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RFC 3597 - Handling of Unknown DNS Resource Record (RR) Types

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Network Working Group                                      A. Gustafsson
Request for Comments: 3597                                  Nominum Inc.
Category: Standards Track                                 September 2003

           Handling of Unknown DNS Resource Record (RR) Types

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   Extending the Domain Name System (DNS) with new Resource Record (RR)
   types currently requires changes to name server software.  This
   document specifies the changes necessary to allow future DNS
   implementations to handle new RR types transparently.

1.  Introduction

   The DNS is designed to be extensible to support new services through
   the introduction of new resource record (RR) types.  In practice,
   deploying a new RR type currently requires changes to the name server
   software not only at the authoritative DNS server that is providing
   the new information and the client making use of it, but also at all
   slave servers for the zone containing it, and in some cases also at
   caching name servers and forwarders used by the client.

   Because the deployment of new server software is slow and expensive,
   the potential of the DNS in supporting new services has never been
   fully realized.  This memo proposes changes to name servers and to
   procedures for defining new RR types aimed at simplifying the future
   deployment of new RR types.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

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

   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 [RFC1123] without defining the
   term "well-known"; it is hereby specified that only the RR types
   defined in [RFC1035] are to be considered "well-known".

   The specifications of a few existing RR types have explicitly allowed
   compression contrary to this specification: [RFC2163] specified that
   compression applies to the PX RR, and [RFC2535] allowed compression
   in SIG RRs and NXT RRs records.  Since this specification disallows
   compression in these cases, it is an update to [RFC2163] (section 4)
   and [RFC2535] (sections 4.1.7 and 5.2).

   Receiving servers MUST decompress domain names in RRs of well-known
   type, and SHOULD also decompress RRs of type RP, AFSDB, RT, SIG, PX,
   NXT, NAPTR, and SRV (although the current specification of the SRV RR
   in [RFC2782] prohibits compression, [RFC2052] mandated it, and some
   servers following that earlier specification are still in use).

   Future specifications for new RR types that contain domain names
   within their RDATA MUST NOT allow the use of name compression for
   those names, and SHOULD explicitly state that the embedded domain
   names MUST NOT be compressed.

   As noted in [RFC1123], the owner name of an RR is always eligible for

5.  Text Representation

   In the "type" field of a master file line, an unknown RR type is
   represented by the word "TYPE" immediately followed by the decimal RR
   type number, with no intervening whitespace.  In the "class" field,
   an unknown class is similarly represented as the word "CLASS"
   immediately followed by the decimal class number.

   This convention allows types and classes to be distinguished from
   each other and from TTL values, allowing the "[<TTL>] [<class>]
   <type> <RDATA>" and "[<class>] [<TTL>] <type> <RDATA>" forms of
   [RFC1035] to both be unambiguously parsed.

   The RDATA section of an RR of unknown type is represented as a
   sequence of white space separated words as follows:

      The special token \# (a backslash immediately followed by a hash
      sign), which identifies the RDATA as having the generic encoding
      defined herein rather than a traditional type-specific encoding.

      An unsigned decimal integer specifying the RDATA length in octets.

      Zero or more words of hexadecimal data encoding the actual RDATA
      field, each containing an even number of hexadecimal digits.

   If the RDATA is of zero length, the text representation contains only
   the \# token and the single zero representing the length.

   An implementation MAY also choose to represent some RRs of known type
   using the above generic representations for the type, class and/or
   RDATA, which carries the benefit of making the resulting master file
   portable to servers where these types are unknown.  Using the generic
   representation for the RDATA of an RR of known type can also be
   useful in the case of an RR type where the text format varies
   depending on a version, protocol, or similar field (or several)
   embedded in the RDATA when such a field has a value for which no text
   format is known, e.g., a LOC RR [RFC1876] with a VERSION other than

   Even though an RR of known type represented in the \# format is
   effectively treated as an unknown type for the purpose of parsing the
   RDATA text representation, all further processing by the server MUST
   treat it as a known type and take into account any applicable type-
   specific rules regarding compression, canonicalization, etc.

   The following are examples of RRs represented in this manner,
   illustrating various combinations of generic and type-specific
   encodings for the different fields of the master file format:

      a.example.   CLASS32     TYPE731         \# 6 abcd (
                                               ef 01 23 45 )
      b.example.   HS          TYPE62347       \# 0
      e.example.   IN          A               \# 4 0A000001
      e.example.   CLASS1      TYPE1 

6.  Equality Comparison

   Certain DNS protocols, notably Dynamic Update [RFC2136], require RRs
   to be compared for equality.  Two RRs of the same unknown type are
   considered equal when their RDATA is bitwise equal.  To ensure that
   the outcome of the comparison is identical whether the RR is known to
   the server or not, specifications for new RR types MUST NOT specify
   type-specific comparison rules.

