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<!ENTITY date "02/10/2009">
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<article id="index" lang="en-GB">
  <articleinfo>
    <title>GNOME Display Manager Reference Manual</title>

    <revhistory>
      <revision lang="en">
        <revnumber>0.0</revnumber>
        <date>2008-09</date>
      </revision>
    </revhistory>

    <abstract role="description">
      <para>GDM is the GNOME Display Manager, a graphical login program.</para>
    </abstract>

    <authorgroup>
      <author lang="en">
        <firstname>Martin</firstname><othername>K.</othername>
           <surname>Petersen</surname>
        <affiliation>
          <address><email>mkp@mkp.net</email></address>
        </affiliation>
      </author>
      <author lang="en">
        <firstname>George</firstname><surname>Lebl</surname>
        <affiliation>
          <address><email>jirka@5z.com</email></address>
        </affiliation>
      </author>
      <author lang="en">
        <firstname>Jon</firstname><surname>McCann</surname>
        <affiliation>
          <address><email>mccann@jhu.edu</email></address>
        </affiliation>
      </author>
      <author lang="en">
        <firstname>Ray</firstname><surname>Strode</surname>
        <affiliation>
          <address><email>rstrode@redhat.com</email></address>
        </affiliation>
      </author>
      <author role="maintainer" lang="en">
        <firstname>Brian</firstname><surname>Cameron</surname>
        <affiliation>
          <address><email>Brian.Cameron@Oracle.COM</email></address>
        </affiliation>
      </author>
    </authorgroup>
    <copyright lang="en">
      <year>1998</year>
      <year>1999</year>
      <holder>Martin K. Petersen</holder>
    </copyright>
    <copyright lang="en">
      <year>2001</year>
      <year>2003</year>
      <year>2004</year>
      <holder>George Lebl</holder>
    </copyright>
    <copyright lang="en">
      <year>2003</year>
      <year>2007</year>
      <year>2008</year>
      <holder>Red Hat, Inc.</holder>
    </copyright>
    <copyright lang="en">
      <year>2003</year>
       <year>2011</year>
       <holder>Oracle and/or its affiliates. All rights reserved.</holder>
    </copyright>

      <legalnotice id="legalnotice">
	<para>Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation Licence (GFDL), Version 1.1 or any later version published by the Free Software Foundation with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. You can find a copy of the GFDL at this <ulink type="help" url="ghelp:fdl">link</ulink> or in the file COPYING-DOCS distributed with this manual.</para>
         <para>This manual is part of a collection of GNOME manuals distributed under the GFDL. If you want to distribute this manual separately from the collection, you can do so by adding a copy of the licence to the manual, as described in section 6 of the licence.</para>

	<para>Many of the names used by companies to distinguish their products and services are claimed as trademarks. Where those names appear in any GNOME documentation, and the members of the GNOME Documentation Project are made aware of those trademarks, then the names are in capital letters or initial capital letters.</para>

	<para lang="en">
	  DOCUMENT AND MODIFIED VERSIONS OF THE DOCUMENT ARE PROVIDED
	  UNDER  THE TERMS OF THE GNU FREE DOCUMENTATION LICENSE
	  WITH THE FURTHER UNDERSTANDING THAT:

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		  <para lang="en">DOCUMENT IS PROVIDED ON AN "AS IS" BASIS,
                    WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR
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                    THAT THE DOCUMENT OR MODIFIED VERSION OF THE
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                    SERVICING, REPAIR OR CORRECTION. THIS DISCLAIMER
                    OF WARRANTY CONSTITUTES AN ESSENTIAL PART OF THIS
                    LICENSE. NO USE OF ANY DOCUMENT OR MODIFIED
                    VERSION OF THE DOCUMENT IS AUTHORIZED HEREUNDER
                    EXCEPT UNDER THIS DISCLAIMER; AND
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		  <para lang="en">UNDER NO CIRCUMSTANCES AND UNDER NO LEGAL
                       THEORY, WHETHER IN TORT (INCLUDING NEGLIGENCE),
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                       INITIAL WRITER, ANY CONTRIBUTOR, OR ANY
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  </legalnotice>



    <releaseinfo>This manual describes version 2.26.0 of the GNOME Display Manager. It was last updated on 02/10/2009.</releaseinfo>  
  </articleinfo>

  <!-- ============= Preface ================================== -->

  <sect1 id="preface">
    <title>Terms and Conventions Used in This Manual</title>

    <para>This manual describes version 2.26.0 of the GNOME Display Manager. It was last updated on 02/10/2009.</para>  

    <para>Chooser - A program used to select a remote host for managing a display remotely on the attached display (<command>gdm-host-chooser</command>).</para>

    <para>FreeDesktop - The organisation providing desktop standards, such as the Desktop Entry Specification used by GDM. <ulink type="http" url="http://www.freedesktop.org/"> http://www.freedesktop.org</ulink>.</para>
    <para>GDM - GNOME Display Manager. Used to describe the software package as a whole.</para>

    <para lang="en">
      Greeter - The graphical login window (provided by <command>gnome-shell</command>).
    </para>

    <para>PAM - Pluggable Authentication Mechanism</para>

    <para>XDMCP - X Display Manage Protocol</para>

    <para lang="en">
      Xserver - An implementation of the X Window System.  For example the
      Xorg Xserver provided by the X.org Foundation
      <ulink type="http" url="http://www.x.org/">http://www.x.org</ulink>.
    </para>

    <para>Paths that start with a word in angle brackets are relative to the installation prefix. I.e. <filename>&lt;share&gt;/pixmaps/</filename> refers to <filename>/usr/share/pixmaps</filename> if GDM was configured with <command>--prefix=/usr</command>.</para>
  </sect1>

  <!-- ============= Overview ================================= -->

  <sect1 id="overview">
    <title>Overview</title>

    <sect2 id="introduction">
      <title>Introduction</title>

      <para>The GNOME Display Manager (GDM) is a display manager that implements all significant features required for managing attached and remote displays. GDM was written from scratch and does not contain any XDM or X Consortium code.</para>

      <para>Note that GDM is configurable, and many configuration settings have an impact on security. Issues to be aware of are highlighted in this document.</para> 

      <para>Please note that some Operating Systems configure GDM to behave differently than the default values as described in this document. If GDM does not seem to behave as documented, then check to see if any related configuration may be different than described here.</para>

      <para lang="en">
        For further information about GDM, refer to the project website at
        <ulink type="http" url="http://wiki.gnome.org/Projects/GDM/">
        http://wiki.gnome.org/Projects/GDM</ulink>.
      </para>

      <para>For discussion or queries about GDM, refer to the <address><email>gdm-list@gnome.org</email></address> mail list. This list is archived, and is a good resource to check to seek answers to common questions. This list is archived at <ulink type="http" url="http://mail.gnome.org/archives/gdm-list/"> http://mail.gnome.org/archives/gdm-list/</ulink> and has a search facility to look for messages with keywords.</para>

      <para>Please submit any bug reports or enhancement requests to the "gdm" category in <ulink type="http" url="http://bugzilla.gnome.org/"> http://bugzilla.gnome.org</ulink>.</para>
    </sect2>

    <sect2 id="stability">
      <title>Interface Stability</title>

      <para lang="en">
        GDM 2.20 and earlier supported stable configuration interfaces.
        However, the codebase was completely rewritten for GDM 2.22, and
        is not completely backward compatible with older releases. This is
        in part because things work differently, so some options just don't
        make sense, in part because some options never made sense, and in
        part because some functionality has not been reimplemented yet.
      </para>

      <para>Interfaces which continue to be supported in a stable fashion include the Init, PreSession, PostSession, PostLogin, and Xsession scripts. Some daemon configuration options in the <filename>&lt;etc&gt;/gdm/custom.conf</filename> file continue to be supported. Also, the <filename>~/.dmrc</filename>, and face browser image locations are still supported.</para>

      <para lang="en">
        GDM 2.20 and earlier supported the ability to manage multiple displays
        with separate graphics cards, such as used in terminal server
        environments, login in a window via a program like Xnest or Xephyr, the
        gdmsetup program, XML-based greeter themes, and the ability to run the
        XDMCP chooser from the login screen.  These features were not 
        added back during the 2.22 rewrite.
      </para>

    </sect2>

    <sect2 id="functionaldesc">
      <title>Functional Description</title>

<!--
<para>
        TODO - Would be good to discuss D-Bus, perhaps the new GObject model,
               and to explain the reasons why the rewrite made GDM better.
               From a high-level overview perspective, rather than the
               technical aspects.
</para>
-->

