The Problem
The user is unable to run X applications through SSH on a CentOS/RHEL system. The following message is displayed:
# xclock Error: Can't open display:
The Solution
1. X11 forwarding is disabled on SSH login unless “-X” option is specified:
# ssh -X user@address
2. It may have been disabled in “/etc/ssh/sshd_config” with the below setting:
# vi /etc/ssh/sshd_config ... X11Forwarding no ...
3. Please make sure X11 forwarding is enabled in “/etc/ssh/sshd_config”:
# vi /etc/ssh/sshd_config ... X11Forwarding yes ...
Then, use the “-X” option while logging in.