Linux XDMCP HOWTO

Thomas Chao

           <tomchao@lucent.com>
        
Revision History                                                             
Revision v1.3                    2 January 2003                              
Adding info for Red Hat 7.3 & 8.0, Mandrake 8.2 & 9.0, SuSE Linux            
configuration and contents update.                                           
Revision v1.2                    15 March 2002                               
Adding more info for Red Hat 7.2, Mandrake 8.1 and Slackware 8.0 Linux       
configuration and SSH X11 Forwarding.                                        
Revision v1.1                    20 March 2001                               
Revision and adding RH 7.0.                                                  
Revision v1.0                    01 November 2000                            
Initial revision and release.                                                


This HOWTO describes how you can use the combination of X Display Manager
(xdm, kdm and gdm) and XDMCP (X Display Manager Control Protocol) to provide
a solution for the X-Terminal and to provide a platform of efficient Remote X
Apps environment. This document will takes the focus on how to setup the X
connection using XDMCP.

-----------------------------------------------------------------------------
Table of Contents
1. Introduction
    1.1. Disclaimer
    1.2. Feedback
   
   
2. The Procedure
    2.1. Before you begin, some background
    2.2. Security Reminder
    2.3. The System I use
    2.4. Remote piece
    2.5. Server Preparation
    2.6. Steps to Complete the Procedures
    2.7. Testing
   
   
3. X11 Forwarding using SSH
4. Troubleshooting
5. XDMCP and GDM (Gnome Display Manager)
6. Additional References
7. Authors
8. Copyright Information

1. Introduction

XDMCP stands for "X Display Manager Control Protocol" and is a network
protocol. It provides a way of running the X-Terminal to run on your PC (or
MAC) and it uses the X Server to provide a client/server interface between
display hardware (the mouse, keyboard, and video displays) and the desktop
environment while also providing both the windowing infrastructure and a
standardized application interface (quoted from XFree86 Project home page).
The X-Terminal can be displayed with an individual window or multiple
windows, based on your X window system's software capabilities and setup.

I am always looking for the best way to use Linux, both at home and in work.
One of the biggest advantages among all is the ability to re-use the old
systems (like 486 and Pentium, Pentium II CPUs) as a Xterminal (by using the
Win32 apps; like Hummingbird's Exceed, Reflection X, X-Win32 or X-ThinPro.
For MAC, try eXodus) to run from any of your PC remotely. I found out,
somehow very surprising, that there are many documents on the INTERNET that
can help you to set it up, but not with a step by step HOW-TO format! This is
how I came up with this document as a way to share my experiences with all
users. By using X and XDMCP, you can build a good, reliable and not expansive
X- environment for your home or work IT solution.
-----------------------------------------------------------------------------

1.1. 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.
-----------------------------------------------------------------------------

1.2. 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 : <
tomchao@lucent.com>.
-----------------------------------------------------------------------------

2. The Procedure

This section details the procedure for setting up Xterminal using XDMCP. The
pre-requisite is to have a (any) Linux distribution installed and running X.
-----------------------------------------------------------------------------

2.1. Before you begin, some background

Before you begin, it is better to have a basic understanding of how this
works. (More details are at the [#REFS] Resources below and [http://
www.tldp.org] LDP HOWTO page)

The X server is usually started from the X Display Manager (DM). Almost all
the Linux distributions include the xdm, kdm and gdm to you as your choices.
(This document will use gdm and kdm as an example). The Display Manager
provides a nice and consistent interfaces for general users (X-based login,
starting up a window manager, clock, etc.). X Display Manager manages a
collection of X displays, which may be on the local host or remote servers.
It is worth noting that the Xsession file is what runs your environment.

When xdm runs, it offers display management in two different ways. It can
manage X Server running on the local machine and specified in "Xservers", and
/or it can manage remote X Servers (typically Xterminals) using XDMCP as
specified in the "Xaccess" file. (refer to the xdm man page).

