XDM and X Terminal mini-HOWTO
Kevin Taylor
������kevin@northants.lug.org.uk
�����
Revision History
Revision v0.05 14 November 2000 Revised by: kt
Added cross-references to other Howtos
Revision v0.04 6 November 2000 Revised by: kt
Updates after first public draft.
Revision v0.03 3 July 2000 Revised by: kt
Minor updates from first comments
Revision v0.02 28 June 2000 Revised by: kt
First SGML source draft from HTML source
Revision v0.01 27 June 2000 Revised by: kt
First HTML source draft
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.
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 Section 7).
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Table of Contents
1. Introduction
1.1. Copyright Information
1.2. Disclaimer
1.3. New Versions
1.4. Credits
1.5. Feedback
1.6. To do
2. Basic Concepts
2.1. What is covered
2.2. About this document
3. XDM
3.1. What is XDM
3.2. What is an X terminal
4. Configuring XDM
4.1. Configuration Files
4.2. Configuring XDM to Manage X Servers
4.3. Configuring XDM to Receive Queries
4.4. Starting X
4.5. The Chooser Application
5. Advanced Configuration Options
5.1. Configuration Sets
5.2. X Resources
6. Common Configurations
6.1. Linux to Linux
6.2. Linux to Other Systems
6.3. Other Systems to Linux
7. Resources
1. Introduction
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1.1. Copyright Information
This document is copyrighted (c) 2000 Kevin Taylor and is distributed under
the terms of the Linux Documentation Project (LDP) license, stated below.
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.
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.
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.
If you have any questions, please contact <linux-howto@metalab.unc.edu>
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1.2. Disclaimer
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.
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.
Naming of particular products or brands should not be seen as endorsements.
You are strongly recommended to take a backup of your system before major
installation and backups at regular intervals.
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1.3. New Versions
The latest version number of this document may be obtained from The Northants
LUG, UK Project Pages .
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1.4. Credits
Thanks go to the following people for help with information and proof reading
of the document.
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.
Members of the Northants LUG, UK for proof reading the document.
The writers of the XDM, Xserver man pages and the default XDM scripts.
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
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1.5. Feedback
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 : <
xdm-mini@northants.lug.org.uk>.
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1.6. To do
I probably ought to mention some details about the following at some point
...
��*�Debugging problems with XDM.
��*�Replacements for XDM - ie KDM and GDM.
��*�Integration with other Unix and Windows
��*�FAQs and common mistakes
Any offers ?
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2. Basic Concepts
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2.1. What is covered
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.
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.
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.
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 Section 7).
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.
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.
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.
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 Section 7).
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2.2. About this document
This document came about because I wanted to experiment with Linux on a 486
PC as an X terminal to my main Linux box.
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.
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.
Oh, and in case you are wondering, a 486dx2/66 with 16 Mb RAM makes a fine X
terminal !
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3. XDM
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3.1. What is XDM
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.
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.
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.
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 Section 7 for further information
on the topics discussed here.
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 Section 7).
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3.2. What is an X terminal
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.
There are several configurations of 'X terminal' with varying levels of
functionality, ranging from completely diskless terminals to full X
workstations.
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3.1. Some Terminology
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.
Diskless X terminal
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 Section 7 for some references that detail how to build diskless
clients.
Dumb X terminal
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.
X Workstation
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.
Application Server
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.
X Server
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 Section 7 for more details.
X Client
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.
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.
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.
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3.2. What can XDM do
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.
XDM has 2 main modes of operation :
��*�Query from an X server
��*�XDM managed X server
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3.2.1. Query from an X Server
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'.
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.
The query from the X server can take one of 3 forms :
��*�Direct query : the X server contacts a named host, requesting that the
server presents a login prompt on its display
��*�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
��*�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.
There are several other options, but these will not be described here - refer
to the XDM and XDMCP documentation in Section 7 for more details.
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3.2.2. XDM Managed X Server
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.
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.
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.
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.
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.
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4. Configuring XDM
This section covers what needs to be configured for XDM to perform the
functions described so far in this document.
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 Section 7 for additional information about securing XDM and X terminals
(in particular the 'Running Remote X Applications Howto').
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4.1. Configuration Files
This describes the following scheme of XDM configuration files :
��*�xdm-config
��*�Xaccess
��*�Xservers
��*�Xresources
This will be found in (Debian 2.1. Mandrake 7.0.2, RedHat 6.2) :
/etc/X11/xdm
or (SuSE 6.4) :
/usr/X11R6/lib/X11/xdm
xdm-config
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).
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.
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).
Xaccess
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.
Xservers
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.
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.
When running as a stand-alone 'X Workstation', there is usually a single
entry in this file, listing just the localhost.
Xresources
Details of the X properties used by the XDM widgets (eg size of the login
'box', colours, bitmap backgrounds, etc).
