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Versions: 00 01 02 03 04 05 06 RFC 3597

INTERNET-DRAFT                                       Andreas Gustafsson
draft-ietf-dnsext-unknown-rrs-04.txt                       Nominum Inc.
                                                         September 2002

                    Handling of Unknown DNS RR Types

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   Extending the Domain Name System with new Resource Record 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

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   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, such that it cannot be converted to a
   type-specific text format, compressed, or otherwise handled in a
   type-specific way, and whose type is not an assigned QTYPE or Meta-
   TYPE in RFC2929 section 3.1 nor within the range reserved in that
   section for assignment only to QTYPEs and Meta-TYPEs.

   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

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   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".

   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

      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.

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   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.

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7. DNSSEC Canonical Form and Ordering

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

   To ensure backwards compatibility, this canonical form remains
   unchanged for any RR types defined in RFC2931 or earlier.  That is,
   the domain names embedded in RRs of type NS, MD, MF, CNAME, SOA, MB,
   NAPTR, KX, SRV, DNAME, and A6 are converted to lower case according
   to the DNS rules for character comparisons.

   For all other RR types, the canonical form is hereby changed such
   that no downcasing of embedded domain names takes place.  The owner
   name is always set to lower case according to the DNS rules for
   character comparisons, regardless of the RR type.

   The canonical ordering is as specified in RFC2535 section 8.3, where
   the octet sequence is the canonical form as revised by this

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

   The IANA is hereby requested to verify that specifications for new RR
   types requesting an RR type number comply with this specification.
   In particular, the IANA MUST NOT assign numbers to new RR types whose
   specification allows embedded domain names to be compressed.

10. Security Considerations

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


   [RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
   November 1987.

   [RFC1035] - Domain Names - Implementation and Specifications, P.
   Mockapetris, November 1987.

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   [RFC1123] - Requirements for Internet Hosts -- Application and
   Support, R. Braden, Editor, October 1989.

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

   [RFC2052] - A DNS RR for specifying the location of services (DNS
   SRV), A. Gulbrandsen, P. Vixie, October 1996.  Obsoleted by RFC2782.

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

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

   [RFC2535] - Domain Name System Security Extensions. D. Eastlake,
   March 1999.

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

   [RFC2929] - Domain Name System (DNS) IANA Considerations.  D.
   Eastlake, E. Brunner-Williams, B. Manning, September 2000.

Author's Address

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

   Phone: +1 650 381 6004

   Email: gson@nominum.com

Full Copyright Statement

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   document itself may not be modified in any way, such as by removing
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