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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5155

Network Working Group                                          B. Laurie
Internet-Draft                                                 G. Sisson
Expires: December 3, 2005                                        Nominet
                                                               R. Arends
                                                    Telematica Instituut
                                                               june 2005


             DNSSEC Hash Authenticated Denial of Existence
                       draft-ietf-dnsext-nsec3-02

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on December 3, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The DNS Security (DNSSEC) NSEC resource record (RR) is intended to be
   used to provide authenticated denial of existence of DNS ownernames
   and types; however, it permits any user to traverse a zone and obtain
   a listing of all ownernames.

   A complete zone file can be used either directly as a source of



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   probable e-mail addresses for spam, or indirectly as a key for
   multiple WHOIS queries to reveal registrant data which many
   registries (particularly in Europe) may be under strict legal
   obligations to protect.  Many registries therefore prohibit copying
   of their zone file; however the use of NSEC RRs renders policies
   unenforceable.

   This document proposes a scheme which obscures original ownernames
   while permitting authenticated denial of existence of non-existent
   names.  Non-authoritative delegation point NS RR types may be
   excluded.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Rationale  . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2   Reserved Words . . . . . . . . . . . . . . . . . . . . . .  4
     1.3   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The NSEC3 Resource Record  . . . . . . . . . . . . . . . . . .  5
     2.1   NSEC3 RDATA Wire Format  . . . . . . . . . . . . . . . . .  5
       2.1.1   The Authoritative Only Flag Field  . . . . . . . . . .  6
       2.1.2   The Hash Function Field  . . . . . . . . . . . . . . .  6
       2.1.3   The Iterations Field . . . . . . . . . . . . . . . . .  7
       2.1.4   The Salt Length Field  . . . . . . . . . . . . . . . .  7
       2.1.5   The Salt Field . . . . . . . . . . . . . . . . . . . .  7
       2.1.6   The Next Hashed Ownername Field  . . . . . . . . . . .  7
       2.1.7   The list of Type Bit Map(s) Field  . . . . . . . . . .  8
     2.2   The NSEC3 RR Presentation Format . . . . . . . . . . . . .  9
   3.  Creating Additional NSEC3 RRs for Empty Non Terminals  . . . .  9
   4.  Calculation of the Hash  . . . . . . . . . . . . . . . . . . . 10
   5.  Including NSEC3 RRs in a Zone  . . . . . . . . . . . . . . . . 10
   6.  Special Considerations . . . . . . . . . . . . . . . . . . . . 11
     6.1   Delegation Points  . . . . . . . . . . . . . . . . . . . . 11
       6.1.1   Unsigned Delegations . . . . . . . . . . . . . . . . . 11
     6.2   Proving Nonexistence . . . . . . . . . . . . . . . . . . . 12
     6.3   Salting  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.4   Hash Collision . . . . . . . . . . . . . . . . . . . . . . 13
       6.4.1   Avoiding Hash Collisions during generation . . . . . . 14
       6.4.2   Second Preimage Requirement Analysis . . . . . . . . . 14
       6.4.3   Possible Hash Value Truncation Method  . . . . . . . . 14
   7.  Performance Considerations . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 16
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 16
     10.2  Informative References . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
   A.  Example Zone . . . . . . . . . . . . . . . . . . . . . . . . . 18



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   B.  Example Responses  . . . . . . . . . . . . . . . . . . . . . . 23
     B.1   answer . . . . . . . . . . . . . . . . . . . . . . . . . . 23
       B.1.1   Authenticating the Example DNSKEY RRset  . . . . . . . 25
     B.2   Name Error . . . . . . . . . . . . . . . . . . . . . . . . 26
     B.3   No Data Error  . . . . . . . . . . . . . . . . . . . . . . 28
       B.3.1   No Data Error, Empty Non-Terminal  . . . . . . . . . . 29
     B.4   Referral to Signed Zone  . . . . . . . . . . . . . . . . . 30
     B.5   Referral to Unsigned Zone using Opt-In . . . . . . . . . . 31
     B.6   Wildcard Expansion . . . . . . . . . . . . . . . . . . . . 32
     B.7   Wildcard No Data Error . . . . . . . . . . . . . . . . . . 34
     B.8   DS Child Zone No Data Error  . . . . . . . . . . . . . . . 35
       Intellectual Property and Copyright Statements . . . . . . . . 37







































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1.  Introduction

   The DNS Security Extensions (DNSSEC) introduced the NSEC Resource
   Record (RR) for authenticated denial of existence.  This document
   introduces a new RR as an alternative to NSEC that provides measures
   against zone traversal and allows for gradual expansion of
   delegation-centric zones.

1.1  Rationale

   The DNS Security Extensions included the NSEC RR to provide
   authenticated denial of existence.  Though the NSEC RR meets the
   requirements for authenticated denial of existence, it introduced a
   side-effect in that the contents of a zone can be enumerated.  This
   property introduces undesired policy issues.

   A second problem was the requirement that the existence of all record
   types in a zone - including delegation point NS record types - must
   be accounted for, despite the fact that delegation point NS RRsets
   are not authoritative and not signed.  This requirement has a side-
   effect that the overhead of delegation-centric signed zones is not
   related to the increase in security of subzones.  This requirement
   does not allow delegation-centric zones size to grow in relation to
   the growth of signed subzones.

   In the past, solutions have been proposed as a measure against these
   side effects but at the time were regarded as secondary over the need
   to have a stable DNSSEC specification.  With (draft-vixie-dnssec-ter)
   a graceful transition path to future enhancements is introduced,
   while current DNSSEC deployment can continue.  This document presents
   the NSEC3 Resource Record which mitigates these issues with the NSEC
   RR.

   The reader is assumed to be familiar with the basic DNS concepts
   described in RFC1034 [RFC1034], RFC1035 [RFC1035] and subsequent RFCs
   that update them:  RFC2136 [RFC2136], RFC2181 [RFC2181] and RFC2308
   [RFC2308].

1.2  Reserved Words

   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 [RFC2119].

1.3  Terminology

   In this document the term "original ownername" refers to a standard
   ownername.  Because this proposal uses the result of a hash function



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   over the original (unmodified) ownername, this result is referred to
   as "hashed ownername".

   "Canonical ordering of the zone" means the order in which hashed
   ownernames are arranged according to their numerical value, treating
   the leftmost (lowest numbered) byte as the most significant byte.

2.  The NSEC3 Resource Record

   The NSEC3 RR provides Authenticated Denial of Existence for DNS
   Resource Record Sets.

   The NSEC3 Resource Record lists RR types present at the NSEC3 RR's
   original ownername.  It includes the next hashed ownername in the
   canonical ordering of the zone.  The complete set of NSEC3 RRs in a
   zone indicates which RRsets exist for the original ownername of the
   RRset and form a chain of hashed ownernames in the zone.  This
   information is used to provide authenticated denial of existence for
   DNS data, as described in RFC 4035 [RFC4035].  Unsigned delegation
   point NS RRsets can optionally be excluded.  To provide protection
   against zone traversal, the ownernames used in the NSEC3 RR are
   cryptographic hashes of the original ownername prepended to the name
   of the zone.  The NSEC3 RR indicates which hash function is used to
   construct the hash, which salt is used, and how many iterations of
   the hash function are performed over the original ownername.