For kdm (which comes with the KDE desktop), it is a replacement of xdm and
configures the same way, except its files are in /etc/X11/kdm in Caldera, /
etc/kde/kdm in Red Hat and /usr/share/config/kdm in Mandrake.

The gdm (Gnome Display Manager) is a re-implementation of the well known xdm.
gdm has similar functions to xdm and kdm, gdm is the Gnome Display Manager,
and its configuration files are found in /etc/X11/gdm/gdm.conf. The gdm.conf
file contains sets of variables and many options for gdm, and the Sessions
directory contains a script for each session option; each script calls /etc/
X11/xdm/Xsession with the appropriate option. gdm has similar functions to
xdm and kdm, but was written from scratch and does not contain any original
XDM / X Consortium code.

RH 8.0 introduces the new graphical interface called "Bluecurve". The new
interface is aimed for XP feel and styles. The setup makes no difference in
this case!

Other good references for the similar setup can be found in the following
documents:

  * The [http://ibiblio.org/pub/Linux/docs/HOWTO/mini/other-formats/
    html_single/XDM-Xterm.html] XDM and Xterminal mini-HOWTO, by Kevin Taylor
   
  * Linux [http://www.ibiblio.org/pub/Linux/docs/HOWTO/mini/other-formats/
    html_single/Remote-X-Apps.html] Remote X Apps mini HOWTO A very good
    reference for Remote X in both theoretical and practical view. By Vincent
    Zweije
   
  * The [http://www.ibiblio.org/pub/Linux/docs/HOWTO/unmaintained/mini/
    Xterminal] Xterminal mini-HOWTO, by Scot W. Stevenson
   

-----------------------------------------------------------------------------
2.2. Security Reminder

Using XDMCP is inherently insecure, therefore, most of the distributions
shipped as it's XDMCP default turned off. If you must use XDMCP, be sure to
use it only in a trusted networks, such as corporate network within a
firewall. Unfortunately, XDMCP uses UDP port 177 and TCP port 6000;
therefore, it is not natively able to use it with SSH. Currently, SSH1 and
SSH2 are not implemented to securely forward the UDP communication.

To secure the connection with SSH, the technique is called X11 TCP/IP Port
Forwarding. Check this [http://www.ox.compsoc.net/~steve/portforwarding.html]
Why Port Forwarding? site and the [#REFS] Resources area for additional
HOW-TO information. If you would like to experiment this, I have add a little
section below to show you how it works. I will give you only the basic idea
how it works, and I will leave the more advanced way of running it to other
experts and/or HOWTOs.
-----------------------------------------------------------------------------

2.3. The System I use

I have tested the setup running a GNOME (gdm), as well as KDE (kdm) on Red
Hat 6.0, 6.2 and Red Hat 7.x and 8.0. I also had a chance to test this on
Mandrake 7.2, 8.0, 8.2 and 9.0. SuSE 7.2 and Slakware 8.0's setup are tested
by the users, thanks to Peter Van Eerten and others, who helps the test for
this HOW-TO. I would like to thank all users who help me on this project).
The other I have tried on is Caldera eDesktop 2.4, which is similar to RH's
setup, except that it uses KDE. I have not had a chance to test it on other
Linux flavors like Debian and Slackware (Slackware users had told me it works
the same way as mentioned in this document). However, the setup should be
similar and should works fine. If you have successfully setup one other than
the Red Hat, Caldera and Mandrake platform, please share it with me. I will
add them into this document.

