window manageres and xdmcp


I heard that window managers like xdm, kdm and gdm etc use xdmcp protocol which is inherently unsafe. Now I have an Xclient that has to be displayed on multiple Xterminals. Can I do this without gdm/xdmcp. Shouldn’t the regular X work in this case. What exaclty is the purpose of gdm/xdmcp on top of X. So if gdm provides the complete desktop manager, does this mean it is unsecure since it uses xdmcp?


It’s insecure in the sense that the traffic is non-encrypted and conceivably someone could sniff the traffic. Are your users likely to do this? Recent LTSP distros use ssh as the transport for a X session.

xdm (and friends) are not windows managers. They are display managers (that is where the dm part of their names come from). They handle the keyboard/mouse actions and display pixels.

Window managers handle windows, among other things, they provide the borders and the top bar (where the maximize and close buttons are) and do the actions that belong to them (resizing, iconizing, moving, bring to the forefromt). You can have wiindows without a window manager (looks very naked), but you need a display manager, else you would not see very much.

I explain this because it is not very easy to understand and when one does not have the slightest idea about what is what it will all look like magic.

As the whole idea behind the general term X-windows is bases on client server and thus differnet components can reside on different systems in the network, the display manager can be on one system and the windows manager on another (and the apllications that show windows on other systems again). The protocol used between the dm and the wm is xdmcp. And then ken_yap is correct and you heard correct that that is not a very save protocol. When you use it in a network you trust and has a strong firewall to protect it from outside, I guess it could be used. But more save (and when you have a slow network, more efficient) solutions are available. ssh being prominent.

Use Wikipedia for more information about dm, wm and X in general.

So how would I stop and enable xdmcp? How do I enable xdmcp using ssh only. I have ssh enabled on my system and I also see xdmcp running on it. I’d like my kdm to use “ssh” instead of “xdmcp”. So how do I stop it? I took the following measures:

I always notice that xdmcp always runs on my system. I don’t want it to. These are my settings and I tried to reboot. xdmcp still runs. How do I stop it once for all.


DIsplayManager.requestPort: 0

the file /opt/kde3/share/config/kdm/kdmrc has the following:

/etc/X11/xdm/Xaccess shows this:

  • #any host can get a login window