This is simple guide about enable xdmcp service on Ubuntu 12.04 for user/root remote login. Precise uses the LightDM, so this method also works on ubuntu 11.10 with default LightDM display manager.

To enable xdmcp:

Edit lightdm config file with your favorite editor:

sudo vi /etc/lightdm/lightdm.conf

Add two lines at the end and save the file:

[XDMCPServer]
enabled=true

Restart and you can use Xmanager to remote login now!

If you would like root login in 12.04

First, enable root by giving it password:

sudo passwd root

Then add this line to ‘/etc/lightdm/lightdm.conf’:

greeter-show-manual-login=true

Restart and you can login by input USERNAEM and PASSWORD.