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].
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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-
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.
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
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
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".
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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
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.
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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:
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 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
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,
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]
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
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.
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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.
[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",
[RFC2163] Allocchio, C., "Using the Internet DNS to Distribute
MIXER Conformant Global Address Mapping (MCGAM)", RFC
[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
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
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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,
13. Intellectual Property Statement
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pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
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proprietary rights by implementors or users of this specification can
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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
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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
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Funding for the RFC Editor function is currently provided by the
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