The PC hardware I use is an IBM PC clone running an AMD Athelon XP 1800+ with
384 MB memory and a 60 GB ATA-100 Hard Drive. This machine has been since
upgraded from Intel Pentium II 500 MHz PC. (I found out that my old Pentium
100 MHz PC runs this just fine). I use a built-in Fast Ethernet NIC in my new
AMD type M/B. In my old machine, I use the 3Com 10/100 (3C509B) NIC with an
ATAPI 48X CD-ROM and an IOMEGA ZIP drive. I have also test it on my Toshiba
Tecra 8100 laptop connecting using my Agere Wireless LAN card.
-----------------------------------------------------------------------------

2.4. Remote piece

I use the Hummingbird Exceed 7.0 (Exceed 6.x also works fine) on my PC and
have tested them on Windows 98 SE, Windows NT 4.0 and Windows 2000 Pro. I
found out that other popular choices are X-Win32 and X-ThinPro. There are
also many open-source apps as well as commercial one available.
-----------------------------------------------------------------------------

2.5. Server Preparation

In RH 7.x, you need to setup DNS lookup, in order for some networking
function to work properly (such as telnet that we will use to test the
setup). You can use "netstat -r" and/or "arp -a" command to verify your DNS
setup or response time. If you are in a small environment (like home or small
office, etc.) that do not have your own DNS and is relying on your ISP's DNS
Server, then add the entry of your DNS Server name(s) in the "resolv.conf"
file. If you are only use it in the lab or at home, then, you can add the
host name of all workstations in your local "host" table.

To prepare your X Server for XDMCP session, you need to make sure the
following are properly installed:

 1. Install your Linux OS. In my case, I installed Red Hat 7.3 (Custom
    Installation). If you plan to use SSH Port Forwarding, you need to
    install the OpenSSH package or compile SSH with your kernel. Also, RH 7.x
    comes with firewall installed by default (unless you choose not to). You
    may encounter problem, if you do not add firewall rules or temporary
    disable it in setting up XDMCP. I will not cover the firewall rules here
    in details, since this is not the focus of this document. I will share
    only how to make it works first and you can fine-tune it yourself.
   
    To show your firewall rules, in kernel 2.2x, use the command ipchains -L
    to list your default rule sets. To temporary disable it, use this command
    ipchains -F to flush the rules (Don't worry, it will restore by
    re-loading or re-boot). For kernel 2.4x, replace command ipchains with 
    iptables. One user shared with me that by adding this rule, you can do it
    without disable your firewall and can allow yourself to access the X
    Server (you can verify this yourself).
    +---------------------------------------------------------------+
    |ipchains -A input -p udp -i $extint --dport 177 -j ACCEPT      |
    +---------------------------------------------------------------+
   
    You should be able to use the iptables in the similar way. (Check for
    iptables references at the [#REFS] Resources area).
   
    For more firewall details, check the [http://www.ibiblio.org/pub/Linux/
    docs/HOWTO/other-formats/html_single/IP-Masquerade-HOWTO.html] IP
    Masquerade HOWTO page.
   
    One other easy way is to add rules that only accept certain IP address
    (es) from your trusted workstations. For using the command iptables,
    please feel free to experiment it. Again, I will not cover it here. I am
    the lucky one, because I use my company's firewall to protect me.
   
 2. Setup your Networking. To test it out, ping, ftp and telnet are good
    commands to use to determine if your network works. RH 7.x and up do not
    have telnet daemon turn on by default (for security reason). Remember to
    enable it, if you prefer to use it for your test. You can always turn it
    off when you are done (Using ntsysv with root privilege). One other thing
    is to remember firewall rules are there. Add your own rules or temporary
    disable it (as mentioned above) to make these commands work.
   
 3. Setup X. Do not setup with a resolution higher than what the remote users
    are able to use for their display. Test the X Server by typing either 
    startx or telinit 5. Make sure X is running properly.
   
 4. Creates the necessary user accounts (and associated groups) for user who
    will access via the Xterminal.
   

-----------------------------------------------------------------------------
2.6. Steps to Complete the Procedures

These are steps I used to setup the X Server for accepting XDMCP requests:

 1. In Linux X environment, you need to provide font using either X font
    server (xfs) or hard coded font path in XF86Config and XF86Config-4
    configuration files. If you plan to use xfs font server (check here to
    see the [http://www.redhat.com/docs/manuals/linux/RHL-7.3-Manual/
    ref-guide/s1-x-fonts.html] xfs advantages), do this in RH 6.2 and
    Mandrake 8.x and 9.0, modify /etc/rc.d/init.d/xfs and make the following
    changes. Change all (this is where the Font Server port):
    +---------------------------------------------------------------+
    |daemon xfs -droppriv -daemon -port -1                          |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |daemon xfs -droppriv -daemon -port 7100                        |
    +---------------------------------------------------------------+
   
