Here's my scenario: my host is a Windows XP, and I have a guest OS of Ubuntu 8.04. I'm using VirtualBox for this.
I have successfully configured my DNS server in my Ubuntu OS. And if I do the following commands, all of them will give me the corresponding IP address of my DNS server.
dig example.com nslookup ns1.example.com host ns1.example.com
But when I try to ping my DNS server from the host (Windows XP), it says it cannot find the DNS server. Also, if I try to ping my DNS server from another PC, it does not seem to find the DNS server as well.
Check that the virtual network adapter is configured as bridged (not NAT)