RFC 3845 (rfc3845) - Page 2 of 7
DNS Security (DNSSEC) NextSECure (NSEC) RDATA Format
Alternative Format: Original Text Document
RFC 3845 DNSSEC NSEC RDATA Format August 2004
1. Introduction
The DNS [6][7] NSEC [5] Resource Record (RR) is used for
authenticated proof of the non-existence of DNS owner names and
types. The NSEC RR is based on the NXT RR as described in RFC 2535
[2], and is similar except for the name and typecode. The RDATA
format for the NXT RR has the limitation in that the RDATA could only
carry information about the existence of the first 127 types. RFC
2535 did reserve a bit to specify an extension mechanism, but the
mechanism was never actually defined.
In order to avoid needing to develop an extension mechanism into a
deployed base of DNSSEC aware servers and resolvers once the first
127 type codes are allocated, this document redefines the wire format
of the "Type Bit Map" field in the NSEC RDATA to cover the full RR
type space.
This document introduces a new format for the type bit map. The
properties of the type bit map format are that it can cover the full
possible range of typecodes, that it is relatively economical in the
amount of space it uses for the common case of a few types with an
owner name, that it can represent owner names with all possible types
present in packets of approximately 8.5 kilobytes, and that the
representation is simple to implement. Efficient searching of the
type bitmap for the presence of certain types is not a requirement.
For convenience and completeness, this document presents the syntax
and semantics for the NSEC RR based on the specification in RFC 2535
[2] and as updated by RFC 3755 [5], thereby not introducing changes
except for the syntax of the type bit map.
This document updates RFC 2535 [2] and RFC 3755 [5].
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 BCP 14, RFC 2119 [1].
2. The NSEC Resource Record
The NSEC resource record lists two separate things: the owner name of
the next RRset in the canonical ordering of the zone, and the set of
RR types present at the NSEC RR's owner name. The complete set of
NSEC RRs in a zone indicate which RRsets exist in a zone, and form a
chain of owner names in the zone. This information is used to
provide authenticated denial of existence for DNS data, as described
in RFC 2535 [2].
The type value for the NSEC RR is 47.
Schlyter, Ed. Standards Track