    In Mandrake 7.2, the port is already set to 7100. Also, in RH 7.x, it is
    by default, for security enhancement, not listening to TCP port any
    longer! If you like to setup X font server, do the following steps:
   
    Change this line in /etc/rc.d/init.d/xfs:
    +---------------------------------------------------------------+
    |daemon xfs -droppriv -daemon                                   |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |daemon xfs -droppriv -daemon -port 7100                        |
    +---------------------------------------------------------------+
   
    Then, in /etc/X11/fs/config, comment out this line:
    +---------------------------------------------------------------+
    |# don't listen to TCP ports by default for security reasons    |
    |#no-listen = tcp                                               |
    +---------------------------------------------------------------+
   
    If you change or add the port, use this command to restart your X font
    server (requires root):
    +---------------------------------------------------------------+
    |service xfs restart                                            |
    +---------------------------------------------------------------+
   
    You do not have to use port 7100. You can set a different port, as long
    as you carefully plan it first to make sure no conflicts in using the
    port number and change it accordingly. It is better to consult your Linux
    admin before doing so, so that he/she knows the port has been taken!
    Different Linux distribution may put the xfs in different folder under /
    etc/rc.d. You may search for it if that's the case.
   
 2. Modify /etc/X11/xdm/xdm-config and make the following change. Be default
    (in most Linux distributions), this line is set, so that it is not
    listening to XDMCP connection. This is for security reason. For Caldera
    using kdm, this file is at /etc/X11/kdm. Find this line:
    +---------------------------------------------------------------+
    |DisplayManager.requestPort:     0                              |
    +---------------------------------------------------------------+
   
    and comment it out as:
    +---------------------------------------------------------------+
    |! DisplayManager.requestPort:     0                            |
    +---------------------------------------------------------------+
   
    Remember, this does not affects gdm. For gdm setup, it is in the
    following section.
   
 3. In /etc/X11/xdm/Xaccess, change this. (this allow all hosts to connect).
    For Caldera using kdm, this file is at /etc/X11/kdm. Set the security to
    644 (chmod 644):
    +---------------------------------------------------------------+
    |#*    # any host can get a login window                        |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |*     # any host can get a login window                        |
    +---------------------------------------------------------------+
   
    The above setup is in a Broadcast mode, which will list all the X Server
    that are listening and willing to manage your X connection. If you only
    want to allow certain connections, use the CHOOSER section in this same
    file. An example can be found in the [#REFS] Resources.
   
 4. I use the gdm as default and use gdm login window to switch between KDE
    and GNOME. For gdm, edit /etc/X11/gdm/gdm.conf. This activates XDMCP,
    causing it to listen to the request. For kdm (if you pick KDE as your DM
    in your installation), edit /usr/share/config/kdm/kdmrc for Mandrake and
    /etc/kde/kdm/kdmrc for Red Hat or /opt/kde2/share/config/kdm/kdmrc for
    Slackware version (KDE2). Change this line:
    +---------------------------------------------------------------+
    |[xdmcp]                                                        |
    |Enable=false (may shown as 0 in some distributions)            |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |Enable=true (or 1 in some distributions)                       |
    +---------------------------------------------------------------+
   
    Make sure "Port=177" is at the end of this block.
   
 5. Now edit /etc/inittab and change the following line:
    +---------------------------------------------------------------+
    |id:3:initdefault:                                              |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |id:5:initdefault:                                              |
    +---------------------------------------------------------------+
   
    In Slackware, the X11 mode is number "4", not "5".
   
