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:
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’:
Restart and you can login by input USERNAEM and PASSWORD.