LDP/LDP/howto/docbook/XDM-Xterm.sgml

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<article id="index">

<!-- Header -->

<artheader>
  <title>XDM and X Terminal mini-HOWTO</title>

  <author>
   <firstname>Kevin</firstname>
   <surname>Taylor</surname>
   <affiliation>
     <address>
      <email>kevin@northants.lug.org.uk</email>
     </address>
   </affiliation>
  </author>

<revhistory>
  <revision>
    <revnumber>v0.05</revnumber>
    <date>14 November 2000</date>
    <authorinitials>kt</authorinitials>
    <revremark>
      Added cross-references to other Howtos
    </revremark>
  </revision>

  <revision>
    <revnumber>v0.04</revnumber>
    <date>6 November 2000</date>
    <authorinitials>kt</authorinitials>
    <revremark>
      Updates after first public draft.
    </revremark>
  </revision>

  <revision>
    <revnumber>v0.03</revnumber>
    <date>3 July 2000</date>
    <authorinitials>kt</authorinitials>
    <revremark>
      Minor updates from first comments
    </revremark>
  </revision>

  <revision>
    <revnumber>v0.02</revnumber>
    <date>28 June 2000</date>
    <authorinitials>kt</authorinitials>
    <revremark>
      First SGML source draft from HTML source
    </revremark>
  </revision>

  <revision>
    <revnumber>v0.01</revnumber>
    <date>27 June 2000</date>
    <authorinitials>kt</authorinitials>
    <revremark>
      First HTML source draft
    </revremark>
  </revision>
</revhistory>

  <abstract>
    <indexterm>
      <primary>template</primary>
    </indexterm>

    <para>
     This document describes the basic ideas for using XDM to manage
     X terminals.  It is not meant to be a comprehensive discussion
     of all the features of XDM, but a gentle introduction to what XDM
     can do for X terminals.
    </para>

    <para>
     For a full discussion about the installation and configuration
     of X terminals, please refer to the 'Thin-client' HOWTO, from
     the Linux Documentation Project or the Linux Terminal Server Project
     (see <xref linkend="resources">).
    </para>
  </abstract>

</artheader>

<!-- Section1: intro -->

<sect1 id="intro">
  <title>Introduction</title>

  <indexterm>
    <primary>disk!introduction</primary>
  </indexterm>

<!-- Section2: copyright -->

  <sect2 id="copyright">
   <title>Copyright Information</title>

   <para>
    This document is copyrighted (c) 2000 Kevin Taylor and is
    distributed under the terms of the Linux Documentation Project
    (LDP) license, stated below.
   </para>

   <para>
    Unless otherwise stated, Linux HOWTO documents are
    copyrighted by their respective authors. Linux HOWTO documents may
    be reproduced and distributed in whole or in part, in any medium
    physical or electronic, as long as this copyright notice is
    retained on all copies. Commercial redistribution is allowed and
    encouraged; however, the author would like to be notified of any
    such distributions.
   </para>

   <para>
    All translations, derivative works, or aggregate works
    incorporating any Linux HOWTO documents must be covered under this
    copyright notice. That is, you may not produce a derivative work
    from a HOWTO and impose additional restrictions on its
    distribution. Exceptions to these rules may be granted under
    certain conditions; please contact the Linux HOWTO coordinator at
    the address given below.
   </para>

   <para>
    In short, we wish to promote dissemination of this
    information through as many channels as possible. However, we do
    wish to retain copyright on the HOWTO documents, and would like to
    be notified of any plans to redistribute the HOWTOs.
   </para>

   <para>
    If you have any questions, please contact 
    <email>linux-howto@metalab.unc.edu</email>
   </para>
  </sect2>

<!-- Section2: disclaimer -->

  <sect2 id="disclaimer">
   <title>Disclaimer</title>

   <para>
    No liability for the contents of this documents can be accepted.
    Use the concepts, examples and other content at your own risk.
    As this is a new edition of this document, there may be errors
    and inaccuracies, that may of course be damaging to your system.
    Proceed with caution, and although this is highly unlikely,
    the author(s) do not take any responsibility for that.
   </para>

   <para>
    All copyrights are held by their by their respective owners, unless
    specifically noted otherwise.  Use of a term in this document
    should not be regarded as affecting the validity of any trademark
    or service mark.
   </para>

   <para>
    Naming of particular products or brands should not be seen 
    as endorsements.
   </para>

   <para>
    You are strongly recommended to take a backup of your system 
    before major installation and backups at regular intervals.
   </para>
  </sect2>