   The ownername for the NSEC3 RR is the base32 encoding of the hashed
   ownername.

   The type value for the NSEC3 RR is XX.

   The NSEC3 RR RDATA format is class independent.

   The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL
   field.  This is in the spirit of negative caching [RFC2308].

2.1  NSEC3 RDATA Wire Format

   The RDATA of the NSEC3 RR is as shown below:












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                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |A|Hash Function|                  Iterations                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Salt Length  |                     Salt                      /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                     Next Hashed Ownername                     /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                         Type Bit Maps                         /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.1.1  The Authoritative Only Flag Field

   The Authoritative Only Flag field indicates whether the Type Bit Maps
   include delegation point NS record types.

   If the flag is set to 1, the NS RR type bit for a delegation point
   ownername SHOULD be clear when the NSEC3 RR is generated.  The NS RR
   type bit MUST be ignored during processing of the NSEC3 RR.  The NS
   RR type bit has no meaning in this context (it is not authoritative),
   hence the NSEC3 does not contest the existence of a NS RRset for this
   ownername.  When a delegation is not secured, there exist no DS RR
   type nor any other authoritative types for this delegation, hence the
   unsecured delegation has no NSEC3 record associated.  Please see the
   Special Consideration section for implications for unsigned
   delegations.

   If the flag is set to 0, the NS RR type bit for a delegation point
   ownername MUST be set if the NSEC3 covers a delegation, even though
   the NS RR itself is not authoritative.  This implies that all
   delegations, signed or unsigned, have an NSEC3 record associated.
   This behaviour is identical to NSEC behaviour.

2.1.2  The Hash Function Field

   The Hash Function field identifies the cryptographic hash function
   used to construct the hash-value.

   This document defines Value 1 for SHA-1 and Value 127 for
   experimental.  All other values are reserved.

   On reception, a resolver MUST discard an NSEC3 RR with an unknown
   hash function value.






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2.1.3  The Iterations Field

   The Iterations field defines the number of times the hash has been
   iterated.  More iterations results in greater resiliency of the hash
   value against dictionary attacks, but at a higher cost for both the
   server and resolver.

2.1.4  The Salt Length Field

   The salt length field defines the length of the salt in octets.

2.1.5  The Salt Field

   The Salt field is not present when the Salt Length Field has a value
   of 0.

   The Salt field is prepended to the original ownername before hashing
   in order to defend against precalculated dictionary attacks.

   The salt is also prepended during iterations of the hash function.

   Note that although it is theoretically possible to cover the entire
   possible ownername space with different salt values, it is
   computationally infeasible to do so, and so there MUST be at least
   one salt which is the same for all NSEC3 records.  This means that no
   matter what name is asked for in a query, it is guaranteed to be
   possible to find a covering NSEC3 record.  Note that this does not
   preclude the use of two different salts at the same time - indeed
   this may well occur naturally, due to rolling the salt value
   periodically.

   The salt value SHOULD be changed from time to time - this is to
   prevent the use of a precomputed dictionary to reduce the cost of
   enumeration.

2.1.6  The Next Hashed Ownername Field

   The Next Hashed Ownername field contains the hash of the ownername of
   the next RR in the canonical ordering of the hashed ownernames of the
   zone.  The value of the Next Hashed Ownername Field in the last NSEC3
   record in the zone is the same as the ownername of the first NSEC3 RR
   in the zone in canonical order.

   Hashed ownernames of RRsets not authoritative for the given zone
   (such as glue records) MUST NOT be listed in the Next Hashed
   Ownername unless at least one authoritative RRset exists at the same
   ownername.




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   Note that the Next Hashed Ownername field is not encoded, unlike the
   NSEC3 RR's ownername.  It is the unmodified binary hash value.

2.1.7  The list of Type Bit Map(s) Field

   The Type Bit Maps field identifies the RRset types which exist at the
   NSEC3 RR's ownername.

   The Type bits for the NSEC3 RR and RRSIG RR MUST be set during
   generation, and MUST be ignored during processing.

   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 NSEC3 RR RDATA in increasing numerical
   order.

   "|" 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 NSEC3
   RR's ownername.  If a bit is set to 0, it indicates that no RRset of
   that type is present for the NSEC3 RR's ownername.

   The RR type 2 (NS) is authoritative at the apex of a zone and is not
   authoritative at delegation points.  If the Authoritative Only Flag
   is set to 1, the delegation point NS RR type MUST NOT be included in
   the type bit maps.  If the Authoritative Only Flag is set to 0, the
   NS RR type at a delegation point MUST be included in the type bit
   maps.

   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
   [RFC2929] (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



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   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
   NSEC3 RR's actual ownername.  Trailing zero octets not specified MUST
   be interpreted as zero octets.

2.2  The NSEC3 RR Presentation Format

   The presentation format of the RDATA portion is as follows:

   The Authoritative Only Field is represented as an unsigned decimal
   integer.  The value are either 0 or 1.

   The Hash field is presented as the name of the hash or as an unsigned
   decimal integer.  The value has a maximum of 127.

   The Iterations field is presented as an unsigned decimal integer.

   The Salt Length field is not presented.

   The Salt field is represented as a sequence of case-insensitive
   hexadecimal digits.  Whitespace is not allowed within the sequence.
   The Salt Field is represented as 00 when the Salt Length field has
   value 0.

   The Next Hashed Ownername field is represented as a sequence of case-
   insensitive base32 digits.  Whitespace is allowed within the
   sequence.

   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 [RFC3597] (section 5) MUST be
   used.

3.  Creating Additional NSEC3 RRs for Empty Non Terminals

   In order to prove the non-existence of a record that might be covered
   by a wildcard, it is necessary to prove the existence of its closest
   encloser.  A closest encloser might be an Empty Non Terminal.

   Additional NSEC3 RRs are synthesized which cover every existing
   intermediate label level.  Additional NSEC3 RRs are identical in
   format to NSEC3 RRs that cover existing RRs in the zone.  The
   difference is that the type-bit-maps only indicate the existence of



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   an NSEC3 RR type and an RRSIG RR type.

4.  Calculation of the Hash

   Define H(x) to be the hash of x using the hash function selected by
   the NSEC3 record and || to indicate concatenation.  Then define:

   IH(salt,x,0)=H(x || salt)

   IH(salt,x,k)=H(IH(salt,x,k-1) || salt) if k > 0

   Then the calculated hash of an ownername is
   IH(salt,ownername,iterations-1), where the ownername is the canonical
   form.

