Erlydns is an authoritative DNS server written in Erlang. It was a university project I did during my undergraduate studies. It has fairly limited functionalty, but may still be of use for someone studying the DNS system, due to its simplicity. It is also a small example of an OTP application.
All the code is now under the Simplified BSD license instead of GPLv3; I admit that I had no idea what I was doing, when I originally picked a license back in 2008.