A teaching lab is built from diskless X terminals. Each terminal runs only an X server and asks a central Linux host for a graphical login screen, and the applications then run on that central host. Which protocol carries that request, and what is the main objection to using it beyond the lab's own switch?
LPIC-1 Exam 102-500, objective 106. User interfaces and desktops medium
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The options
Correct XDMCP, and the objection is that the protocol is unencrypted, so credentials and screen contents cross the network in the clear.
Correct. XDMCP lets an X server query a display manager for a session; the display manager answers by connecting back and drawing the greeter. Neither XDMCP itself nor the X traffic that follows is encrypted, so it belongs on a trusted segment only.
Not correct VNC, and the objection is that each terminal would need its own dedicated framebuffer on the server.
Wrong. VNC exports an existing desktop or a virtual framebuffer to a viewer; it is not how an X server obtains a login greeter. The description given is the classic XDMCP arrangement.
Not correct RDP, and the objection is that it requires a Windows licence per connected seat.
Wrong. RDP is Microsoft's protocol, served on Linux only by add-on software such as xrdp. It plays no part in an X server requesting a session from a display manager.
Not correct SSH X11 forwarding, and the objection is that the tunnel cannot carry a login screen.
Wrong on the first half. SSH forwarding is a genuinely good way to run remote X clients, and it is encrypted, but it presupposes that you have already logged in — which is exactly the step XDMCP performs.
Why
XDMCP is the query protocol between an X server and a display manager such as xdm, gdm or lightdm; the X server broadcasts or directly queries, the display manager replies and manages the session. It listens on UDP port 177 and is disabled by default on modern distributions precisely because it is unauthenticated and unencrypted. The encrypted alternative for running remote graphical applications is ssh -X, which forwards X clients over an already authenticated channel.
Where this comes from
- Cited
- manual page xdm(1)
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
- 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? 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