      <para>GDM is responsible for managing displays on the system. This includes authenticating users, starting the user session, and terminating the user session. GDM is configurable and the ways it can be configured are described in the "Configuring GDM" section of this document. GDM is also accessible for users with disabilities.</para>

      <para>GDM provides the ability to manage the main console display, and displays launched via VT. It is integrated with other programs, such as the Fast User Switch Applet (FUSA) and gnome-screensaver to manage multiple displays on the console via the Xserver Virtual Terminal (VT) interface. It also can manage XDMCP displays.</para>

      <para>Regardless of the display type, GDM will do the following when it manages the display. It will start an Xserver process, then run the <filename>Init</filename> script as the root user, and start the greeter program on the display.</para>

      <para lang="en">
        The greeter program is run as the unprivileged "gdm"
        user/group.  This user and group are described in the
        "Security" section of this document.  The main functions of
        the greeter program are to provide a mechanism for selecting
        an account for log in and to drive the dialogue between
        the user and system when authenticating that account. The authentication
        process is driven by Pluggable Authentication Modules (PAM).  The PAM
        modules determine what prompts (if any) are shown to the user to
        authenticate.  On the average system, the greeter program will request
        a username and password for authentication.  However some systems may
        be configured to use supplemental mechanisms such as a fingerprint or
        SmartCard readers. GDM can be configured to support these
        alternatives in parallel with greeter login extensions and the
        <command>--enable-split-authentication</command>
        <filename>./configure</filename> option, or one at a
        time via system PAM configuration.
      </para>

      <para lang="en">
         The smartcard extension can be enabled or disabled via the
         <filename>org.gnome.display-manager.extensions.smartcard.active</filename>
         gsettings key.
      </para>

      <para lang="en">
         Likewise, the fingerprint extension can be enabled or disabled via the
         <filename>org.gnome.display-manager.extensions.fingerprint.active</filename>
         gsettings key.
      </para>

      <para lang="en">
        GDM and PAM can be configured to not require any
        input, which will cause GDM to automatically log in and simply
        start a session, which can be useful for some environments, such as
        single user systems or kiosks.
      </para>

      <para lang="en">
        In addition to authentication, the greeter program allows the user to
        select which session to start and which language to use.  Sessions are
        defined by files that end in the .desktop suffix and more information
        about these files can be found in the "GDM User Session and Language
        Configuration" section of this document.  By default, GDM is configured
        to display a face browser so the user can select their user account by
        clicking on an image instead of having to type their username. GDM
        keeps track of the user's default session and language in the user's
        <filename>~/.dmrc</filename> and will use these defaults if the user
        did not pick a session or language in the login GUI.
      </para>

      <para>After authenticating a user, the daemon runs the <filename>PostLogin</filename> script as root, then runs the <filename>PreSession</filename> script as root. After running these scripts, the user session is started. When the user exits their session, the <filename>PostSession</filename> script is run as root. These scripts are provided as hooks for distributions and end-users to customise how sessions are managed. For example, using these hooks you could set up a machine which creates the user's $HOME directory on the fly, and erases it on logout. The difference between the <filename>PostLogin</filename> and <filename>PreSession</filename> scripts is that <filename>PostLogin</filename> is run before the pam_open_session call so is the right place to do anything which should be run before the user session is initialised. The <filename>PreSession</filename> script is called after session initialisation.</para>
    </sect2>

    <sect2 id="greeterpanel">
      <title>Greeter Panel</title>
      <para lang="en">
        The GDM greeter program displays a panel docked at the bottom of the
        screen which provides additional functionality.  When a user is
        selected, the panel allows the user to select which session, language,
        and keyboard layout to use after logging in.  The keyboard layout
        selector also changes the keyboard layout used when typing your
        password.  The panel also contains an area for login services to leave
        status icons.  Some example status icons include a battery icon for
        current battery usage, and an icon for enabling accessibility features.
        The greeter program also provides buttons which allow the user to
        shutdown or restart the system.  It is possible to configure GDM to not
        provide the shutdown and restart buttons, if desired.  GDM can also be
        configured via PolicyKit (or via RBAC on Oracle Solaris) to require the
        user have appropriate authorization before accepting the shutdown or
        restart request.
      </para>

      <para>Note that keyboard layout features are only available on systems that support libxklavier.</para>
    </sect2>

    <sect2 id="accessibility">
      <title>Accessibility</title>

        <para>GDM supports "Accessible Login", allowing users to log into their desktop session even if they cannot easily use the screen, mouse, or keyboard in the usual way. Accessible Technology (AT) features such as an on-screen keyboard, screen reader, screen magnifier, and Xserver AccessX keyboard accessibility are available. It is also possible to enable large text or high contrast icons and controls, if needed. Refer to the "Accessibility Configuration" section of the document for more information how various accessibility features can be configured.</para>

        <para>On some Operating Systems, it is necessary to make sure that the GDM user is a member of the "audio" group for AT programs that require audio output (such as text-to-speech) to be functional.</para>
    </sect2>

    <sect2 id="facebrowser">
      <title>The GDM Face Browser</title>

      <para lang="en">
        The Face Browser is the interface which allows users to select their
        username by clicking on an image.  This feature can be enabled or
        disabled via the org.gnome.login-screen disable-user-list GSettings 
        key and is on by default.  When disabled, users must type their
        complete username by hand.  When enabled, it displays all local users
        which are available for login on the system (all user accounts defined
        in the /etc/passwd file that have a valid shell and sufficiently high
        UID) and remote users that have recently logged in.
        The face browser in GDM 2.20 and earlier would attempt to display all
        remote users, which caused performance problems in large,
        enterprise deployments.
      </para>

      <para>The Face Browser is configured to display the users who log in most frequently at the top of the list. This helps to ensure that users who log in frequently can quickly find their login image.</para>

      <para>The Face Browser supports "type-ahead search" which dynamically moves the face selection as the user types to the corresponding username in the list. This means that a user with a long username will only have to type the first few characters of the username before the correct item in the list gets selected.</para>

      <para>The icons used by GDM can be installed globally by the sysadmin or can be located in the user's home directories. If installed globally they should be in the <filename>&lt;share&gt;/pixmaps/faces/</filename> directory and the filename should be the name of the user. Face image files should be a standard image that GTK+ can read, such as PNG or JPEG. Face icons placed in the global face directory must be readable to the GDM user.</para>

<!--
<para>
        TODO - In the old GDM the ~/gnome2/gdm file is used, but the new code
               seems to use ~/.gnome/gdm.  Error?
</para>
-->
      <para>If there is no global icon for the user, GDM will look in the user's $HOME directory for the image file. GDM will first look for the user's face image in <filename>~/.face</filename>. If not found, it will try <filename>~/.face.icon</filename>. If still not found, it will use the value defined for "face/picture=" in the <filename>~/.gnome2/gdm</filename> file.</para>

      <para>If a user has no defined face image, GDM will use the "stock_person" icon defined in the current GTK+ theme. If no such image is defined, it will fallback to a generic face image.</para>

      <para>Please note that loading and scaling face icons located in remote user home directories can be a very time-consuming task. Since it not practical to load images over NIS or NFS, GDM does not attempt to load face images from remote home directories.</para>

      <para>When the browser is turned on, valid usernames on the computer are exposed for everyone to see. If XDMCP is enabled, then the usernames are exposed to remote users. This, of course, limits security somewhat since a malicious user does not need to guess valid usernames. In some very restrictive environments the face browser may not be appropriate.</para>

    </sect2>

    <sect2 id="xdmcp">
      <title>XDMCP</title>

<!--
<para>
        TODO - What XDMCP features actually work?   I know that the
               chooser is missing.
</para>
-->

      <para>The GDM daemon can be configured to listen for and manage X Display Manage Protocol (XDMCP) requests from remote displays. By default XDMCP support is turned off, but can be enabled if desired. If GDM is built with TCP Wrapper support, then the daemon will only grant access to hosts specified in the GDM service section in the TCP Wrappers configuration file.</para>

      <para lang="en">
        GDM includes several measures making it more resistant to denial of
        service attacks on the XDMCP service.  A lot of the protocol
        parameters, handshaking timeouts, etc. can be fine tuned. The default
        configuration should work reasonably on most systems. 
      </para>

      <para>GDM by default listens for XDMCP requests on the normal UDP port used for XDMCP, port 177, and will respond to QUERY and BROADCAST_QUERY requests by sending a WILLING packet to the originator.</para>

      <para>GDM can also be configured to honour INDIRECT queries and present a host chooser to the remote display. GDM will remember the user's choice and forward subsequent requests to the chosen manager. GDM also supports an extension to the protocol which will make it forget the redirection once the user's connection succeeds. This extension is only supported if both daemons are GDM. It is transparent and will be ignored by XDM or other daemons that implement XDMCP.</para>

