7Network Working Group A. Gustafsson
8Request for Comments: 3597 Nominum Inc.
9Category: Standards Track September 2003
12 Handling of Unknown DNS Resource Record (RR) Types
16 This document specifies an Internet standards track protocol for the
17 Internet community, and requests discussion and suggestions for
18 improvements. Please refer to the current edition of the "Internet
19 Official Protocol Standards" (STD 1) for the standardization state
20 and status of this protocol. Distribution of this memo is unlimited.
24 Copyright (C) The Internet Society (2003). All Rights Reserved.
28 Extending the Domain Name System (DNS) with new Resource Record (RR)
29 types currently requires changes to name server software. This
30 document specifies the changes necessary to allow future DNS
31 implementations to handle new RR types transparently.
35 The DNS is designed to be extensible to support new services through
36 the introduction of new resource record (RR) types. In practice,
37 deploying a new RR type currently requires changes to the name server
38 software not only at the authoritative DNS server that is providing
39 the new information and the client making use of it, but also at all
40 slave servers for the zone containing it, and in some cases also at
41 caching name servers and forwarders used by the client.
43 Because the deployment of new server software is slow and expensive,
44 the potential of the DNS in supporting new services has never been
45 fully realized. This memo proposes changes to name servers and to
46 procedures for defining new RR types aimed at simplifying the future
47 deployment of new RR types.
49 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
50 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
51 document are to be interpreted as described in [RFC 2119].
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60RFC 3597 Handling of Unknown DNS RR Types September 2003
65 An "RR of unknown type" is an RR whose RDATA format is not known to
66 the DNS implementation at hand, and whose type is not an assigned
67 QTYPE or Meta-TYPE as specified in [RFC 2929] (section 3.1) nor
68 within the range reserved in that section for assignment only to
69 QTYPEs and Meta-TYPEs. Such an RR cannot be converted to a type-
70 specific text format, compressed, or otherwise handled in a type-
73 In the case of a type whose RDATA format is class specific, an RR is
74 considered to be of unknown type when the RDATA format for that
75 combination of type and class is not known.
79 To enable new RR types to be deployed without server changes, name
80 servers and resolvers MUST handle RRs of unknown type transparently.
81 That is, they must treat the RDATA section of such RRs as
82 unstructured binary data, storing and transmitting it without change
85 To ensure the correct operation of equality comparison (section 6)
86 and of the DNSSEC canonical form (section 7) when an RR type is known
87 to some but not all of the servers involved, servers MUST also
88 exactly preserve the RDATA of RRs of known type, except for changes
89 due to compression or decompression where allowed by section 4 of
90 this memo. In particular, the character case of domain names that
91 are not subject to compression MUST be preserved.
934. Domain Name Compression
95 RRs containing compression pointers in the RDATA part cannot be
96 treated transparently, as the compression pointers are only
97 meaningful within the context of a DNS message. Transparently
98 copying the RDATA into a new DNS message would cause the compression
99 pointers to point at the corresponding location in the new message,
100 which now contains unrelated data. This would cause the compressed
101 name to be corrupted.
103 To avoid such corruption, servers MUST NOT compress domain names
104 embedded in the RDATA of types that are class-specific or not well-
105 known. This requirement was stated in [RFC1123] without defining the
106 term "well-known"; it is hereby specified that only the RR types
107 defined in [RFC1035] are to be considered "well-known".
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116RFC 3597 Handling of Unknown DNS RR Types September 2003
119 The specifications of a few existing RR types have explicitly allowed
120 compression contrary to this specification: [RFC2163] specified that
121 compression applies to the PX RR, and [RFC2535] allowed compression
122 in SIG RRs and NXT RRs records. Since this specification disallows
123 compression in these cases, it is an update to [RFC2163] (section 4)
124 and [RFC2535] (sections 4.1.7 and 5.2).
126 Receiving servers MUST decompress domain names in RRs of well-known
127 type, and SHOULD also decompress RRs of type RP, AFSDB, RT, SIG, PX,
128 NXT, NAPTR, and SRV (although the current specification of the SRV RR
129 in [RFC2782] prohibits compression, [RFC2052] mandated it, and some
130 servers following that earlier specification are still in use).
