How to ssh to a running X instance remotely?

Question

I have an X instance running on my server computer, and on occasion I connect to it remotely via TeamViewer. Once in a while TeamViewer crashes and I cannot restart it remotely. Is there a way to login to my running X instance through ssh, and restart TeamViewer remotely?

Answer 1

An X program needs two pieces of information in order to connect to an X display.

• It needs the address of the display, which is typically :0 when you're logged in locally or :10, :11, etc. when you're logged in remotely (but the number can change depending on how many X connections are active). The address of the display is normally indicated in the DISPLAY environment variable.

• It needs the password for the display. X display passwords are called magic cookies. Magic cookies are not specified directly: they are always stored in X authority files, which are a collection of records of the form “display :42 has cookie 123456”. The X authority file is normally indicated in the XAUTHORITY environment variable. If $XAUTHORITY is not set, programs use ~/.Xauthority.

You're trying to act on the windows that are displayed on your desktop. If you're the only person using your desktop machine, it's very likely that the display name is :0. Finding the location of the X authority file is harder, because with gdm as set up under Debian squeeze or Ubuntu 10.04, it's in a file with a randomly generated name. (You had no problem before because earlier versions of gdm used the default setting, i.e. cookies stored in ~/.Xauthority.)

If this is a one-shot, you can detect the values of DISPLAY and XAUTHORITY from a running process. This is awkward to automate. You have to figure out the PID of a process that's connected to the display you want to work on, then get the environment variables from /proc/$pid/environ (eval export $(</proc/$pid/environ tr \\0 \\n | grep -E '^(DISPLAY|XAUTHORITY)=')).

A long-term, automatic solution is to copy cookies when you log into your desktop X session. Add the following lines to ~/.profile (or some other script that is read when you log in):

case $DISPLAY:$XAUTHORITY in
  :*:?*)
    # DISPLAY is set and points to a local display, and XAUTHORITY is
    # set, so merge the contents of `$XAUTHORITY` into ~/.Xauthority.
    XAUTHORITY=~/.Xauthority xauth merge "$XAUTHORITY";;
esac

Then you can run programs on your remote X display simply by setting DISPLAY:

ssh foo.example.com 'DISPLAY=:0 restart teamviewer'

This answer is adapted from a more complete answer to a similar question on Unix Stack Exchange.

Answer 2

Probably the easiest way is to remotely (via SSH) start a vncserver on the relevant display. After that use SSH tunneling to connect a vnc client from your remote machine through the SSH connection to the VNC server.