This howto is old, unsupported, and relies on a broken package. This should be used as reference only.
So here's the complete list of steps that are required to set the VNC server that any user can login into and start a session. It is also persistent, meanning that even if you disconnect the VNC client your X session will not end (unless you explicitly log out) and you can reconnect to the same session again. The VNC server uses a separate display (:1) than your regular X server, which works with your physical display (:0). So two sessions can be active at the same time (one person sitting at the physical display and another remotely connecting using VNC).
1. Enable XDMCP
System->Administration->Login Screen Setup
Tab Security->Enable XDMCP
Tab XDMCP--> You can disable "Honor Indirect Requests"
Note: Before doing the next step, you need to make sure the extra repositories (e.g. universe) are enabled:
2. Install required packages (vncserver and xinetd)
Note to AMD64 users: The current version of vnc4server in the repositories has a bug, so you need to download and install the fixed vnc4 packages as shown below:
sudo apt-get install vnc4server xinetd
sudo dpkg -i vnc4server_4.0-7.3_amd64.deb
sudo dpkg -i xvnc4viewer_4.0-7.3_amd64.deb
3. Set the VNC passwd
4. Add vnc service to xinetd:
sudo vncpasswd /root/.vncpasswd
Enter this into the new file:
sudo gedit /etc/xinetd.d/Xvnc
5. Restart xinetd (usually there is no need to reboot, but occasionally it might be required)
type = UNLISTED
disable = no
socket_type = stream
protocol = tcp
wait = yes
user = root
server = /usr/bin/Xvnc
server_args = -inetd :1 -query localhost -geometry 1024x768 -depth 16 -once -fp /usr/share/X11/fonts/misc -DisconnectClients=0 -NeverShared passwordFile=/root/.vncpasswd
port = 5901
6. That's it! To test that this is working first try to connect from the same machine (the machine we just set up the VNC server on):
sudo /etc/init.d/xinetd stop
sudo killall Xvnc
sudo /etc/init.d/xinetd start
You should be prompted for the VNC password, and then see the GDM login screen where you can login and start a new X session. If that works, you can now go ahead and try to connect from remote machine using your favorite VNC client (remember to first close the local vncviewer we started above). Remember to use the VNC server machine's domain name or IP address, followed by :1 (e.g. 192.168.0.100:1). If connecting locally as shown above works, but connecting remotely fails, then this means you have a problem with a firewall which is blocking some ports. See the notes below about how to deal with that.
Note about ports: The VNC server set up as shown uses TCP port 5901. If you are using firewall software (e.g. firestarter) on that machine, you need to allow incoming connections on this port. If you are using a router which assigns your machine a private address (e.g. 192.168.0.100) which is not accessible from the internet, then you need to forward TCP port 5901 from the router to this machine.
Note about security: This setup allows any user to start an X-session remotely by logging in using his regular password (after starting the VNC connection using the VNC password), so if the user disconnects without logging out, any other user which knows the VNC password can connect afterwards and resume the same session that the first user started. So if you do not want to log out before disconnecting, it's advisable to at least lock your VNC X-session screen. Also note that while a remote user is connected thru VNC, no other connection will be accepted. An idle VNC client will be disconnected after one hour, but this can be changed by using the "-IdleTimeout" option in the server_args line in /etc/xinetd.d/Xvnc. For example, you can add "-IdleTimeout 300" to change it to 5 minutes.