wierd DNS problem

Hi folks,

the setup:
We have primary internal IP net (10.x.x.x) and a wan (192.168.x.x) We have an internal DNS server, which resolves both internal and internet namelookups for the clients.

the problem:
DNS lookup is not working over the wan connection. I can ping the dns server with success from the wan, but it refuses to answer a dns lookup.

The dns service works perfectly for the clients on the internal net.

I have tested the route tabels, and the are working as exspected.

all other tcp/ip communication over the wan is working.

So what the heck is wrong???
What have I forgotten?