<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<book id="sag">
<bookinfo>
<title>The Linux-PAM System Administrators' Guide</title>
<authorgroup>
<author>
<firstname>Andrew G.</firstname>
<surname>Morgan</surname>
<email>morgan@kernel.org</email>
</author>
<author>
<firstname>Thorsten</firstname>
<surname>Kukuk</surname>
<email>kukuk@thkukuk.de</email>
</author>
</authorgroup>
<releaseinfo>Version 1.1.2, 31. August 2010</releaseinfo>
<abstract>
<para>
This manual documents what a system-administrator needs to know about
the <emphasis remap='B'>Linux-PAM</emphasis> library. It covers the
correct syntax of the PAM configuration file and discusses strategies
for maintaining a secure system.
</para>
</abstract>
</bookinfo>
<chapter id='sag-introduction'>
<title>Introduction</title>
<para>
<emphasis remap='B'>Linux-PAM</emphasis> (Pluggable Authentication
Modules for Linux) is a suite of shared libraries that enable the
local system administrator to choose how applications authenticate users.
</para>
<para>
In other words, without (rewriting and) recompiling a PAM-aware
application, it is possible to switch between the authentication
mechanism(s) it uses. Indeed, one may entirely upgrade the local
authentication system without touching the applications themselves.
</para>
<para>
Historically an application that has required a given user to be
authenticated, has had to be compiled to use a specific authentication
mechanism. For example, in the case of traditional UN*X systems, the
identity of the user is verified by the user entering a correct
password. This password, after being prefixed by a two character
``salt'', is encrypted (with crypt(3)). The user is then authenticated
if this encrypted password is identical to the second field of the
user's entry in the system password database (the
<filename>/etc/passwd</filename> file). On such systems, most if
not all forms of privileges are granted based on this single
authentication scheme. Privilege comes in the form of a personal
user-identifier (UID) and membership of various groups. Services and
applications are available based on the personal and group identity
of the user. Traditionally, group membership has been assigned based
on entries in the <filename>/etc/group</filename> file.
</para>
<para>
It is the purpose of the <emphasis remap='B'>Linux-PAM</emphasis>
project to separate the development of privilege granting software
from the development of secure and appropriate authentication schemes.
This is accomplished by providing a library of functions that an
application may use to request that a user be authenticated. This
PAM library is configured locally with a system file,
<filename>/etc/pam.conf</filename> (or a series of configuration
files located in <filename>/etc/pam.d/</filename>) to authenticate a
user request via the locally available authentication modules. The
modules themselves will usually be located in the directory
<filename>/lib/security</filename> or
<filename>/lib64/security</filename> and take the form of dynamically
loadable object files (see <citerefentry>
<refentrytitle>dlopen</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>).
</para>
</chapter>
<chapter id="sag-text-conventions">
<title>Some comments on the text</title>
<para>
Before proceeding to read the rest of this document, it should be
noted that the text assumes that certain files are placed in certain
directories. Where they have been specified, the conventions we adopt
here for locating these files are those of the relevant RFC (RFC-86.0,
see <link linkend="sag-see-also">bibliography"</link>). If you are
using a distribution of Linux (or some other operating system) that
supports PAM but chooses to distribute these files in a different way
you should be careful when copying examples directly from the text.
</para>
<para>
As an example of the above, where it is explicit, the text assumes
that PAM loadable object files (the
<emphasis remap='B'>modules</emphasis>) are to be located in
the following directory: <filename>/lib/security/</filename> or
<filename>/lib64/security</filename> depending on the architecture.
This is generally the location that seems to be compatible with the
Filesystem Hierarchy Standard (FHS). On Solaris, which has its own
licensed version of PAM, and some other implementations of UN*X,
these files can be found in <filename>/usr/lib/security</filename>.
Please be careful to perform the necessary transcription when using
the examples from the text.
</para>
</chapter>
<chapter id="sag-overview">
<title>Overview</title>
<para>
For the uninitiated, we begin by considering an example. We take an
application that grants some service to users;
<command>login</command> is one such program.
<command>Login</command> 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
<command>login</command> the service is a command shell
(bash, tcsh, zsh, etc.) running with the identity of the user.
</para>
<para>
Traditionally, the former step is achieved by the
<command>login</command> 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 <emphasis remap='B'>Linux-PAM</emphasis>.
</para>
<para>
From the perspective of the application programmer (in this case
the person that wrote the <command>login</command> application),
<emphasis remap='B'>Linux-PAM</emphasis> takes care of this
authentication task -- verifying the identity of the user.
</para>
<para>
The flexibility of <emphasis remap='B'>Linux-PAM</emphasis> is
that <emphasis>you</emphasis>, 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
<emphasis>simple trust</emphasis> (<command>pam_permit</command>)
to something as paranoid as a combination of a retinal scan, a
voice print and a one-time password!
</para>
<para>
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
<emphasis>multiplication tables</emphasis>. As they mature, the
authentication can be upgraded to include (long) division!
