A Common Operational Problem in DNS Servers

The DNS , is a query / response protocol. Failure to respond to queries causes both immediate operational problems and long term problems with protocol development. Failure to respond to a query is indistinguishable from a packet loss without doing a analysis of query response patterns and results in unnecessary additional queries being made by DNS clients and unnecessary delays being introduced to the resolution process. Allowing servers which fail to respond to queries to remain in the DNS hierarchy for extended periods results in developers being afraid to deploy new type codes. Such servers need to be identified and corrected / replaced. The DNS has response codes that cover almost any conceivable query response. A nameserver should be able to respond to any conceivable query using them. Unless a nameserver is under attack, it should respond to all queries directed to it as a result of following delegations. Additionally code should not assume that there isn't a delegation to the server even if it is not configured to serve the zone. Broken delegation are a common occurrence in the DNS and receiving queries for zones that you are not configured for is not a necessarily a indication that you are under attack.

There are two common query class that result in non responses today. These are EDNS queries and queries for unknown (unallocated) or unsupported types.

Identifying servers that fail to respond to EDNS queries can be done by first identifying that the server responds to regular DNS queries then making a series otherwise identical responses using EDNS, then making the original query again. A series of EDNS queries is needed as at least one DNS implementation responds to the first EDNS query with FORMERR but fails to respond to subsequent queries from the same address for a period until a regular DNS query is made. If the server responds to the first and last queries but fails to respond to most or all of the EDNS queries it is probably faulty. The test should be repeated a number of times to eliminate the likely hood of a false positive due to packet loss.

Identifying servers that fail to respond to unknown or unsupported types can be done by making a initial DNS query for a A record, making a number of queries for unallocated type, them making a query for a A record again. IANA maintains a registry of allocated types. If the server responds to the first and last queries but fails to respond to the queries for the unallocated type it is probably faulty. The test should be repeated a number of times to eliminate the likely hood of a false positive due to packet loss.

While the first step in remediating this problem is to get the offending nameserver code corrected, there is a very long tail problem with DNS servers in that it can often take over a decade between the code being corrected and a nameserver being upgraded with corrected code. With that in mind it is requested that TLD, and other similar zone operators, take steps to identify and inform their customers, directly or indirectly through registrars, that they are running such servers and that the customers need to correct the problem. TLD operators should construct a list of servers child zones are delegated to along with a delegated zone name. This name shall be the query name used to test the server as it is supposed to exist. For each server the TLD operator shall make a SOA query the delegated zone name. This should result in the SOA record being returned in the answer section. If the SOA record is not return but some other response is returned this is a indication of a bad delegation and the TLD operator should take whatever steps it normally takes to rectify a bad delegation. If more that one zone is delegated to the server it should choose another zone until it finds a zone which responds correctly or it exhausts the list of zones delegated to the server. If it fails to get a response to a SOA query the TLD operator should make a A query as some nameservers fail to respond to SOA queries but respond to A queries. If it gets no response to the A query another delegated zone should be queried for as some nameservers fail to respond to zones they are not configured for. If subsequent queries find a responding zone all delegation to this server need to be checked and rectified using the TLD's normal procedures. Having identified a working <server, query name> tuple the TLD operator should now check that the server responds to both EDNS and unknown query type tests as described above. If the TLD operator finds that server fails either or both tests, the TLD operator shall take steps to inform the operator of the server that they are running a fault nameserver and that they need to take steps to correct the matter. The TLD operator shall also record the <server, query name> for followup testing. If repeated attempts to inform and get the customer to correct / replace the fault server are unsuccessful the TLD operator shall remove all delegations to said server from the zone.