      <para>If XDMCP seems to not be working, make sure that all machines are specified in <filename>/etc/hosts</filename>.</para>

      <para>Refer to the "Security" section for information about security concerns when using XDMCP.</para>
    </sect2>

    <sect2 id="logging">
      <title>Logging</title>

      <para lang="en">
        GDM uses syslog to log errors and status.  It can also log debugging
        information, which can be useful for tracking down problems if GDM is
        not working properly.  Debug output can be enabled by setting the
        debug/Enable key to "true" in the
        <filename>&lt;etc&gt;/gdm/custom.conf</filename> file.
      </para>

      <para>Output from the various Xservers is stored in the GDM log directory, which is normally <filename>&lt;var&gt;/log/gdm/</filename>. Any Xserver messages are saved to a file associated with the display value, <filename>&lt;display&gt;.log</filename>.</para>

      <para lang="en">
        The session output is piped through the GDM daemon to the
        <filename>~/<replaceable>$XDG_CACHE_HOME</replaceable>/gdm/session.log</filename>
        file which usually expands to <filename>~/.cache/gdm/session.log</filename>.
        The file is overwritten on each login, so logging out and logging back
        into the same user via GDM will cause any messages from the previous
        session to be lost.
      </para>

      <para lang="en">
        Note that if GDM can not create this file for some reason, then a
        fallback file will be created named <filename>~/<replaceable>$XDG_CACHE_HOME</replaceable>/gdm/session.log.XXXXXXXX</filename>
        where the <filename>XXXXXXXX</filename> are some random characters.
      </para>
    </sect2>

    <sect2 id="fusa">
      <title>Fast User Switching</title>

      <para lang="en">
        GDM allows multiple users to be logged in at the same time.  After one
        user is logged in, additional users can log in via the User Switcher
        on the GNOME Panel, or from the "Switch User" button in Lock Screen dialog
        of GNOME Screensaver.  The active session can be changed back and forth using
        the same mechanism.  Note that some distributions may not add the User Switcher
        to the default panel configuration.  It can be added using the panel context
        menu.
      </para>
      <para>Note this feature is available on systems that support Virtual Terminals. This feature will not function if Virtual Terminals is not available.</para>
    </sect2>
  </sect1>

  <!-- ============= Security ================================= -->

  <sect1 id="security">
    <title>Security</title>

    <sect2 id="gdmuser">
      <title>The GDM User and Group</title>

      <para>For security reasons a dedicated user and group ID are recommended for proper operation. This user and group are normally "gdm" on most systems, but can be configured to any user or group. All GDM GUI programs are run as this user, so that the programs which interact with the user are run in a sandbox. This user and group should have limited privileges.</para>

      <para>The only special privilege the "gdm" user requires is the ability to read and write Xauth files to the <filename>&lt;var&gt;/run/gdm</filename> directory. The <filename>&lt;var&gt;/run/gdm</filename> directory should have root:gdm ownership and 1777 permissions.</para>

      <para lang="en">
        You should not, under any circumstances, configure the GDM user/group
        to a user which a user could easily gain access to, such as the user
        <filename>nobody</filename>.  Any user who gains access to an Xauth
        key can snoop on and control running GUI programs running in the
        associated session or perform a denial-of-service attack on it.  It
        is important to ensure that the system is configured properly so that
        only the "gdm" user has access to these files and that it
        is not easy to login to this account.  For example, the account should
        be setup to not have a password or allow non-root users to login to the
        account.
      </para>

      <para>The GDM greeter configuration is stored in GConf. To allow the GDM user to be able to write configuration, it is necessary for the "gdm" user to have a writable $HOME directory. Users may configure the default GConf configuration as desired to avoid the need to provide the "gdm" user with a writable $HOME directory. However, some features of GDM may be disabled if it is unable to write state information to GConf configuration.</para>
    </sect2>

    <sect2 id="PAM">
      <title>PAM</title>

      <para>GDM uses PAM for login authentication. PAM stands for Pluggable Authentication Module, and is used by most programs that request authentication on your computer. It allows the administrator to configure specific authentication behavior for different login programs (such as SSH, login GUI, screensaver, etc.)</para>

      <para>PAM is complicated and highly configurable, and this documentation does not intend to explain this in detail. Instead, it is intended to give an overview of how PAM configuration relates with GDM, how PAM is commonly configured with GDM, and known issues. It is expected that a person needing to do PAM configuration would need to do further reading of PAM documentation to understand how to configure PAM and to understand terms used in this section.</para>
        
      <para lang="en">
        PAM configuration has different, but similar, interfaces on different
        Operating Systems, so check the
        <ulink type="help" url="man:pam.d">pam.d</ulink> or
        <ulink type="help" url="man:pam.conf">pam.conf</ulink> man page for
        details.  Be sure you read the PAM documentation and are comfortable
        with the security implications of any changes you intend to make to
        your configuration.
      </para>

      <para>Note that, by default, GDM uses the "gdm" PAM service name for normal login and the "gdm-autologin" PAM service name for automatic login. These services may not be defined in your pam.d or pam.conf configured file. If there is no entry, then GDM will use the default PAM behavior. On most systems this should work fine. However, the automatic login feature may not work if the gdm-autologin service is not defined.</para>

      <para>The <filename>PostLogin</filename> script is run before pam_open_session is called, and the <filename>PreSession</filename> script is called after. This allows the system administrator to add any scripting to the login process either before or after PAM initialises the session.</para>

      <para>If you wish to make GDM work with other types of authentication mechanisms (such as a fingerprint or SmartCard reader), then you should implement this by using a PAM service module for the desired authentication type rather than by trying to modify the GDM code directly. Refer to the PAM documentation on your system. How to do this is frequently discussed on the <address><email>gdm-list@gnome.org</email></address> mail list, so you can refer to the list archives for more information.</para>

      <para>PAM does have some limitations regarding being able to work with multiple types of authentication at the same time, like supporting the ability to accept either SmartCard and the ability to type the username and password into the login program. There are techniques that are used to make this work, and it is best to research how this problem is commonly solved when setting up such a configuration.</para>

      <para>If automatic login does not work on a system, check to see if the "gdm-autologin" PAM stack is defined in the PAM configuration. For this to work, it is necessary to use a PAM module that simply does no authentication, or which simply returns PAM_SUCCESS from all of its public interfaces. Assuming your system has a pam_allow.so PAM module which does this, a PAM configuration to enable "gdm-autologin" would look like this:</para>

<screen>
       gdm-autologin auth  required    pam_unix_cred.so.1
       gdm-autologin auth  sufficient  pam_allow.so.1
       gdm-autologin account  sufficient  pam_allow.so.1
       gdm-autologin session  sufficient  pam_allow.so.1
       gdm-autologin password  sufficient  pam_allow.so.1
</screen>

      <para lang="en">
        The above setup will cause no lastlog entry to be generated.  If a
        lastlog entry is desired, then use the following for the session:
      </para>

<screen>
       gdm-autologin session required pam_unix_session.so.1
</screen>

      <para lang="en">
        If the computer is used by several people, which makes automatic login
        unsuitable, you may want to allow some users to log in without entering
        their password. This feature can be enabled as a per-user option in
        the users-admin tool from the gnome-system-tools; it is achieved by
        checking that the user is member a Unix group called
        "nopasswdlogin" before asking for a password. For this to work,
        the PAM configuration file for the "gdm" service must include
        a line such as:
      </para>

<screen lang="en">
      gdm auth  sufficient  pam_succeed_if.so  user ingroup nopasswdlogin
</screen>

    </sect2>

    <sect2 id="utmpwtmp">
      <title>utmp and wtmp</title>

      <para>GDM generates utmp and wtmp User Accounting Database entries upon session login and logout. The utmp database contains user access and accounting information that is accessed by commands such as <command>finger</command>, <command>last</command>, <command>login</command>, and <command>who</command>. The wtmp database contains the history of user access and accounting information for the utmp database. Refer to the <ulink type="help" url="man:utmp">utmp</ulink> and <ulink type="help" url="man:wtmp">wtmp</ulink> man pages on your system for more information.</para>
    </sect2>

    <sect2 id="xauth">
      <title lang="en">Xserver Authentication Scheme</title>

      <para lang="en">
        Xserver authorization files are stored in a newly created subdirectory
        of <filename>&lt;var&gt;/run/gdm</filename> at start up.  These files
        are used to store and share a "password" between X clients
        and the Xserver.  This "password" is unique for each session
        logged in, so users from one session can't snoop on users from another.
      </para>