   The canonical form of the ownername is the wire format of the
   ownername where:
   1.  The ownername is fully expanded (no DNS name compression) and
       fully qualified;
   2.  All uppercase US-ASCII letters are replaced by the corresponding
       lowercase US-ASCII letters;
   3.  If the ownername is a wildcard name, the ownername is in its
       original unexpanded form, including the "*" label (no wildcard
       substitution);

5.  Including NSEC3 RRs in a Zone

   Each owner name in the zone which has authoritative data or a secured
   delegation point NS RRset MUST have an NSEC3 resource record.

   An unsecured delegation point NS RRset MAY have an NSEC3 resource
   record.  This is different from NSEC records where an unsecured
   delegation point NS RRset MUST have an NSEC record.

   The TTL value for any NSEC3 RR SHOULD be the same as the minimum TTL
   value field in the zone SOA RR.

   The type bitmap of every NSEC3 resource record in a signed zone MUST
   indicate the presence of both the NSEC3 RR type itself and its
   corresponding RRSIG RR type.

   The bitmap for the NSEC3 RR at a delegation point requires special
   attention.  Bits corresponding to the delegation NS RRset and any
   RRsets for which the parent zone has authoritative data MUST be set;
   bits corresponding to any non-NS RRset for which the parent is not
   authoritative MUST be clear.

   The following steps describe the proper construction of NSEC3



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   records.
   1.  For each unique original owner name in the zone, add an NSEC3
       RRset.  This includes NSEC3 RRsets for unsigned delegation point
       NS RRsets, unless the policy is to have Authoritative Only NSEC3
       RRsets.  The ownername of the NSEC3 RR is the hashed equivalent
       of the original owner name, prepended to the zone name.
   2.  For each RRset at the original owner, set the corresponding bit
       in the type bit map.
   3.  If the difference in number of labels between the apex and the
       original ownername is greater then 1, additional NSEC3s need to
       be added for every empty non-terminal between the apex and the
       original ownername.
   4.  Sort the set of NSEC3 RRs.
   5.  In each NSEC3 RR, insert the Next Hashed Ownername.  The Next
       Hashed Ownername of the last NSEC3 in the zone contains the value
       of the hashed ownername of the first NSEC3 in the zone.
   6.  If the policy is to have authoritative only, set the
       Authoritative Only bit in those NSEC3 RRs that cover unsecured
       delegation points.

6.  Special Considerations

   The following paragraphs clarify specific behaviour explain special
   considerations for implementations.

6.1  Delegation Points

   This proposal introduces the Authoritative Only Flag which indicates
   whether non authoritative delegation point NS records are included in
   the type bit Maps.  As discussed in paragraph 2.1.1, a flag value of
   0 indicates that the interpretation of the type bit maps is identical
   to NSEC records.

   The following subsections describe behaviour when the flag value is
   1.

6.1.1  Unsigned Delegations

   Delegation point NS records are not authoritative.  They are
   authoritative in the delegated zone.  No other data exists at the
   ownername of an unsigned delegation point.

   Since no authoritative data exist at this ownername, it is excluded
   from the NSEC3 chain.  This is an optimization, since it relieves the
   zone of including an NSEC3 record and its associated signature for
   this name.

   An NSEC3 that denies existence of ownernames between X and X' with



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   the Authoritative Only Flag set to 1 can not be used to prove the
   presence or the absence of delegation point NS records for unsigned
   delegations in the interval (X, X').  The Authoritative Only Flag
   effectively states No Contest on the presence of delegation point NS
   resource records.

   Since proof is absent, there exists a new attack vector.  Unsigned
   delegation point NS records can be deleted during a man in the middle
   attack, effectively denying existence of the delegation.  This is a
   form of Denial of Service, where the victim has no information it is
   under attack, since all signatures are valid and the fabricated
   response form is a known type of response.

   The only possible mitigation is to either not use this method, hence
   proving existence or absence of unsigned delegations, or to sign all
   delegations, regardless of whether the delegated zone is signed or
   not.

   A second attack vector exists in that an adversary is able to
   successfully fabricate an (unsigned) response claiming a nonexistent
   delegation exists.

   The only possible mitigation is to mandate the signing of all
   delegations.

6.2  Proving Nonexistence

   If a wildcard resource record appears in a zone, its asterisk label
   is treated as a literal symbol and is treated in the same way as any
   other ownername for purposes of generating NSEC3 RRs.  RFC 4035
   [RFC4035] describes the impact of wildcards on authenticated denial
   of existence.

   In order to prove there exist no RRs for a domain, as well as no
   source of synthesis, an RR must be shown for the closest encloser,
   and non-existence must be shown for all closer labels and for the
   wildcard at the closest encloser.

   This can be done as follows.  If the QNAME in the query is
   omega.alfa.beta.example, and the closest encloser is beta.example
   (the nearest ancestor to omega.alfa.beta.example), then the server
   should return an NSEC3 that demonstrates the nonexistence of
   alfa.beta.example, an NSEC3 that demonstrates the nonexistence of
   *.beta.example, and an NSEC3 that demonstrates the existence of
   beta.example.  This takes between one and three NSEC3 records, since
   a single record can, by chance, prove more than one of these facts.

   When a verifier checks this response, then the existence of



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   beta.example together with the non-existence of alfa.beta.example
   proves that the closest encloser is indeed beta.example.  The non-
   existence of *.beta.example shows that there is no wildcard at the
   closest encloser, and so no source of synthesis for
   omega.alfa.beta.example.  These two facts are sufficient to satisfy
   the resolver that the QNAME cannot be resolved.

   In practice, since the NSEC3 owner and next names are hashed, if the
   server responds with an NSEC3 for beta.example, the resolver will
   have to try successively longer names, starting with example, moving
   to beta.example, alfa.beta.example, and so on, until one of them
   hashes to a value that matches the interval (but not the ownername
   nor next owner name) of one of the returned NSEC3s (this name will be
   alfa.beta.example).  Once it has done this, it knows the closest
   encloser (i.e. beta.example), and can then easily check the other two
   required proofs.

   Note that it is not possible for one of the shorter names tried by
   the resolver to be denied by one of the returned NSEC3s, since, by
   definition, all these names exist and so cannot appear within the
   range covered by an NSEC3.  Note, however, that the first name that
   the resolver tries MUST be the apex of the zone, since names above
   the apex could be denied by one of the returned NSEC3s.

6.3  Salting

   Augmenting original ownernames with salt before hashing increases the
   cost of a dictionary of pre-generated hash-values.  For every bit of
   salt, the cost of the dictionary doubles.  The NSEC3 RR can use a
   maximum of 2040 bits of salt, multiplying the cost by 2^2040.

   There MUST be a complete set of NSEC3s for the zone using the same
   salt value.  The salt value for each NSEC3 RR MUST be equal for a
   single version of the zone.

   The salt SHOULD be changed every time the zone is resigned to prevent
   precomputation using a single salt.

6.4  Hash Collision

   Hash collisions occur when different messages have the same hash
   value.  The expected number of domain names needed to give a 1 in 2
   chance of a single collision is about 2^(n/2) for a hash of length n
   bits (i.e. 2^80 for SHA-1).  Though this probability is extremely
   low, the following paragraphs deal with avoiding collisions and
   assessing possible damage in the event of an attack using hash
   collisions.