<!-- Section2: newversions-->

  <sect2 id="newversions">
   <title>New Versions</title>

    <indexterm>
     <primary>(your index root)!news on</primary>
    </indexterm>

   <para>
    The latest version number of this document may be 
    obtained from 
    <ulink url="http://www.northants.lug.org.uk/projects/xdm-mini/">
    The Northants LUG, UK Project Pages
    </ulink>.
   </para>
  </sect2>

<!-- Section2: credits -->

  <sect2 id="credits">
   <title>Credits</title>

   <para>
    Thanks go to the following people for help with information and
    proof reading of the document.
   </para>

   <para>
    Scot W Stevenson for the original X teriminal mini-howto document,
    from 1995, on which some of the material for the section on advanced
    xdm-config configurations was obtained.
   </para>

   <para>
    Members of the Northants LUG, UK for proof reading the document.
   </para>

   <para>
    The writers of the XDM, Xserver man pages and the default XDM scripts.
   </para>

   <para>
    Everyone who commented on the earlier versions, including Neil Zanella,
    Rafael Herrera, Paul Hornshaw, Clive Jones, Robert de Geus, Alex Schenkman,
    Richard Kaszeta, Malcolm Herbet
   </para>

  </sect2>

<!-- Section2: feedback -->

  <sect2 id="feedback">
   <title>Feedback</title>

   <para>
    Feedback is most certainly welcome for this document. Without
    your submissions and input, this document wouldn't exist. Please
    send your additions, comments and criticisms to the following
    email address : <email>xdm-mini@northants.lug.org.uk</email>.
   </para>
 </sect2>

<!-- Section2: todo -->

  <sect2 id="todo">
    <title>To do</title>

    <para>
    I probably ought to mention some details about the following at
    some point ...
    <itemizedlist>
    <listitem>
      <para>
      Debugging problems with XDM.
      </para>
    </listitem>
    <listitem>
      <para>
      Replacements for XDM - ie KDM and GDM.
      </para>
    </listitem>
    <listitem>
      <para>
      Integration with other Unix and Windows
      </para>
    </listitem>
    <listitem>
      <para>
      FAQs and common mistakes
      </para>
    </listitem>
    </itemizedlist>
    Any offers ?
    </para>
  </sect2>

</sect1>

<!-- Section1: intro: END -->

<!-- Section1: Basics -->

<sect1 id="basics">
  <title>Basic Concepts</title>

  <indexterm>
    <primary>disk!basics</primary>
  </indexterm>

<!-- Section2: Coverage -->

  <sect2 id="coverage">
    <title>What is covered</title>
 
    <para>
    This document describes the basic concepts behind using XDM
    (the X display manager) to manage X terminals and X servers,
    in order to provide 'thin-client' computing, using Linux.
    </para>

    <para>
    X (or the 'X Window System') is the windowing and graphics environment
    of choice for Unix systems.  Its particular strength (and the key bit
    that we are interested in for this document) is that it separates the
    running applications (web browser, word processor, etc) from the actual
    graphics screen and input devices (mouse, keyboard, etc) via a network
    communications mechanism.
    </para>

    <para>
    Essentially, this means that you can be running
    an application on one machine, but have its input and output redirected
    to another machine via a network.  This is the key feature that makes
    an X terminal possible.
    </para>

    <para>
    This document does not discuss the installation or
    configuration of a network or X on Linux.  Please refer to the
    appropriate HOWTO documents from the Linux Documentation Project for
    more details (see <xref linkend="resources">).
    </para>
    
    <para>
    This document should be treated as a 'getting started with XDM' document,
    in that it describes the basic terms and concepts for using XDM and
    X terminals, with simple examples that provide the minimum amount of
    security.
    </para>
    
    <para>
    The reader is advised to consult the list of resources provided at
    the end of the document in order to proceed beyound these basic
    facilities - in particular, the configuration of the 'authentication'
    and security settings should be examined, as the examples given in
    this document utilise the least secure modes of operation.
    </para>
    
    <para>
    Please note - the information in this document was obtained from
    systems running Debian 2.1, SuSE 6.4, Mandrake 7.0 and RedHat 6.0.
    </para>

    <para>
    This document does not attempt to describe how to install and configure
    Linux for an X terminal.  For this information, please refer to the
    'thin-client' HOWTO document, provided as part of the Linux Documentation
    Project, or the Linux Terminal Server Project (see <xref linkend="resources">).
    </para>
  </sect2>

<!-- Section2: About -->

  <sect2 id="about">
    <title>About this document</title>

    <para>
    This document came about because I wanted to experiment with Linux
    on a 486 PC as an X terminal to my main Linux box.
    </para>
    
    <para>
    After reading the man pages, specifications and current howto documents
    relating to XDM and X terminals, I ended getting really confused
    about where XDM was supposed to run and confusing XDM servers with
    X servers and the like, and so after an evening or two of
    experimentation, this document was born.
    </para>
    
