>
<para>The facility is one of the four facility keywords
described in <xref linkend="pam-facilities-primitives"/>.</para>
<para>Likewise, the control flag is one of the four keywords
described in <xref linkend="pam-chains-policies"/>,
describing how to interpret the return code from the module.
Linux-PAM supports an alternate syntax that lets you specify
the action to associate with each possible return code, but
this should be avoided as it is non-standard and closely tied
in with the way Linux-PAM dispatches service calls (which
differs greatly from the way &solaris; and OpenPAM do it.)
Unsurprisingly, OpenPAM does not support this syntax.</para>
</section>
<section xml:id="pam-policies">
<title xml:id="pam-policies.title">Policies</title>
<para>To configure PAM correctly, it is essential to understand
how policies are interpreted.</para>
<para>When an application calls &man.pam.start.3;, the PAM
library loads the policy for the specified service and
constructs four module chains (one for each facility.) If one
or more of these chains are empty, the corresponding chains
from the policy for the <literal>other</literal> service are
substituted.</para>
<para>When the application later calls one of the six PAM
primitives, the PAM library retrieves the chain for the
corresponding facility and calls the appropriate service
function in each module listed in the chain, in the order in
which they were listed in the configuration. After each call
to a service function, the module type and the error code
returned by the service function are used to determine what
happens next. With a few exceptions, which we discuss below,
the following table applies:</para>
<table>
<title>PAM chain execution summary</title>
<tgroup cols="4">
<colspec colwidth="1*" colname="type"/>
<colspec colwidth="1*" colname="success"/>
<colspec colwidth="1*" colname="ignore"/>
<colspec colwidth="1*" colname="other"/>
<thead>
<row>
<entry colname="type"/>
<entry colname="success"><literal>PAM_SUCCESS</literal></entry>
<entry colname="ignore"><literal>PAM_IGNORE</literal></entry>
<entry colname="other"><literal>other</literal></entry>
</row>
</thead>
<tbody>
<row>
<entry colname="type">binding</entry>
<entry colname="success">if (!fail) break;</entry>
<entry colname="ignore">-</entry>
<entry colname="other">fail = true;</entry>
</row>
<row>
<entry colname="type">required</entry>
<entry colname="success">-</entry>
<entry colname="ignore">-</entry>
<entry colname="other">fail = true;</entry>
</row>
<row>
<entry colname="type">requisite</entry>
<entry colname="success">-</entry>
<entry colname="ignore">-</entry>
<entry colname="other">fail = true; break;</entry>
</row>
<row>
<entry colname="type">sufficient</entry>
<entry colname="success">if (!fail) break;</entry>
<entry colname="ignore">-</entry>
<entry colname="other">-</entry>
</row>
<row>
<entry colname="type">optional</entry>
<entry colname="success">-</entry>
<entry colname="ignore">-</entry>
<entry colname="other">-</entry>
</row>
</tbody>
</tgroup>
</table>
<para>If <varname>fail</varname> is true at the end of a chain,
or when a <quote>break</quote> is reached, the dispatcher
returns the error code returned by the first module that
failed. Otherwise, it returns
<literal>PAM_SUCCESS</literal>.</para>
<para>The first exception of note is that the error code
<literal>PAM_NEW_AUTHTOK_REQD</literal> is treated like a
success, except that if no module failed, and at least one
module returned <literal>PAM_NEW_AUTHTOK_REQD</literal>, the
dispatcher will return
<literal>PAM_NEW_AUTHTOK_REQD</literal>.</para>
<para>The second exception is that &man.pam.setcred.3; treats
<literal>binding</literal> and
<literal>sufficient</literal> modules as if they were
<literal>required</literal>.</para>
<para>The third and final exception is that
&man.pam.chauthtok.3; runs the entire chain twice (once for
preliminary checks and once to actually set the password), and
in the preliminary phase it treats
<literal>binding</literal> and
<literal>sufficient</literal> modules as if they were
<literal>required</literal>.</para>
</section>
</section>
<section xml:id="pam-freebsd-modules">
<title xml:id="pam-freebsd-modules.title">FreeBSD PAM Modules</title>
<section xml:id="pam-modules-deny">
<title xml:id="pam-modules-deny.title">&man.pam.deny.8;</title>
