~darkxst/ubuntu/trusty/gdm/nokill

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<!ENTITY version "2.26.0">

<!ENTITY date "02/10/2009">

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<article id="index" lang="oc">

  <articleinfo>

    <title lang="en">GNOME Display Manager Reference Manual</title>

    <revhistory>

      <revision lang="en">

        <revnumber>0.0</revnumber>

        <date>2008-09</date>

      </revision>

    </revhistory>

    <abstract role="description">

      <para lang="en">

        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 lang="en">

	  Permission is granted to copy, distribute and/or modify this

	  document under the terms of the GNU Free Documentation

	  License (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 lang="en"> 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 license to the manual, as described in

          section 6 of the license.

	</para>

	<para lang="en">

	  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:

	  <orderedlist>

		<listitem>

		  <para lang="en">DOCUMENT IS PROVIDED ON AN "AS IS" BASIS,

                    WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR

                    IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES

                    THAT THE DOCUMENT OR MODIFIED VERSION OF THE

                    DOCUMENT IS FREE OF DEFECTS MERCHANTABLE, FIT FOR

                    A PARTICULAR PURPOSE OR NON-INFRINGING. THE ENTIRE

                    RISK AS TO THE QUALITY, ACCURACY, AND PERFORMANCE

                    OF THE DOCUMENT OR MODIFIED VERSION OF THE

                    DOCUMENT IS WITH YOU. SHOULD ANY DOCUMENT OR

                    MODIFIED VERSION PROVE DEFECTIVE IN ANY RESPECT,

                    YOU (NOT THE INITIAL WRITER, AUTHOR OR ANY

                    CONTRIBUTOR) ASSUME THE COST OF ANY NECESSARY

                    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

		  </para>

		</listitem>

		<listitem>

		  <para lang="en">UNDER NO CIRCUMSTANCES AND UNDER NO LEGAL

                       THEORY, WHETHER IN TORT (INCLUDING NEGLIGENCE),

                       CONTRACT, OR OTHERWISE, SHALL THE AUTHOR,

                       INITIAL WRITER, ANY CONTRIBUTOR, OR ANY

                       DISTRIBUTOR OF THE DOCUMENT OR MODIFIED VERSION

                       OF THE DOCUMENT, OR ANY SUPPLIER OF ANY OF SUCH

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                       EVEN IF SUCH PARTY SHALL HAVE BEEN INFORMED OF

                       THE POSSIBILITY OF SUCH DAMAGES.

		  </para>

		</listitem>

	  </orderedlist>

	</para>

  </legalnotice>

    <releaseinfo lang="en">

       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 lang="en">Terms and Conventions Used in This Manual</title>

    <para lang="en">

      This manual describes version 2.26.0 of the GNOME Display Manager.

      It was last updated on 02/10/2009.

    </para>  

    <para lang="en">

      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 lang="en">

      FreeDesktop - The organization 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 lang="en">

      GDM - GNOME Display Manager. Used to describe the software package as a

      whole.

    </para>

    <para lang="en">

      Greeter - The graphical login window (<command>gdm-simple-greeter</command>).

    </para>

    <para lang="en">

      PAM - Pluggable Authentication Mechanism

    </para>

    <para lang="en">

      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 lang="en">

      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 lang="en">Overview</title>

    <sect2 id="introduction">

      <title lang="en">Introduction</title>

      <para lang="en"> 

        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 lang="en">

        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 lang="en">

        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://www.gnome.org/projects/gdm/">

        http://www.gnome.org/projects/gdm</ulink> and the project

        Wiki <ulink type="http" url="http://live.gnome.org/GDM">

        http://live.gnome.org/GDM</ulink>.

      </para>

      <para lang="en">

        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 lang="en">

        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 lang="en">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 lang="en">

        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 lang="en">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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en"> 

        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 customize 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 initialized.  The <filename>PreSession</filename>

        script is called after session initialization.

      </para>

    </sect2>

    <sect2 id="greeterpanel">

      <title lang="en">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 lang="en">

        Note that keyboard layout features are only available on systems that

        support libxklavier.

      </para>

    </sect2>

    <sect2 id="accessibility">

      <title lang="en">Accessibility</title>

        <para lang="en">

          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 lang="en">

          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 lang="en">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 /apps/gdm/simple-greeter/disable_user_list GConf

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">XDMCP</title>

<!--

<para>

        TODO - What XDMCP features actually work?   I know that the

               chooser is missing.

</para>

-->

      <para lang="en">

        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 lang="en">

        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 lang="en">

        GDM can also be configured to honor 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 lang="en">

        If XDMCP seems to not be working, make sure that all machines are

        specified in <filename>/etc/hosts</filename>.

      </para>

      <para lang="en">

        Refer to the "Security" section for information about

        security concerns when using XDMCP.

      </para>

    </sect2>

    <sect2 id="logging">

      <title lang="en">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 lang="en">

        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 lang="en">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 lang="en">

        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 lang="en">Security</title>

    <sect2 id="gdmuser">

      <title lang="en">The GDM User And Group</title>

      <para lang="en">

        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 privilege.

      </para>

      <para lang="en">

        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 lang="en">

        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 lang="en">PAM</title>

      <para lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

        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 

        initializes the session.

      </para>

      <para lang="en">

        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 lang="en">

        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 lang="en">

        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 lang="en">

       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 lang="en">

       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 lang="en">utmp and wtmp</title>

      <para lang="en">

        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 lang="en">XDMCP Security</title>

      <para lang="en">

        Even though your display is protected by cookies, XEvents and thus

        keystrokes typed when entering passwords will still go over the wire in

        clear text.  It is trivial to capture these.

      </para>

      <para lang="en">

        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 lang="en">XDMCP Access Control</title>

      <para lang="en">

        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 lang="en">

        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 lang="en">

gdm: .evil.domain

</screen>

      <para lang="en">

        to <filename>&lt;etc&gt;/hosts.deny</filename>.  You may also need

        to add

      </para>

<screen lang="en">

gdm: .your.domain

</screen>

      <para lang="en">

        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 lang="en">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 lang="en">PolicyKit</title>

<!--

<para>

        TODO - Should we say more?

</para>

-->

      <para lang="en">

        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 lang="en">

        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 lang="en">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 lang="en">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 lang="en">

      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 lang="en">

      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 lang="en">Configuration</title>

    <para lang="en">

      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 lang="en">Scripting Integration Points</title>

      <para lang="en">

        The GDM script integration points can be found in the

        <filename>&lt;etc&gt;/gdm/</filename> directory:

      </para>

<screen lang="en">

Xsession

Init/

PostLogin/

PreSession/

PostSession/

</screen>

      <para lang="en">

        The <filename>Init</filename>, <filename>PostLogin</filename>,

        <filename>PreSession</filename>, and <filename>PostSession</filename>

        scripts all work as described below.

      </para>

      <para lang="en">

        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 lang="en">

        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 initialization that needs to

        happen before the session starts.  For example, you might setup the

        user's $HOME directory if needed.

      </para>

      <para lang="en">

        After the user session has been initialized, GDM will run the

        <filename>PreSession</filename> script.  This script is useful for

        doing any session initialization that needs to happen after the 

        session has been initialized.  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 lang="en">

        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>