    <para>
    Once the basic terminology has been sorted out, the documentation
    for XDM and self-documenting sample files makes very good reading
    - I just could not find a simple introduction to the basic concepts
    anywhere to get me started.  Hopefully this document could prove
    to be a suitable introduction to someone in a similar position to
    me.
    </para>
    
    <para>
    Oh, and in case you are wondering, a 486dx2/66 with 16 Mb RAM
    makes a fine X terminal !
    </para>
  </sect2>
</sect1>


<!-- Section1: Terms -->

<sect1 id="xdm">
  <title>XDM</title>

  <indexterm>
    <primary>disk!xdm</primary>
  </indexterm>

<!-- Section2: What is XDM -->

  <sect2 id="xdm-what">
    <title>What is XDM</title>

    <para>
    Put simply, XDM (the X Display Manager) can be thought of as a graphical
    replacement for the command line 'login' prompt.  In reality, it can
    actually do much more than that.
    </para>
    
    <para>
    Typcially, it would be started by the 'root' user (or the system startup
    scripts) on power up, and would present a user with a graphical login
    prompt.  It will then manage the users X session once they login -
    ie it will initiate the running of their window manager and applications.
    </para>

    <para>
    This could be considered a typical 'simple local machine login'
    configuration, as may be found installed by many Linux distributions
    by default.  However, XDM can also manage remote X servers and provide
    login prompts to remote 'X terminals'.  In short, it is not limited to
    the local machine - it can easily manage other machines connected via
    a network.
    </para>
    
    <para>
    XDM is a very configurable utility and this document will only just
    'scratch the surface' of what may be acheived.  This document aims to
    provide enough information to configure your X terminals and application
    servers to connect to each other.  The reader is refered to the
    <xref linkend="resources"> for further information on the topics
    discussed here.
    </para>

    <para>
    A note on security : X (in its default configuration) and XDMCP are not
    particularly secure.  I am assuming that you are running X on a
    'trusted' network and that security is not an issue.
    For details of how to toughen up your X connections
    (and more details about using the networking capabilities of X) please
    refer to the 'Running Remote X Applications' Howto document, which is
    also part of the LDP (see <xref linkend="resources">).
    </para>
  </sect2>

<!-- Section2: What is an X Terminal -->

  <sect2 id="xdm-xterm">
    <title>What is an X terminal</title>

    <para>
    This term could be used to cover various configurations, but at its
    simplest form, is a machine with a network connection, keyboard,
    mouse and monitor, configured to run the X windows system to connect
    to an application server somewhere on the network.
    </para>

    <para>
    There are several configurations of 'X terminal' with varying levels
    of functionality, ranging from completely diskless terminals to full
    X workstations.
    </para>
    
<!-- Section2: Terminolgy -->

  <sect2 id="xdm-terms">
    <title>Some Terminology</title>

    <para>
    Before I go any further, I ought to explain the terms I will be using
    in this document.  When talking about X, there is quite a lot of
    confusion over what is serving facilities to what.  This is especially
    true when you are considering distributed sessions over a network
    involving X terminals.  I will be using the terms described below.
    </para>

    <para>
    <variablelist>
      <VarListentry>
      <term> Diskless X terminal
      </term>
      <listitem>
        <para>
        These would be a machine with no local disks, which would perform its
        boot up from an EPROM (or similar) which utilises a network connection to a
        server.  This means obtaining the
        network configuration, operating system, system configuration and all
        applications from the server.  Once booted however, this
        would be the same as a 'dumb X terminal' (see below).  Typically this
        configuration would use a combination of the following network protocols
        in order to boot : BOOTP, DHCP, TFTP, etc.  Refer to
        <xref linkend="resources">
        for some references that detail how to build diskless clients.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> Dumb X terminal
      </term>
      <listitem>
        <para>
        This would be a machine that boots from its local disk into an operating
        system, and starts the 'X server' program and nothing more.  Somehow, a login
        prompt would be provided on the machine, to enable a user to login to an
        'application server' somewhere on the network.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> X Workstation
      </term>
      <listitem>
        <para>
        This would be similar to a dumb X terminal, but would provide the option
        of logging on to the local machine itself, hence would be quite capable of
        becoming a standalone workstation (ie no network connectivity) if required.
        Most distributions can be configured 'out of the box' as a stand-alone
        X Workstation, with a graphical login prompt.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> Application Server
      </term>
      <listitem>
        <para>
        In the context of this document, I use the term 'application server' to
        describe a machine that will provide the applications (X clients) that our X
        terminal will want to run.  This can include everything from editors and browsers,
        through to the actual 'Window Manager' itself.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> X Server
      </term>
      <listitem>
        <para>
        This is the program that manages the display of the machine with a physical
        console (display, keyboard, mouse, etc).  It can be thought of as a combined
        graphics card, keyboard and mouse 'driver'.
        This will provide these facilities as a service to X clients (hence the term
        'server').  Please refer to the X User Howto in
        <xref linkend="resources">
        for more details.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> X Client
      </term>
      <listitem>
        <para>
        This is an application that requires the use of an X server to provide
        input (keyboard and mouse) and output (display).  An X client cannot produce
        output without the services of the X server. The X server could be
        running locally (on the same machine) or elsewhere on the network.
        </para>
      </listitem>
      </VarListentry>
    </variablelist>
    </para>
    
