You log into a remote server over SSH and start the graphical tool xterm, which fails because the DISPLAY variable is not set. Which client option lets the remote program draw on your local X server?
LPIC-1 Exam 102-500, objective 110. Security medium
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How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct ssh -X server
Correct. -X enables X11 forwarding: sshd sets DISPLAY on the remote side to a proxy display such as localhost:10.0 and tunnels the X protocol back through the encrypted connection.
Not correct ssh -A server
Wrong. -A forwards the authentication agent so that keys held locally can be used for onward connections. It does nothing for graphics.
Not correct ssh -N server
Wrong. -N tells ssh not to run a remote command at all, which is used when the connection exists only to carry forwarded ports. It would not even give you a shell to start xterm from.
Not correct ssh -D 6000 server
Wrong. -D opens a local SOCKS proxy. Port 6000 happens to be where the first X display listens, but a SOCKS proxy does not set DISPLAY or carry the X protocol for the remote side.
Why
X11 forwarding needs both ends to agree: X11Forwarding yes in the server's /etc/ssh/sshd_config, and -X or ForwardX11 yes on the client. When it is on, sshd allocates a display number, exports DISPLAY into the remote session and relays the traffic inside the SSH channel, so no X port is exposed on the network. -Y is the trusted variant that skips the X11 SECURITY extension restrictions, which some applications need but which gives the remote host far more power over your display.
Where this comes from
- Cited
- manual page ssh(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 110 and space the ones you get wrong.
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