path: root/doc/sag/Linux-PAM_SAG.xml

<?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-[----&gt;--\--] Linux-   |--&lt;--| PAM config file|
  |       +        [----&lt;--/--]   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 &lt;kukuk@thkukuk.de&gt;
Copyright (c) 1996-2002 Andrew G. Morgan &lt;morgan@kernel.org&gt;
    </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>