RFC 1536 (rfc1536) - Page 2 of 12

Common DNS Implementation Errors and Suggested Fixes

RFC 1536            Common DNS Implementation Errors        October 1993


   retransmission policy, RFC 1035 does recommend that the resolvers
   should cycle through a list of servers. Both name servers and stub
   resolvers should, therefore, implement some kind of a retransmission
   policy based on round trip time estimates of the name servers. The
   client should back-off exponentially, probably to a maximum timeout
   value.

   However, clients might not implement either of the two. They might
   not wait a sufficient amount of time before retransmitting or they
   might not back-off their inter-query times sufficiently.

   Thus, what the server would see will be a series of queries from the
   same querying entity, spaced very close together. Of course, a
   correctly implemented server discards all duplicate queries but the
   queries contribute to wide-area traffic, nevertheless.

   We classify a retransmission of a query as a pure Fast retry timeout
   problem when a series of query packets meet the following conditions.

      a. Query packets are seen within a time less than a "reasonable
         waiting period" of each other.

      b. No response to the original query was seen i.e., we see two or
         more queries, back to back.

      c. The query packets share the same query identifier.

      d. The server eventually responds to the query.

A GOOD IMPLEMENTATION:

   BIND (we looked at versions 4.8.3 and 4.9) implements a good
   retransmission algorithm which solves or limits all of these
   problems.  The Berkeley stub-resolver queries servers at an interval
   that starts at the greater of 4 seconds and 5 seconds divided by the
   number of servers the resolver queries. The resolver cycles through
   servers and at the end of a cycle, backs off the time out
   exponentially.

   The Berkeley full-service resolver (built in with the program
   "named") starts with a time-out equal to the greater of 4 seconds and
   two times the round-trip time estimate of the server.  The time-out
   is backed off with each cycle, exponentially, to a ceiling value of
   45 seconds.







Kumar, Postel, Neuman, Danzig & Miller