RFC 3845 (rfc3845) - Page 2 of 7

DNS Security (DNSSEC) NextSECure (NSEC) RDATA Format

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