what the hell is dns enumeration? well, according to this pdf book DNS enumeration is
... the process of locating all the DNS servers and their corresponding records for an organization. A company may have both internal and external DNS servers that can yield information such as usernames, computer names, and IP addresses of potential target systems. so, this just little tricks on how to prevent dns enumeration on bind dns server, some dns enumeration tools will look at the zone transfer located on your dns server, and when it was misconfigured, it will leak all dns info to the attacker, like this: ...