DNS Extensions Working Group                            J. Schlyter, Ed.

Updates: RFC 2535, RFC TCR (if approved)
Expires: June 15, 18, 2004

                        DNSSEC NSEC RDATA Format

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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Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   This document defines updates the NSEC resource record RDATA format
   to cover all type codes.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    The NSEC Resource Record . . . . . . . . . . . . . . . . . .  3
   2.1   NSEC RDATA Wire Format . . . . . . . . . . . . . . . . . . .  4
   2.1.1 The Next Domain Name Field . . . . . . . . . . . . . . . . .  4
   2.1.2 The List of Type Bit Map(s) Field  . . . . . . . . . . . . .  4
   2.1.3 Inclusion of Wildcard Names in NSEC RDATA  . . . . . . . . .  5
   2.2   The NSEC RR Presentation Format  . . . . . . . . . . . . . .  5
   2.3   NSEC RR Example  . . . . . . . . . . . . . . . . . . . . . .  5
   3.    IANA Considerations  . . . . . . . . . . . . . . . . . . . .  6
   4.    Security Considerations  . . . . . . . . . . . . . . . . . .  6
         Normative References . . . . . . . . . . . . . . . . . . . .  6
         Informational References . . . . . . . . . . . . . . . . . .  7
         Author's Address . . . . . . . . . . . . . . . . . . . . . .  7
   A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  7
         Intellectual Property and Copyright Statements . . . . . . .  8

1. Introduction

   The NSEC [5] Resource Record (RR) is used for authenticated proof of
   the non-existence of DNS owner names and types.  The RDATA format for
   the NSEC RR, as described in RFC 2535 [2], had a limitation in that,
   without using a yet undefined extension mechanism, the the RDATA
   could only carry information about the existence of the first 127

   To prevent the introduction of 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

   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 economic 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 type
   present in packets of approximately 8.5 kilobytes; 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 TCR [5], thereby not introducing changes
   except for the syntax of the type bit map.

   [Editors note: this is the text that is to be copied into

   This document updates RFC 2535 [2] and RFC TCR [5].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

2. The NSEC Resource Record

   The NSEC resource record lists two separate things: the owner name of
   the next authoritative 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 both indicate which authoritative
   RRsets exist in a zone and also form a chain of authoritative 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.

   The NSEC RR RDATA format is class independent and defined for all

   The NSEC RR has no special TTL requirements.

2.1 NSEC RDATA Wire Format

   The RDATA of the NSEC RR is as shown below:

                        1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   /                      Next Domain Name                         /
   /                   List of Type Bit Map(s)                     /

2.1.1 The Next Domain Name Field

   The Next Domain Name field contains the owner name of the next
   authoritative RRset in the canonical ordering of the zone. The value
   of the Next Domain Name field in the last NSEC record in the zone is
   the name of the zone apex (the owner name of the zone's SOA RR).

2.1.2 The List of Type Bit Map(s) Field

   The RR type space is split into 256 window blocks, each representing
   the low-order 8 bits of the 16-bit RR type space. Each block that has
   at least one active RR type is encoded using a single octet window
   number (from 0 to 255), a single octet bitmap length (from 1 to 32)
   indicating the number of octets used for the window block's bitmap,
   and up to 32 octets (256 bits) of bitmap.

   Blocks are present in the NSEC RR RDATA in increasing numerical

   "|" denotes concatenation

   Type Bit Map(s) Field = ( Window Block # | Bitmap Length | Bitmap ) +

   Each bitmap encodes the low-order 8 bits of RR types within the
   window block, in network bit order.  The first bit is bit 0.  For
   window block 0, bit 1 corresponds to RR type 1 (A), bit 2 corresponds
   to RR type 2 (NS), and so forth.  For window block 1, bit 1
   corresponds to RR type 257, bit 2 to RR type 258.  If a bit is set to
   1, it indicates that an RRset of that type is present for the NSEC
   RR's owner name.  If a bit is set to 0, it indicates that no RRset of
   that type is present for the NSEC RR's owner name.