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6.4.1  Avoiding Hash Collisions during generation

   During generation of NSEC3 RRs, hash values are supposedly unique.
   In the (academic) case of a collision occurring, an alternative salt
   SHOULD be chosen and all hash values SHOULD be regenerated.

   If hash values are not regenerated on collision, the NSEC3 RR MUST
   list all authoritative RR types that exist for both owners, to avoid
   a replay attack, spoofing an existing type as non-existent.

6.4.2  Second Preimage Requirement Analysis

   A cryptographic hash function has a second-preimage resistance
   property.  The second-preimage resistance property means that it is
   computationally infeasible to find another message with the same hash
   value as a given message, i.e. given preimage X, to find a second
   preimage X' <> X such that hash(X) = hash(X').  The work factor for
   finding a second preimage is of the order of 2^160 for SHA-1.  To
   mount an attack using an existing NSEC3 RR, an adversary needs to
   find a second preimage.

   Assuming an adversary is capable of mounting such an extreme attack,
   the actual damage is that a response message can be generated which
   claims that a certain QNAME (i.e. the second pre-image) does exist,
   while in reality QNAME does not exist (a false positive), which will
   either cause a security aware resolver to re-query for the non-
   existent name, or to fail the initial query.  Note that the adversary
   can't mount this attack on an existing name but only on a name that
   the adversary can't choose and does not yet exist.

6.4.3  Possible Hash Value Truncation Method

   The previous sections outlined the low probability and low impact of
   a second-preimage attack.  When impact and probability are low, while
   space in a DNS message is costly, truncation is tempting.  Truncation
   might be considered to allow for shorter ownernames and rdata for
   hashed labels.  In general, if a cryptographic hash is truncated to n
   bits, then the expected number of domains required to give a 1 in 2
   probability of a single collision is approximately 2^(n/2) and the
   work factor to produce a second preimage resistance is 2^n.

   An extreme hash value truncation would be truncating to the shortest
   possible unique label value.  Considering that hash values are
   presented in base32, which represents 5 bits per label character,
   truncation must be done on a 5 bit boundary.  This would be unwise,
   since the work factor to produce collisions would then approximate
   the size of the zone.




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   Though the mentioned truncation can be maximized to a certain
   extreme, the probability of collision increases exponentially for
   every truncated bit.  Given the low impact of hash value collisions
   and limited space in DNS messages, the balance between truncation
   profit and collision damage may be determined by local policy.  Of
   course, the size of the corresponding RRSIG RR is not reduced, so
   truncation is of limited benefit.

   Truncation could be signalled simply by reducing the length of the
   first label in the ownername.  Note that there would have to be a
   corresponding reduction in the length of the Next Hashed Ownername
   field.

7.  Performance Considerations

   Iterated hashes will obviously impose a performance penalty on both
   authoritative servers and resolvers.  Therefore, the number of
   iterations should be carefully chosen.  In particular it should be
   noted that a high value for iterations gives an attacker a very good
   denial of service attack, since the attacker need not bother to
   verify the results of their queries, and hence has no performance
   penalty of his own.

   On the other hand, nameservers with low query rates and limited
   bandwidth are already subject to a bandwidth based denial of service
   attack, since responses are typically an order of magnitude larger
   than queries, and hence these servers may choose a high value of
   iterations in order to increase the difficulty of offline attempts to
   enumerate their namespace without significantly increasing their
   vulnerability to denial of service attacks.

8.  IANA Considerations

   IANA has to create a new registry for NSEC3 Hash Functions.  The
   range for this registry is 0-127.  Value 0 is the identity function.
   Value 1 is SHA-1.  Values 2-126 are Reserved For Future Use. Value
   127 is marked as Experimental.

9.  Security Considerations

   The NSEC3 records are still susceptible to dictionary attacks (i.e.
   the attacker retrieves all the NSEC3 records, then calculates the
   hashes of all likely domain names, comparing against the hashes found
   in the NSEC3 records, and thus enumerating the zone).  These are
   substantially more expensive than traversing the original NSEC
   records would have been, and in any case, such an attack could also
   be used directly against the name server itself by performing queries
   for all likely names, though this would obviously be more detectable.



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   The expense of this off-line attack can be chosen by setting the
   number of iterations in the NSEC3 RR.

   High-value domains are also susceptible to a precalculated dictionary
   attack - that is, a list of hashes for all likely names is computed
   once, then NSEC3 is scanned periodically and compared against the
   precomputed hashes.  This attack is prevented by changing the salt on
   a regular basis.

   Walking the NSEC3 RRs will reveal the total number of records in the
   zone, and also what types they are.  This could be mitigated by
   adding dummy entries, but certainly an upper limit can always be
   found.

   Hash collisions may occur.  If they do, it will be impossible to
   prove the non-existence of the colliding domain - however, this is
   fantastically unlikely, and, in any case, DNSSEC already relies on
   SHA-1 to not collide.

10.  References

10.1  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, March 1998.

   [RFC2929]  Eastlake, D., Brunner-Williams, E., and B. Manning,
              "Domain Name System (DNS) IANA Considerations", BCP 42,
              RFC 2929, September 2000.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.



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   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

10.2  Informative References

   [I-D.ietf-dnsext-trustupdate-threshold]
              Ihren, J., "An In-Band Rollover Mechanism and an Out-Of-
              Band Priming Method for DNSSEC  Trust Anchors.",
              draft-ietf-dnsext-trustupdate-threshold-00 (work in
              progress), October 2004.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2418]  Bradner, S., "IETF Working Group Guidelines and
              Procedures", BCP 25, RFC 2418, September 1998.


Authors' Addresses

   Ben Laurie
   Nominet
   17 Perryn Road
   London  W3 7LR
   England

   Phone: +44 (20) 8735 0686
   Email: ben@algroup.co.uk


   Geoffrey Sisson
   Nominet










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   Roy Arends
   Telematica Instituut
   Brouwerijstraat 1
   7523 XC  Enschede
   The Netherlands

   Phone: +31 (53) 485 0485
   Email: roy.arends@telin.nl

Appendix A.  Example Zone

   This is a zone showing its NSEC3 records.  They can also be used as
   test vectors for the hash algorithm.