    <para>
    From the above descriptions, an X Workstation could be thought of
    as consisting of a dumb X terminal and application server running on the same
    machine.
    </para>
    
    <para>
    This document will be looking at the architecture of the various options
    listed above and will describe the role that xdm can play in configuring
    them.
    </para>

  </sect2>

<!-- Section2: What can XDM do -->

  <sect2 id="xdm-do">
    <title>What can XDM do</title>

    <para>
    XDM is responsible for providing the user with a login
    prompt and initiating their X session.  It can manage local sessions (ie people
    logging into an X workstation) or sessions on remote machines, via a connection
    to an application server from a diskless or dumb X terminal.
    </para>
    
    <para>
    XDM has 2 main modes of operation :
    <itemizedlist>
      <listitem>
        <para>
        Query from an X server
        </para>
      </listitem>
      <listitem>
        <para>
        XDM managed X server
        </para>
      </listitem>
    </itemizedlist>
    </para>

    <sect3 id="xserv-query">
      <title>Query from an X Server</title>

      <para>
      The communications between XDM and the actual 'X server' (the machines
      with the physical screen/keyboard/mouse/etc) are handled via XDMCP the
      'X display manager control protocol'.
      </para>

      <para>
      This permits X servers to send out queries to servers running XDM.
      Effectively, the X server has to say 'I have someone wanting to login -
      please give me a login prompt'.  In this mode of operation, XDM will
      not do anything unless it is asked to by your X server.
      </para>
      
      <para>
      The query from the X server can take one of 3 forms :
      <itemizedlist>
        <listitem>
          <para>
          Direct query : the X server contacts a named host, requesting that the
          server presents a login prompt on its display
          </para>
        </listitem>
        <listitem>
          <para>
          Broadcast : the X server sends out a broadcast message to the network,
          and the first server running XDM that responds to the broadcast will
          be the one to present the login prompt on its display
          </para>
        </listitem>
        <listitem>
          <para>
          Indirect query : the X server contacts a named host, but asks it which
          other hosts it knows about on the network.  The named host will then
          present the user with a list of hosts to choose from, and will then
          go on to initiate communications with the selected host resulting in
          the selected host presenting a login prompt on the X servers display.
          </para>
        </listitem>
      </itemizedlist>
      </para>
      
      <para>
      There are several other options, but these will not be described here
      - refer to the XDM and XDMCP documentation in <xref linkend="resources">
      for more details.
      </para>
    </sect3>

    <sect3 id="xdm-server">
      <title>XDM Managed X Server</title>

      <para>
      If you have a set of machines (eg diskless or dumb X terminals) that just
      end up running an X server, all designed to provide a login prompt to a
      single application server, then it is possible to configure XDM on the
      application server to connect back to each X server and present its login
      prompt on each display automatically.
      </para>

      <para>
      In this mode of operation, XDM will actively 'push out' a login prompt
      to any listed X server that it knows about, without waiting for
      a query from the X servers themselves.
      </para>
      
      <para>
      In this case, the configuration file 'Xservers' lists each
      machine (including the local display, if required) to which XDM should
      connect, to display its login prompt.
      </para>
      
      <para>
      This configuration, when used with no remote X servers listed in the
      configuration, is the typical configuration used for a X workstation,
      in order to present a user with a graphical login to the local machine
      he is working on.  As stated earlier, most distributions will support
      this configuration 'out of the box' in order to present the user with
      a local graphical login prompt.
      </para>
      
      <para>
      Note: XDM must be permitted to connect to the X servers in question - so
      the access control on the X servers must be configured accordingly.
      </para>
    </sect3>

  </sect2>

</sect1>

<!-- Section1: config -->

<sect1 id="config">
  <title>Configuring XDM</title>

  <indexterm>
    <primary>disk!config</primary>
  </indexterm>

  <para>
  This section covers what needs to be configured for XDM to perform
  the functions described so far in this document.
  </para>
   
