Stupid SSH Tricks: Yet Another X11 Protocol Tunneling Tutorial

24Jul08
Ingredients:
– One X server running on the machine in front of you. (Think of an X server as a virtual video monitor with thousands of input jacks and the remote applications as different video sources)
– One OpenSSH server on the system you are connecting to, that must be configured to (tunnel) forward X protocol

NOTE: If you are on the Windows platform, you can either purchase a commercial X server, or use Cygwin’s excellent free X server. Again… try to imagine that the X server on the machine in front of you “virtualizes” your monitor and makes it “networkable”. When you run the X server, it opens a connection on your machine listening for traffic coming from either the network, or (on *nix) shared memory. When you run an X application, it is instructed to connect to the X server so the Xserver can display it’s output. (Just like plugging in a game console to one of your TV/Monitor’s composite inputs)

Today we will talk about running X applications remotely using OpenSSH. Normally if you run X applications remotely, your X protocol traffic is going over your network connection out in the open. This is all well and good if you can trust the network that your X traffic is travelling on. But, what if you can’t? This is where ssh and X make a pretty good team. You still run your X server on the machine in front of you like usual. But instead of instructing the remote application to connect directly to it, you use OpenSSH’s X Protocol Forwarding so that all X traffic is sent through an encrypted TCP tunnel.

SSH Server Side Preparations
You will need to edit your sshd_config file which controls how your SSH server works. You would make these changes on the machine you are connecting to. First, find your sshd_config file. Typically, it’s in /usr/local/etc if you compiled the OpenSSH suite yourself or if the package maintainer went with defaults. In other cases it could be in /etc/openssh or /usr/local/etc/openssh. To verify for your distribution, you can run the ‘find’ command:

find /usr -name sshd_config

Once you’ve located it, make sure you add these lines to it for TCP and X Forwarding:

AllowTcpForwarding yes
X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes

You will then need to restart your ssh server so that it reads the new configuration.

SSH Client Side Preparations:

A quick aside about the .ssh directory in your home directory. Not everyone is familiar with the purpose of this directory, but to simplify using OpenSSH, it’s another essential tool. If you don’t already have one, create a text file in ~/.ssh called ‘config’ and add something like this to it, and save it:

host work
hostname 192.168.1.10
User george

Now, if I type ‘ssh work’ it will automatically try to connect to 192.168.1.10 passing ‘george’ as the user name. Obviously, you will need to adjust this appropriately to your correct IP and username. Combine that with a shared key for passwordless connection, and your life with OpenSSH becomes a lot easier. Now back to the task at hand.

Testing from the workstation in front of you:
Now we can test and see if we can forward a simple X app over the ssh tunnel. Assume the existence of ‘~/.ssh/config’ file and connection profile I create above. Also assume that the remote system has the simple X application ‘xeyes’ in /usr/bin:

ssh -X work “/usr/bin/xeyes”

We should now be seeing the familiar googly eyes peeking at us. All the application execution is happening remotely, but displaying in front of us and it’s coming over an encrypted ssh tunnel to boot!

Add X Forwarding to Your ~/.ssh/config File:
Instead of having to type ‘ssh -X work [some app]’, you can instead enable X forwarding from your ~/.ssh/config profiles. For each connection profile you create, you can add:

ForwardX11 yes

This means that all you would have to do to run a remote X app is either log into a shell using that profile and type the name of the X app you want to use, or… You can create a script to run the app using ssh and make an icon for it on your Windows Quick Launch bar or Gnome Panel. A sample script in Linux would be:

#!/bin/bash

ssh work “/usr/local/bin/gimp”

Pretty simple, huh? In Windows, you can write a CMD file using about the same syntax:

C:\cygwin\usr\local\bin\ssh work -F “C:\Documents and Settings\george\.ssh\config” “/usr/local/bin/gimp”

You can argue that this is a form of “application publishing” to use a friendly term. But it’s really a way of exploiting the features of X in a more secure way and without needing to open anything other than port 22 for OpenSSH. Once everything is configured, it works pretty seamlessly as well.

Compression:
This X traffic can take up a good deal of bandwidth since it is quite chatty back and forth, and I personally don’t use it unless I have a fast connection (DSL 1.5M or better). In the past I used to prefer ‘vnc’ over ssh for most instances and these days I use Nomachine NX protocol (which I will discuss at a later date) for remote desktop access. However, there is something you can do which might help out a little in terms of speed with X if you really don’t have any other options. You can compress your ssh traffic. Just add these lines to each host profile in your ~/.ssh/config file:

Compression yes
CompressionLevel 9

You can set your CompressionLevel to anything between 1 and 9 with ‘1’ being the fastest but worst compression, and 9 being slow but better. There is a slight improvement in X application performance. This compression applies across the board to any ssh traffic for that connection profile though, so it’s handy to add it to your slower connections.

Final Words:
Again, I don’t pretend to know everything there is about ssh or X and I am sure there are other ways that this can be done better. If you know of any, I am hoping that some of you will have more suggestions that readers here can share.  Please comment.

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