Domain system changes and observations
RFC 973

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Network Working Group                                   Paul Mockapetris
Request for Comments: 973                                            ISI
                                                            January 1986

                 Domain System Changes and Observations

STATUS OF THIS MEMO

   This RFC documents updates to Domain Name System specifications
   RFC-882 [1] and RFC-883 [2], suggests some operational guidelines,
   and discusses some experiences and problem areas in the present
   system.  Distribution of this memo is unlimited.

   This document includes all changes to the Domain System through
   January, 1986.  Change notices and additional discussion are
   available online in file [USC-ISIB.ARPA]<DOMAIN>DOMAIN.CHANGES.

OVERVIEW

   This memo is divided into four major sections:

      "UPDATES" which discusses changes to the domain specification
      which are in widespread use and should be regarded as being part
      of the specification.

      "OPERATION GUIDELINES" which suggests rules-of-thumb for using the
      domain system and configuring your database which are appropriate
      in most cases, but which may have rare exceptions.

      "EXPERIENCES" which discusses some unusual situations and common
      bugs which are encountered in the present system, and should be
      helpful in problem determination and tuning.

      "PROBLEM AREAS" which discusses some shortcomings in the present
      system which may be addressed in future versions.

UPDATES

   This section discusses changes to the specification which are final,
   and should be incorporated in all domain system software.

   TTL timeouts too small

      The 16 bit TTL field in RRs could not represent a large enough
      time interval.  The 16 bit field, using seconds for units, has a
      maximum period of approximately 18 hours.

      All time values, including all TTLs and the MINIMUM field of the
      SOA RR, are expanded to 32 bits.

Mockapetris                                                     [Page 1]



RFC 973                                                     January 1986
Domain System Changes and Observations

   CLASS changes

      Class 2, originally reserved for CSNET, is obsolete.  Class 3 has
      been assigned for use by CHAOS.

   CNAME usage

      The specification allows CNAME RRs to exist with other RRs at the
      same node.  This creates difficulties since the other RRs stored
      with the CNAME at the alias might not agree with the RRs stored at
      the primary name.

      If a node has a CNAME RR, it should have no other RRs.

   * semantics

      The use of * to represent a single label wildcard, along with the
      possibility of multiple * labels, led to difficult server
      implementations and complicated search algorithms.  There were
      also questions regarding whether a * based specification could
      refer to names that were not contained in the zone which had the *
      specification.

      While we might want the "inheritability" for some cases, it leads
      to implementation difficulties.  The first of these is that
      whenever we can't find a RR in a particular zone, we have to
      search all parent zones to look for a suitable * result.
      (Alternatively we could develop some automatic method for insuring
      consistency or insist on careful duplication of inherited data.)
      We also must deal with conflicts, i.e. what if a subdomain doesn't
      want to inherit defaults.

      Given these difficulties, the solution is to insist that
      delegation of authority cancels the * defaults.  This is quite
      simple to implement; all you need to do is to check for delegation
      before looking for * RRs.

      A second difficulty is the restriction that * match a single
      label.  Thus if a name server is looking for RRs for the name
      A.B.C.D.E.F, it must check for *.B.C.D.E.F, *.*.C.D.E.F,
      *.*.*.D.E.F, etc.  This check must also be careful of zone
      boundaries and multiplies the effort to handle a query.

      The solution adopted is to allow a single * label in the leftmost
      part of a name stored in a zone, and to allow this label to match

Mockapetris                                                     [Page 2]



RFC 973                                                     January 1986
Domain System Changes and Observations

      any number of unknown labels or a single known label in the query
      name.  However, the * match is only taken for parts of the tree
      which are neither delegated or explicitly represented.

      The algorithm for performing the search in a tree structured
      database has the following steps:

      1) Descend in the tree matching labels from right to left.  If a
      delegation is found return that;  if the specified node is found
      go to step 2, if the tree ends go to step 3.

      2) Look for RRs that answer the query.  If any are found, return