Open Recursive Name Servers
Makes sure that the name servers do not respond to open recursive queries.
A recursive name server is one that will respond to any query for any domain name. These are the DNS servers that you put into your computer's network settings so your browser can lookup domain names. The authoritative name server is different. It should only respond to queries for domain names for which it has the authoritative answer. If an authoritative name sever responds to a DNS lookup for a domain that it does not have the authoritative answer for, then this test will indicate that there is a problem.
If an authoritative name server responds to open recursive queries, then it can be used to launch DNS amplification attacks which can cause your name servers to crash or operate slowly. You would also be enabling an attack on someone else. If your name severs respond to open recursive queries, then this should be fixed.