  <para>
  In each case, the configuration described is the minimum necessary to
  accomplish each goal.  In most cases this means that the configuration
  is also the least secure.  Please refer to some of the additional
  documentation listed in <xref linkend="resources">
  for additional information about securing XDM and X terminals
  (in particular the 'Running Remote X Applications Howto').
  </para>

<!-- Section2: Config Files -->

  <sect2 id="config-files">
    <title>Configuration Files</title>

    <para>
    This describes the following scheme of XDM configuration files :
    <itemizedlist>
      <listitem>
        <para>
        xdm-config
        </para>
      </listitem>
      <listitem>
        <para>
        Xaccess
        </para>
      </listitem>
      <listitem>
        <para>
        Xservers
        </para>
      </listitem>
      <listitem>
        <para>
        Xresources
        </para>
      </listitem>
    </itemizedlist>
    </para>
    
    <para>
    This will be found in (Debian 2.1. Mandrake 7.0.2, RedHat 6.2) :
      <screen>
      /etc/X11/xdm
      </screen>
    or (SuSE 6.4) :
      <screen>
      /usr/X11R6/lib/X11/xdm
      </screen>
    </para>

    <para>
    <variablelist>
      <VarListentry>
      <term> xdm-config
      </term>
      <listitem>
        <para>
        Defines the locations of the other configuration files and the basic
        access permissions.  For all distributions considered for this document,
        the file names were as listed here (but sometimes the locations varied).
        </para>

        <para>
        This also defines the scripts to be run for the various state
        transitions for an X session, ie on startup, etc.  You should
        not need to change these, as most distributions would appear to
        come with this pre-configured for you.
        </para>

        <para>
        Note that XDM managed X sessions have a different set of startup
        and configuration scripts to X sessions started via xinit or startx
        (ie non-XDM managed X sessions).
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> Xaccess
      </term>
      <listitem>
        <para>
        Determines which machines can connect to XDM - ie
        from which other machines on the network we are accepting XDMCP
        queries.  If a machine is not listed in this file, then it will not
        be able to request a login prompt from XDM.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> Xservers
      </term>
      <listitem>
        <para>
        Contains a list of machines that XDM will connect to, to provide a
        login prompt, automatically - ie those machines already running an X
        server, but would like this machine to provide the login prompt.
        </para>

        <para>
        This is only required for 'XDM Managed X Servers'.  You do not need
        any entries in this file if you will be relying on remote X servers
        to query XDM.
        </para>

        <para>
        When running as a stand-alone 'X Workstation', there is usually a
        single entry in this file, listing just the localhost.
        </para>
      </listitem>
      </VarListentry>
      
      <VarListentry>
      <term> Xresources
      </term>
      <listitem>
        <para>
        Details of the X properties used by the XDM widgets (eg size of the
        login 'box', colours, bitmap backgrounds, etc).
        </para>
      </listitem>
      </VarListentry>
    </variablelist>
    </para>
  </sect2>

<!-- Section2: XDM Managing X Servers -->

  <sect2 id="xdm-xserv">
    <title>Configuring XDM to Manage X Servers</title>

    <para>
    An entry must be placed in the Xservers file for each X server
    that XDM will be presenting a login prompt on.  This could include
    the local machine and/or a list of remote machines.
    </para>
    
    <para>
    Example
      <screen>
      # First the local host
      :0 local /usr/bin/X11/X vt7
      #
      # Then the remote hosts
      emma:0 foreign
      alex:0 foreign
      </screen>
    </para>
    
    <para>
    This will start XDM on the local machine and also present a login screen
    on the X servers running on the hosts 'emma' and 'alex' (assuming that
    permissions have been setup on 'emma' and 'alex' such that this machine
    is permitted to connect to the running X servers).
    </para>
    
    <para>
    Note that it is possible to specify the host and display (:0, :1, etc)
    if required, for example, if you are running multiple X servers on a
    single machine, etc.
    </para>
  </sect2>

<!-- Section2: XDM for queries -->

  <sect2 id="xdm-queries">
    <title>Configuring XDM to Receive Queries</title>

    <para>
    The file Xaccess determines which hosts may query XDM on this machine,
    in order to request a login prompt.
    </para>
    
    <para>
    Example
      <screen>
      # First line for direct queries
      *
      # Following line for indirect queries
      * CHOOSER BROADCAST
      </screen>
    </para>
    
    <para>
    This means that any host may request a login prompt via XDM (the first '*')
    using a direct query.
    </para>

    <para>
    The 'CHOOSER' line specifies which hosts can connect to XDM using
    indirect queries - in this case, any host may query this machine for
    a list of potential hosts to connect to (the second '*' line).
    </para>
    
    <para>
    'BROADCAST' means that the 'chooser' application on this machine
    will obtain its list of available servers (that will also be running XDM)
    via network broadcast queries.  I will talk about the 'chooser' later.
    </para>
    