</para>
<para>
<emphasis remap='B'>Linux-PAM</emphasis> deals with four
separate types of (management) task. These are:
<emphasis>authentication management</emphasis>;
<emphasis>account management</emphasis>;
<emphasis>session management</emphasis>; and
<emphasis>password management</emphasis>.
The association of the preferred management scheme with the behavior
of an application is made with entries in the relevant
<emphasis remap='B'>Linux-PAM</emphasis> configuration file.
The management functions are performed by <emphasis>modules</emphasis>
specified in the configuration file. The syntax for this
file is discussed in the section
<link linkend="sag-configuration">below</link>.
</para>
<para>
Here is a figure that describes the overall organization of
<emphasis remap='B'>Linux-PAM</emphasis>:
<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 ]
+--------------+
</programlisting>
By way of explanation, the left of the figure represents the
application; application X. Such an application interfaces with the
<emphasis remap='B'>Linux-PAM</emphasis> library and knows none of
the specifics of its configured authentication method. The
<emphasis remap='B'>Linux-PAM</emphasis> 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 <emphasis remap='B'>Linux-PAM</emphasis>, 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 <emphasis>conversation</emphasis>
function.
</para>
<para>
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.
</para>
</chapter>
<chapter id="sag-configuration">
<title>The Linux-PAM configuration file</title>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="../man/pam.conf-desc.xml"
xpointer='xpointer(//section[@id = "pam.conf-desc"]/*)' />
<section id='sag-configuration-file'>
<title>Configuration file syntax</title>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="../man/pam.conf-syntax.xml"
xpointer='xpointer(//section[@id = "pam.conf-syntax"]/*)' />
</section>
<section id='sag-configuration-directory'>
<title>Directory based configuration</title>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="../man/pam.conf-dir.xml"
xpointer='xpointer(//section[@id = "pam.conf-dir"]/*)' />
</section>
<section id='sag-configuration-example'>
<title>Example configuration file entries</title>
<para>
In this section, we give some examples of entries that can
be present in the <emphasis remap='B'>Linux-PAM</emphasis>
configuration file. As a first attempt at configuring your
system you could do worse than to implement these.
</para>
<para>
If a system is to be considered secure, it had better have a
reasonably secure '<emphasis remap='B'>other</emphasis> entry.
The following is a paranoid setting (which is not a bad place
to start!):
</para>
<programlisting>
#
# default; deny access
#
other auth required pam_deny.so
other account required pam_deny.so
other password required pam_deny.so
other session required pam_deny.so
</programlisting>
<para>
Whilst fundamentally a secure default, this is not very
sympathetic to a misconfigured system. For example, such
a system is vulnerable to locking everyone out should the
rest of the file become badly written.
</para>
<para>
The module <command>pam_deny</command> (documented in a
<link linkend="sag-pam_deny">later section</link>) is not very
sophisticated. For example, it logs no information when it
is invoked so unless the users of a system contact the
administrator when failing to execute a service application,
the administrator may go for a long while in ignorance of the
fact that his system is misconfigured.
</para>
<para>
The addition of the following line before those in the above
example would provide a suitable warning to the administrator.
</para>
<programlisting>
#
# default; wake up! This application is not configured
#
other auth required pam_warn.so
other password required pam_warn.so
</programlisting>
<para>
Having two '<command>other auth</command>' lines is an
example of stacking.
</para>
<para>
On a system that uses the <filename>/etc/pam.d/</filename>
configuration, the corresponding default setup would be
achieved with the following file:
</para>
<programlisting>
#
# default configuration: /etc/pam.d/other
#
auth required pam_warn.so
auth required pam_deny.so
account required pam_deny.so
password required pam_warn.so
password required pam_deny.so
session required pam_deny.so
</programlisting>
<para>
This is the only explicit example we give for an
<filename>/etc/pam.d/</filename> file. In general, it
should be clear how to transpose the remaining examples
to this configuration scheme.
</para>
<para>
On a less sensitive computer, one on which the system
administrator wishes to remain ignorant of much of the
power of <emphasis remap='B'>Linux-PAM</emphasis>, the
following selection of lines (in
<filename>/etc/pam.d/other</filename>) is likely to
mimic the historically familiar Linux setup.
</para>
<programlisting>
#
# default; standard UN*X access
#
auth required pam_unix.so
account required pam_unix.so
password required pam_unix.so
session required pam_unix.so
</programlisting>
<para>
In general this will provide a starting place for most applications.
</para>
</section>
</chapter>
<chapter id='sag-security-issues'>
<title>Security issues</title>
<section id='sag-security-issues-wrong'>
<title>If something goes wrong</title>
<para>
<emphasis remap='B'>Linux-PAM</emphasis> has the potential
to seriously change the security of your system. You can
choose to have no security or absolute security (no access
permitted). In general, <emphasis remap='B'>Linux-PAM</emphasis>
errs towards the latter. Any number of configuration errors
can disable access to your system partially, or completely.
</para>
<para>
The most dramatic problem that is likely to be encountered when
configuring <emphasis remap='B'>Linux-PAM</emphasis> is that of
<emphasis>deleting</emphasis> the configuration file(s):
<filename>/etc/pam.d/*</filename> and/or
<filename>/etc/pam.conf</filename>. This will lock you out of
your own system!