   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600 )
                  3600 RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
                  3600 NS     ns1.example.
                  3600 NS     ns2.example.
                  3600 RRSIG  NS 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
                              m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
                              1SH5r/wfjuCg+g== )
                  3600 MX     1 xx.example.
                  3600 RRSIG  MX 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              L/ZDLMSZJKITmSxmM9Kni37/wKQsdSg6FT0l
                              NMm14jy2Stp91Pwp1HQ1hAMkGWAqCMEKPMtU
                              S/o/g5C8VM6ftQ== )
                  3600 DNSKEY 257 3 5 (
                              AQOnsGyJvywVjYmiLbh0EwIRuWYcDiB/8blX
                              cpkoxtpe19Oicv6Zko+8brVsTMeMOpcUeGB1
                              zsYKWJ7BvR2894hX
                              ) ; Key ID = 21960
                  3600 DNSKEY 256 3 5 (
                              AQO0gEmbZUL6xbD/xQczHbnwYnf+jQjwz/sU
                              5k44rHTt0Ty+3aOdYoome9TjGMhwkkGby1TL
                              ExXT48OGGdbfIme5



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                              ) ; Key ID = 62699
                  3600 RRSIG  DNSKEY 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              e6EB+K21HbyZzoLUeRDb6+g0+n8XASYe6h+Z
                              xtnB31sQXZgq8MBHeNFDQW9eZw2hjT9zMClx
                              mTkunTYzqWJrmQ== )
                  3600 RRSIG  DNSKEY 5 1 3600 20050712112304 (
                              20050612112304 21960 example.
                              SnWLiNWLbOuiKU/F/wVMokvcg6JVzGpQ2VUk
                              ZbKjB9ON0t3cdc+FZbOCMnEHRJiwgqlnncik
                              3w7ZY2UWyYIvpw== )
   5pe7ctl7pfs2cilroy5dcofx4rcnlypd.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              7nomf47k3vlidh4vxahhpp47l3tgv7a2
                              NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              PTWYq4WZmmtgh9UQif342HWf9DD9RuuM4ii5
                              Z1oZQgRi5zrsoKHAgl2YXprF2Rfk1TLgsiFQ
                              sb7KfbaUo/vzAg== )
   7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              dw4o7j64wnel3j4jh7fb3c5n7w3js2yb
                              MX NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              YTcqole3h8EOsTT3HKnwhR1QS8borR0XtZaA
                              ZrLsx6n0RDC1AAdZONYOvdqvcal9PmwtWjlo
                              MEFQmc/gEuxojA== )
   a.example.     3600 IN NS  ns1.a.example.
                  3600 IN NS  ns2.a.example.
                  3600 DS     58470 5 1 3079F1593EBAD6DC121E202A8B
                              766A6A4837206C )
                  3600 RRSIG  DS 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              QavhbsSmEvJLSUzGoTpsV3SKXCpaL1UO3Ehn
                              cB0ObBIlex/Zs9kJyG/9uW1cYYt/1wvgzmX2
                              0kx7rGKTc3RQDA== )
   ns1.a.example. 3600 IN A   192.0.2.5
   ns2.a.example. 3600 IN A   192.0.2.6
   ai.example.    3600 IN A   192.0.2.9
                  3600 RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              plY5M26ED3Owe3YX0pBIhgg44j89NxUaoBrU
                              6bLRr99HpKfFl1sIy18JiRS7evlxCETZgubq
                              ZXW5S+1VjMZYzQ== )
                  3600 HINFO  "KLH-10" "ITS"
                  3600 RRSIG  HINFO 5 2 3600 20050712112304 (



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                              20050612112304 62699 example.
                              AR0hG/Z/e+vlRhxRQSVIFORzrJTBpdNHhwUk
                              tiuqg+zGqKK84eIqtrqXelcE2szKnF3YPneg
                              VGNmbgPnqDVPiA== )
                  3600 AAAA   2001:db8:0:0:0:0:f00:baa9
                  3600 RRSIG  AAAA 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              PNF/t7+DeosEjhfuL0kmsNJvn16qhYyLI9FV
                              ypSCorFx/PKIlEL3syomkYM2zcXVSRwUXMns
                              l5/UqLCJJ9BDMg== )
   b.example.     3600 IN NS  ns1.b.example.
                  3600 IN NS  ns2.b.example.
   ns1.b.example. 3600 IN A   192.0.2.7
   ns2.b.example. 3600 IN A   192.0.2.8
   dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              gmnfcccja7wkax3iv26bs75myptje3qk
                              MX DNSKEY NS SOA NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              VqEbXiZLJVYmo25fmO3IuHkAX155y8NuA50D
                              C0NmJV/D4R3rLm6tsL6HB3a3f6IBw6kKEa2R
                              MOiKMSHozVebqw== )
   gmnfcccja7wkax3iv26bs75myptje3qk.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              jt4bbfokgbmr57qx4nqucvvn7fmo6ab6
                              DS NS NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              ZqkdmF6eICpHyn1Cj7Yvw+nLcbji46Qpe76/
                              ZetqdZV7K5sO3ol5dOc0dZyXDqsJp1is5StW
                              OwQBGbOegrW/Zw== )
   jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              kcll7fqfnisuhfekckeeqnmbbd4maanu
                              NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              FXyCVQUdFF1EW1NcgD2V724/It0rn3lr+30V
                              IyjmqwOMvQ4G599InTpiH46xhX3U/FmUzHOK
                              94Zbq3k8lgdpZA== )
   kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 NSEC3  1 1 1 (
                              deadbeaf
                              n42hbhnjj333xdxeybycax5ufvntux5d
                              MX NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              d0g8MTOvVwByOAIwvYV9JrTHwJof1VhnMKuA



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                              IBj6Xaeney86RBZYgg7Qyt9WnQSK3uCEeNpx
                              TOLtc5jPrkL4zQ== )
   n42hbhnjj333xdxeybycax5ufvntux5d.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              nimwfwcnbeoodmsc6npv3vuaagaevxxu
                              A NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              MZGzllh+YFqZbY8SkHxARhXFiMDPS0tvQYyy
                              91tj+lbl45L/BElD3xxB/LZMO8vQejYtMLHj
                              xFPFGRIW3wKnrA== )
   nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              vhgwr2qgykdkf4m6iv6vkagbxozphazr
                              HINFO A AAAA NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              c3zQdK68cYTHTjh1cD6pi0vblXwzyoU/m7Qx
                              z8kaPYikbJ9vgSl9YegjZukgQSwybHUC0SYG
                              jL33Wm1p07TBdw== )
   ns1.example.   3600 A      192.0.2.1
                  3600 RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              QLGkaqWXxRuE+MHKkMvVlswg65HcyjvD1fyb
                              BDZpcfiMHH9w4x1eRqRamtSDTcqLfUrcYkrr
                              nWWLepz1PjjShQ== )
   ns2.example.   3600 A      192.0.2.2
                  3600 RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              UoIZaC1O6XHRWGHBOl8XFQKPdYTkRCz6SYh3
                              P2mZ3xfY22fLBCBDrEnOc8pGDGijJaLl26Cz
                              AkeTJu3J3auUiA== )
   vhgwr2qgykdkf4m6iv6vkagbxozphazr.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              wbyijvpnyj33pcpi3i44ecnibnaj7eiw
                              HINFO A AAAA NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              leFhoF5FXZAiNOxK4OBOOA0WKdbaD5lLDT/W
                              kLoyWnQ6WGBwsUOdsEcVmqz+1n7q9bDf8G8M
                              5SNSHIyfpfsi6A== )
   *.w.example.   3600 MX     1 ai.example.
                  3600 RRSIG  MX 5 3 3600 20050712112304 (
                              20050612112304 62699 example.
                              sYNUPHn1/gJ87wTHNksGdRm3vfnSFa2BbofF
                              xGfJLF5A4deRu5f0hvxhAFDCcXfIASj7z0wQ
                              gQlgxEwhvQDEaQ== )
   x.w.example.   3600 MX     1 xx.example.



