How to remotely display X11 application on an Ubuntu 20 desktop

How to remotely display X11 application on an Ubuntu 20 desktop

On the client Ubuntu Desktop system

An Ubuntu 20 desktop is by default not supporting displaying remote X11 based applications as Ubuntu X Windows system is based on Wayland instead of the one from X.org. You can easily check this with the command:

$ echo $XDG_SESSION_TYPE
wayland

When you see as output wayland instead of x11 then you know we have some work to perform to allow remote X11 application to be able to pop-up on our Ubuntu desktop. To modify this please edit the /etc/gdm3/custom.conf file and uncomment the WaylandEnable line as follows and restart the system:

# Uncomment the line below to force the login screen to use Xorg
WaylandEnable=false

Finally, to accept x11 applications add the following to your personal Secure Shell config file ($HOME/.ssh/config) as global variables:

ForwardAgent yes
ForwardX11 yes
ForwardX11Trusted yes

On the remote system where the x11 application runs

To allow X11 forwarding on the remote system we also need to make sure that in the /etc/ssh/ssh_config file (not your local Ubuntu desktop system) we uncomment or add the following lines:

#   ForwardAgent no
    ForwardX11 yes
    ForwardX11Trusted yes

Furthermore, also edit the /etc/ssh/sshd_config file and define the following settings:

X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost no

The X11UseLocalhost no setting is required to avoid the error message X11 forwarding request failed on channel 0 [2].

And finally, restart the Secure Shell daemon (sshd) with the command : systemctl restart sshd

The Remote X11 test

From your Ubuntu desktop (via a terminal window) login on the remote server from where you want to launch a x11 application and get it visible on your Ubuntu desktop. Therefore, use the following command:

DISPLAY=:1 ssh -X n5

The -X flag means we want to forward X11 displays and when you type the command echo $DISPLAY (on the remote system) you will see it echos:

n5:10.0

Now, you can launch any available x11 application and get it projected on your Ubuntu desktop, e.g. xmessage "Hello from system $(hostname)"

References

[1] Displaying Ubuntu 20.04 Applications Remotely (X11 Forwarding)

[2] X11 forwarding request failed on channel 0 Error and Solution