Commit 3a2a2463 authored by Mark Andrews's avatar Mark Andrews
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new draft

parent 91c6a867
Araneus Information Systems Oy
September 23, 2009
Intended status: Draft Standard
Obsoletes: RFC3597
Handling of Unknown DNS Resource Record (RR) Types
Status of this Memo
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Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Extending the Domain Name System (DNS) with new Resource Record (RR)
types should not requires changes to name server software. This
document specifies how new RR types are transparently handled by DNS
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1. Introduction
The DNS [RFC1034] is designed to be extensible to support new
services through the introduction of new resource record (RR) types.
Nevertheless, DNS implementations have historically required software
changes to support new RR types, not only at the authoritative DNS
server 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 DNS software is slow and expensive,
this has been a significant impediment to supporting new services in
the DNS.
[RFC3597] defined DNS implementation behavior and procedures for
defining new RR types aimed at simplifying the deployment of new RR
types by allowing them to be treated transparently by existing
implementations. Thanks to the widespread adoption of that
specification, much of the DNS is now capable of handling new record
types without software changes.
This document is a self-contained revised specification supplanting
and obsoleting [RFC3597].
2. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [RFC2119].
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 [RFC5395] (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
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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 document. 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".
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 to ensure interoperability with implementations
predating [RFC3597].
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
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
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[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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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
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 C0000201
e.example. CLASS1 TYPE1
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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7. DNSSEC Considerations
The rules for the DNSSEC canonical form and ordering were updated to
support transparent treatment of unknown types in [RFC3597]. Those
updates have subsequently been integrated into the base DNSSEC
specification, such that the DNSSEC canonical form and ordering are
now specified in [RFC4034] or its successors rather than in this
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.
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.
[RFC5395] Eastlake, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 5395, November 2008.
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.
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[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
14. Author's Address
Andreas Gustafsson
Araneus Information Systems Oy
PL 110
02321 Espoo
Phone: +358 40 547 2099
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