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                  3600 RRSIG  MX 5 3 3600 20050712112304 (
                              20050612112304 62699 example.
                              s1XQ/8SlViiEDik9edYs1Ooe3XiXo453Dg7w
                              lqQoewuDzmtd6RaLNu52W44zTM1EHJES8ujP
                              U9VazOa1KEIq1w== )
   x.y.w.example. 3600 MX     1 xx.example.
                  3600 RRSIG  MX 5 4 3600 20050712112304 (
                              20050612112304 62699 example.
                              aKVCGO/Fx9rm04UUsHRTTYaDA8o8dGfyq6t7
                              uqAcYxU9xiXP+xNtLHBv7er6Q6f2JbOs6SGF
                              9VrQvJjwbllAfA== )
   wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui
                              A NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              ledFAaDCqDxapQ1FvBAjjK2DP06iQj8AN6gN
                              ZycTeSmobKLTpzbgQp8uKYYe/DPHjXYmuEhd
                              oorBv4xkb0flXw== )
   xx.example.    3600 A      192.0.2.10
                  3600 RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              XSuMVjNxovbZUsnKU6oQDygaK+WB+O5HYQG9
                              tJgphHIX7TM4uZggfR3pNM+4jeC8nt2OxZZj
                              cxwCXWj82GVGdw== )
                  3600 HINFO  "KLH-10" "TOPS-20"
                  3600 RRSIG  HINFO 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              ghS2DimOqPSacG9j6KMgXSfTMSjLxvoxvx3q
                              OKzzPst4tEbAmocF2QX8IrSHr67m4ZLmd2Fk
                              KMf4DgNBDj+dIQ== )
                  3600 AAAA   2001:db8:0:0:0:0:f00:baaa
                  3600 RRSIG  AAAA 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              rto7afZkXYB17IfmQCT5QoEMMrlkeOoAGXzo
                              w8Wmcg86Fc+MQP0hyXFScI1gYNSgSSoDMXIy
                              rzKKwb8J04/ILw== )
   zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 NSEC3  0 1 1 (
                              deadbeaf
                              5pe7ctl7pfs2cilroy5dcofx4rcnlypd
                              MX NSEC3 RRSIG )
                  3600 RRSIG  NSEC3 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
                              7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
                              OcFlrPGPMm48/A== )




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Appendix B.  Example Responses

   The examples in this section show response messages using the signed
   zone example in Appendix A.

B.1  answer

   A successful query to an authoritative server.











































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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   x.w.example.        IN MX

   ;; Answer
   x.w.example.   3600 IN MX  1 xx.example.
   x.w.example.   3600 IN RRSIG  MX 5 3 3600 20050712112304 (
                              20050612112304 62699 example.
                              s1XQ/8SlViiEDik9edYs1Ooe3XiXo453Dg7w
                              lqQoewuDzmtd6RaLNu52W44zTM1EHJES8ujP
                              U9VazOa1KEIq1w== )

   ;; Authority
   example.       3600 IN NS  ns1.example.
   example.       3600 IN NS  ns2.example.
   example.       3600 IN RRSIG  NS 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
                              m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
                              1SH5r/wfjuCg+g== )

   ;; Additional
   xx.example.    3600 IN A   192.0.2.10
   xx.example.    3600 IN RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              XSuMVjNxovbZUsnKU6oQDygaK+WB+O5HYQG9
                              tJgphHIX7TM4uZggfR3pNM+4jeC8nt2OxZZj
                              cxwCXWj82GVGdw== )
   xx.example.    3600 IN AAAA   2001:db8::f00:baaa
   xx.example.    3600 IN RRSIG  AAAA 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              rto7afZkXYB17IfmQCT5QoEMMrlkeOoAGXzo
                              w8Wmcg86Fc+MQP0hyXFScI1gYNSgSSoDMXIy
                              rzKKwb8J04/ILw== )
   ns1.example.   3600 IN A   192.0.2.1
   ns1.example.   3600 IN RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              QLGkaqWXxRuE+MHKkMvVlswg65HcyjvD1fyb
                              BDZpcfiMHH9w4x1eRqRamtSDTcqLfUrcYkrr
                              nWWLepz1PjjShQ== )
   ns2.example.   3600 IN A   192.0.2.2
   ns2.example.   3600 IN RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              UoIZaC1O6XHRWGHBOl8XFQKPdYTkRCz6SYh3
                              P2mZ3xfY22fLBCBDrEnOc8pGDGijJaLl26Cz
                              AkeTJu3J3auUiA== )




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   The query returned an MX RRset for "x.w.example".  The corresponding
   RRSIG RR indicates that the MX RRset was signed by an "example"
   DNSKEY with algorithm 5 and key tag 62699.  The resolver needs the
   corresponding DNSKEY RR in order to authenticate this answer.  The
   discussion below describes how a resolver might obtain this DNSKEY
   RR.

   The RRSIG RR indicates the original TTL of the MX RRset was 3600,
   and, for the purpose of authentication, the current TTL is replaced
   by 3600.  The RRSIG RR's labels field value of 3 indicates that the
   answer was not the result of wildcard expansion.  The "x.w.example"
   MX RRset is placed in canonical form, and, assuming the current time
   falls between the signature inception and expiration dates, the
   signature is authenticated.

B.1.1  Authenticating the Example DNSKEY RRset

   This example shows the logical authentication process that starts
   from a configured root DNSKEY RRset (or DS RRset) and moves down the
   tree to authenticate the desired "example" DNSKEY RRset.  Note that
   the logical order is presented for clarity.  An implementation may
   choose to construct the authentication as referrals are received or
   to construct the authentication chain only after all RRsets have been
   obtained, or in any other combination it sees fit.  The example here
   demonstrates only the logical process and does not dictate any
   implementation rules.

   We assume the resolver starts with a configured DNSKEY RRset for the
   root zone (or a configured DS RRset for the root zone).  The resolver
   checks whether this configured DNSKEY RRset is present in the root
   DNSKEY RRset (or whether a DS RR in the DS RRset matches some DNSKEY
   RR in the root DNSKEY RRset), whether this DNSKEY RR has signed the
   root DNSKEY RRset, and whether the signature lifetime is valid.  If
   all these conditions are met, all keys in the DNSKEY RRset are
   considered authenticated.  The resolver then uses one (or more) of
   the root DNSKEY RRs to authenticate the "example" DS RRset.  Note
   that the resolver may have to query the root zone to obtain the root
   DNSKEY RRset or "example" DS RRset.