132 Future specifications for new RR types that contain domain names
133 within their RDATA MUST NOT allow the use of name compression for
134 those names, and SHOULD explicitly state that the embedded domain
135 names MUST NOT be compressed.
137 As noted in [RFC1123], the owner name of an RR is always eligible for
1405. Text Representation
142 In the "type" field of a master file line, an unknown RR type is
143 represented by the word "TYPE" immediately followed by the decimal RR
144 type number, with no intervening whitespace. In the "class" field,
145 an unknown class is similarly represented as the word "CLASS"
146 immediately followed by the decimal class number.
148 This convention allows types and classes to be distinguished from
149 each other and from TTL values, allowing the "[<TTL>] [<class>]
150 <type> <RDATA>" and "[<class>] [<TTL>] <type> <RDATA>" forms of
151 [RFC1035] to both be unambiguously parsed.
153 The RDATA section of an RR of unknown type is represented as a
154 sequence of white space separated words as follows:
156 The special token \# (a backslash immediately followed by a hash
157 sign), which identifies the RDATA as having the generic encoding
158 defined herein rather than a traditional type-specific encoding.
160 An unsigned decimal integer specifying the RDATA length in octets.
162 Zero or more words of hexadecimal data encoding the actual RDATA
163 field, each containing an even number of hexadecimal digits.
165 If the RDATA is of zero length, the text representation contains only
166 the \# token and the single zero representing the length.
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172RFC 3597 Handling of Unknown DNS RR Types September 2003
175 An implementation MAY also choose to represent some RRs of known type
176 using the above generic representations for the type, class and/or
177 RDATA, which carries the benefit of making the resulting master file
178 portable to servers where these types are unknown. Using the generic
179 representation for the RDATA of an RR of known type can also be
180 useful in the case of an RR type where the text format varies
181 depending on a version, protocol, or similar field (or several)
182 embedded in the RDATA when such a field has a value for which no text
183 format is known, e.g., a LOC RR [RFC1876] with a VERSION other than
186 Even though an RR of known type represented in the \# format is
187 effectively treated as an unknown type for the purpose of parsing the
188 RDATA text representation, all further processing by the server MUST
189 treat it as a known type and take into account any applicable type-
190 specific rules regarding compression, canonicalization, etc.
192 The following are examples of RRs represented in this manner,
193 illustrating various combinations of generic and type-specific
194 encodings for the different fields of the master file format:
196 a.example. CLASS32 TYPE731 \# 6 abcd (
198 b.example. HS TYPE62347 \# 0
199 e.example. IN A \# 4 0A000001
200 e.example. CLASS1 TYPE1 10.0.0.2
2026. Equality Comparison
204 Certain DNS protocols, notably Dynamic Update [RFC2136], require RRs
205 to be compared for equality. Two RRs of the same unknown type are
206 considered equal when their RDATA is bitwise equal. To ensure that
207 the outcome of the comparison is identical whether the RR is known to
208 the server or not, specifications for new RR types MUST NOT specify
209 type-specific comparison rules.
211 This implies that embedded domain names, being included in the
212 overall bitwise comparison, are compared in a case-sensitive manner.
214 As a result, when a new RR type contains one or more embedded domain
215 names, it is possible to have multiple RRs owned by the same name
216 that differ only in the character case of the embedded domain
217 name(s). This is similar to the existing possibility of multiple TXT
218 records differing only in character case, and not expected to cause
219 any problems in practice.
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228RFC 3597 Handling of Unknown DNS RR Types September 2003
2317. DNSSEC Canonical Form and Ordering
233 DNSSEC defines a canonical form and ordering for RRs [RFC2535]
234 (section 8.1). In that canonical form, domain names embedded in the
235 RDATA are converted to lower case.
237 The downcasing is necessary to ensure the correctness of DNSSEC
238 signatures when case distinctions in domain names are lost due to
239 compression, but since it requires knowledge of the presence and
240 position of embedded domain names, it cannot be applied to unknown
243 To ensure continued consistency of the canonical form of RR types
244 where compression is allowed, and for continued interoperability with
245 existing implementations that already implement the [RFC2535]
246 canonical form and apply it to their known RR types, the canonical
247 form remains unchanged for all RR types whose whose initial
248 publication as an RFC was prior to the initial publication of this
249 specification as an RFC (RFC 3597).
