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I have a Red Hat 7.3 server running XVnc. On a Windows 10 desktop I have Putty and Xming installed. Putty is configured for X11 forwarding. When I SSH in as my standard/non-privileged user and launch an X application, it displays on my Windows 10 desktop without issue.

Now within same session, if I su to a more privileged account and try to run an X application, it fails with "error: can't open display".

In my standard user session if I echo $DISPLAY it is automatically set for me as "IP_ADDRESS:10.0". Under my su session, $DISPLAY is null. I tried exporting the DISPLAY variable to the same value but it now a different error appears: "PuTTY X11 proxy: Unsupported authorization protocol Error: Can't open display:server_IP:10.0".

How can I configure the X11 forwarding to work under the context of the other user?

  • The easiest way is to login with that user. – Ipor Sircer Oct 18 '18 at 12:11
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The below steps should fix the issue for you.

Say it's working for user1 and you want to use it for user2

For user1:

$ xauth list $DISPLAY
<output1>
$ echo $DISPLAY
<outoput2>

Switch to other user , i.e user2

$ xauth add <output1> 
$ export DISPLAY=<output2>

Try:

$ xclock
-1

That's because the the X11 display connection belongs to the user with which you log in via SSH. X11Forwarding mechanism does not allow anyone else to use the display.

Fortunately, there's a workaround. After using su and become the othe user, issue this command:

xauth add $(xauth -f ~username/.Xauthority list | tail -1)

where ~username is your original user, i.e. the user with which you connected to the server.

Obviously, this works if your new user has enough privileges to read ~username/.Xauthority (i.e. if you su to root then there are no problems, if to another user it might not work and you need to check/modify permissions).

  • Thanks for the explanation - had success with the suggested workaround. – tickermcse76 Oct 18 '18 at 18:20

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