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Old 03-31-2003, 01:56 AM   #1
Registered: Aug 2002
Distribution: SME Server, CentOS
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XDMCP: How do I get this working ... and what is it?

One of the many joys of using Linux and computing in general is being able to control another computer over a network. Remote desktop functionality existed in Windows 98 SE and exists now in XP Pro. But I've never seen anything as completely transparent as being able to run a piece of software on another computer in my LAN with the GUI showing up on my local unit like you can in Linux. I used to use kdessh (I think that is the command) to start up kppp on another machine that had my modem connection to the internet and get the graphical interface on the other computer I was sitting at. I could then use that little traffic monitor to see how my connection was going.

But the ultimate would be to get an entire desktop for that other machine on the computer I was sitting at. I think this is the basic mission of the X Windows system and its open source implementation in XFree86. I have yet to get even a clue as to where to start or even if it is possible as I imagine.

Now in Red Hat 8.0 there is a tab in the GUI Login manager; -> Systems Settings -> Login Screen; that is for XDMCP. I've done some searching and found a HOWTO on it here: It sounds like some way of getting your login window to provide with prompts to log in to another computer on a network using a complete desktop. But even with the few HOWTOs I've read and applied, I have yet to see a prompt to do this on my own system.

Has anyone ever successfully managed to create a login window for remote systems using the XDMCP technology? Is it actually possible? How do you do it?

Any and all tips and clues would be appreciated.
Old 03-31-2003, 05:14 PM   #2
Registered: Sep 2002
Location: .at
Distribution: SuSE, Knoppix
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XDMCP is useful to send a X11 login/session to an otherwise "unamanaged" X-server (e.g. X-Terminal, X-Emulator, ... )

in your case you already have a window manager running on your server, so you just need to log on yout target machine (ssh/telnet), set the $DISPLAY shell variable and start any program you need.
Old 03-31-2003, 05:48 PM   #3
Registered: Aug 2002
Distribution: SME Server, CentOS
Posts: 219

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But what about this posed relationship with the graphical login display? Something should be happening there. In fact, you can configure the login display so that you can choose to configure it from that login point. To me that means that somehow you can login to a desktop from the graphical login and not just some piece of software with an X display. And, it sounds like you should be able to choose which machine to log into from the graphical login window.
Old 03-31-2003, 08:43 PM   #4
Registered: Dec 2002
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>it sounds like you should be able to choose which machine to log into from the graphical login window

It is indeed...

XDMCP is basically a method of 'broadcasting' the availability of a desktop able to be 'logged into' across a network. When you run XDMCP on a machine it allows you to 'load up the desktop' of that machine on another. It's very similar, in that regard, to the remote desktop function of Windows XP / terminal server.

The reason for the tie with the graphical manager, is precisely because you're logging into a *desktop* rather than just an X-windows application. The desktop is responsible for um.. presenting the desktop.

That doesn't mean, though, that you can't login to a Gnome desktop from a KDE desktop. You can, it just means that if you have a Gnome desktop advertising itself over XDMCP that's what you'll get when you connect to it.

Now, here's the rub. It's kinda hard to set up. It's also inherently insecure. And it's hard to set up *because* it's insecure, and comes turned off in Redhat by default. To turn it on you have to xauth machines you want to use to connect to your desktop on the server.

You can do that most insecurely by running xhost+ on your server, which will allow *any* machine to connect. Read up on xhost.

You'll also need to unblock port 6000 (xwindows) to allow the Xwindows traffic to and from the serving machine - again, something that's most often turned *off* on various firewalls - X is a very insecure protocol.

You'll need to run um.. xdmcpd.. I think it is, to do the broadcasting on your network and allow the machine to be seen from your chooser.

When you're finally done you should be able to choose your machine from your login prompt there, and see your serving machine as a choice, pick that and you get a desktop on *that* machine (albeit a slow one).

I have it working, but not on a linux machine, on an HPUX machine (running Ximian's Gnome desktop) which I manage from a Linux box.

Old 04-09-2003, 08:10 AM   #5
LQ Newbie
Registered: Apr 2003
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Hello, I have Xfree broadcasting via XDMCP. I can see the system from my XP workstation that is running Hummingbird Exceed. I can get a log in screen and log in but.... thats where it ends.. I see the Red Hat splash screen then it just hangs, If I use failsafe I can connect so I know I can connect. It seems like some kind of graphical problem. Any Idea what the problem could be?


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