    This is switching from Text Mode login to Graphical Mode using Display
    Manager. Before changing this line, you can use the telinit command to
    test prior to modifying the line. Use either telinit 3 to set to level 3,
    or telinit 5 to set to level 5, graphics mode (you can issue this command
    on the second machine that telnets into this server).
   
 6. Make sure the proper security of the file /etc/X11/xdm/Xservers is set to
    444 (chmod 444).
   
 7. Locate /etc/X11/xdm/Xsetup_0 and chmod 755 this file.
   
 8. Edit the XF86Config file (if you are using XFree86 4.x, the file is
    XF86Config-4) at /etc/X11 and change the line:
    +---------------------------------------------------------------+
    |FontPath    "unix/:-1"                                         |
    +---------------------------------------------------------------+
   
    to:
    +---------------------------------------------------------------+
    |FontPath    "unix/:7100"                                       |
    +---------------------------------------------------------------+
   
    If you decide to use the port number other than the usual 7100, be sure
    to change both in "/etc/rc.d/init.d/xfs" file and here!
   
    To save your time and energy, I recommend you to add the FontPath in the
    XF86Config and XF86Config-4 configuration files. If you are not sure what
    fonts are available to you, you can use this command to check it out
    (requires root):
    +---------------------------------------------------------------+
    |chkfontpath --list                                             |
    +---------------------------------------------------------------+
   
    The following are some of the example fonts for your reference. Make sure
    you have these fonts before editing these path.
    +---------------------------------------------------------------+
    |         FontPath  "/usr/X11R6/lib/X11/fonts/75dpi/"           |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/misc/"            |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/CID/"             |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/Speedo/"          |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/100dpi/"          |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/Type1/"           |
    +---------------------------------------------------------------+
   
 9. (You do not have to make this change. You can keep the default setting,
    but this is what I prefer. If you are not sure, leave this alone.) Change
    this line to the end of /etc/inittab:
    +---------------------------------------------------------------+
    |x:5:respawn:/usr/bin/gdm                                       |
    +---------------------------------------------------------------+
   
    If you decided not to change this line, it is fine! This is not a
    required step, but of a personal preference!
   

You are now ready to run a test.

One other thing to know (that some users have asked) is how to display with 
Willing to manage message with load info As I know this is available in xdm
by adding the following to the /etc/X11/xdm/xdm-config.
+---------------------------------------------------------------------------+
|DisplayManager.willing:  su noboby -c /etc/X11/xdm/Xwilling                |
+---------------------------------------------------------------------------+
and the XWilling script must exist. For gdm, add this line to the /etc/X11/
gdm/gdm.conf in [security] section:
+---------------------------------------------------------------------------+
|Willing=/etc/X11/gdm/Xwilling                                              |
+---------------------------------------------------------------------------+

A sample of [http://www.penguinlovers.net/linux/Xwilling] Xwilling script is
here for your reference. Adding this script or not is your preference. It is
not required step here!
-----------------------------------------------------------------------------

2.7. Testing

To test if your XDMCP with X Server is ready to accept connection(s), do
these steps. I find it easier using the X Server and another machine to test
it:

 1. (Re-)Start your X (which is in runlevel 5). If you are not sure how to do
    this, simply reboot your system (but this is really not necessary, if you
    know how to restart it using command line. That's the beauty of Linux,
    when comparing it to my Windows).
   
 2. If you have not modify your firewall rules, you need to temporary disable
    it by using iptables -F (or ipchains -F).
   
 3. Make sure the Graphical login page comes up. Make sure the display
    resolution and mouse work. Log in from the console to see if the local
    access is OK. If OK, do not log off.
   
 4. Setup Hummingbird Exceed (or other X Client software) to either query
    this machine (using the IP address or fully qualified DNS name) or set to
    use XDMCP-Broadcast and try to connect to the X Server. You should see
    the X Session come up and the login screen appear.
   

