xdm not responding to XDMCP queries

[Brian Morley <b.morley%rocketmail.com@localhost>]

I've bee playing with this for a few days now and I seem to be getting 
precisely nowhere with it.  xdm flatly refuses to respond to any queries it is 
sent.  Tracing with Wireshark shows the queries are entering the system but it 
does not appear to be doing anything with them.  Playing with xdm's no-deamon 
and debug options does not reveal anything at all either - it is as if the 
packets are somehow not reaching xdm at all.

/usr/pkg/lib/X11/xdm/Xaccess reads:
*               # Allow any machine
LISTEN *        # Listen on all interfaces

The xdm-config file has the following line commented out:
!DisplayManager.requestPort:    0

and explicitly setting it to 177 also does nothing.  Even on the local machine 
an "Xnest -query localhost :1" displays an empty window, no login prompt.

I'm beginning to wonder if there is some kind of packet filtering at work - 
does NetBSD ship with something like that by default since it is certainly 
nothing I have added.  I have vague memories or being completely unable to get 
NTP working on this machine either but I simply gave up on that - I need to get 
this one working.  The common factor for both is privileged UDP ports.