I'm trying to have two BIND servers keep their zones in sync. Usually easy enough. The tricky bit is that while both servers have external IPs, they are also on the same local subnet with internal IPs (I know name servers should be geographically separated... just humour me). They are both behind the same NAT router forwarding all traffic respectively. However when one server sends packets to the other external address they actually appear on the internal interface. In this situation Bind refuses to acknowledge the response and tries the transfer again.
The notifications from master to slave works fine, it's only when the slave requests a zone transfer from (eg.) 18.104.22.168 and gets a response from 192.168.0.1 that there is a problem.
Is there any way to allow this behaviour in bind?