<para>The &man.pam.deny.8; module is one of the simplest modules
available; it responds to any request with
<literal>PAM_AUTH_ERR</literal>. It is useful for quickly
disabling a service (add it to the top of every chain), or for
terminating chains of <literal>sufficient</literal>
modules.</para>
</section>
<section xml:id="pam-modules-echo">
<title xml:id="pam-modules-echo.title">&man.pam.echo.8;</title>
<para>The &man.pam.echo.8; module simply passes its arguments to
the conversation function as a
<literal>PAM_TEXT_INFO</literal> message. It is mostly useful
for debugging, but can also serve to display messages such as
<quote>Unauthorized access will be prosecuted</quote> before
starting the authentication procedure.</para>
</section>
<section xml:id="pam-modules-exec">
<title xml:id="pam-modules-exec.title">&man.pam.exec.8;</title>
<para>The &man.pam.exec.8; module takes its first argument to be
the name of a program to execute, and the remaining arguments
are passed to that program as command-line arguments. One
possible application is to use it to run a program at login
time which mounts the user's home directory.</para>
</section>
<section xml:id="pam-modules-ftpusers">
<title xml:id="pam-modules-ftpusers.title">&man.pam.ftpusers.8;</title>
<para>The &man.pam.ftpusers.8; module</para>
</section>
<section xml:id="pam-modules-group">
<title xml:id="pam-modules-group.title">&man.pam.group.8;</title>
<para>The &man.pam.group.8; module accepts or rejects applicants
on the basis of their membership in a particular file group
(normally <literal>wheel</literal> for &man.su.1;). It is
primarily intended for maintaining the traditional behavior
of BSD &man.su.1;, but has many other uses, such as excluding
certain groups of users from a particular service.</para>
</section>
<section xml:id="pam-modules-guest">
<title xml:id="pam-modules-guest.title">&man.pam.guest.8;</title>
<para>The &man.pam.guest.8; module allows guest logins using
fixed login names. Various requirements can be placed on the
password, but the default behavior is to allow any password
as long as the login name is that of a guest account. The
&man.pam.guest.8; module can easily be used to implement
anonymous FTP logins.</para>
</section>
<section xml:id="pam-modules-krb5">
<title xml:id="pam-modules-krb5.title">&man.pam.krb5.8;</title>
<para>The &man.pam.krb5.8; module</para>
</section>
<section xml:id="pam-modules-ksu">
<title xml:id="pam-modules-ksu.title">&man.pam.ksu.8;</title>
<para>The &man.pam.ksu.8; module</para>
</section>
<section xml:id="pam-modules-lastlog">
<title xml:id="pam-modules-lastlog.title">&man.pam.lastlog.8;</title>
<para>The &man.pam.lastlog.8; module</para>
</section>
<section xml:id="pam-modules-login-access">
<title xml:id="pam-modules-login-access.title">&man.pam.login.access.8;</title>
<para>The &man.pam.login.access.8; module provides an
implementation of the account management primitive which
enforces the login restrictions specified in the
&man.login.access.5; table.</para>
</section>
<section xml:id="pam-modules-nologin">
<title xml:id="pam-modules-nologin.title">&man.pam.nologin.8;</title>
<para>The &man.pam.nologin.8; module refuses non-root logins
when <filename>/var/run/nologin</filename> exists. This file
is normally created by &man.shutdown.8; when less than five
minutes remain until the scheduled shutdown time.</para>
</section>
<section xml:id="pam-modules-opie">
<title xml:id="pam-modules-opie.title">&man.pam.opie.8;</title>
<para>The &man.pam.opie.8; module implements the &man.opie.4;
authentication method. The &man.opie.4; system is a
challenge-response mechanism where the response to each
challenge is a direct function of the challenge and a
passphrase, so the response can be easily computed <quote>just
in time</quote> by anyone possessing the passphrase,
eliminating the need for password lists. Moreover, since
&man.opie.4; never reuses a challenge that has been correctly
answered, it is not vulnerable to replay attacks.</para>
</section>
<section xml:id="pam-modules-opieaccess">
<title xml:id="pam-modules-opieaccess.title">&man.pam.opieaccess.8;</title>
<para>The &man.pam.opieaccess.8; module is a companion module to
&man.pam.opie.8;. Its purpose is to enforce the restrictions
codified in &man.opieaccess.5;, which regulate the conditions
