RFC 3597 (rfc3597) - Page 2 of 8

Handling of Unknown DNS Resource Record (RR) Types

RFC 3597            Handling of Unknown DNS RR Types      September 2003


2.  Definition

   An "RR of unknown type" is an RR whose RDATA format is not known to
   the DNS implementation at hand, and whose type is not an assigned
   QTYPE or Meta-TYPE as specified in [RFC 2929] (section 3.1) nor
   within the range reserved in that section for assignment only to
   QTYPEs and Meta-TYPEs.  Such an RR cannot be converted to a type-
   specific text format, compressed, or otherwise handled in a type-
   specific way.

   In the case of a type whose RDATA format is class specific, an RR is
   considered to be of unknown type when the RDATA format for that
   combination of type and class is not known.

3.  Transparency

   To enable new RR types to be deployed without server changes, name
   servers and resolvers MUST handle RRs of unknown type transparently.
   That is, they must treat the RDATA section of such RRs as
   unstructured binary data, storing and transmitting it without change
   [RFC 1123].

   To ensure the correct operation of equality comparison (section 6)
   and of the DNSSEC canonical form (section 7) when an RR type is known
   to some but not all of the servers involved, servers MUST also
   exactly preserve the RDATA of RRs of known type, except for changes
   due to compression or decompression where allowed by section 4 of
   this memo.  In particular, the character case of domain names that
   are not subject to compression MUST be preserved.

4.  Domain Name Compression

   RRs containing compression pointers in the RDATA part cannot be
   treated transparently, as the compression pointers are only
   meaningful within the context of a DNS message.  Transparently
   copying the RDATA into a new DNS message would cause the compression
   pointers to point at the corresponding location in the new message,
   which now contains unrelated data.  This would cause the compressed
   name to be corrupted.

   To avoid such corruption, servers MUST NOT compress domain names
   embedded in the RDATA of types that are class-specific or not well-
   known.  This requirement was stated in [RFC 1123] without defining the
   term "well-known"; it is hereby specified that only the RR types
   defined in [RFC 1035] are to be considered "well-known".






Gustafsson                  Standards Track