The DNS interface in Server Admin.app is not suitable for doing a split-horizon DNS configuration. It simply doesn’t expose all of the flexibility of bind that you need to pull off such a configuration.

If you poke around the bind config files on your OS X Server, you’ll be able to see how apple has set them up so that you can edit them directly without confusing the GUI. /var/named contains zone files that you may edit, and they include corresponding files in /var/named/zones which you should not edit. They’ve done something similar for /etc/named.conf and the files in /etc/dns/.

Having said that, I recommend not doing both internal and external resolving for split-horizon DNS on your server, mainly because:

1. It’s kind of complicated, and you lose any convenience you had when you were able to use the GUI exclusively
2. You have NAT, which makes it even more complicated
3. There are solutions available from third parties that are better-performing, cheap/free, and more robust

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