2. The Procedure

This section details the procedures for setting up X-Terminal using XDMCP.

2.1. Before you begin, some backgrounds

Before you begin, it is better to have a basic understanding of how this works. (More details are at the Resources below and LDP HOWTO page)

The X server is usually started from the X Display Manager program (xdm, kdm and gdm. This document will use gdm as an example). It 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.

When xdm runs, it is usually run as a local copy of X, also xdm can listen for requests from remote hosts over a network. 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. The gdm ( Gnome Display Manager) is a reimplementation of the xdm program. gdm has similar funtions to xdm and kdm, but was written from scratch and does not contain any original XDM / X Consortium code.

In the case of xdm, it offers display management in two different ways. It can manage X servers running on the local machine and specified in Xservers, and it can manage remote X servers (typically X terminals) using XDMCP (the XDM Control Protocol) as specified in the Xaccess file. (Courtesy of xdm man page).

The XDM and Xterminal mini-HOWTO
Linux Remote X Apps mini HOWTO A very good reference for Remote X in both theoretical and practical view.
The Xterminal mini-HOWTO

2.2. Security Reminder

Using XDMCP is inherently insecure, therefore, most of the distributions shipped as its' XDMCP default turned off. If you must use XDMCP, be sure to use it only on a trusted networks, such as corporate network within a firewall. Unfortunely, XDMCP uses UDP, not TCP, therefore, it is not possible to forward XDMCP over SSH. Some people has success in X11 TCP/IP port Forwarding. Check this UC Berkeley Howto site for more info.

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 7.0. The other I have tried on are on Caldera eDesktop 2.4, which is similar to RH's setup. I have not had a chance to test it on other Linux flavors (but plan to do so for Debian and Slackware in the future). If you have successfully setup one other than the Red Hat platform, please share it with me. I will add them into this document.

My server hardware is an IBM PC clone running an Intel Pentium II 400 MHz with 128 MB memory and 30 MB ATA-66 Hard Drive. (I found out that 486 PC and my other Pentium 100 MHz PC runs this just fine). I use a 3COM 10/100 Fast Ethernet (3C509B) NIC.

2.4. Remote piece

I use the Hummingbird Exceed 6.x (with Service Pack) and have tested them on Windows 98 SE, Windows NT 4.0 and Windows 2000 Pro. I found out that another popular choice are X-Win32 and VNC. However, there are many open-source apps as well as commercial one available.

2.5. Server Preparation

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

Install your Linux OS. In my case, I installed Red Hat 6.2 (Custom Installation). I also tried on RH 7.0.
Setup your Networking. To test it out, ping and telnet are good comamnds to use to determine if your network works.
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.
Creates the necessary user accounts (and associated groups) for user who will access via the X-Terminal.

2.6. Steps to Complete the Procedures

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

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 RH 7.0, you do not need to do this, since by default, it is, for security enhancement, not listening to TCP port any longer! If you need to setup default font server to use, do it in /etc/X11/fs/config and add the setting there.
In /etc/X11/xdm/Xaccess, change (this allow all hosts to connect):
#* # any host can get a login window
to:
* # any host can get a login window
xdm usually run as a local copy of X and can listen for requests from remote hosts over a network. xdm reads its configuration files /etc/X11/xdm/xdm-config for all configuration and log files that xdm uses. For kdm, it is a replacement of xdm and configures the same way, except its files are in /etc/X11/kdm. It is worth noting that the Xsession file is what runs your environment.
The gdm (Gnome Display Manager) is a reimplementation of the well known xdm. gdm has similar funtions 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.
I use the gdm as default and use gdm login window to switch between KDE and GNOME. Edit /etc/X11/gdm/gdm.conf. This activates XDMCP, causing it to listen to the request. Change this:
[xdmcp] Enable=0
to:
Enable=1
Make sure "Port=177" is at the end of this block.
Now edit /etc/inittab and change the following line:
id:3:initdefault:
to:
id:5:initdefault:
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).
Make sure the proper security of the file /etc/X11/xdm/XServers set to 444 (chmod 444).
Locate /etc/X11/xdm/Xsetup_0 and chmod 755 this file.
Edit the XF86Config file in /etc/X11 and change the line, if you are using RH 6.2:
FontPath "unix:-1"
to:
FontPath "unix:7100"
(You do not have to make this change. You can keep the default setting, but this is what I use. If you are not sure, leave this alone.) Add this line to the end of /etc/inittab:
x:5:respawn:/usr/bin/gdm

You are now ready to run a test.

2.7. Testing

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

(Though you don't need to; it doesn't hurt...) Reboot the machine (I am assuming you are running level 5).
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.
Setup Hummingbird Exceed 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.