under which a user who would normally authenticate herself
using &man.opie.4; is allowed to use alternate methods. This
is most often used to prohibit the use of password
authentication from untrusted hosts.</para>
<para>In order to be effective, the &man.pam.opieaccess.8;
module must be listed as <literal>requisite</literal>
immediately after a <literal>sufficient</literal> entry for
&man.pam.opie.8;, and before any other modules, in the
<literal>auth</literal> chain.</para>
</section>
<section xml:id="pam-modules-passwdqc">
<title xml:id="pam-modules-passwdqc.title">&man.pam.passwdqc.8;</title>
<para>The &man.pam.passwdqc.8; module</para>
</section>
<section xml:id="pam-modules-permit">
<title xml:id="pam-modules-permit.title">&man.pam.permit.8;</title>
<para>The &man.pam.permit.8; module is one of the simplest
modules available; it responds to any request with
<literal>PAM_SUCCESS</literal>. It is useful as a placeholder
for services where one or more chains would otherwise be
empty.</para>
</section>
<section xml:id="pam-modules-radius">
<title xml:id="pam-modules-radius.title">&man.pam.radius.8;</title>
<para>The &man.pam.radius.8; module</para>
</section>
<section xml:id="pam-modules-rhosts">
<title xml:id="pam-modules-rhosts.title">&man.pam.rhosts.8;</title>
<para>The &man.pam.rhosts.8; module</para>
</section>
<section xml:id="pam-modules-rootok">
<title xml:id="pam-modules-rootok.title">&man.pam.rootok.8;</title>
<para>The &man.pam.rootok.8; module reports success if and only
if the real user id of the process calling it (which is
assumed to be run by the applicant) is 0. This is useful for
non-networked services such as &man.su.1; or &man.passwd.1;,
to which the <literal>root</literal> should have automatic
access.</para>
</section>
<section xml:id="pam-modules-securetty">
<title xml:id="pam-modules-securetty.title">&man.pam.securetty.8;</title>
<para>The &man.pam.securetty.8; module</para>
</section>
<section xml:id="pam-modules-self">
<title xml:id="pam-modules-self.title">&man.pam.self.8;</title>
<para>The &man.pam.self.8; module reports success if and only if
the names of the applicant matches that of the target account.
It is most useful for non-networked services such as
&man.su.1;, where the identity of the applicant can be easily
verified.</para>
</section>
<section xml:id="pam-modules-ssh">
<title xml:id="pam-modules-ssh.title">&man.pam.ssh.8;</title>
<para>The &man.pam.ssh.8; module provides both authentication
and session services. The authentication service allows users
who have passphrase-protected SSH secret keys in their
<filename>~/.ssh</filename> directory to authenticate
themselves by typing their passphrase. The session service
starts &man.ssh-agent.1; and preloads it with the keys that
were decrypted in the authentication phase. This feature is
particularly useful for local logins, whether in X (using
&man.xdm.1; or another PAM-aware X login manager) or at the
console.</para>
</section>
<section xml:id="pam-modules-tacplus">
<title xml:id="pam-modules-tacplus.title">&man.pam.tacplus.8;</title>
<para>The &man.pam.tacplus.8; module</para>
</section>
<section xml:id="pam-modules-unix">
<title xml:id="pam-modules-unix.title">&man.pam.unix.8;</title>
<para>The &man.pam.unix.8; module implements traditional &unix;
password authentication, using &man.getpwnam.3; to obtain the
target account's password and compare it with the one provided
by the applicant. It also provides account management
services (enforcing account and password expiration times) and
password-changing services. This is probably the single most
useful module, as the great majority of admins will want to
maintain historical behavior for at least some
services.</para>
</section>
</section>
<section xml:id="pam-appl-prog">
<title xml:id="pam-appl-prog.title">PAM Application Programming</title>
<para><!--XXX-->This section has not yet been written.</para>
<!--
Note that while the original PAM paper includes a sample PAM
application that calls pam_open_session() before pam_setcred(),
the Linux-PAM documentation states that pam_setcred() must be
called first, which makes more sense.
Also note that the example in the paper calls setgid(),
initgroups() and setuid() itself rather than rely on
pam_setcred() to do it.
