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Network Working Group                                      A. GustafssonRequest for Comments: 3597                                  Nominum Inc.Category: Standards Track                                 September 2003Handling of Unknown DNS Resource Record (RR) TypesStatus 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 20032.  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 bysection 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] (sections4.1.7 and5.2).   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           10.0.0.26.  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 20037.  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 thanRFC 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]section8.3, 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 20039.  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)",RFC2163, 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 inBCP-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.comGustafsson                  Standards Track                     [Page 7]

RFC 3597            Handling of Unknown DNS RR Types      September 200315.  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]