How to use X11-Forwarding in SSH as root or sudo user

X11-Forwarding allows you to execute graphical programs on a remote SSH host and display them locally.

If you're connecting to a remote SSH server for system administration work, chances are you'll be running programs using sudo or su to switch to another user such as root. You will get Can't open display, or X11 connection rejected because of wrong authentication error when running GUI program in SSH when switching users via sudo or su.

$ sudo xclock
[sudo] password for user: 
X11 connection rejected because of wrong authentication.
Error: Can't open display: localhost:10.0

This is because SSH X11-Forwarding requires authorization for connecting to the X server which is stored in ~/.Xauthority file and with the correct value for DISPLAY environment variable.

1. Make sure you're already able to run graphical program via SSH X tunneling as normal user.

   Related: How to run GUI application via SSH

2. Connect to SSH server with X11-Forwarding enabled.

   ~$ ssh -X remote-host
   user@remote-host's password: 
   Welcome to Ubuntu 20.10 (GNU/Linux 5.8.0-26-generic x86_64)
   
    * Documentation:  https://help.ubuntu.com
    * Management:     https://landscape.canonical.com
    * Support:        https://ubuntu.com/advantage
   
   0 updates can be installed immediately.
   0 of these updates are security updates.
   
   Last login: Sun Nov  1 21:17:13 2020 from 192.168.111.27

3. Get X authorization entry for current display.

   $ xauth list $DISPLAY
   host/unix:10  MIT-MAGIC-COOKIE-1  742d024faeb3d29a15ff06f1b8c3b21e

   This info is stored in ~/.Xauthority file.

   $ cat ~/.Xauthority 
   host10MIT-MAGIC-COOKIE-1t-O��Қ��ò

4. Get DISPLAY environment variable value.

   $ echo $DISPLAY
   localhost:10.0

5. Switch to root or other user using sudo and/or su.

   $ sudo su -
   [sudo] password for user: 
   root@host:~#

6. Generate ~/.Xauthority file using xauth command.

   # xauth add host/unix:10  MIT-MAGIC-COOKIE-1  742d024faeb3d29a15ff06f1b8c3b21e

7. Check .Xauthority file to confirm.

   # cat ~/.Xauthority 
   host10MIT-MAGIC-COOKIE-1t-O��Қ��ò

8. Export DISPLAY environment variable value for current user.

   # export DISPLAY=localhost:10.0

9. Run X program.

   # xclock