</para>
<para>
To recover, your best bet is to restore the system from a
backup or boot the system into a rescue system and correct
things from there.
</para>
</section>
<section id='sag-security-issues-other'>
<title>Avoid having a weak `other' configuration</title>
<para>
It is not a good thing to have a weak default
(<emphasis remap='B'>other</emphasis>) entry.
This service is the default configuration for all PAM aware
applications and if it is weak, your system is likely to be
vulnerable to attack.
</para>
<para>
Here is a sample "other" configuration file. The
<command>pam_deny</command> module will deny access and the
<command>pam_warn</command> module will send a syslog message
to <emphasis>auth.notice</emphasis>:
</para>
<programlisting>
#
# The PAM configuration file for the `other' service
#
auth required pam_deny.so
auth required pam_warn.so
account required pam_deny.so
account required pam_warn.so
password required pam_deny.so
password required pam_warn.so
session required pam_deny.so
session required pam_warn.so
</programlisting>
</section>
</chapter>
<chapter id='sag-module-reference'>
<title>A reference guide for available modules</title>
<para>
Here, we collect together the descriptions of the various modules
coming with Linux-PAM.
</para>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_access.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_cracklib.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_debug.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_deny.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_echo.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_env.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_exec.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_faildelay.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_filter.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_ftp.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_group.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_issue.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_keyinit.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_lastlog.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_limits.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_listfile.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_localuser.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_loginuid.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_mail.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_mkhomedir.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_motd.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_namespace.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_nologin.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_permit.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_pwhistory.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_rhosts.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_rootok.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_securetty.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_selinux.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_shells.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_succeed_if.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_tally.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_tally2.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_time.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_timestamp.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_umask.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_unix.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_userdb.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_warn.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_wheel.xml"/>
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude"
href="pam_xauth.xml"/>
</chapter>
<chapter id="sag-see-also">
<title>See also</title>
<itemizedlist>
<listitem>
<para>
The Linux-PAM Application Writers' Guide.
</para>
</listitem>
<listitem>
<para>
The Linux-PAM Module Writers' Guide.
</para>
</listitem>
<listitem>
<para>
The V. Samar and R. Schemers (SunSoft), ``UNIFIED LOGIN WITH
PLUGGABLE AUTHENTICATION MODULES'', Open Software Foundation
Request For Comments 86.0, October 1995.
</para>
</listitem>
</itemizedlist>
</chapter>
<chapter id='sag-author'>
<title>Author/acknowledgments</title>
<para>
This document was written by Andrew G. Morgan (morgan@kernel.org)
with many contributions from
Chris Adams, Peter Allgeyer, Tim Baverstock, Tim Berger,
Craig S. Bell, Derrick J. Brashear, Ben Buxton, Seth Chaiklin,
Oliver Crow, Chris Dent, Marc Ewing, Cristian Gafton,
Emmanuel Galanos, Brad M. Garcia, Eric Hester, Michel D'Hooge,
Roger Hu, Eric Jacksch, Michael K. Johnson, David Kinchlea,
Olaf Kirch, Marcin Korzonek, Thorsten Kukuk, Stephen Langasek,
Nicolai Langfeldt, Elliot Lee, Luke Kenneth Casson Leighton,
Al Longyear, Ingo Luetkebohle, Marek Michalkiewicz,
Robert Milkowski, Aleph One, Martin Pool, Sean Reifschneider,
Jan Rekorajski, Erik Troan, Theodore Ts'o, Jeff Uphoff, Myles Uyema,
Savochkin Andrey Vladimirovich, Ronald Wahl, David Wood, John Wilmes,
Joseph S. D. Yao and Alex O. Yuriev.
</para>
<para>
Thanks are also due to Sun Microsystems, especially to Vipin Samar and
Charlie Lai for their advice. At an early stage in the development of
<emphasis remap='B'>Linux-PAM</emphasis>, Sun graciously made the
documentation for their implementation of PAM available. This act
greatly accelerated the development of
<emphasis remap='B'>Linux-PAM</emphasis>.
</para>
</chapter>
<chapter id='sag-copyright'>
<title>Copyright information for this document</title>
<programlisting>
Copyright (c) 2006 Thorsten Kukuk <kukuk@thkukuk.de>
Copyright (c) 1996-2002 Andrew G. Morgan <morgan@kernel.org>
</programlisting>
<para>
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
</para>
<programlisting>
1. Redistributions of source code must retain the above copyright
notice, and the entire permission notice in its entirety,
including the disclaimer of warranties.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. The name of the author may not be used to endorse or promote
products derived from this software without specific prior
written permission.
</programlisting>
<para>
Alternatively, this product may be distributed under the terms of
the GNU General Public License (GPL), in which case the provisions
of the GNU GPL are required instead of the above restrictions.
(This clause is necessary due to a potential bad interaction between
the GNU GPL and the restrictions contained in a BSD-style copyright.)
</para>
<programlisting>
THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR
TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
</programlisting>
</chapter>
</book>