How to run a GUI application over SSH with X11 forwarding

Remote GUI tools become awkward when the software is installed on a server but the display is on a workstation. X11 forwarding lets OpenSSH carry the display connection through the SSH session so a remote program opens on the local desktop while it continues to run on the remote host.

The local computer provides the X server or Xwayland display endpoint. The remote host runs the application, while ssh -X creates a temporary DISPLAY value such as localhost:10.0 and uses xauth to install an authorization cookie for that session.

The server must allow X11Forwarding and have xauth available before a forwarded window can open. Use ordinary -X forwarding first, and reserve -Y trusted forwarding for servers and applications that genuinely need the extra display access. A forwarded GUI program can observe or interact with the local display, so use it only with servers that should be allowed that access.

Steps to run a GUI application over SSH with X11 forwarding:

Start the local X server on the client workstation.

Linux desktop sessions usually provide Xorg or Xwayland already. On macOS, start XQuartz before connecting. On Windows, start the local X server used by the SSH client.
Check the remote server's effective X11 forwarding settings.
```
$ sudo sshd -T
##### snipped #####
x11displayoffset 10
x11forwarding yes
x11uselocalhost yes
xauthlocation /usr/bin/xauth
##### snipped #####
```
If x11forwarding is no, or xauthlocation is none, enable the server side before testing a GUI program.
Related: How to enable or disable X11 forwarding on an SSH server

Install xauth and a small X11 test application on the remote server when they are missing.

$ sudo apt install --assume-yes xauth x11-apps
Reading package lists...
Building dependency tree...
Reading state information...
The following NEW packages will be installed:
  x11-apps xauth
##### snipped #####
Setting up xauth (1:1.1.2-1.1build1)
Setting up x11-apps (7.7+11build4)

On RHEL, Fedora, and similar systems, install the xorg-x11-xauth package and an X11 test-app package from the distribution repositories.

Run a forwarded-display check from the client.
```
$ ssh -X user@server.example.net 'printf "%s\n" "$DISPLAY"'
localhost:10.0
```
A blank value means the client did not request forwarding or no local X display was available. X11 forwarding request failed on channel 0 points to the server-side policy or xauth setup.
Start a test GUI application through the forwarded session.
```
$ ssh -X user@server.example.net xclock
```
The terminal stays attached until the window closes. Replace xclock with the remote graphical program after the test window opens correctly.
Use trusted forwarding only when the application refuses ordinary X11 forwarding and the remote server is trusted.
```
$ ssh -Y user@server.example.net xclock
```
-Y disables the X11 security restrictions applied to -X forwarding. Do not use trusted forwarding for untrusted servers.