-----------------------------------------------------------------------------
3. X11 Forwarding using SSH

As I have explained earlier, using XDMCP to display X across Internet is
basically a no-no, due to it's lack of encryption across the Internet. One
way to enforce the traffic security is to use the SSH by the way of X11
tunnelling or port forwarding. SSH (Secure Shell) is developed in 1995 by
Tatu Ylonen to replace the insecure telnet, ftp, scp, rcp, rlogin, rsh, etc.
The first thing you need to know is that X11 forwarding using SSH is
different from your regular, non-secure way of running X Window.

To start this setup, you need an additional piece of information. First, you
must have your SSH package installed. In Linux, they are the OpenSSH
packages. Check your distribution to decide what package you need to install
(some installed it as standard packages). Secondly, you need a Windows SSH
Client (other OS version, like MAC, are also available). I recommend PuTTY.
It is a wonderful free SSH client and you can download them from [http://
www.chiark.greenend.org.uk/~sgtatham/putty/] this link. Remember to download
the document and read them carefully. The other good free SSH clients are:
Tera Term Pro + TTSSH: An SSH Extension to Tera Term, SSH Secure Shell Client
by SSH.com (only free for non-commercial use). I will break down again into
steps, so it is easy for you to follow.

 1. Open up the command putty.exe by double-click it. It will brings up the
    interface. First, setup the connection info in Host Name (or use IP)
    field and select SSH (SSH is using port 22). In Connection Category, find
    the Connection tree. In SSH, expand it and you will see Tunnels window.
    Click "Enable X11 forwarding". It is setting the default to X display at
    "localhost:0". Now, go back to Session and save this session with a name
    you like. I normally use the Host Name to make me easily remember where I
    am connecting to.
   
 2. In the example of Hummingbird Exceed, this is what you need to do. (For
    other X client, the setup is similar). Open up the Xconfig from your
    Exceed folder. In your "Screen Definition", change to "Multiple" Window
    mode and save it. Next, open up your "Communication" icon and set the
    Startup mode to "Passive".
   
 3. Now you are done. To test it, first using PuTTY (or other SSH client) to
    connect to your server. The first time connection, it will ask you
    whether you want to cache the Security Key or not. (Yes is normal
    choice). Once log in is done, fire up your Exceed. It will stay in the
    background. Now you can execute any of your X application and it should
    forward the X application via SSH to your local screen. For example:
    +---------------------------------------------------------------+
    |$ xclock &                                                     |
    +---------------------------------------------------------------+
   
    We should now see the Xclock is running on your local screen.
   

Now you see the difference is that you do not see all your X Window. You are
simply running X application one by one and forwarding via SSH to your local
screen. Therefore, you need to know the command for running each X
application. All the control are done via SSH client window. To me, the
security is worthy than the slightly inconvenience!

If you are using X-Win32 and you want to use [http://www.starnet.com/products
/ssh.htm] SSH with Port Forwarding, you can use this reference to set it up.
-----------------------------------------------------------------------------

4. Troubleshooting

  * If X cannot come up and is broken:
   
    If X is broken and the connection fails, most of the time it has this
    error messages:
    +----------------------------------------------------------------+
    |       _ FontTransSocketUNIXConnect: Can't connect: errno = 111 |
    |       failed to set dafault font path 'unix:-1'                |
    |       Fatal server error:                                      |
    |       could not open default font 'fixed'                      |
    +----------------------------------------------------------------+
   
    This is likely due to xfs not finding the correct port for the Font
    Server or the font path is not set correctly! To resolve this, check
    steps 1 and 8 above. Make sure the configuration are pointing to (port)
    7100 and make sure you have the following fonts installed (if not
    re-install the XFree86 font packages from your CD). Check the listing in
    XF86Config file (if you are using XFree86 4.x, the file is XF86Config-4)
    at /etc/X11:
    +---------------------------------------------------------------+
    |         FontPath  "/usr/X11R6/lib/X11/fonts/75dpi/"           |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/misc/"            |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/CID/"             |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/Speedo/"          |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/100dpi/"          |
    |         FontPath  "/usr/X11R6/lib/X11/fonts/Type1/"           |
    +---------------------------------------------------------------+
   
    Use the command startx (on local) to restart the X server (or use telinit
    5 to switch the run-level). To restart xfs, use the command in step 1.
   