    <para>
    It is possible to place specific host names in these entries (and there
    are also other indirect queries possible, without using the chooser)
    but this is not described here (refer to <xref linkend="resources">
    for some links to more information).
    </para>
  </sect2>

<!-- Section2: Starting X -->

  <sect2 id="xdm-startx">
    <title>Starting X</title>

    <para>
    The way you start the X server itself, will depend upon how you want it to
    interact with XDM locally and remotely.
    </para>
    
    <para>
    <variablelist>
      <varlistentry>
      <term> X Workstation : XDM and local X server
      </term>
      <listitem>
        <para>
        XDM will normally start X automatically for you and XDM will usually
        be configured to run as part of the startup process (via the init scripts).
        </para>

        <para>
        The Xservers file would typically contain a single entry - that of the
        local host, and the Xaccess file would only need to permit access from
        the local host.
        </para>
      </listitem>
      </varlistentry>
    
      <varlistentry>
      <term> X Terminal : Remote XDM
      </term>
      <listitem>
        <para>
        Just start X with no clients, with access permissions such that the
        remote XDM is able to connect when it starts up.  The following will start
        X with no access control :
          <screen>
          /usr/X11R6/bin/X -ac
          </screen>
        </para>

        <para>
        When the remote XDM is started, it will 'push out' a login prompt to
        all such configured X servers (as listed in its Xservers file).
        </para>
      </listitem>
      </varlistentry>
    
      <varlistentry>
      <term> X Terminal : Query a remote XDM
      </term>
      <listitem>
        <para>
        Recall there are 3 modes for queries : direct, indirect and broadcast
        (direct for a single host, broadcast for the first host that replies or
        indirect for a list of hosts) :
          <screen>
          /usr/X11R6/bin/X -query the.remote.host
          /usr/X11R6/bin/X -broadcast
          /usr/X11R6/bin/X -indirect the.remote.host
          </screen>
        </para>
      </listitem>
      </varlistentry>
    </variablelist>
    </para>
    
    <para>
    In each case, X will probably have to be started as root, and could be
    configured to be started up via the init scripts, if X is required
    automatically on startup.
    </para>
  </sect2>

<!-- Section2: The Chooser -->

  <sect2 id="xdm-chooser">
    <title>The Chooser Application</title>

    <para>
    When XDM receives an indirect query, and assuming that the option
    has been specified in Xaccess for the 'chooser' application, it can
    provide the user with a list of other XDM managed servers that it
    knows about.
    </para>

    <para>
    In this mode of operation, instead of the normal XDM login prompt,
    the user will be presented with a 'chooser' application, which will
    provide a list of detected hosts on the network that are currently
    accepting XDM connections.
    </para>

    <para>
    When I first tried the use the chooser, I found that the Xresources
    files that came with my SuSE and Debian systems, specified a size for
    the chooser widget that was too big for the screens ...  The following
    line from the Xresources file fixed that :
      <screen>
      Chooser*geometry:      700x500+300+200
      </screen>
    </para>

    <para>
    The chooser will obtain its lists of hosts by one of two methods :
    <itemizedlist>
    <listitem>
      <para>
      Broadcast Query : In this mode a request is broadcast over the network,
      and a list is built up from the replies received from other application
      servers running XDM.
      </para>
    </listitem>
    <listitem>
      <para>
      Explicit Listing : It is possible to provide a list of hosts for
      the chooser in the Xaccess file, as follows :
        <screen>
        %hostlist      emma alex liam
        *              CHOOSER %hostlist
        </screen>
      This will mean that the hosts emma, alex and liam will all be
      listed as candidates - even if one of the machines is down
      (there is often a button to 'ping' the host to see if it is
      running, before trying to connect to it).
      </para>
    </listitem>
    </itemizedlist>
    </para>

  </sect2>

</sect1>

<!-- Section1: Advanced Configuration -->

<sect1 id="xdm-advconfig">

  <title>Advanced Configuration Options</title>

  <indexterm>
    <primary>disk!advanced</primary>
  </indexterm>

<!-- Section2: Config Sets -->

  <sect2 id="adv-configsets">
    <title>Configuration Sets</title>

    <para>
    The xdm-config file provides a rich set of options, when it comes
    to defined scripts and other configuration files.  In many cases,
    the defaults provided with your distribution should be fine, but
    for those of you who want more ...
    </para>

    <para>
    The names of the startup scripts and configuration files used by
    XDM are determined by a series of statements in the top-level
    xdm-config file.  This permits you to configure a different set
    of files for different X servers and X terminals, with different
    abilities.
    </para>

