Most people today are used to having a nice, intuitive graphical environment when they sit down to use a computer. Gone are the days of using a DOS machine or being lucky enough to have a dial-up account at 300 baud on a UNIX mainframe. Today, most people expect to be able to point and click their way to work nirvana, even when that work is being done on some other machine over a network. In this article, I look at some of the available options, what the relative costs and benefits are, and hopefully by the end, you should have enough information to make a choice as to what would be the best option for you.
The overwhelming majority of Linux users will be using the X11 Window System (typically shortened to just X11),built on a client-server model. In this model, the server is the part that actually draws on a physical screen. As such, it is the part you end up running on your desktop in front of you. The client portion in this model is the user-space program that has output that needs to be seen by the end user.
The client does this by sending requests to the X11 server. These are high level and of the form "Draw a window", "Add text to the window border", "Move the window to the right by x pixels". Because of this, there potentially can be a lot of communication if a lot is going on.
When most people experience X11, both parts of this model are running on the desktop in front of them. But, this isn't the only option. There is no reason for the client and server to be on the same machine, or even on the same continent. Because everything is done by sending messages, those messages easily can be sent over a network connection.
The first thought that should come to you is "Hey, I can open a window on my friend's machine and display rude pictures there." Unfortunately, the writers of the X11 specification have beaten you there and closed that particular hole. By default, X11 servers are configured not to accept connections from clients on the network. You have to allow this explicitly. You can use two separate mechanisms to allow external connections. The older one is called xhosts. With xhosts, you tell the X11 server to accept connections from the servers that you permit. The command looks like this:
xhost +111.222.333.444
The newer authentication mechanism is called xauth. In this method, the X11 server creates a cookie that is required in order to connect to the X11 server. If you want to connect to the X11 server from a remote machine, you need to copy this cookie over to the remote machine. An example of doing this, from the xauth man page is:
xauth extract - $DISPLAY | ssh otherhost xauth merge -
This command pulls out the cookie for the current X11 server and "SSHes" it over to the remote machine and merges it into the user's xauth file.
Once you have done this, how do client applications go ahead and connect to your X11 server? All programs that run under X11 accept certain command-line options. One of these is -display. With this option, you can tell your client program to which X11 server to connect and send its output. The general form of this option is:
-display hostname:display#.screen#
The reason for all of the parts of this option is that a given machine could be running more than one X11 server, and each server could be running more than one display screen. The first server and screen is labeled as 0.0, so in most cases you will use:
-display hostname:0.0
As an aside, you can do this on your desktop. If you open a terminal, you can run an X11 client program with:
-display :0.0
and it will show up on your default desktop. Cool and useful right? Keep reading about it here!