Network Working Group A. Gustafsson
Request for Comments: 3597 Nominum Inc.
Category: Standards Track September 2003
Handling of Unknown DNS Resource Record (RR) Types
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2003). All Rights Reserved.
Extending the Domain Name System (DNS) with new Resource Record (RR)
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.
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].
Gustafsson Standards Track [Page 1]RFC 3597 Handling of Unknown DNS RR Types September 20032. Definition
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
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".
Gustafsson Standards Track [Page 2]RFC 3597 Handling of Unknown DNS RR Types September 2003
The specifications of a few existing RR types have explicitly allowed