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4.2. Configuring XDM to Manage X Servers
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.
Example
# First the local host
:0 local /usr/bin/X11/X vt7
#
# Then the remote hosts
emma:0 foreign
alex:0 foreign
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).
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.
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4.3. Configuring XDM to Receive Queries
The file Xaccess determines which hosts may query XDM on this machine, in
order to request a login prompt.
Example
# First line for direct queries
*
# Following line for indirect queries
* CHOOSER BROADCAST
This means that any host may request a login prompt via XDM (the first '*')
using a direct query.
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).
'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.
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 Section 7 for some links to more information).
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4.4. Starting X
The way you start the X server itself, will depend upon how you want it to
interact with XDM locally and remotely.
X Workstation : XDM and local X server
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).
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.
X Terminal : Remote XDM
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 :
/usr/X11R6/bin/X -ac
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).
X Terminal : Query a remote XDM
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) :
/usr/X11R6/bin/X -query the.remote.host
/usr/X11R6/bin/X -broadcast
/usr/X11R6/bin/X -indirect the.remote.host
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.
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4.5. The Chooser Application
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.
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.
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 :
Chooser*geometry: 700x500+300+200
The chooser will obtain its lists of hosts by one of two methods :
��*�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.
��*�Explicit Listing : It is possible to provide a list of hosts for the
chooser in the Xaccess file, as follows :
%hostlist emma alex liam
* CHOOSER %hostlist
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).
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5. Advanced Configuration Options
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5.1. Configuration Sets
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 ...
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.
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 :
DisplayManager._0.resources /etc/X11/xdm/Xres_0
DisplayManager*resources /etc/X11/xdm/Xresources
This will use Xres_0 for the local display (_0 is the XDM way of saying :0)
and Xresources for everything else (the '*').
Note that XDM configuration files use the terminology host_0, where you would
normally use host:0, to designate 'display 0 on host'.
Similarly, if you wanted a particular resource file for a specific host, you
would use an entry like the following :
DisplayManager.host_0.resources /etc/X11/xdm/Xres_host_0
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 :
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
and the following for remote servers :
DisplayManager*resources: /etc/X11/xdm/Xresources
DisplayManager*setup: /etc/X11/xdm/Xsetup
DisplayManager*startup: /etc/X11/xdm/Xstartup
DisplayManager*reset: /etc/X11/xdm/Xreset
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5.2. X Resources
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.
The following may all be changed if required :
��*�Fonts, login prompt sizes
��*�Background graphics
��*�Window Titles, etc
There is a more detailed discussion of XDM resources, on Richard Kaszeta's
web site (see Section 7)
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6. Common Configurations
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6.1. Linux to Linux
6.1.1. Stand-alone X Workstation
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)
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6.1.2. X Terminal and Application Server
XDM runs on the application server :
��*�Xserver : Contains no entries
��*�Xaccess : Must permit the X terminal to connect
X terminal runs X using a direct query to the application server :
/usr/X11R6/bin/X -query the.application.server
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6.1.3. Group of Managed X Terminals
XDM runs on an application server :
��*�Xserver : Lists each X terminal to be managed
��*�Xaccess : Must permit each X terminal to connect
Each X terminal, just runs X, with suitable access control to permit XDM to
connect to it.
/usr/X11R6/bin/X -ac
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6.2. Linux to Other Systems
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.
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6.2.1. Linux and Solaris
You can run X on a Linux box, instructing it to query a Solaris machine as
previously described :
/usr/X11R6/bin/X -query the.solaris.server
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 ...):
tcp/solaris.box:7200
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6.3. Other Systems to Linux
If you have an X server for your system, it should be able to connect to a
Linux XDM application server.
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6.3.1. Solaris and Linux
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 ?
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6.3.2. Windows and Linux
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 ?
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7. Resources
This section lists some resources that have been consulted in order to
construct this document and which provide further details to the concepts
described.
Many of the references listed below form part of the Linux Documentation
Project (LDP) : http://www.linuxdoc.org/
The X Window System
��+�X User Howto (from the LDP)
��+�Running Remote X Applications Mini Howto (from the LDP)
��+�Man pages : X (main concepts), Xserver (X server concepts)
��+�X FAQ (on http://www.x.org/)
Thin-clients/X terminals
��+�Thin-client Howto Document (from the LDP)
��+�Linux Terminal Server Project : http://www.ltsp.org/
��+�Diskless Howto Document (from the LDP)
��+�X terminal pages : http://dns.solucorp.qc.ca/xterminals
��+�Richard Kaszeta : Converting old PCs into Standalone X Terminals :
http://www.menet.umn.edu/~kaszeta/unix/xterminal/
XDM, XDMCP, etc
��+�Man pages : xdm
��+�XDMCP Howto Document (from the LDP)