RFC 973 (rfc973) - Page 2 of 10

Domain system changes and observations

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      Class 2, originally reserved for CSNET, is obsolete.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




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