Everything we have looked at so far has been about command-line remoting, with no mention so far of how to bring up a graphical user interface. There are two ways of doing this in Linux: the X Display Manager Control Protocol (XDMCP) and Virtual Network Computing (VNC). The former is specific to X Windows and is very tightly integrated with the rest of the graphical system but is also very insecure. VNC is more modern and very widespread but insecure in some implementations. Both are being used with Ubuntu, so we will cover both here.
Unless you have Ubuntu configured to log in a specific user automatically, you will be familiar with the user login screen that appears at bootup. What you are seeing is the Gnome Display Manager (GDM), which runs your X sessions, checks passwords, and so forth. What you are doing is logging in to the local machine because that is the default configuration.
However, GDM is also equipped to allow other network users to connect to your machine through the XDMCP protocol. There are various reasons for using XDMCP, of which the most popular is that many modern machines are large and noisy. They have big hard drives, CPUs with huge fans, and powerful graphics cards, and so do not fit into a peaceful living room. On the flip side, a thin client (a machine with very little CPU power and no hard disk of its own) is silent but not powerful enough to run Gnome or OpenOffice.org.
The solution is to have your powerful machine locked away in a cupboard somewhere with a Wi-Fi connection attached and your quiet thin client sitting in the lounge also on the Wi-Fi link. The thin client connects to the powerful machine and runs all its programs from there, with all the graphics being relayed over the network.
With Ubuntu, this is easy to do. Starting with the server side first, select System > Administration > Login Window; then select the Remote tab and change the Style option to "Plain with face browser". On the client side, go to the same dialog and make sure the Show Actions Menu box is checked from the Local tab.
Now, from the client side, log out from your desktop so you return to the Ubuntu login screen. When it prompts you for your username, look for the Options button and select Remote Login with XDMCP. A new dialog box appears with a list of local XDMCP servers that are willing to accept your connectionyou should see your server in there. Select it and click Connect; you will see a login screen from that server, inviting you to log in. You will, of course, need a valid account on the server to be able to log in; however, that is the only thing you need.
As you can see, because XDMCP is so core to the X Windows system, it is easy to set up. However, as you will find as you use it, XDMCP is very sloweven on a Gigabit Ethernet network, it will chew up a substantial percentage of bandwidth. It is also insecure. Anyone can monitor what you are doing with very little work. Because of these two flaws, XDMCP should never be used outside a trusted network.
The next step up from XDMCP is VNC, which was developed at AT&T's Cambridge Research Laboratory in England. VNC is widespread in the Linux world and, to a lesser extent, in the Windows world. Its main advantage is its widespread nature: Nearly all Linux distros bundle VNC, and clients are available for a wide selection of platforms.
To set up VNC, start Synaptic and install the vnc-common and xvncviewer packages. With that done, all that remains is to tell Ubuntu who should be allowed to connect to your session. This is done from the Remote Desktop option on the Preferences menu. By default, your desktop is not shared, so check Allow Other Users to View Your Desktop to share it. You should also check Allow Other Users to Control Your Desktop; otherwise, people will be able to see what you are doing but not interact with the desktopwhich is not very helpful.
The second set of options on that screen is important. If you are using this as a remote way to connect to your own desktop, deselect Ask You for Confirmation. If this is not done, when you try to connect from your remote location, Ubuntu will pop up a message box on the local machine asking should this person be allowed to connect? Because you are not there to click Yes, the connection will fail. If you want to let someone else remotely connect to your system, keep this box enabled so you know when people are connecting. You should always enter a password, no matter who it is that will connect. VNC, like XDMCP, should not be considered secure over the Internet, or even on untrusted networks.
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