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INTERNET-DRAFT Andreas Gustafsson
draft-ietf-dnsext-unknown-rrs-00.txt Nominum Inc.
November 2000
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.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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. Definitions
In this document, a "well known" RR type means one defined in
RFC1035.
An "RR of unknown type" is an RR type whose RDATA format is not known
to the DNS implementation at hand, and which therefore cannot be
converted to a type-specific text format, compressed, or otherwise
handled in any type-specific way. This includes the case where the
RR's type is recognized but its RDATA format is class specific and
the RR is of a class for which the format 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.
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 not 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 SRV RR is clearly defined to not
allow compression of the target field, some existing name servers
compress it anyway).
Future specifications for new RR types that contain domain names
within their RDATA MUST NOT allow the use of name compression for
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those names, and SHOULD explicitly state that the embedded domain
names MUST NOT be compressed.
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.
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
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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 compatilbility, this canonical form remains
unchanged for any RR types defined in RFC2931 or earlier. That is,
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
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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 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.
[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.
Author's Address
Andreas Gustafsson
Nominum Inc.
950 Charter Street
Redwood City, CA 94063
USA
Phone: +1 650 779 6004
Email: Andreas.Gustafsson@nominum.com
Full Copyright Statement
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Copyright (C) The Internet Society (2000). 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 implmentation 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 assigns.
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."
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