      <para lang="en">
        GDM only supports the MIT-MAGIC-COOKIE-1 Xserver authentication
        scheme.  Normally little is gained from the other schemes, and no
        effort has been made to implement them so far.  Be especially
        careful about using XDMCP because the Xserver authentication cookie
        goes over the wire as clear text.  If snooping is possible, then an
        attacker could simply snoop your authentication password as you log in,
        regardless of the authentication scheme being used.  If snooping is
        possible and undesirable, then you should use ssh for tunneling an X
        connection rather then using XDMCP.  You could think of XDMCP as a sort
        of graphical telnet, having the same security issues.  In most cases,
        ssh -Y should be preferred over GDM's XDMCP features.
      </para>

    </sect2>

    <sect2 id="xdmcpsecurity">
      <title>XDMCP Security</title>

      <para>Even though your display is protected by cookies, XEvents and keystrokes typed when entering passwords will still go over the network in clear text. It is trivial to capture these.</para>

      <para>XDMCP is primarily useful for running thin clients such as in terminal labs. Those thin clients will only ever need the network to access the server, and so it seems like the best security policy to have those thin clients on a separate network that cannot be accessed by the outside world, and can only connect to the server. The only point from which you need to access outside is the server. This type of set up should never use an unmanaged hub or other sniffable network.</para>

    </sect2>

    <sect2 id="xdmcpaccess">
      <title>XDMCP Access Control</title>

      <para>XDMCP access control is done using TCP wrappers. It is possible to compile GDM without TCP wrapper support, so this feature may not be supported on some Operating Systems.</para>

      <para>You should use the daemon name <command>gdm</command> in the <filename>&lt;etc&gt;/hosts.allow</filename> and <filename>&lt;etc&gt;/hosts.deny</filename> files. For example, to deny computers from <filename>.evil.domain</filename> from logging in, then add</para>
<screen>
gdm: .evil.domain
</screen>
      <para>to <filename>&lt;etc&gt;/hosts.deny</filename>. You may also need to add</para>
<screen>
gdm: .your.domain
</screen>
      <para>to your <filename>&lt;etc&gt;/hosts.allow</filename> if you normally disallow all services from all hosts. See the <ulink type="help" url="man:hosts.allow">hosts.allow(5)</ulink> man page for details.</para>
    </sect2>

    <sect2 id="firewall">
      <title>Firewall Security</title>

      <para lang="en">
        Even though GDM tries to outsmart potential attackers trying to take
        advantage of XDMCP, it is still advised that you block the XDMCP port
        (normally UDP port 177) on your firewall unless really needed.  GDM
        guards against denial of service attacks, but the X protocol is still
        inherently insecure and should only be used in controlled environments.
        Also each remote connection takes up lots of resources, so it is much
        easier to do a denial of service attack via XDMCP than attacking a
        webserver.
      </para>

      <para lang="en">
        It is also wise to block all of the Xserver ports.  These are TCP
        ports 6000+ (one for each display number) on your firewall.  Note that
        GDM will use display numbers 20 and higher for flexible on-demand
        servers.
      </para>

      <para lang="en">
         X is not a very safe protocol when using it over the Internet, and
         XDMCP is even less safe.
      </para>
    </sect2>

    <sect2 id="policykit">
      <title>PolicyKit</title>

<!--
<para>
        TODO - Should we say more?
</para>
-->

      <para>GDM may be configured to use PolicyKit to allow the system administrator to control whether the login screen should provide the shutdown and restart buttons on the greeter screen.</para>

      <para>These buttons are controlled by the <filename>org.freedesktop.consolekit.system.stop-multiple-users</filename> and <filename>org.freedesktop.consolekit.system.restart-multiple-users</filename> actions respectively. Policy for these actions can be set up using the polkit-gnome-authorization tool, or the polkit-auth command line program.</para>
 
    </sect2>

    <sect2 id="rbac">
      <title>RBAC (Role Based Access Control)</title>

      <para lang="en">
        GDM may be configured to use RBAC instead of PolicyKit.  In this
        case the RBAC configuration is used to control whether the login screen
        should provide the shutdown and restart buttons on the greeter screen.
      </para>

      <para lang="en">
        For example, on Oracle Solaris, the "solaris.system.shutdown"
        authorization is used to control this.  Simply modify the 
        <filename>/etc/user_attr</filename> file so that the "gdm"
        user has this authorization.
      </para>
    </sect2>

  </sect1>

  <!-- ============= ConsoleKit ================================ -->

  <sect1 id="consolekit">
    <title>Support for ConsoleKit</title>

<!--
<para>
    TODO - Should we update these docs?  Probably should mention any 
           configuration that users may want to do for using it with GDM?
           If so, perhaps this section should be moved to a subsection of
           the "Configure" section?
</para>
-->

    <para>GDM includes support for publishing user login information with the user and login session accounting framework known as ConsoleKit. ConsoleKit is able to keep track of all the users currently logged in. In this respect, it can be used as a replacement for the utmp or utmpx files that are available on most Unix-like Operating Systems.</para>

    <para lang="en">
      When GDM is about to create a new login process for a user it will call
      a privileged method of ConsoleKit in order to open a new session for this
      user.  At this time GDM also provides ConsoleKit with information about
      this user session such as: the user ID, the X11 Display name that will be
      associated with the session, the host-name from which the session
      originates (useful in the case of an XDMCP session), whether or not this
      session is attached, etc.  As the entity that initiates the user process,
      GDM is in a unique position to know about the user session and to be 
      trusted to provide these bits of information. The use of this privileged 
      method is restricted by the use of the D-Bus system message bus security 
      policy.
    </para>

    <para lang="en">
      In case a user with an existing session has authenticated
      at GDM and requests to resume that existing session, GDM calls a
      privileged method of ConsoleKit to unlock that session.  The exact
      details of what happens when the session receives this unlock signal are
      undefined and session-specific.  However, most sessions will unlock a
      screensaver in response.
    </para>

    <para>When the user chooses to log out, or if GDM or the session quit unexpectedly the user session will be unregistered from ConsoleKit.</para>
  </sect1>

  <!-- ============= Configuration ============================= -->

  <sect1 id="configuration">
    <title>Configuration</title>

    <para>GDM has a number of configuration interfaces. These include scripting integration points, daemon configuration, greeter configuration, general session settings, integration with gnome-settings-daemon configuration, and session configuration. These types of integration are described in detail below.</para>

    <sect2 id="scripting">
      <title>Scripting Integration Points</title>
      
      <para>The GDM script integration points can be found in the <filename>&lt;etc&gt;/gdm/</filename> directory:</para>

<screen>
Xsession
Init/
PostLogin/
PreSession/
PostSession/
</screen>

      <para>The <filename>Init</filename>, <filename>PostLogin</filename>, <filename>PreSession</filename>, and <filename>PostSession</filename> scripts all work as described below.</para>

      <para>For each type of script, the default one which will be executed is called "Default" and is stored in a directory associated with the script type. So the default <filename>Init</filename> script is <filename>&lt;etc&gt;/gdm/Init/Default</filename>. A per-display script can be provided, and if it exists it will be run instead of the default script. Such scripts are stored in the same directory as the default script and have the same name as the Xserver DISPLAY value for that display. For example, if the <filename>&lt;Init&gt;/:0</filename> script exists, it will be run for DISPLAY ":0".</para>

     <para lang="en">
        All of these scripts are run with root privilege and return 0 if run
        successfully, and a non-zero return code if there was any failure that
        should cause the login session to be aborted.  Also note that GDM will
        block until the scripts finish, so if any of these scripts hang, this
        will cause the login process to also hang.
      </para>

      <para lang="en">
        When the Xserver for the login GUI has been successfully started, but
        before the login GUI is actually displayed, GDM will run the
        <filename>Init</filename> script.  This script is useful for starting
        programs that should be run while the login screen is showing, or for
        doing any special initialization if required.
      </para>

      <para>After the user has been successfully authenticated GDM will run the <filename>PostLogin</filename> script. This is done before any session setup has been done, including before the pam_open_session call. This script is useful for doing any session initialisation that needs to happen before the session starts. For example, you might set up the user's $HOME directory if needed.</para>

      <para>After the user session has been initialised, GDM will run the <filename>PreSession</filename> script. This script is useful for doing any session initialisation that needs to happen after the session has been initialised. It can be used for session management or accounting, for example.</para>

      <para lang="en"> 
        When a user terminates their session, GDM will run the
        <filename>PostSession</filename> script.  Note that the Xserver will
        have been stopped by the time this script is run, so it should not be
        accessed.  
      </para>

