XDM and X Terminal mini-HOWTO
Kevin Taylor
kevin@northants.lug.org.uk
Revision History
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 to 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 (see [1]Section 6).
_________________________________________________________________
Table of Contents
1. [2]Introduction
1.1. [3]Copyright Information
1.2. [4]Disclaimer
1.3. [5]New Versions
1.4. [6]Credits
1.5. [7]Feedback
1.6. [8]To do
2. [9]Basic Concepts
2.1. [10]What is covered
2.2. [11]About this document
3. [12]XDM
3.1. [13]What is XDM
3.2. [14]What is an X terminal
4. [15]Configuring XDM
4.1. [16]Configuration Files
4.2. [17]Configuring XDM to manage X servers
4.3. [18]Configuring XDM for queries
4.4. [19]Starting X
4.5. [20]The Chooser Application
5. [21]Advanced Configuration Options
5.1. [22]Configuration Sets
6. [23]Resources
1. Introduction
_________________________________________________________________
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
<[24]linux-howto@metalab.unc.edu>
_________________________________________________________________
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.
_________________________________________________________________
1.3. New Versions
This is the initial release.
The latest version number of this document may be obtained from
[25]The Northants LUG, UK Project Pages .
_________________________________________________________________
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.
_________________________________________________________________
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 :
<[26]xdm-mini@northants.lug.org.uk>.
_________________________________________________________________
1.6. To do
I probably ought to mention some details about the following at some
point ...
* Security and authentication schemes.
* Debugging problems with XDM.
* Replacements for XDM - ie KDM and GDM.
* LBX ?
Any offers ?
_________________________________________________________________
2. Basic Concepts
_________________________________________________________________
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 [27]Section
6).
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 (see [28]Section 6).
_________________________________________________________________
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 !
_________________________________________________________________
3. XDM
_________________________________________________________________
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 [29]Section 6 for further information on the topics discussed
here.
_________________________________________________________________
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.
_________________________________________________________________
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 via 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 [30]Section 6 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 an 'X server' 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 loging on to the local machine itself, hence
would be quite capable of becoming a standalone workstation (ie
no network connectivity) if required.
Application Server
In the context of this document, I use the term 'application
server' to describe a machine that will provide a running X
session and applications (but not necessarily an actual
graphical console login prompt and local screen and keyboard).
X Server
This is a machine with a physical console (display, keyboard,
mouse, etc) that is able to provided these facilities as a
service to X clients. Please refer to the X User Howto in
[31]Section 6 for more details.
X Client
This is an application that requires the use of the X server to
provide a display and input.
Note that from the above descriptions, an X Workstation could be
thought of as consisting of a dumb X terminal and application server
running on a single 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 these setups.
_________________________________________________________________
3.2. What can XDM do
XDM is the utility that will be responsible for providing the users
login prompt and initiating their X session. This could be a local
session (in the case of an X workstation) or via a connection to an
application server from a diskless or dumb X terminal.
There are 2 main methods that can result with the user being presented
with a login prompt :
* X server query
* XDM server managed
_________________________________________________________________
3.2.1. X Server Query
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,
basically saying 'I have someone wanting to login - someone please
give me a login prompt'.
This facility 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 processes, but these will not be described
here - refer to the XDM and XDMCP documentation in [32]Section 6 for
more details.
_________________________________________________________________
3.2.2. XDM Server Managed
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, 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.
Note this assumes that the access control on the X server is such that
the application server XDM is able to connect back to the appropriate
display.
_________________________________________________________________
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 [33]Section 6 for additional information about
securing XDM and X terminals.
_________________________________________________________________
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.
Xaccess
Determines which machines can connect to XDM on this machine -
ie from which other machines on the network we are accepting
XDMCP queries.
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.
Xresources
Details of the X properties used by the XDM widgets (eg size of
the login 'box', colours, bitmap backgrounds, etc).
_________________________________________________________________
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.
_________________________________________________________________
4.3. Configuring XDM for queries
The file Xaccess determines which hosts may query XDM on this machine,
in order to be presented with a login prompt.
Example
# First line for direct queries
*
# Following line for indirect queries
* CHOOSER BROADCAST
This means that any host may connect to XDM directly (the first '*').
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 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 [34]Section 6 for
some links to more information).
_________________________________________________________________
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).
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
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.
_________________________________________________________________
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 given the 'chooser' widget, which will provide a list of
detected hosts on the network.
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 host 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 available (presumably even if one of the machines is
down ? Need to check this ...)
_________________________________________________________________
5. Advanced Configuration Options
_________________________________________________________________
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
_________________________________________________________________
6. 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.
The X Window System
+ X User Howto (from the [35]LDP)
+ Man pages : X (main concepts), Xserver (X server concepts)
+ X FAQ (on [36]www.x.org)
Thin-clients/X terminals
+ Thin-client Howto Document (from the [37]LDP)
+ Diskless Howto Document (from the [38]LDP)
+ [39]X terminal pages
XDM
+ Man pages : xdm
References
1. XDM-Xterm.html#RESOURCES
2. XDM-Xterm.html#INTRO
3. XDM-Xterm.html#COPYRIGHT
4. XDM-Xterm.html#DISCLAIMER
5. XDM-Xterm.html#NEWVERSIONS
6. XDM-Xterm.html#CREDITS
7. XDM-Xterm.html#FEEDBACK
8. XDM-Xterm.html#TODO
9. XDM-Xterm.html#BASICS
10. XDM-Xterm.html#COVERAGE
11. XDM-Xterm.html#ABOUT
12. XDM-Xterm.html#XDM
13. XDM-Xterm.html#XDM-WHAT
14. XDM-Xterm.html#XDM-XTERM
15. XDM-Xterm.html#CONFIG
16. XDM-Xterm.html#CONFIG-FILES
17. XDM-Xterm.html#XDM-XSERV
18. XDM-Xterm.html#XDM-QUERIES
19. XDM-Xterm.html#XDM-STARTX
20. XDM-Xterm.html#XDM-CHOOSER
21. XDM-Xterm.html#XDM-ADVCONFIG
22. XDM-Xterm.html#ADV-CONFIGSETS
23. XDM-Xterm.html#RESOURCES
24. mailto:linux-howto@metalab.unc.edu
25. http://www.northants.lug.org.uk/projects/xdm-mini/
26. mailto:xdm-mini@northants.lug.org.uk
27. XDM-Xterm.html#RESOURCES
28. XDM-Xterm.html#RESOURCES
29. XDM-Xterm.html#RESOURCES
30. XDM-Xterm.html#RESOURCES
31. XDM-Xterm.html#RESOURCES
32. XDM-Xterm.html#RESOURCES
33. XDM-Xterm.html#RESOURCES
34. XDM-Xterm.html#RESOURCES
35. http://www.linuxdoc.org/
36. http://www.x.org/
37. http://www.linuxdoc.org/
38. http://www.linuxdoc.org/
39. http://dns.solucorp.qc.ca/xterminals