Open a window on a remote X display (why “Cannot open display”)?

Once upon a time,
DISPLAY=:0.0 totem /path/to/movie.avi
after ssh 'ing into my desktop from my laptop would cause totem to play movie.avi on my desktop.
Now it gives the error:
No protocol specified
Cannot open display:
I reinstalled Debian squeeze when it went stable on both computers, and I guess I broke the config.
I've googled on this, and cannot for the life of me figure out what I'm supposed to be doing.
(VLC has an HTTP interface that works, but it isn't as convenient as ssh.)



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 DISPLAYenvironment 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.)

Getting the values of the variables

Here are a few ways to obtain the values of DISPLAY and XAUTHORITY:
  • You can systematically start a screen session from your desktop, perhaps automatically in your login scripts (from ~/.profile; but do it only if logging in under X: test if DISPLAY is set to a value beginning with : (that should cover all the cases you're likely to encounter)). In~/.profile:
    case $DISPLAY in
      :*) screen -S local -d -m;;
    Then, in the ssh session:
    screen -d -r local
  • You could also save the values of DISPLAY and XAUTHORITY in a file and recall the values. In~/.profile:
    case $DISPLAY in
      :*) export | grep -E ' (DISPLAY|XAUTHORITY)=' >~/;;
    In the ssh session:
    . ~/
  • You could detect the values of DISPLAY and XAUTHORITY from a running process. This is harder to automate. You have to figure out the PID of a process that's connected to the display you want to work on, then get the environment variables from /proc/$pid/environ (eval export $(</proc/$pid/environ tr \\0 \\n | grep -E '^(DISPLAY|XAUTHORITY)=')¹).

Copying the cookies

Another approach (following a suggestion by Arrowmaster) is to not try to obtain the value of$XAUTHORITY in the ssh session, but instead to make the X session copy its cookies into~/.Xauthority. Since the cookies are generated each time you log in, it's not a problem if you keep stale values in ~/.Xauthority.
There can be a security issue if your home directory is accessible over NFS or other network file system that allows remote administrators to view its contents. They'd still need to connect to your machine somehow, unless you've enabled X TCP connections (Debian has them off by default). So for most people, this either does not apply (no NFS) or is not a problem (no X TCP connections).
To copy cookies when you log into your desktop X session, add the following lines to ~/.xprofileor ~/.profile (or some other script that is read when you log in):
    # DISPLAY is set and points to a local display, and XAUTHORITY is
    # set, so merge the contents of `$XAUTHORITY` into ~/.Xauthority.
    XAUTHORITY=~/.Xauthority xauth merge "$XAUTHORITY";;
¹ In principle this lacks proper quoting, but in this specific instance $DISPLAY and $XAUTHORITY won't contain any shell metacharacter.


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