    I found out in my RH 7.3 that if my xfs is not setup, it will crash the
    Exceed connection if I use the GNOME. (Using KDE is fine and it does not
    affect my Mandrake GNOME). After I fix it and start up my xfs, it works
    fine!
   
  * If Exceed has no respond (in blank screen):
   
    In this case, most likely your xdm (or gdm, depending upon which is used
    in /etc/inittab) is not starting correctly. Issue the command: ps -ef |
    grep gdm (or xdm or kdm, replace it in the command). Also, if your box
    has udp port turned on for XDMCP, you can type netstat -l | grep xdmcp
    and you should see this:
    +---------------------------------------------------------------+
    |         udp    0    0  *:xdmcp         *:*                    |
    +---------------------------------------------------------------+
   
    If the process is not running, check the steps on the setup above (make
    sure there are no typo's and that the correct path is given). Restart X
    using the command telinit 5. If the udp port is not there for XDMCP, do
    step 2 as above.
   
    Another possibilities are that your DNS setup is incorrect and/or
    firewall is enabled. An easy way to find out is simply ping or telnet
    your host and if the reply takes a long time, then that's DNS problem. If
    by using telnet and you got a "Connection Refused", then this is a
    firewall problem (assuming that you have your telnet daemon turned on
    already)! Check the section above for details how to resolve this.
   
  * PC Box with PPPoE (PPP over Ethernet):
   
    A user using PPPoE told me that if you have PPPoE, you might experience
    problem using XDMCP. After uninstall it, he then is able to get XDMCP
    working. I personally do not have the environment to test this, so you
    can test it yourself.
   
  * Linux to Linux Display export:
   
    If you are using another Linux with X, you do not need to use XDMCP to
    manage your display. You can actually export your display right from your
    X box. To do this, you must enable your access control to allow other to
    make connection to the X Server. The common error you will get without
    doing so are:
    +--------------------------------------------------------------------+
    |xlib: Connection refused (error 111): unable to connect to X server |
    |xlib: No such process (error 3): Server error                       |
    +--------------------------------------------------------------------+
   
    To resolve the problem, use the command below:
    +---------------------------------------------------------------+
    |$ xhost +                                                      |
    |$ export DISPLAY=(your local host IP):0.0                      |
    +---------------------------------------------------------------+
   
    Always remember to enable access control by using the command "xhost -"
    again. One thing to remind you, you do not need this, if you are using PC
    as X-Terminal using XDMCP. This is only required when you have Linux to
    Linux or Linux to UNIX connection.
   
    If you are using many Linux X boxes and you would like to setup the
    Chooser to pick from which X to login, you need to enable the following
    in the /etc/X11/gdm/gdm.conf:
    +-------------------------------------------------------------------+
    |[daemon]                                                           |
    |Chooser=/usr/bin/gdmchooser --disable-sound --disable-crash-dialog |
    |...                                                                |
    |[xdmcp]                                                            |
    |Enable=1                                                           |
    |HonorIndirect=1                                                    |
    +-------------------------------------------------------------------+
   
  * I got a "Signal 11" error:
   
    The "Signal 11" error, also called "Segmentation Fault", can sometimes be
    a problem of your hardware and/or software. If you have this problem in
    bring up the X Server, you need to fix it before configuring XDMCP.
    Unfortunately, there is no simple way to fix the problem due to many
    possible causes. For details, please check this [http://www.bitwizard.nl/
    sig11/] SIG 11 while compiling the Kernel.
   

