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Versions: 00 01 02 03 04 05 06 RFC 3845

DNS Extensions Working Group                            J. Schlyter, Ed.
Internet-Draft                                              May 10, 2004
Updates: RFC 2535, RFC TCR (if approved)
Expires: November 8, 2004


                         DNSSEC NSEC RDATA Format
                   draft-ietf-dnsext-nsec-rdata-06.txt

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
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    This Internet-Draft will expire on November 8, 2004.

Copyright Notice

    Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

    This document redefines the wire format of the "Type Bit Map" field
    in the NSEC resource record RDATA format to cover the full RR type
    space.











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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


































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1. Introduction

    The 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 the need 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 TCR [5], thereby not introducing changes
    except for the syntax of the type bit map.

    This document updates RFC 2535 [2] and RFC TCR [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 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 both indicate which RRsets exist in a zone and
    also 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.



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

    The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
    field. This is in the spirit of negative caching [8].

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 RR 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).

    A sender MUST NOT use DNS name compression on the Next Domain Name
    field when transmitting an NSEC RR.

    Owner names of RRsets not authoritative for the given zone (such as
    glue records) MUST NOT be listed in the Next Domain Name unless at
    least one authoritative RRset exists at the same owner name.

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.

    Window blocks are present in the NSEC RR RDATA in increasing
    numerical order.

    "|" denotes concatenation

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



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    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 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



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    alfa.example.com.

    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, NSEC
    and TYPE1234 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 update of the RDATA format and encoding does not affect the
    security of the use of NSEC RRs.

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



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         2000.

    [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.

    [8]  Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC
         2308, March 1998.


Author's Address

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

    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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    HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

    Funding for the RFC Editor function is currently provided by the
    Internet Society.











































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