      <para>Note that the <filename>PostSession</filename> script will be run even when the display fails to respond due to an I/O error or similar. Thus, there is no guarantee that X applications will work during script execution.</para>

      <para>All of the above scripts will set the <filename>$RUNNING_UNDER_GDM</filename> environment variable to <filename>yes</filename>. If the scripts are also shared with other display managers, this allows you to identify when GDM is calling these scripts, so you can run specific code when GDM is used.</para>
    </sect2>

    <sect2 id="autostart">
      <title>Autostart Configuration</title>
      
      <para lang="en">
        The <filename>&lt;share&gt;/gdm/autostart/LoginWindow</filename>
        directory contains files in the format specified by the
        "FreeDesktop.org Desktop Application Autostart
        Specification".  Standard features in the specification may be
        used to specify programs that should auto-restart or only be launched
        if a GConf configuration value is set, etc.
      </para>

      <para>Any <filename>.desktop</filename> files in this directory will cause the associated program to automatically start with the login GUI greeter. By default, GDM is shipped with files which will autostart the gdm-simple-greeter login GUI greeter itself, the gnome-power-manager application, the gnome-settings-daemon, and the Metacity window manager. These programs are needed for the greeter program to work. In addition, desktop files are provided for starting various AT programs if the configuration values specified in the Accessibility Configuration section below are set.</para>
    </sect2>

    <sect2 id="xsessionscript">
      <title>Xsession Script</title>

      <para lang="en">
        There is also an <filename>Xsession</filename> script located at
        <filename>&lt;etc&gt;/gdm/Xsession</filename> which is called between
        the <filename>PreSession</filename> and the
        <filename>PostSession</filename> scripts.  This script does not
        support per-display like the other scripts.  This script is used for
        actually starting the user session.  This script is run as the user,
        and it will run whatever session was specified by the Desktop session
        file the user selected to start. 
      </para>
    </sect2>

    <sect2 id="daemonconfig">
      <title>Daemon Configuration</title>

      <para lang="en">
        The GDM daemon is configured using the
        <filename>&lt;etc&gt;/gdm/custom.conf</filename> file.  Default
        values are stored in GConf in the <filename>gdm.schemas</filename>
        file.  It is recommended that end-users modify the
        <filename>&lt;etc&gt;/gdm/custom.conf</filename> file because the
        schemas file may be overwritten when the user updates their system to
        have a newer version of GDM.
      </para>

      <para>Note that older versions of GDM supported additional configuration options which are no longer supported in the latest versions of GDM.</para>

      <para>The <filename>&lt;etc&gt;/gdm/custom.conf</filename> file is in the <filename>keyfile</filename> format. Keywords in brackets define group sections, strings before an equal sign (=) are keys and the data after equal sign represents their value. Empty lines or lines starting with the hash mark (#) are ignored.</para>

      <para lang="en">
        The file <filename>&lt;etc&gt;/gdm/custom.conf</filename> supports the
        "[daemon]", "[security]", and "[xdmcp]"
        group sections.  Within each group, there are particular key/value
        pairs that can be specified to modify how GDM behaves.  For example,
        to enable timed login and specify the timed login user to be a user
        named "you", you would modify the file so it contains the
        following lines:
      </para>
     
<screen>
[daemon]
TimedLoginEnable=true
TimedLogin=you
</screen>

      <para>A full list of supported configuration keys follow:</para>

      <sect3 id="choosersection">
        <title>[chooser]</title>
        <variablelist>

          <varlistentry>
            <term>Multicast</term>
            <listitem>
              <synopsis>Multicast=false</synopsis>
              <para>If true and IPv6 is enabled, the chooser will send a multicast query to the local network and collect responses from the hosts who have joined the multicast group.</para>
            </listitem>
          </varlistentry>
         
          <varlistentry>
            <term>MulticastAddr</term>
            <listitem>
              <synopsis>MulticastAddr=ff02::1</synopsis>
              <para lang="en">
                This is the Link-local multicast address.
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="daemonsection">
        <title>[daemon]</title>
        <variablelist>
          <varlistentry>
            <term>TimedLoginEnable</term>
            <listitem>
              <synopsis>TimedLoginEnable=false</synopsis>
              <para>If the user given in <filename>TimedLogin</filename> should be logged in after a number of seconds (set with <filename>TimedLoginDelay</filename>) of inactivity on the login screen. This is useful for public access terminals or perhaps even home use. If the user uses the keyboard or browses the menus, the timeout will be reset to <filename>TimedLoginDelay</filename> or 30 seconds, whichever is higher. If the user does not enter a username but just hits the ENTER key while the login program is requesting the username, then GDM will assume the user wants to log in immediately as the timed user. Note that no password will be asked for this user so you should be careful, although if using PAM it can be configured to require password entry before allowing login. Refer to the "Security-&gt;PAM" section of the manual for more information, or for help if this feature does not seem to work.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>TimedLogin</term>
            <listitem>
              <synopsis>TimedLogin=</synopsis>
              <para>This is the user that should be logged in after a specified number of seconds of inactivity.</para>
              <para lang="en">
                If the value ends with a vertical bar | (the pipe symbol),
                then GDM will execute the program specified and use whatever
                value is returned on standard out from the program as the user.
                The program is run with the DISPLAY environment variable set so
                that it is possible to specify the user in a per-display
                fashion.  For example if the value is "/usr/bin/getloginuser|",
                then the program "/usr/bin/getloginuser" will be run to get the
                user value.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>TimedLoginDelay</term>
            <listitem>
              <synopsis>TimedLoginDelay=30</synopsis>
              <para>Delay in seconds before the <filename>TimedLogin</filename> user will be logged in.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>AutomaticLoginEnable</term>
            <listitem>
              <synopsis>AutomaticLoginEnable=false</synopsis>
              <para lang="en">
                If true, the user given in <filename>AutomaticLogin</filename>
                should be logged in immediately. This feature is like timed
                login with a delay of 0 seconds.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>AutomaticLogin</term>
            <listitem>
              <synopsis>AutomaticLogin=</synopsis>
              <para>This is the user that should be logged in immediately if <filename>AutomaticLoginEnable</filename> is true.</para>
              <para lang="en">
                If the value ends with a vertical bar | (the pipe symbol),
                then GDM will execute the program specified and use whatever
                value is returned on standard out from the program as the user.
                The program is run with the DISPLAY environment variable set so
                that it is possible to specify the user in a per-display
                fashion.  For example if the value is "/usr/bin/getloginuser|",
                then the program "/usr/bin/getloginuser" will be run to get the
                user value.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>User</term>
            <listitem>
              <synopsis>User=gdm</synopsis>
              <para>The username under which the greeter and other GUI programs are run. Refer to the <filename>Group</filename> configuration key and to the "Security-&gt;GDM User And Group" section of this document for more information.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>Group</term>
            <listitem>
              <synopsis>Group=gdm</synopsis>
              <para>The group name under which the greeter and other GUI programs are run. Refer to the <filename>User</filename> configuration key and to the "Security-&gt;GDM User And Group" section of this document for more information.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="debugsection">
        <title lang="en">Debug Options</title>
      
        <variablelist>
          <title lang="en">[debug]</title>
          
          <varlistentry>
            <term>Enable</term>
            <listitem>
              <synopsis>Enable=false</synopsis>
              <para lang="en">
                To enable debugging, set the debug/Enable key to
                "true" in the
                <filename>&lt;etc&gt;/gdm/custom.conf</filename>
                file and restart GDM.  Then debug output will be sent to the
                system log file (<filename>&lt;var&gt;/log/messages</filename>
                or <filename>&lt;var&gt;/adm/messages</filename> depending on
                your Operating System).
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="greetersection">
        <title lang="en">Greeter Options</title>
      
        <variablelist>
          <title lang="en">[greeter]</title>
          
          <varlistentry>
            <term lang="en">IncludeAll</term>
            <listitem>
              <synopsis lang="en">IncludeAll=true</synopsis>
              <para lang="en">
                If true, then the face browser will show all users on the local
                machine.  If false, the face browser will only show users who
                have recently logged in.
              </para>

              <para lang="en">
                When this key is true, GDM will call fgetpwent() to get a list
                of local users on the system.  Any users with a user id less
                than 500 (or 100 if running on Oracle Solaris) are filtered
                out.  The Face Browser also will display any users that have
                previously logged in on the system  (for example NIS/LDAP
                users).  It gets this list via calling the
                <command>ck-history</command> ConsoleKit interface.  It will
                also filter out any users which do not have a valid shell
                (valid shells are any shell that getusershell() returns -
                /sbin/nologin or /bin/false are considered invalid shells even
                if getusershell() returns them).
              </para>

