X11: Which Display Does a GUI Client Talk To?

MediumKernel Security & Rootkits2-5 min

The brief

You SSH into a workstation and launch a graphical app, but it exits with 'cannot open display'. X11 is a client/server system: the X server owns the screen, keyboard, and mouse, and each GUI program is a client that must know which server (and screen) to connect to. Which environment variable carries that address (e.g. :0 for the local server, or host:10.0 for a forwarded one) so an X client knows where to draw?

What you'll do

Understand the X11 client/server split
Know the variable that points a client at an X server
Recognise why a headless/SSH session has no display set

Knowledge check: answer from what you've learned, graded on the server.