Currently gdm has no option to just serve remote xdmcp connections, which means you can't run it on a server without a local display configured, since the lack of a display causes gdm to exit without trying to start xdmcp. Thus you can only run gdm locally, or locally and remotely, there is no option only to have it listening to remote connections.
please see https://bugzilla.redhat.com/show_bug.cgi?id=446224 Miroslav
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This is still an issue in Fedora 11.
This bug still exists in Fedora 12. Because of this, GDM is unusable in a headless server configuration: I have a HP server with iLO remote management, and I want to be able to log in to the text console remotely using ssh to the iLO address, while still being able to serve XDMCP clients.
This should be fixed in gnome 2.30 (Fedora 13)
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I have checked this in Fedora 14, and it is at least partially fixed, because you can now run xdmcp gdm on a system without a graphics card. There still doesn't seem to be an option to stop it trying to start a display on a graphics card if one does exist.
How many servers have you seen recently without a graphics card? As for me - exactly one (SGI Altix). Yet I need to run a display manager on many head-less (= monitor-less, not graphics card-less) servers. So I don't think this bug is fixed.