-----------------------------------------------------------------------------
5. XDMCP and GDM (Gnome Display Manager)

The following is taken from the [http://www.oswg.org/oswg-nightly/oswg/
en_US.ISO_8859-1/articles/gdm-reference/gdm-reference/index.html] Gnome
Display Manager Reference Manual:

GDM also supports the X Display Manager Protocol (XDMCP) for managing remote
displays. GDM listens to UDP port 177 and will respond to QUERY and
BROADCAST_QUERY requests by sending a WILLING packet to the originator. GDM
can also be configured to honor INDIRECT queries and present a host chooser
to the remote display. GDM will remember the user's choice and forward
subsequent requests to the chosen manager. GDM only supports the
MIT-MAGIC-COOKIE-1 authentication system. Little is gained from the other
schemes, and no effort has been made to implement them so far. Since it is
fairly easy to do denial of service attacks on the XDMCP service, GDM
incorporates a few features to guard against attacks. Please read the XDMCP
reference section below for more information.

Even though GDM tries to outsmart potential attackers, it is still advised
that you block UDP port 177 on your firewall unless you really need it. GDM
guards against DoS attacks, but the X protocol is still inherently insecure
and should only be used in controlled environments. Even though your display
is protected by cookies the XEvents and thus the keystrokes typed when
entering passwords will still go over the wire in clear text. It is trivial
to capture these. You should also be aware that cookies, if placed on an NFS
mounted directory, are prone to eavesdropping too.
-----------------------------------------------------------------------------

6. Additional References

Some additional references on this subject include:

  * Your local xdm man page.
   
  * Your local gdm man page.
   
  * [http://www.linuxgazette.com/issue43/nielsen.xdm.html] Configuring XDM
   
  * [http://www.me.umn.edu/~kaszeta/unix/xterminal/config.html] Configuring
    Chooser through X Resources
   
  * [http://sun3.gs.uni-heidelberg.de/~malsburg/files/ports/xdmcp_udp.html]
    xdmcp/udp
   
  * [ftp://ftp.x.org/pub/R6.4/xc/doc/hardcopy/XDMCP/xdmcp.PS.gz] XDMCP
    Documentation (File download)
   
  * [http://www-uxsup.csx.cam.ac.uk/security/probing/about/xdmcp.html] Should
    you be running XDMCP?
   
  * [http://www.linuxgazette.com/issue27/kaszeta.html] X Window System
    Terminals
   
  * [http://www.tcu-inc.com/mark/projects/xdm/index2.html] A second way of
    using XDM
   
  * [http://www.linuxworld.com/linuxworld/lw-2000-09/lw-09-legacy_1.html]
    Accessing Xterms from Windows
   
  * [http://www.umanitoba.ca/campus/acn/support/xwin/xwininst.html] How to
    install X-Win32
   
  * [http://www.rru.com/~meo/pubsntalks/xrj/xdm.html] Taming the X Display
    Manager
   
  * [http://www.ox.compsoc.net/~steve/portforwarding.html] Why Port
    Forwarding?; [http://www.ssh.com/products/ssh/administrator30/
    Port_Forwarding.html] Port Forwarding; [http://www.ssh.com/products/ssh/
    administrator30/X11_Forwarding.html] SSH: X11 Forwarding
   
  * [http://www.5z.com/jirka/gdm.html] GNOME Display Manager
   
  * [http://www.linuxworld.com/site-stories/2001/0920.ipchains.html] 10
    minutes to an iptables-base Linux firewall; [http://www.onlamp.com/linux/
    cmd/i/iptables.html] iptables command introduction
   
  * [http://www.macworld.com/1996/05/features/2023.html] Through the X Window
    (for MAC)
   
  * [http://www.linuxsecurity.com/resource_files/host_security/
    securing-debian-howto/ch-sec-services.en.html] Securing Services on your
    system (Debian)
   
  * [http://linux-sxs.org/remotexkdm.html] Remote X using KDM (Caldera)
   

-----------------------------------------------------------------------------
7. Authors

Current: Thomas Chao, Lucent Technologies. <tomchao@lucent.com>
-----------------------------------------------------------------------------

8. Copyright Information

This document is copyrighted (c) 2000 - 2003 Thomas Chao and is distributed
under the terms of the Linux Documentation Project (LDP) license, stated
below.

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