   Once the DS RRset has been authenticated using the root DNSKEY, the
   resolver checks the "example" DNSKEY RRset for some "example" DNSKEY
   RR that matches one of the authenticated "example" DS RRs.  If such a
   matching "example" DNSKEY is found, the resolver checks whether this
   DNSKEY RR has signed the "example" DNSKEY RRset and the signature
   lifetime is valid.  If these conditions are met, all keys in the
   "example" DNSKEY RRset are considered authenticated.

   Finally, the resolver checks that some DNSKEY RR in the "example"



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   DNSKEY RRset uses algorithm 5 and has a key tag of 62699.  This
   DNSKEY is used to authenticate the RRSIG included in the response.
   If multiple "example" DNSKEY RRs match this algorithm and key tag,
   then each DNSKEY RR is tried, and the answer is authenticated if any
   of the matching DNSKEY RRs validate the signature as described above.

B.2  Name Error

   An authoritative name error.  The NSEC3 RRs prove that the name does
   not exist and that no covering wildcard exists.









































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   ;; Header: QR AA DO RCODE=3
   ;;
   ;; Question
   a.c.x.w.example.         IN A

   ;; Answer
   ;; (empty)

   ;; Authority
   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600
                              )
   example.       3600 IN RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
   7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              dw4o7j64wnel3j4jh7fb3c5n7w3js2yb
                              MX NSEC3 RRSIG )
   7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              YTcqole3h8EOsTT3HKnwhR1QS8borR0XtZaA
                              ZrLsx6n0RDC1AAdZONYOvdqvcal9PmwtWjlo
                              MEFQmc/gEuxojA== )
   nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              vhgwr2qgykdkf4m6iv6vkagbxozphazr
                              HINFO A AAAA NSEC3 RRSIG )
   nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              c3zQdK68cYTHTjh1cD6pi0vblXwzyoU/m7Qx
                              z8kaPYikbJ9vgSl9YegjZukgQSwybHUC0SYG
                              jL33Wm1p07TBdw== )
   ;; Additional
   ;; (empty)

   The query returned two NSEC3 RRs that prove that the requested data
   does not exist and no wildcard applies.  The negative reply is
   authenticated by verifying both NSEC3 RRs.  The NSEC3 RRs are
   authenticated in a manner identical to that of the MX RRset discussed



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   above.  At least one of the owner names of the NSEC3 RRs will match
   the closest encloser.  At least one of the NSEC3 RRs prove that there
   exists no longer name.  At least one of the NSEC3 RRs prove that
   there exists no wildcard RRsets that should have been expanded.  The
   closest encloser can be found by hasing the apex ownername (The SOA
   RR's ownername, or the ownername of the DNSKEY RRset referred by an
   RRSIG RR), matching it to the ownername of one of the NSEC3 RRs, and
   if that fails, continue by adding labels.

   In the above example, the name 'x.w.example' hashes to
   '7nomf47k3vlidh4vxahhpp47l3tgv7a2'.  This indicates that this might
   be the closest encloser.  To prove that 'c.x.w.example' and
   '*.x.w.example' do not exists, these names are hashed to respectively
   'qsgoxsf2lanysajhtmaylde4tqwnqppl' and
   'cvljzyf6nsckjowghch4tt3nohocpdka'.  The two NSEC3 records prove that
   these hashed ownernames do not exists, since the names are within the
   given intervals.

B.3  No Data Error

   A "no data" response.  The NSEC3 RR proves that the name exists and
   that the requested RR type does not.





























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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   ns1.example.        IN MX

   ;; Answer
   ;; (empty)

   ;; Authority
   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600
                              )
   example.       3600 IN RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
   wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui
                              A NSEC3 RRSIG )
   wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              ledFAaDCqDxapQ1FvBAjjK2DP06iQj8AN6gN
                              ZycTeSmobKLTpzbgQp8uKYYe/DPHjXYmuEhd
                              oorBv4xkb0flXw== )
   ;; Additional
   ;; (empty)

   The query returned an NSEC3 RR that proves that the requested name
   exists ("ns1.example." hashes to "wbyijvpnyj33pcpi3i44ecnibnaj7eiw"),
   but the requested RR type does not exist (type MX is absent in the
   type code list of the NSEC RR).  The negative reply is authenticated
   by verifying the NSEC3 RR.  The NSEC3 RR is authenticated in a manner
   identical to that of the MX RRset discussed above.

B.3.1  No Data Error, Empty Non-Terminal

   A "no data" response because of an empty non-terminal.  The NSEC3 RR
   proves that the name exists and that the requested RR type does not.






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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   y.w.example.        IN A

   ;; Answer
   ;; (empty)

   ;; Authority
   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600
                              )
   example.       3600 IN RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
   jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              kcll7fqfnisuhfekckeeqnmbbd4maanu
                              NSEC3 RRSIG )
   jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              FXyCVQUdFF1EW1NcgD2V724/It0rn3lr+30V
                              IyjmqwOMvQ4G599InTpiH46xhX3U/FmUzHOK
                              94Zbq3k8lgdpZA== )

   The query returned an NSEC3 RR that proves that the requested name
   exists ("y.w.example." hashes to "jt4bbfokgbmr57qx4nqucvvn7fmo6ab6"),
   but the requested RR type does not exist (Type A is absent in the
   type-bit-maps of the NSEC3 RR).  The negative reply is authenticated
   by verifying the NSEC3 RR.  The NSEC3 RR is authenticated in a manner
   identical to that of the MX RRset discussed above.  Note that, unlike
   generic empty non terminal proof using NSECs, this is identical to
   proving a No Data Error.  This example is solely mentioned to be
   complete.

B.4  Referral to Signed Zone

   Referral to a signed zone.  The DS RR contains the data which the
   resolver will need to validate the corresponding DNSKEY RR in the
   child zone's apex.




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   ;; Header: QR DO RCODE=0
   ;;

   ;; Question
   mc.a.example.       IN MX

   ;; Answer
   ;; (empty)

   ;; Authority
   a.example.     3600 IN NS  ns1.a.example.
   a.example.     3600 IN NS  ns2.a.example.
   a.example.     3600 IN DS  58470 5 1 (
                              3079F1593EBAD6DC121E202A8B766A6A4837
                              206C )
   a.example.     3600 IN RRSIG  DS 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              QavhbsSmEvJLSUzGoTpsV3SKXCpaL1UO3Ehn
                              cB0ObBIlex/Zs9kJyG/9uW1cYYt/1wvgzmX2
                              0kx7rGKTc3RQDA== )

   ;; Additional
   ns1.a.example. 3600 IN A   192.0.2.5
   ns2.a.example. 3600 IN A   192.0.2.6

   The query returned a referral to the signed "a.example." zone.  The
   DS RR is authenticated in a manner identical to that of the MX RRset
   discussed above.  This DS RR is used to authenticate the "a.example"
   DNSKEY RRset.

