INTERNET-DRAFT Andreas Gustafsson
draft-ietf-dnsext-unknown-rrs-03.txt Nominum Inc.
June 2002
Handling of Unknown DNS RR Types
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
Extending the Domain Name System with new Resource Record 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
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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].
2. Definition
An "RR of unknown type" is an RR whose RDATA format is not known to
the DNS implementation at hand, such that it cannot be converted to a
type-specific text format, compressed, or otherwise handled in a
type-specific way, and whose type is not an assigned QTYPE or Meta-
TYPE in RFC2929 section 3.1 nor within the range reserved in that
section for assignment only to QTYPEs and Meta-TYPEs.
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].
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 (although the current specification of the SRV RR
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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.
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
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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.
7. DNSSEC Canonical Form and Ordering
DNSSEC [RFC2535] defines a canonical form and ordering for RRs. In
the canonical form, domain names embedded in the RDATA are converted
to lower case.
To ensure backwards compatibility, this canonical form remains
unchanged for any RR types defined in RFC2931 or earlier. That is,
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the domain names embedded in RRs of type 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 are converted to lower case. For all
other RR types, the canonical form is hereby changed such that no
downcasing of embedded domain names takes place. The owner name is
still set to lower case.
The canonical 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.
9. IANA Considerations
The IANA is hereby requested to verify that specifications for new RR
types requesting an RR type number comply with this specification.
In particular, the IANA MUST NOT assign numbers to new RR types whose
specification allows embedded domain names to be compressed.
10. Security Considerations
This specification is not believed to cause any new security
problems, nor to solve any existing ones.
References
[RFC1034] - Domain Names - Concepts and Facilities, P. Mockapetris,
November 1987.
[RFC1035] - Domain Names - Implementation and Specifications, P.
Mockapetris, November 1987.
[RFC1123] - Requirements for Internet Hosts -- Application and
Support, R. Braden, Editor, October 1989.
[RFC1876] - A Means for Expressing Location Information in the Domain
Name System, C. Davis, P. Vixie, T. Goodwin, I. Dickinson, January
1996.
[RFC2052] - A DNS RR for specifying the location of services (DNS
SRV), A. Gulbrandsen, P. Vixie, October 1996. Obsoleted by RFC2782.
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[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] - Dynamic Updates in the Domain Name System (DNS UPDATE).
P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, April 1997.
[RFC2535] - Domain Name System Security Extensions. D. Eastlake,
March 1999.
[RFC2782] - A DNS RR for specifying the location of services (DNS
SRV). A. Gulbrandsen, P. Vixie, L. Esibov, February 2000.
[RFC2929] - Domain Name System (DNS) IANA Considerations. D.
Eastlake, E. Brunner-Williams, B. Manning, September 2000.
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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