DNS server library implementing just zone transfer.
Having your DNS records dynamically generated is fun, but why put your hodgepdoge DNS implementation on the public internet when you can rely on high quality, battle tested software such as NSD?
Zonemaster is a library that deliberately implements only a small part of the DNS protocol, namely the part that let's you synchronize with your secondary servers running a proper DNS implementation.
The zonemaster interface is small, providing the bare minimum to generate your DNS records on-the-fly, and notify slaves of updates.
Hide your zonemaster server from the public internet. Only your secondary servers should be able to reach it. Let them deal with the heavy lifting!
Documentation is still a bit lacking, but take a look at the example.