   Once the "a.example" DS RRset has been authenticated using the
   "example" DNSKEY, the resolver checks the "a.example" DNSKEY RRset
   for some "a.example" DNSKEY RR that matches the DS RR.  If such a
   matching "a.example" DNSKEY is found, the resolver checks whether
   this DNSKEY RR has signed the "a.example" DNSKEY RRset and whether
   the signature lifetime is valid.  If all these conditions are met,
   all keys in the "a.example" DNSKEY RRset are considered
   authenticated.

B.5  Referral to Unsigned Zone using Opt-In

   Referral to an unsigned zone using Opt-In.  The NSEC3 RR proves that
   nothing for this delegation was signed in the parent zone.  There is
   no proof that the delegation exists







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   ;; Header: QR DO RCODE=0
   ;;
   ;; Question
   mc.b.example.       IN MX

   ;; Answer
   ;; (empty)

   ;; Authority
   b.example.     3600 IN NS  ns1.b.example.
   b.example.     3600 IN NS  ns2.b.example.
   kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 IN NSEC3  1 1 1 (
                              deadbeaf
                              n42hbhnjj333xdxeybycax5ufvntux5d
                              MX NSEC3 RRSIG )
   kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              d0g8MTOvVwByOAIwvYV9JrTHwJof1VhnMKuA
                              IBj6Xaeney86RBZYgg7Qyt9WnQSK3uCEeNpx
                              TOLtc5jPrkL4zQ== )

   ;; Additional
   ns1.b.example. 3600 IN A   192.0.2.7
   ns2.b.example. 3600 IN A   192.0.2.8

   The query returned a referral to the unsigned "b.example." zone.  The
   NSEC3 proves that no authentication leads from "example" to
   "b.example", since the hash of "b.example"
   ("ldjpfcucebeks5azmzpty4qlel4cftzo") is within the NSEC3 interval and
   the NSEC3 opt-in bit is set.  The NSEC3 RR is authenticated in a
   manner identical to that of the MX RRset discussed above.

B.6  Wildcard Expansion

   A successful query that was answered via wildcard expansion.  The
   label count in the answer's RRSIG RR indicates that a wildcard RRset
   was expanded to produce this response, and the NSEC3 RR proves that
   no closer match exists in the zone.












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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   a.z.w.example.      IN MX

   ;; Answer
   a.z.w.example. 3600 IN MX  1 ai.example.
   a.z.w.example. 3600 IN RRSIG  MX 5 3 3600 20050712112304 (
                              20050612112304 62699 example.
                              sYNUPHn1/gJ87wTHNksGdRm3vfnSFa2BbofF
                              xGfJLF5A4deRu5f0hvxhAFDCcXfIASj7z0wQ
                              gQlgxEwhvQDEaQ== )
   ;; Authority
   example.       3600 NS     ns1.example.
   example.       3600 NS     ns2.example.
   example.       3600 IN RRSIG  NS 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
                              m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
                              1SH5r/wfjuCg+g== )
   zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              5pe7ctl7pfs2cilroy5dcofx4rcnlypd
                              MX NSEC3 RRSIG )
   zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
                              7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
                              OcFlrPGPMm48/A== )
   ;; Additional
   ai.example.    3600 IN A   192.0.2.9
   ai.example.    3600 IN RRSIG  A 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              plY5M26ED3Owe3YX0pBIhgg44j89NxUaoBrU
                              6bLRr99HpKfFl1sIy18JiRS7evlxCETZgubq
                              ZXW5S+1VjMZYzQ== )
   ai.example.    3600 AAAA   2001:db8::f00:baa9
   ai.example.    3600 IN RRSIG  AAAA 5 2 3600 20050712112304 (
                              20050612112304 62699 example.
                              PNF/t7+DeosEjhfuL0kmsNJvn16qhYyLI9FV
                              ypSCorFx/PKIlEL3syomkYM2zcXVSRwUXMns
                              l5/UqLCJJ9BDMg== )

   The query returned an answer that was produced as a result of
   wildcard expansion.  The answer section contains a wildcard RRset
   expanded as it would be in a traditional DNS response, and the
   corresponding RRSIG indicates that the expanded wildcard MX RRset was



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   signed by an "example" DNSKEY with algorithm 5 and key tag 62699.
   The RRSIG indicates that the original TTL of the MX RRset was 3600,
   and, for the purpose of authentication, the current TTL is replaced
   by 3600.  The RRSIG labels field value of 2 indicates that the answer
   is the result of wildcard expansion, as the "a.z.w.example" name
   contains 4 labels.  The name "a.z.w.example" is replaced by
   "*.w.example", the MX RRset is placed in canonical form, and,
   assuming that the current time falls between the signature inception
   and expiration dates, the signature is authenticated.

   The NSEC3 proves that no closer match (exact or closer wildcard)
   could have been used to answer this query, and the NSEC3 RR must also
   be authenticated before the answer is considered valid.

B.7  Wildcard No Data Error

   A "no data" response for a name covered by a wildcard.  The NSEC3 RRs
   prove that the matching wildcard name does not have any RRs of the
   requested type and that no closer match exists in the zone.
































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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   a.z.w.example.      IN AAAA

   ;; Answer
   ;; (empty)

   ;; Authority
   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600
                              )
   example.       3600 IN RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
   zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              5pe7ctl7pfs2cilroy5dcofx4rcnlypd
                              MX NSEC3 RRSIG )
   zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
                              7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
                              OcFlrPGPMm48/A== )
   ;; Additional
   ;; (empty)

   The query returned NSEC3 RRs that prove that the requested data does
   not exist and no wildcard applies.  The negative reply is
   authenticated by verifying both NSEC3 RRs.

B.8  DS Child Zone No Data Error

   A "no data" response for a QTYPE=DS query that was mistakenly sent to
   a name server for the child zone.









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   ;; Header: QR AA DO RCODE=0
   ;;
   ;; Question
   example.            IN DS

   ;; Answer
   ;; (empty)

   ;; Authority
   example.       3600 IN SOA ns1.example. bugs.x.w.example. (
                              1
                              3600
                              300
                              3600000
                              3600
                              )
   example.       3600 IN RRSIG  SOA 5 1 3600 20050712112304 (
                              20050612112304 62699 example.
                              RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
                              mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
                              qYIt90txzE/4+g== )
   dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 IN NSEC3  0 1 1 (
                              deadbeaf
                              gmnfcccja7wkax3iv26bs75myptje3qk
                              MX DNSKEY NS SOA NSEC3 RRSIG )
   dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 IN RRSIG  NSEC3 (
                              5 2 3600 20050712112304
                              20050612112304 62699 example.
                              VqEbXiZLJVYmo25fmO3IuHkAX155y8NuA50D
                              C0NmJV/D4R3rLm6tsL6HB3a3f6IBw6kKEa2R
                              MOiKMSHozVebqw== )

   ;; Additional
   ;; (empty)

   The query returned NSEC RRs that shows the requested was answered by
   a child server ("example" server).  The NSEC RR indicates the
   presence of an SOA RR, showing that the answer is from the child .
   Queries for the "example" DS RRset should be sent to the parent
   servers ("root" servers).











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Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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Laurie, et al.          Expires December 3, 2005               [Page 37]


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