Here is something that I did not know - DNS servers use TCP port 53 to do zone transfers (axfrs). Make sure to open that port up in your firewall if you are allowing zone transfers from your DNS server. Knowing that could have saved me a lot of time

So for future reference:
Incoming open ports:
TCP/53
UDP/53

Outgoing open ports:
TCP/53
UDP/53
TCP/1024-65535
UDP/1024-65535

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