How to enable X11 forwarding for remote access

Environment

Linux/macOS/Windows with X11 server

OpenSSH client

X11 applications

Issue

When trying to run X11 applications on remote systems, encountered error message
username@slogin-01:~$ xclock
Error: Can't open display:

Resolution

Enable X11 forwarding in SSH client when connecting to the remote system.

Enable X11 Forwarding

Note

X11 forwarding must be enabled when connecting to the remote system via SSH. Please disconnect your current SSH session and reconnect after applying the changes.

X11 forwarding can be enabled ad-hoc at command-line or persistently by modifying SSH client config.

One-time Connection

Use -X flag when connecting via SSH

ssh -X username@hpcname.ust.hk

Persistent Configuration

Add X11 forwarding settings to your SSH config file

Create or edit ~/.ssh/config

mkdir -p ~/.ssh
chmod 700 ~/.ssh
nano ~/.ssh/config

Add the following configuration

Host hpcname.ust.hk
    ForwardX11 yes
    ForwardX11Trusted yes

Set proper permissions
chmod 600 ~/.ssh/config

Verify X11 Connection

Test the connection by running a simple X11 application

Warning

Ensure you have an X11 server installed and running on your local machine

Linux: Usually pre-installed (xorg)
macOS: Install XQuartz
Windows: Install VcXsrv or Xming

xclock

Or check the DISPLAY environment variable

echo $DISPLAY

Root Cause

X11 forwarding requires client support, which is disabled by default.