    <para>
    For example, say you are using XDM to manage your local display, but
    also want it to accept queries from other X terminals on the network.
    It is possible to specify a different Xresources file for each of these
    cases, by using the following 2 lines in xdm-config :
      <screen>
      DisplayManager._0.resources            /etc/X11/xdm/Xres_0
      DisplayManager*resources               /etc/X11/xdm/Xresources
      </screen>
    This will use Xres_0 for the local display (_0 is the XDM way of
    saying :0) and Xresources for everything else (the '*').
    </para>

    <para>
    Note that XDM configuration files use the terminology host_0, where you
    would normally use host:0, to designate 'display 0 on host'.
    </para>

    <para>
    Similarly, if you wanted a particular resource file for a specific
    host, you would use an entry like the following :
      <screen>
      DisplayManager.host_0.resources       /etc/X11/xdm/Xres_host_0
      </screen>
    </para>

    <para>
    If you look over your default xdm-config file, you will probably
    find that it has been setup so that your local X server has different
    files to the remote ones anyway, as different things must be performed
    on startup and reset of the server.  My debian file has the following
    for local servers :
      <screen>
      DisplayManager._0.resources:    /etc/X11/xdm/Xresources_0
      DisplayManager._0.setup:        /etc/X11/xdm/Xsetup_0
      DisplayManager._0.startup:      /etc/X11/xdm/Xstartup_0
      DisplayManager._0.reset:        /etc/X11/xdm/Xreset_0
      </screen>
    and the following for remote servers :
      <screen>
      DisplayManager*resources:       /etc/X11/xdm/Xresources
      DisplayManager*setup:           /etc/X11/xdm/Xsetup
      DisplayManager*startup:         /etc/X11/xdm/Xstartup
      DisplayManager*reset:           /etc/X11/xdm/Xreset
      </screen>
    </para>

  </sect2>

<!-- Section2: X Resources -->

  <sect2 id="x-resources">
    <title>X Resources</title>

    <para>
    This document has only briefly touched on the available X resources, but
    I should mention that it is possible to fully configure XDM via the Xresources
    file.
    </para>

    <para>
    The following may all be changed if required :
    <itemizedlist>
    <listitem>
      <para> Fonts, login prompt sizes
      </para>
    </listitem>
    <listitem>
      <para> Background graphics
      </para>
    </listitem>
    <listitem>
      <para> Window Titles, etc
      </para>
    </listitem>
    </itemizedlist>
    </para>

    <para>
    There is a more detailed discussion of XDM resources, on Richard Kaszeta's web site
    (see <xref linkend="resources">)
    </para>

  </sect2>

</sect1>

<!-- Section1: Common Configurations -->

<sect1 id="common-configs">

  <title>Common Configurations</title>

  <indexterm>
    <primary>disk!advanced</primary>
  </indexterm>

<!-- Section2: Linux to Linux -->

  <sect2 id="cfgs-linux">
    <title>Linux to Linux</title>

    <sect3 id="cfgs-stdalonex">
      <title>Stand-alone X Workstation</title>

      <para>
        This has already been mentioned several times in this document, but
        in summary, it is a PC runing XDM, with a single entry in the Xservers
        file, relating to the localhost (ie the local machine)
      </para>
  
    </sect3>
    <sect3 id="cfgs-xtermapps">
      <title>X Terminal and Application Server</title>

      <para>
        XDM runs on the application server :
        <itemizedlist>
        <listitem>
          <para>
          Xserver : Contains no entries
          </para>
        </listitem>
        <listitem>
          <para>
          Xaccess : Must permit the X terminal to connect
          </para>
        </listitem>
        </itemizedlist>
      </para>

      <para>
        X terminal runs X using a direct query to the application server :
          <screen>
          /usr/X11R6/bin/X -query the.application.server
          </screen>
      </para>
    </sect3>

    <sect3 id="cfgs-mngexterm">
      <title>Group of Managed X Terminals</title>

      <para>
        XDM runs on an application server :
        <itemizedlist>
        <listitem>
          <para>
          Xserver : Lists each X terminal to be managed
          </para>
        </listitem>
        <listitem>
          <para>
          Xaccess : Must permit each X terminal to connect
          </para>
        </listitem>
        </itemizedlist>
      </para>

      <para>
        Each X terminal, just runs X, with suitable access control to
        permit XDM to connect to it.
        <screen>
          /usr/X11R6/bin/X -ac
        </screen>
      </para>
    </sect3>
  </sect2>

<!-- Section2: Linux to Other -->

  <sect2 id="cfgs-toother">
    <title>Linux to Other Systems</title>

    <para>
    It is possible to use a Linux X terminal to connect to another
    system running XDM.  The same principles as above apply, but
    the specifics of configuring XDM (or its equivalent) will be
    specific to that system.
    </para>