              <para lang="en">
                If false, then GDM more simply only displays users that have
                previously logged in on the system (local or NIS/LDAP users) by
                calling the <command>ck-history</command> ConsoleKit interface.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term lang="en">Include</term>
            <listitem>
              <synopsis lang="en">Include=</synopsis>
              <para lang="en">
                Set to a list of users to always include in the Face Browser.
                This value is set to a list of users separated by commas.  By
                default, the value is empty.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term lang="en">Exclude</term>
            <listitem>
              <synopsis lang="en">Exclude=bin,root,daemon,adm,lp,sync,shutdown,halt,mail,news,uucp,operator,nobody,nobody4,noaccess,postgres,pvm,rpm,nfsnobody,pcap</synopsis>
              <para lang="en">
                Set to a list of users to always exclude in the Face Browser.
                This value is set to a list of users separated by commas.  Note
                that the setting in the <filename>custom.conf</filename>
                overrides the default value, so if you wish to add additional
                users to the list, then you need to set the value to the
                default value with additional users appended to the list.
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="securitysection">
        <title>Security Options</title>
      
        <variablelist>
          <title>[security]</title>
          
          <varlistentry>
            <term>DisallowTCP</term>
            <listitem>
              <synopsis>DisallowTCP=true</synopsis>
              <para>If true, then always append <filename>-nolisten tcp</filename> to the command line when starting attached Xservers, thus disallowing TCP connection. This is a more secure configuration if you are not using remote connections.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="xdmcpsection">
        <title>XDCMP Support</title>

        <variablelist>
          <title>[xdmcp]</title>
          
          <varlistentry>
            <term>DisplaysPerHost</term>
            <listitem>
              <synopsis>DisplaysPerHost=1</synopsis>
              <para>To prevent attackers from filling up the pending queue, GDM will only allow one connection for each remote computer. If you want to provide display services to computers with more than one screen, you should increase this value.</para>

              <para>Note that the number of attached DISPLAYS allowed is not limited. Only remote connections via XDMCP are limited by this configuration option.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>Enable</term>
            <listitem>
              <synopsis>Enable=false</synopsis>
              <para>Setting this to true enables XDMCP support allowing remote displays/X terminals to be managed by GDM.</para>
            
              <para><filename>gdm</filename> listens for requests on UDP port 177. See the Port option for more information.</para>
              
              <para>If GDM is compiled to support it, access from remote displays can be controlled using the TCP Wrappers library. The service name is <filename>gdm</filename></para>
            
              <para lang="en">
                You should add 
<screen lang="en">
gdm:.my.domain
</screen>
                to your <filename>&lt;etc&gt;/hosts.allow</filename>, depending
                on your TCP Wrappers configuration.  See the
                <ulink type="help" url="man:hosts.allow">hosts.allow</ulink>
                man page for details.
              </para>
              
              <para>Please note that XDMCP is not a particularly secure protocol and that it is a good idea to block UDP port 177 on your firewall unless you really need it.</para>
            </listitem>
          </varlistentry>
          
          <varlistentry>
            <term>HonorIndirect</term>
            <listitem>
              <synopsis>HonorIndirect=true</synopsis>
              <para>Enables XDMCP INDIRECT choosing (i.e. remote execution of <filename>gdmchooser</filename>) for X-terminals which do not supply their own display browser.</para>
            </listitem>
          </varlistentry>
        
          <varlistentry>
            <term>MaxPending</term>
            <listitem>
              <synopsis>MaxPending=4</synopsis>
              <para>To avoid denial of service attacks, GDM has fixed size queue of pending connections. Only MaxPending displays can start at the same time.</para>
            
              <para>Please note that this parameter does not limit the number of remote displays which can be managed. It only limits the number of displays initiating a connection simultaneously.</para>
            </listitem>
          </varlistentry>
          
          <varlistentry>
            <term>MaxSessions</term>
            <listitem>
              <synopsis>MaxSessions=16</synopsis>
              <para>Determines the maximum number of remote display connections which will be managed simultaneously. I.e. the total number of remote displays that can use your host.</para>
            </listitem>
          </varlistentry>
        
          <varlistentry>
            <term>MaxWait</term>
            <listitem>
              <synopsis>MaxWait=30</synopsis>
              <para>When GDM is ready to manage a display an ACCEPT packet is sent to it containing a unique session id which will be used in future XDMCP conversations.</para>
            
              <para>GDM will then place the session id in the pending queue waiting for the display to respond with a MANAGE request.</para>
            
              <para>If no response is received within MaxWait seconds, GDM will declare the display dead and erase it from the pending queue freeing up the slot for other displays.</para>
            </listitem>
          </varlistentry>
          
          <varlistentry>
            <term>MaxWaitIndirect</term>
            <listitem>
              <synopsis>MaxWaitIndirect=30</synopsis>
              <para>The MaxWaitIndirect parameter determines the maximum number of seconds between the time where a user chooses a host and the subsequent indirect query where the user is connected to the host. When the timeout is exceeded, the information about the chosen host is forgotten and the indirect slot freed up for other displays. The information may be forgotten earlier if there are more hosts trying to send indirect queries then <filename>MaxPendingIndirect</filename>.</para>
            </listitem>
          </varlistentry>
        
          <varlistentry>
            <term>PingIntervalSeconds</term>
            <listitem>
              <synopsis lang="en">PingIntervalSeconds=60</synopsis>
              <para lang="en">
                If the Xserver does not respond in the specified number of
                seconds, then the connection is stopped and the session ended.
                When this happens the daemon dies with an ALARM signal.
                Note that GDM 2.20 and earlier multiplied this setting by 2,
                so it may be necessary to increase the timeout if upgrading
                from GDM 2.20 and earlier to a newer version.
              </para>

              <para lang="en">
                Note that GDM in the past used to have a
                <filename>PingInterval</filename> configuration key which was
                also in minutes.  For most purposes you'd want this setting
                to be lower than one minute. However since in most cases where
                XDMCP would be used (such as terminal labs), a lag of more
                than 15 or so seconds would really mean that the terminal was
                turned off or restarted and you would want to end the session.
              </para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>Port</term>
            <listitem>
              <synopsis>Port=177</synopsis>
              <para>The UDP port number <filename>gdm</filename> should listen to for XDMCP requests. Do not change this unless you know what you are doing.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>Willing</term>
            <listitem>
              <synopsis>Willing=&lt;etc&gt;/gdm/Xwilling</synopsis>
              <para>When the machine sends a WILLING packet back after a QUERY it sends a string that gives the current status of this server. The default message is the system ID, but it is possible to create a script that displays customised message. If this script does not exist or this key is empty the default message is sent. If this script succeeds and produces some output, the first line of its output is sent (and only the first line). It runs at most once every 3 seconds to prevent possible denial of service by flooding the machine with QUERY packets.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>
    </sect2>

    <sect2 id="greeterconfiguration">
        <title>Simple Greeter Configuration</title>

        <para>The GDM default greeter is called the simple Greeter and is configured via GConf. Default values are stored in GConf in the <filename>gdm-simple-greeter.schemas</filename> file. These defaults can be overridden if the "gdm" user has a writable $HOME directory to store GConf settings. These values can be edited using the <command>gconftool-2</command> or <command>gconf-editor</command> programs. The following configuration options are supported:</para>

        <variablelist>
          <title>Greeter Configuration Keys</title>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/banner_message_enable</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para>Controls whether the banner message text is displayed.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/banner_message_text</term>
            <listitem>
              <synopsis>NULL (string)</synopsis>
              <para>Specifies the text banner message to show on the greeter window.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/disable_restart_buttons</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para>Controls whether to show the restart buttons in the login window.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/disable_user_list</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para>If true, then the face browser with known users is not shown in the login window.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/logo_icon_name</term>
            <listitem>
              <synopsis>computer (string)</synopsis>
              <para>Set to the themed icon name to use for the greeter logo.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/recent-languages</term>
            <listitem>
              <synopsis>[] (string list)</synopsis>
              <para>Set to a list of languages to be shown by default in the login window. Default value is "[]". With the default setting only the system default language is shown and the option "Other..." which pops up a dialogue box showing a full list of available languages which the user can select.</para>

              <para>Users are not intended to change this setting by hand. Instead GDM keeps track of any languages selected in this configuration key, and will show them in the language combo box along with the "Other..." choice. This way, commonly-selected languages are easier to select.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/recent-layouts</term>
            <listitem>
              <synopsis>[] (string list)</synopsis>
              <para>Set to a list of keyboard layouts to be shown by default in the login panel. Default value is "[]". With the default setting only the system default keyboard layout is shown and the option "Other..." which pops up a dialogue box showing a full list of available keyboard layouts which the user can select.</para>

