DNS Server

We have installed windows 2000 PDC with AD integrated DNS server (not a registered domain) and a backup server to PDC. Backup server is configured with IP address of the PDC as primary DNS server. It?s synchronizes with PDC and everything works fine. Now we need to have access to Internet on backup server. For that we need to change IP address of the primary DNS server to the IP of our ISP DNS server (external). Here problem start. If we use the Internal DNS then we are not able to access Internet because it will not resolve to IP address or if we use the external DNS then backup server will not synchronize with PDC because it will not find the PDC using external DNS.

Please Help.