   Since bit 0 in window block 0 refers to the non-existing RR type 0,
   it MUST be set to 0.  After verification, the validator SHOULD MUST ignore
   the value of bit 0 in window block 0.

   Bits representing Meta-TYPEs or QTYPEs as specified in RFC 2929 [3]
   (section 3.1) or within the range reserved for assignment only to
   QTYPEs and Meta-TYPEs MUST be set to 0, since they do not appear in
   zone data.  If encountered, they must be ignored upon reading.

   Blocks with no types present MUST NOT be included.  Trailing zero
   octets in the bitmap MUST be omitted.  The length of each block's
   bitmap is determined by the type code with the largest numerical
   value, within that block, among the set of RR types present at the
   NSEC RR's owner name.  Trailing zero octets not specified MUST be
   interpretted as zero octets.

2.1.3 Inclusion of Wildcard Names in NSEC RDATA

   If a wildcard owner name appears in a zone, the wildcard label ("*")
   is treated as a literal symbol and is treated the same as any other
   owner name for purposes of generating NSEC RRs. Wildcard owner names
   appear in the Next Domain Name field without any wildcard expansion.
   RFC 2535 [2] describes the impact of wildcards on authenticated
   denial of existence.

2.2 The NSEC RR Presentation Format

   The presentation format of the RDATA portion is as follows:

   The Next Domain Name field is represented as a domain name.

   The List of Type Bit Map(s) Field is represented as a sequence of RR
   type mnemonics.  When the mnemonic is not known, the TYPE
   representation as described in RFC 3597 [4] (section 5) MUST be used.

2.3 NSEC RR Example

   The following NSEC RR identifies the RRsets associated with
   alfa.example.com. and identifies the next authoritative name after

   alfa.example.com. 86400 IN NSEC host.example.com. A MX RRSIG NSEC TYPE1234

   The first four text fields specify the name, TTL, Class, and RR type
   (NSEC).  The entry host.example.com. is the next authoritative name
   after alfa.example.com. in canonical order. The A, MX, RRSIG and NSEC
   mnemonics indicate there are A, MX, RRSIG, NSEC and TYPE1234 RRsets
   associated with the name alfa.example.com.

   The RDATA section of the NSEC RR above would be encoded as:

         0x04 'h'  'o'  's'  't'
         0x07 'e'  'x'  'a'  'm'  'p'  'l'  'e'
         0x03 'c'  'o'  'm'  0x00
         0x00 0x06 0x40 0x01 0x00 0x00 0x00 0x03
         0x04 0x1b 0x00 0x00 0x00 0x00 0x00 0x00
         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
         0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
         0x00 0x00 0x00 0x00 0x20

   Assuming that the resolver can authenticate this NSEC record, it
   could be used to prove that beta.example.com does not exist, or could
   be used to prove there is no AAAA record associated with
   alfa.example.com.  Authenticated denial of existence is discussed in
   RFC 2535 [2].

3. IANA Considerations

   This document introduces no new IANA considerations, because all of
   the protocol parameters used in this document have already been
   assigned by RFC TCR [5].

4. Security Considerations

   The change introducted here does not affect security, since it only
   updates the RDATA format and encoding.

Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [3]  Eastlake, D., Brunner-Williams, E. and B. Manning, "Domain Name
        System (DNS) IANA Considerations", BCP 42, RFC 2929, September

   [4]  Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
        Types", RFC 3597, September 2003.

   [5]  Weiler, S., "Legacy Resolver Compatibility for Delegation
        Signer", draft-ietf-dnsext-dnssec-2535typecode-change-05 (work
        in progress), October 2003.

Informational References

   [6]  Mockapetris, P., "Domain names - concepts and facilities", STD
        13, RFC 1034, November 1987.

   [7]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

Author's Address

   Jakob Schlyter (editor)
   Karl Gustavsgatan 15
   Goteborg  SE-411 25

   EMail: jakob@schlyter.se

Appendix A. Acknowledgements

   The encoding described in this document was initially proposed by
   Mark Andrews.  Other encodings where proposed by David Blacka and
   Michael Graff.

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