Linux XDMCP HOWTO
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# 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 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!

Hummingbird Exceed's newer version now support the SSH connection. I am sure other X application may be able to do the same in their latest new version. Check the application web site you are using or the Resources belowfor for more details).

If you are using X-Win32 and you want to use SSH with Port Forwarding, you can use this reference to set it up.

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