GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3597

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].

Gustafsson Standards Track [Page 1] 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".

Gustafsson Standards Track [Page 2] 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.

Gustafsson Standards Track [Page 3] 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.

Gustafsson Standards Track [Page 4] RFC 3597 Handling of Unknown DNS RR Types September 2003

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
 types.
 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,
 HINFO, MINFO, MX, HINFO, RP, AFSDB, RT, SIG, PX, NXT, NAPTR, KX, SRV,
 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.

Gustafsson Standards Track [Page 5] RFC 3597 Handling of Unknown DNS RR Types September 2003

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
             1996.
 [RFC2136]   Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,
             "Dynamic Updates in the Domain Name System (DNS UPDATE)",
             RFC 2136, April 1997.

Gustafsson Standards Track [Page 6] RFC 3597 Handling of Unknown DNS RR Types September 2003

 [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
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 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
 Director.

14. Author's Address

 Andreas Gustafsson
 Nominum, Inc.
 2385 Bay Rd
 Redwood City, CA 94063
 USA
 Phone: +1 650 381 6004
 EMail: gson@nominum.com

Gustafsson Standards Track [Page 7] RFC 3597 Handling of Unknown DNS RR Types September 2003

15. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assignees.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Gustafsson Standards Track [Page 8]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3597.txt · Last modified: 2003/09/08 17:57 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki