You have an idle Ubuntu desktop machine running somewhere in your office, and you want to open-up an X session to run some Linux specific software, but you don’t want to physically be in front of the machine?
Configure the Ubuntu desktop to accept XDMCP clients :
# vi /etc/gdm/custom.conf ... [security] DisallowTCP=false ... [xdmcp] Enable=true ...
Restart GDM :
# service gdm restart
Now, from your Mac OS X desktop, run Xnest :
$ Xnest -geometry 1280x1024 -query ubuntu-desktop.example.net
That will display the GUI login screen from the Ubuntu desktop, and allow you to sign-in with a user, as if you where in front of the machine.
Hint: sound will be played on the remote Ubuntu machine, so you might wan’t to mute the volume in your X session before running something that might blast music at full volume in the remote room 🙂