Most organizations have internal and external DNS servers and in many cases, the default internal DNS zone may differ from the externally referenced DNS zone. For example, the internal DNS zone could be called domain.com and the external DNS zone could be called external.domain.com. The internal DNS zone could be active directory integrated and will try to respond to all queries for external.domain.com and queries will fail since there arent any entries for external.domain.com in the internal DNS server.

In such a situation, instead of maintaining both zones internally and externally, it would be better and safer (avoiding human error) to setup a Split Brain DNS server to automatically forward all DNS queries for external.domain.com to the publicly hosted DNS server.

This is how it is done:

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