              <para>Users are not intended to change this setting by hand. Instead GDM keeps track of any keyboard layouts selected in this configuration key, and will show them in the keyboard layout combo box along with the "Other..." choice. This way, commonly-selected keyboard layouts are easier to select.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>/apps/gdm/simple-greeter/wm_use_compiz</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para>Controls whether Compiz is used as the window manager instead of Metacity.</para>
            </listitem>
          </varlistentry>
        </variablelist>
    </sect2>

    <sect2 id="accessibilityconfiguration">
     <title>Accessibility Configuration</title>

      <para lang="en">
       This section describes the accessibility configuration options available
       in GDM.
      </para>

      <sect3 id="accessibilitydialog">
        <title lang="en">GDM Accessibility Dialog And GConf Keys</title>

        <para lang="en">
         The GDM greeter panel at the login screen displays an accessibility
         icon.  Clicking on that icon opens the GDM Accessibility Dialog.  In
         the GDM Accessibility Dialog, there is a list of checkboxes, so the
         user can enable or disable the associated assistive tools.
        </para>

        <para lang="en">
         The checkboxes that correspond to the on-screen keyboard, screen
         magnifier and screen reader assistive tools act on the three GConf
         keys that are described in the next section of this document. By
         enabling or disabling these checkboxes, the associated GConf key is
         set to "true" or "false".  When the GConf key is set to true, the 
         assistive tools linked to this GConf key are launched.  When the 
         GConf key is set to "false", any running assistive tool linked to
         this GConf key are terminated.  These GConf keys are not automatically
         reset to a default state after the user has logged in.  Consequently,
         the assistive tools that were running during the last GDM login
         session will automatically be launched at the next GDM login session.
        </para>

        <para lang="en">
         The other checkboxes in the GDM Accessibility Dialog do not have
         corresponding GConf keys because no additional program is launched to
         provide the accessibility features that they offer.  These other
         options correspond to accessibility features that are provided by the
         Xserver, which is always running during the GDM session.
        </para>
      </sect3>

      <sect3 id="accessibilitygconfconfiguration">
        <title lang="en">Accessibility GConf Keys</title>

        <para lang="en">
         GDM offers the following GConf keys to control its accessibility
         features:
        </para>

        <variablelist>
          <title lang="en">GDM Configuration Keys</title>

          <varlistentry>
            <term>/desktop/gnome/interface/accessibility</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para lang="en">
                Controls whether the Accessibility infrastructure will be
                started with the GDM GUI.  This is needed for many
                accessibility technology programs to work.
              </para>
            </listitem>
          </varlistentry>
          <varlistentry>
            <term>/desktop/gnome/applications/at/screen_magnifier_enabled</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para lang="en">
                If set, then the assistive tools linked to this GConf key will
                be started with the GDM GUI program.  By default this is a
                screen magnifier application.
              </para>
            </listitem>
          </varlistentry>
          <varlistentry>
            <term>/desktop/gnome/applications/at/screen_keyboard_enabled</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para lang="en">
                If set, then the assistive tools linked to this GConf key will
                be started with the GDM GUI program.  By default this is an
                on-screen keyboard application.
              </para>
            </listitem>
          </varlistentry>
          <varlistentry>
            <term>/desktop/gnome/applications/at/screen_reader_enabled</term>
            <listitem>
              <synopsis>false (boolean)</synopsis>
              <para lang="en">
                If set, then the assistive tools linked to this GConf key will
                be started with the GDM GUI program.  By default this is a
                screen reader application.
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="accessibilitytoolsconfiguration">
        <title lang="en">Linking GConf Keys to Accessibility Tools</title>

        <para lang="en">
         For the screen_magnifier_enabled, the screen_keyboard_enabled, and the
         screen_reader_enabled GConf keys, the assistive tool which gets
         launched depends on the desktop files located in the GDM autostart
         directory as described in the "Autostart Configuration" section of
         this manual.  Any desktop file in the GDM autostart directory can be
         linked to these GConf key via specifying that GConf key in the
         AutostartCondition value in the desktop file.  So the exact
         AutostartCondition line in the desktop file could be one of the
         following:
        </para>

<screen lang="en">
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_magnifier_enabled
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_reader_enabled
</screen>

        <para lang="en">
         When an accessibility key is true, then any program which is linked to
         that key in a GDM autostart desktop file will be launched (unless the
         Hidden key is set to true in that desktop file).  A single GConf key
         can even start multiple assistive tools if there are multiple desktop
         files with this AutostartCondition in the GDM autostart directory.
        </para>
      </sect3>

      <sect3 id="accessibilitytoolexample">
        <title lang="en">Example Of Modifying Accessibility Tool Configuration</title>

        <para lang="en">
         For example, if GNOME is distributed with GOK as the default on-screen
         keyboard, then this could be replaced with a different program if
         desired.  To replace GOK with the on-screen keyboard application
         "onboard" and additionally activate the assistive tool "mousetweaks"
         for dwelling support, then the following configuration is needed.
        </para>

        <para lang="en">
         Create a desktop file for onboard and a second one for mousetweaks;
         for example, onboard.desktop and mousetweaks.desktop. These files
         must be placed in the GDM autostart directory and be in the format
         as explained in the "Autostart Configuration" section of this
         document.
        </para>

        <para lang="en">
         The following is an example <filename>onboard.desktop</filename> file:
        </para>

<screen lang="en">
[Desktop Entry]
Encoding=UTF-8
Name=Onboard Onscreen Keyboard
Comment=Use an on-screen keyboard
TryExec=onboard
Exec=onboard --size 500x180 -x 20 -y 10
Terminal=false
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Accessibility;
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled
</screen>

        <para lang="en">
         The following is an example <filename>mousetweaks.desktop</filename>
         file:
        </para>

<screen lang="en">
[Desktop Entry]
Encoding=UTF-8
Name=Software Mouse-Clicks
Comment=Perform clicks by dwelling with the pointer
TryExec=mousetweaks
Exec=mousetweaks --enable-dwell -m window -c -x 20 -y 240 
Terminal=false
Type=Application
StartupNotify=true
Categories=GNOME;GTK;Accessibility;
AutostartCondition=GNOME /desktop/gnome/applications/at/screen_keyboard_enabled
</screen>

        <para lang="en">
         Note the line with the AutostartCondition that links both desktop
         files to the GConf key for the on-screen keyboard.
        </para>

        <para lang="en">
         To disable GOK from starting, the desktop file for the GOK on-screen
         keyboard must be removed or deactivated.  Otherwise onboard and GOK
         would simultaneously be started.  This can be done by removing the
         gok.desktop file from the GDM autostart directory, or by adding the
         "Hidden=true" key setting to the gok.desktop file.
        </para>

        <para lang="en">
         After making these changes, GOK will no longer be started when the
         user activates the on-screen keyboard in the GDM session; but onboard
         and mousetweaks will instead be launched.
        </para>
      </sect3>
    </sect2>

    <sect2 id="generalsessionconfig">
      <title>General Session Settings</title>
<!--
<para>
          TODO - I think this section should be expanded upon.  What specific
                 keys are of interest, or would some users be likely to want
                 to configure?  Also, would be good to be more specific about
                 how lock down management is handled.
</para>
-->
        <para>The GDM Greeter uses some of the same framework that your desktop session will use. And so, it is influenced by a number of the same GConf settings. For each of these settings the Greeter will use the default value unless it is specifically overridden by a) GDM's installed mandatory policy b) system mandatory policy. GDM installs its own mandatory policy to lock down some settings for security.</para>
    </sect2>

    <sect2 id="gnomesettingsdaemon">
      <title>GNOME Settings Daemon</title>
<!--
<para>
          TODO - I think this section should be expanded upon.  What specific
                 keys are of interest, or would some users be likely to want
                 to configure?  Also, would be good to give a more complete
                 list of plugins that users might want to consider disabling.
                 Also, shouldn't we list the sound/active key in the Greeter
                 configuration setting?  Oddly I do not find this key used
                 in anything but the chooser in SVN.
</para>
-->

        <para>GDM enables the following gnome-settings-daemon plugins: a11y-keyboard, background, sound, xsettings.</para>

        <para>These are responsible for things like the background image, font and theme settings, sound events, etc.</para>