   This implies that embedded domain names, being included in the
   overall bitwise comparison, are compared in a case-sensitive manner.

   As a result, when a new RR type contains one or more embedded domain
   names, it is possible to have multiple RRs owned by the same name
   that differ only in the character case of the embedded domain
   name(s).  This is similar to the existing possibility of multiple TXT
   records differing only in character case, and not expected to cause
   any problems in practice.

7.  DNSSEC Canonical Form and Ordering

   DNSSEC defines a canonical form and ordering for RRs [RFC2535]
   (section 8.1).  In that canonical form, domain names embedded in the
   RDATA are converted to lower case.

   The downcasing is necessary to ensure the correctness of DNSSEC
   signatures when case distinctions in domain names are lost due to
   compression, but since it requires knowledge of the presence and
   position of embedded domain names, it cannot be applied to unknown

   To ensure continued consistency of the canonical form of RR types
   where compression is allowed, and for continued interoperability with
   existing implementations that already implement the [RFC2535]
   canonical form and apply it to their known RR types, the canonical
   form remains unchanged for all RR types whose whose initial
   publication as an RFC was prior to the initial publication of this
   specification as an RFC (RFC 3597).

   As a courtesy to implementors, it is hereby noted that the complete
   set of such previously published RR types that contain embedded
   domain names, and whose DNSSEC canonical form therefore involves
   downcasing according to the DNS rules for character comparisons,
   consists of the RR types NS, MD, MF, CNAME, SOA, MB, MG, MR, PTR,
   DNAME, and A6.

   This document specifies that for all other RR types (whether treated
   as unknown types or treated as known types according to an RR type
   definition RFC more recent than RFC 3597), the canonical form is such
   that no downcasing of embedded domain names takes place, and
   otherwise identical to the canonical form specified in [RFC2535]
   section 8.1.

   Note that the owner name is always set to lower case according to the
   DNS rules for character comparisons, regardless of the RR type.

   The DNSSEC canonical RR ordering is as specified in [RFC2535] section
   8.3, where the octet sequence is the canonical form as revised by
   this specification.

8.  Additional Section Processing

   Unknown RR types cause no additional section processing.  Future RR
   type specifications MAY specify type-specific additional section
   processing rules, but any such processing MUST be optional as it can
   only be performed by servers for which the RR type in case is known.

9.  IANA Considerations

   This document does not require any IANA actions.

10.  Security Considerations

   This specification is not believed to cause any new security
   problems, nor to solve any existing ones.

11.  Normative References

   [RFC1034]   Mockapetris, P., "Domain Names - Concepts and
               Facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]   Mockapetris, P., "Domain Names - Implementation and
               Specifications", STD 13, RFC 1035, November 1987.

   [RFC1123]   Braden, R., Ed., "Requirements for Internet Hosts --
               Application and Support", STD 3, RFC 1123, October 1989.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2535]   Eastlake, D., "Domain Name System Security Extensions",
               RFC 2535, March 1999.

   [RFC2163]   Allocchio, C., "Using the Internet DNS to Distribute
               MIXER Conformant Global Address Mapping (MCGAM)", RFC
               2163, January 1998.

   [RFC2929]   Eastlake, D., Brunner-Williams, E. and B. Manning,
               "Domain Name System (DNS) IANA Considerations", BCP 42,
               RFC 2929, September 2000.

12.  Informative References

   [RFC1876]   Davis, C., Vixie, P., Goodwin, T. and I. Dickinson, "A
               Means for Expressing Location Information in the Domain
               Name System", RFC 1876, January 1996.

   [RFC2052]   Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
               the location of services (DNS SRV)", RFC 2052, October

   [RFC2136]   Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,
               "Dynamic Updates in the Domain Name System (DNS UPDATE)",
               RFC 2136, April 1997.

   [RFC2782]   Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
               specifying the location of services (DNS SRV)", RFC 2782,
               February 2000.

13.  Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   claims of rights made available for publication and any assurances of
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   obtain a general license or permission for the use of such
   proprietary rights by implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive

14.  Author's Address

   Andreas Gustafsson
   Nominum, Inc.
   2385 Bay Rd
   Redwood City, CA 94063

   Phone: +1 650 381 6004
   EMail: gson@nominum.com

15.  Full Copyright Statement

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

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