after very intens investigation I would like to reduce my question to the very essence.
The setup is as follows:
* Hyper-V core 2008R2 with one NIC
* Two guest OSs - 1x W2k3 Std., 1x W2k3SBS
* DNS-Server on W2k3SBS guest
* any DNS-request within the whole network to the internal DNS-Server works fine
* DNS-request from ANY other machines except the two virtual Servers to external DNS-Servers also work fine
* DNS-request from the Hyper-V host to external DNS-Servers are also ok
* DNS-request from any of the virtual servers fail.
* DNS-packets from the virtual servers can be successfuly traced up to the external DNS-server
* It makes no difference if nslookup is using the external DNS-server or the internal one that is then forwarding the request to external DNS-server. In any case the packets do definitly arrive at the external DNS and are answered, but the answers never arrive at the guest OS
* DNS trace on the w2k3SBS guest shows DNS-packets going out to the external DNS-servers but no packets coming in
* no firewall what so ever that could block any traffic (Hyper-V firewall explicitly turned off)
* system time is in sync on all machines
* again: ANY other computer on the network (using the same gaetway/firewall) is OK
The only thing I can think of may be the Hyper-V host not passing DNS packets from outside to the guest OSes.
Anyone ever had such a strange problem ?