rfc3597.txt

bind 9.3结合mysql数据库

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.

Abstract

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].






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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
   [RFC1123].

   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".






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RFC 3597            Handling of Unknown DNS RR Types      September 2003


   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
   compression.

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.



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RFC 3597            Handling of Unknown DNS RR Types      September 2003


   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
   0.

   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           10.0.0.2

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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