    <sect3 id="cfgs-linsol">
      <title> Linux and Solaris </title>

      <para>
         You can run X on a Linux box, instructing it to query
         a Solaris machine as previously described :
         <screen>
          /usr/X11R6/bin/X -query the.solaris.server
         </screen>
      </para>

      <para>
         Note that you may have to configure X on the Linux
         machine to use a font server from the Solaris box.
         I have not got this to work yet, as I don't have a
         Solaris box that I have any control over - but I am
         told that a font entry in /etc/XF86config similar to
         the following should work (I think - WARNING this is
         to be checked ...):
         <screen>
           tcp/solaris.box:7200
         </screen>
      </para>
    </sect3>
  </sect2>

<!-- Section2: Other to Linux -->

  <sect2 id="cfgs-tolinux">
    <title>Other Systems to Linux</title>

    <para>
    If you have an X server for your system, it should be able to connect
    to a Linux XDM application server.
    </para>

    <sect3 id="cfgs-sollin">
      <title>Solaris and Linux </title>

      <para>
         I assume that you would run XDM on the Linux box as usual, and
         instruct X on Solaris to query the Linux box - does anyone have
         any information on this ?
      </para>

    </sect3>

    <sect3 id="winlin">

      <title> Windows and Linux </title>

      <para>
         If you have an X server for windows that supports XDMCP queries, then
         it should be possible to configure it to query the Linux box.  You should
         just run XDM on the Linux box as usual.  Again - any specifics anyone ?
      </para>
    </sect3>
  </sect2>

</sect1>

<!-- Section1: resources -->

<sect1 id="resources">
  <title>Resources</title>

  <indexterm>
    <primary>disk!resources</primary>
  </indexterm>

  <para>
  This section lists some resources that have been consulted in order to
  construct this document and which provide further details to the concepts
  described.
  </para>

  <para>
  Many of the references listed below form part of the Linux Documentation
  Project (LDP) : <ulink url="http://www.linuxdoc.org/">
  http://www.linuxdoc.org/</ulink>
  </para>

  <para>
  <variablelist>
    <varlistentry>
    <term> The X Window System
    </term>
    <listitem>
      <para>
      <itemizedlist>
      <listitem>
        <para>
        X User Howto (from the
        <ulink url="http://www.linuxdoc.org/">LDP</ulink>)
        </para>
      </listitem>
      <listitem>
        <para>
        Running Remote X Applications Mini Howto (from the
        <ulink url="http://www.linuxdoc.org/">LDP</ulink>)
        </para>
      </listitem>
      <listitem>
        <para>
        Man pages : X (main concepts), Xserver (X server concepts)
        </para>
      </listitem>
      <listitem>
        <para>
        X FAQ (on
        <ulink url="http://www.x.org/">http://www.x.org/</ulink>)
        </para>
      </listitem>
      </itemizedlist>
      </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term> Thin-clients/X terminals
    </term>
    <listitem>
      <para>
      <itemizedlist>
      <listitem>
        <para>
        Thin-client Howto Document (from the
        <ulink url="http://www.linuxdoc.org/">LDP</ulink>)
        </para>
      </listitem>
      <listitem>
        <para>
        Linux Terminal Server Project : 
        <ulink url="http://www.ltsp.org/">http://www.ltsp.org/</ulink>
        </para>
      </listitem>
      <listitem>
        <para>
        Diskless Howto Document (from the
        <ulink url="http://www.linuxdoc.org/">LDP</ulink>)
        </para>
      </listitem>
      <listitem>
        <para>
        X terminal pages : 
        <ulink url="http://dns.solucorp.qc.ca/xterminals">http://dns.solucorp.qc.ca/xterminals</ulink>
        </para>
      </listitem>
      <listitem>
        <para>
        Richard Kaszeta : Converting old PCs into Standalone X Terminals :
        <ulink url="http://www.menet.umn.edu/~kaszeta/unix/xterminal/">http://www.menet.umn.edu/~kaszeta/unix/xterminal/
        </ulink>
        </para>
      </listitem>
      </itemizedlist>
      </para>
    </listitem>
    </varlistentry>

    <varlistentry>
    <term> XDM, XDMCP, etc
    </term>
    <listitem>
      <para>
      <itemizedlist>
      <listitem>
        <para>
        Man pages : xdm
        </para>
      </listitem>
      <listitem>
        <para>
        XDMCP Howto Document (from the 
        <ulink url="http://www.linuxdoc.org">LDP</ulink>)
        </para>
      </listitem>
      </itemizedlist>
      </para>
    </listitem>
    </varlistentry>
  </variablelist>
  </para>
</sect1>

</article>