        <para>Plugins can also be disabled using GConf. For example, if you want to disable the sound plugin then unset the following key: <filename>/apps/gdm/simple-greeter/settings-manager-plugins/sound/active</filename>.</para>
    </sect2>

    <sect2 id="sessionconfig">
      <title>GDM Session Configuration</title>

      <para>GDM sessions are specified using the FreeDesktop.org Desktop Entry Specification, which can be referenced at the following URL: <ulink url="http://www.freedesktop.org/wiki/Specifications/desktop-entry-spec"> http://www.freedesktop.org/wiki/Specifications/desktop-entry-spec</ulink>.</para>

      <para lang="en">
        By default, GDM will install desktop files in the
        <filename>&lt;share&gt;/xsessions</filename> directory.  GDM will
        search the following directories in this order to find desktop files:
        <filename>&lt;etc&gt;/X11/sessions/</filename>,
        <filename>&lt;dmconfdir&gt;/Sessions</filename>, 
        <filename>&lt;share&gt;/xsessions</filename>, and
        <filename>&lt;share&gt;/gdm/BuiltInSessions</filename>.  By default the
        <filename>&lt;dmconfdir&gt;</filename> is set to
        <filename>&lt;etc&gt;/dm/</filename> unless GDM is configured to use
        a different directory via the "--with-dmconfdir" option.
      </para>

      <para lang="en">
        A session can be disabled by editing the desktop file and adding a line
        as follows: <filename>Hidden=true</filename>.
      </para>

      <para lang="en">
        GDM desktop files support a GDM-specific extension, a key named
        "X-GDM-BypassXsession".  If the key is not specified in a
        desktop file, the value defaults to "false".  If this key is
        specified to be "true" in a desktop file, then GDM will
        launch the program specified by the desktop file "Exec" key
        directly when starting the user session.  It will not run the program
        via the <filename>&lt;etc&gt;/gdm/Xsession</filename> script, which is
        the normal behavior.  Since bypassing the
        <filename>&lt;etc&gt;/gdm/Xsession</filename> script avoids setting up
        the user session with the normal system and user settings, sessions
        started this way can be useful for debugging problems in the system or
        user scripts that might be preventing a user from being able to start
        a session.
      </para>

    </sect2>

    <sect2 id="userconfig">
      <title>GDM User Session and Language Configuration</title>
      <para>The user's default session and language choices are stored in the <filename>~/.dmrc</filename> file. When a user logs in for the first time, this file is created with the user's initial choices. The user can change these default values by simply changing to a different value when logging in. GDM will remember this change for subsequent logins.</para>

      <para>The <filename>~/.dmrc</filename> file is in the standard <filename>INI</filename> format. It has one section called <filename>[Desktop]</filename> which has two keys: <filename>Session</filename> and <filename>Language</filename>.</para>

      <para>The <filename>Session</filename> key specifies the basename of the session <filename>.desktop</filename> file that the user wishes to normally use without the <filename>.desktop</filename> extension. The <filename>Language</filename> key specifies the language that the user wishes to use by default. If either of these keys is missing, the system default is used. The file would normally look as follows:</para>

<screen>
[Desktop]
Session=gnome
Language=cs_CZ.UTF-8
</screen>
    </sect2>

  </sect1>

  <!-- ============= GDM Commands ============================= -->

  <sect1 id="binaries">
    <title>GDM Commands</title>

    <sect2 id="sbindir_binaries">
      <title>GDM Root User Commands</title>

      <para>The GDM package provides the following commands in <filename>sbindir</filename> intended to be run by the root user:</para>

      <sect3 id="gdmcommandline">
        <title lang="en"><command>gdm</command> Command Line Options</title>

        <para lang="en">
          <command>gdm</command> is the main daemon which sets up
          graphical login environment and starts necessary helpers.
       </para>

        <variablelist>
          <title lang="en"><command>gdm</command> Command Line Options</title>

          <varlistentry>
            <term>-?, --help</term>
            <listitem>
              <para>Gives a brief overview of the command line options.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>--fatal-warnings</term>
            <listitem>
              <para>Make all warnings cause GDM to exit.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>--timed-exit</term>
            <listitem>
              <para>Exit after 30 seconds. Useful for debugging.</para>
            </listitem>
          </varlistentry>

          <varlistentry>
            <term>--version</term>
            <listitem>
              <para>Print the version of the GDM daemon.</para>
            </listitem>
          </varlistentry>
        </variablelist>
      </sect3>

      <sect3 id="gdmrestartcommandline">
        <title><command>gdm-restart</command> Command Line Options</title>

        <para><command>gdm-restart</command> stops and restarts GDM by sending the GDM daemon a HUP signal. This command will immediately terminate all sessions and log out users currently logged in with GDM.</para>
      </sect3>

      <sect3 id="gdmsaferestartcommandline">
        <title><command>gdm-safe-restart</command> Command Line Options</title>
  
        <para><command>gdm-safe-restart</command> stops and restarts GDM by sending the GDM daemon a USR1 signal. GDM will be restarted as soon as all users log out.</para>
      </sect3>

      <sect3 id="gdmstopcommandline">
        <title><command>gdm-stop</command> Command Line Options</title>

        <para><command>gdm-stop</command> stops GDM by sending the GDM daemon a TERM signal.</para>
      </sect3>
    </sect2>
  </sect1>

  <!-- ============= Troubleshooting =========================== -->

  <sect1 id="troubleshooting">
    <title>Troubleshooting</title>
<!--
<para>
      TODO - any other tips we should add?  Might be useful to highlight any
             common D-Bus configuration issues?
</para>
-->

    <para>This section discusses helpful tips for getting GDM working. In general, if you have a problem using GDM, you can submit a bug or send an e-mail to the gdm-list mailing list. Information about how to do this is in the Introduction section of the document.</para>

    <para lang="en">
      If GDM is failing to work properly, it is always a good idea to include
      debug information.  To enable debugging, set the debug/Enable key to
      "true" in the <filename>&lt;etc&gt;/gdm/custom.conf</filename>
      file and restart GDM.  Then use GDM to the point where it fails, and
      debug output will be sent to the system log file
      (<filename>&lt;var&gt;/log/messages</filename> or
      <filename>&lt;var&gt;/adm/messages</filename> depending on your Operating
      System).  If you share this output with the GDM community via a bug
      report or email, please only include the GDM related debug information
      and not the entire file since it can be large.  If you do not see any
      GDM syslog output, you may need to configure syslog (refer to the
      <ulink type="help" url="man:syslog">syslog</ulink> man page).
    </para>

    <sect2 id="wontstart">
      <title>GDM Will Not Start</title>

      <para>There are a many problems that can cause GDM to fail to start, but this section will discuss a few common problems and how to approach tracking down a problem with GDM starting. Some problems will cause GDM to respond with an error message or dialogue when it tries to start, but it can be difficult to track down problems when GDM fails silently.</para>

      <para>First make sure that the Xserver is configured properly. The GDM configuration file contains a command in the [server-Standard] section that is used for starting the Xserver. Verify that this command works on your system. Running this command from the console should start the Xserver. If it fails, then the problem is likely with your Xserver configuration. Refer to your Xserver error log for an idea of what the problem may be. The problem may also be that your Xserver requires different command-line options. If so, then modify the Xserver command in the GDM configuration file so that it is correct for your system.</para>

      <para>Also make sure that the <filename>/tmp</filename> directory has reasonable ownership and permissions, and that the machine's file system is not full. These problems will cause GDM to fail to start.</para>
    </sect2>
  </sect1>

  <!-- ============= Application License ============================= -->

  <sect1 id="license">
    <title>Licence</title>
    <para lang="en">
      This program is free software; you can redistribute it and/or
      modify it under the terms of the  <ulink type="help" url="gnome-help:gpl">
      <citetitle>GNU General Public License</citetitle></ulink> as
      published by the Free Software Foundation; 
      either version 2 of the License, or (at your option) any later
      version.
    </para>
    <para>This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the <citetitle>GNU General Public Licence</citetitle> for more details.</para>
    <para lang="en">
      A copy of the <citetitle>GNU General Public License</citetitle> is
      included as an appendix to the <citetitle>GNOME Users
      Guide</citetitle>.  You may also obtain a copy of the
      <citetitle>GNU General Public License</citetitle> from the Free
      Software Foundation by visiting
      <ulink type="http" url="http://www.fsf.org">their Web site</ulink> or by
      writing to
      <address lang="en">
      Free Software Foundation, Inc.
      <street>51 Franklin Street, Fifth Floor</street>
      <city>Boston</city>, <state>MA</state> <postcode>02110-1301</postcode>
      <country>USA</country>
      </address>
    </para>
  </sect1>
</article>
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