-->
</section>
<section xml:id="pam-module-prog">
<title xml:id="pam-module-prog.title">PAM Module Programming</title>
<para><!--XXX-->This section has not yet been written.</para>
</section>
<appendix xml:id="pam-sample-appl">
<title xml:id="pam-sample-appl.title">Sample PAM Application</title>
<para>The following is a minimal implementation of &man.su.1;
using PAM. Note that it uses the OpenPAM-specific
&man.openpam.ttyconv.3; conversation function, which is
prototyped in <filename>security/openpam.h</filename>. If you wish
build this application on a system with a different PAM library,
you will have to provide your own conversation function. A
robust conversation function is surprisingly difficult to
implement; the one presented in <xref linkend="pam-sample-conv"/> is a good
starting point, but should not be used in real-world
applications.</para>
<programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="su.c" parse="text"/></programlisting>
</appendix>
<appendix xml:id="pam-sample-module">
<title xml:id="pam-sample-module.title">Sample PAM Module</title>
<para>The following is a minimal implementation of
&man.pam.unix.8;, offering only authentication services. It
should build and run with most PAM implementations, but takes
advantage of OpenPAM extensions if available: note the use of
&man.pam.get.authtok.3;, which enormously simplifies prompting
the user for a password.</para>
<programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="pam_unix.c" parse="text"/></programlisting>
</appendix>
<appendix xml:id="pam-sample-conv">
<title xml:id="pam-sample-conv.title">Sample PAM Conversation
Function</title>
<para>The conversation function presented below is a greatly
simplified version of OpenPAM's &man.openpam.ttyconv.3;. It is
fully functional, and should give the reader a good idea of how
a conversation function should behave, but it is far too simple
for real-world use. Even if you are not using OpenPAM, feel free
to download the source code and adapt &man.openpam.ttyconv.3; to
your uses; we believe it to be as robust as a tty-oriented
conversation function can reasonably get.</para>
<programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="converse.c" parse="text"/></programlisting>
</appendix>
<bibliography xml:id="pam-further">
<info><title xml:id="pam-further.title">Further Reading</title>
<abstract>
<para>This is a list of documents relevant to PAM and related
issues. It is by no means complete.</para>
</abstract>
</info>
<bibliodiv>
<title>Papers</title>
<biblioentry>
<citetitle><link xlink:href="http://www.sun.com/software/solaris/pam/pam.external.pdf">
Making Login Services Independent of Authentication
Technologies</link></citetitle>
<authorgroup>
<author><personname><surname>Samar</surname><firstname>Vipin</firstname></personname></author>
<author><personname><surname>Lai</surname><firstname>Charlie</firstname></personname></author>
</authorgroup>
<orgname>Sun Microsystems</orgname>
</biblioentry>
<biblioentry>
<citetitle><link xlink:href="http://www.opengroup.org/pubs/catalog/p702.htm">X/Open
Single Sign-on Preliminary Specification</link></citetitle>
<orgname>The Open Group</orgname>
<biblioid class="isbn">1-85912-144-6</biblioid>
<pubdate>June 1997</pubdate>
</biblioentry>
<biblioentry>
<citetitle><link xlink:href="http://www.kernel.org/pub/linux/libs/pam/pre/doc/current-draft.txt">
Pluggable Authentication Modules</link></citetitle>
<author><personname><surname>Morgan</surname><firstname>Andrew</firstname><othername role="mi">G.</othername></personname></author>
<pubdate>1999-10-06</pubdate>
</biblioentry>
</bibliodiv>
<bibliodiv>
<title>User Manuals</title>
<biblioentry>
<citetitle><link xlink:href="http://www.sun.com/software/solaris/pam/pam.admin.pdf">PAM
Administration</link></citetitle>
<orgname>Sun Microsystems</orgname>
</biblioentry>
</bibliodiv>
<bibliodiv>
<title>Related Web pages</title>
<biblioentry>
<citetitle><link xlink:href="http://openpam.sourceforge.net/">OpenPAM homepage</link></citetitle>
<author><personname><surname>Sm&oslash;rgrav</surname><firstname>Dag-Erling</firstname></personname></author>
<orgname>ThinkSec AS</orgname>
</biblioentry>
<biblioentry>
<citetitle><link xlink:href="http://www.kernel.org/pub/linux/libs/pam/">Linux-PAM homepage</link></citetitle>
<author><personname><surname>Morgan</surname><firstname>Andrew</firstname><othername role="mi">G.</othername></personname></author>
</biblioentry>
<biblioentry>
<citetitle><link xlink:href="http://wwws.sun.com/software/solaris/pam/">Solaris PAM homepage</link></citetitle>
<orgname>Sun Microsystems</orgname>
</biblioentry>
</bibliodiv>
</bibliography>
</article>
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