A query on a localhost name server is processed in a matter of milliseconds, so it's really not going to cause a DoS. When you put your name server on the public internet, it becomes a network address, which you can only get from an authoritative source. But even then, the response is going to be processed by a DNS query processor before it is handed over to your application layer, so you're going to see that it's processed before it is presented to the end user.