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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.
+
+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]
+\f
+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]
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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.
+
+
+
+Gustafsson Standards Track [Page 3]
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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.
+
+
+
+
+
+
+Gustafsson Standards Track [Page 4]
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+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]
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+RFC 3597 Handling of Unknown DNS RR Types September 2003
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+
+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]
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+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
+
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+RFC 3597 Handling of Unknown DNS RR Types September 2003
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+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.
+
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