From your desktop workstation you run `ssh -X admin@fileserver` and then start `xterm` in that session. The xterm window appears on your workstation's screen. Which machine is running the X server in this arrangement, and which is running the X client?
LPIC-1 Exam 102-500, objective 106. User interfaces and desktops 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 The workstation runs the X server; fileserver runs the X client (xterm).
Correct. The screen, keyboard and mouse are on the workstation, so the workstation runs the X server. The xterm process executes on fileserver and connects back to that server as a client through the forwarded channel.
Not correct fileserver runs the X server; the workstation runs the X client (xterm).
Wrong. This is the intuitive-but-backwards reading. xterm is the application, so it is the client, and it is running on fileserver.
Not correct Both machines run an X server, and ssh copies the rendered image between them.
Wrong. ssh forwards X protocol requests, not rendered images or screenshots. fileserver needs no X server at all to run X clients; it only needs the X client libraries.
Not correct Neither machine runs an X server, because -X tells ssh to render the window itself.
Wrong. ssh does no rendering. -X sets up a proxy display on the remote side and tunnels the X protocol to the X server that is already running on the local machine.
Why
With `ssh -X`, sshd on the remote host sets DISPLAY in the remote shell to something like localhost:10.0 and listens on that proxy display. X clients started there connect to the proxy, ssh tunnels their traffic back over the encrypted connection, and the local ssh hands it to the local X server. The remote host needs X client libraries but no X server. `ssh -X` requests untrusted forwarding with X security extension restrictions and `ssh -Y` requests trusted forwarding, although Debian and Ubuntu ship `ForwardX11Trusted yes` in /etc/ssh/ssh_config, which makes -X behave like -Y there. Either way the server side must have X11Forwarding enabled in sshd_config, and xauth must be present on the remote host for the cookie to be set up.
Where this comes from
- Cited
- LPI exam objective 106.1
- What it says
- Understand the client and server model used by X and the use of remote displays.
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 106 and space the ones you get wrong.
More questions on this objective
- In the X Window System architecture, which statement correctly describes the roles of the X server and an X client? machine-checked
- In a bash shell, type the complete command that sets and exports the environment variable X clients read to find their display, so that they connect to screen 0 of display 0 on the local machine. machine-checked
- Type the command, with no arguments and no options, that prints whether host-based access control is currently enabled on your X server together with the list of hosts and users allowed to connect. machine-checked
- An X11-forwarded ssh session authorises the remote client to talk to your local X server without you ever running an xhost command. Which mechanism provides that authorisation? machine-checked
- A display manager is the program that presents the graphical login screen and starts the user's session after authentication. Which three of the following are display managers? (Choose three.) machine-checked
- A user types with one hand and cannot hold Ctrl or Shift down while pressing another key. Which accessibility feature lets a modifier key stay active after it is released, so the next key press is combined with it? machine-checked