251 As a courtesy to implementors, it is hereby noted that the complete
252 set of such previously published RR types that contain embedded
253 domain names, and whose DNSSEC canonical form therefore involves
254 downcasing according to the DNS rules for character comparisons,
255 consists of the RR types NS, MD, MF, CNAME, SOA, MB, MG, MR, PTR,
256 HINFO, MINFO, MX, HINFO, RP, AFSDB, RT, SIG, PX, NXT, NAPTR, KX, SRV,
259 This document specifies that for all other RR types (whether treated
260 as unknown types or treated as known types according to an RR type
261 definition RFC more recent than RFC 3597), the canonical form is such
262 that no downcasing of embedded domain names takes place, and
263 otherwise identical to the canonical form specified in [RFC2535]
266 Note that the owner name is always set to lower case according to the
267 DNS rules for character comparisons, regardless of the RR type.
269 The DNSSEC canonical RR ordering is as specified in [RFC2535] section
270 8.3, where the octet sequence is the canonical form as revised by
2738. Additional Section Processing
275 Unknown RR types cause no additional section processing. Future RR
276 type specifications MAY specify type-specific additional section
277 processing rules, but any such processing MUST be optional as it can
278 only be performed by servers for which the RR type in case is known.
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284RFC 3597 Handling of Unknown DNS RR Types September 2003
2879. IANA Considerations
289 This document does not require any IANA actions.
29110. Security Considerations
293 This specification is not believed to cause any new security
294 problems, nor to solve any existing ones.
29611. Normative References
298 [RFC1034] Mockapetris, P., "Domain Names - Concepts and
299 Facilities", STD 13, RFC 1034, November 1987.
301 [RFC1035] Mockapetris, P., "Domain Names - Implementation and
302 Specifications", STD 13, RFC 1035, November 1987.
304 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts --
305 Application and Support", STD 3, RFC 1123, October 1989.
307 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
308 Requirement Levels", BCP 14, RFC 2119, March 1997.
310 [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
311 RFC 2535, March 1999.
313 [RFC2163] Allocchio, C., "Using the Internet DNS to Distribute
314 MIXER Conformant Global Address Mapping (MCGAM)", RFC
317 [RFC2929] Eastlake, D., Brunner-Williams, E. and B. Manning,
318 "Domain Name System (DNS) IANA Considerations", BCP 42,
319 RFC 2929, September 2000.
32112. Informative References
323 [RFC1876] Davis, C., Vixie, P., Goodwin, T. and I. Dickinson, "A
324 Means for Expressing Location Information in the Domain
325 Name System", RFC 1876, January 1996.
327 [RFC2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
328 the location of services (DNS SRV)", RFC 2052, October
331 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,
332 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
333 RFC 2136, April 1997.
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340RFC 3597 Handling of Unknown DNS RR Types September 2003
343 [RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
344 specifying the location of services (DNS SRV)", RFC 2782,
34713. Intellectual Property Statement
349 The IETF takes no position regarding the validity or scope of any
350 intellectual property or other rights that might be claimed to
351 pertain to the implementation or use of the technology described in
352 this document or the extent to which any license under such rights
353 might or might not be available; neither does it represent that it
354 has made any effort to identify any such rights. Information on the
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359 obtain a general license or permission for the use of such
360 proprietary rights by implementors or users of this specification can
361 be obtained from the IETF Secretariat.
363 The IETF invites any interested party to bring to its attention any
364 copyrights, patents or patent applications, or other proprietary
365 rights which may cover technology that may be required to practice
366 this standard. Please address the information to the IETF Executive
374 Redwood City, CA 94063
377 Phone: +1 650 381 6004
378 EMail: gson@nominum.com
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396RFC 3597 Handling of Unknown DNS RR Types September 2003
39915. Full Copyright Statement
401 Copyright (C) The Internet Society (2003). All Rights Reserved.
403 This document and translations of it may be copied and furnished to
404 others, and derivative works that comment on or otherwise explain it
405 or assist in its implementation may be prepared, copied, published
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