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Versions: 00 01 02 03 04 05 06 RFC 7129

Network Working Group                                          R. Gieben
Internet-Draft                                                 SIDN Labs
Intended status: Informational                                W. Mekking
Expires: February 2, 2013                                     NLnet Labs
                                                                Aug 2012

              Authenticated Denial of Existence in the DNS


   Authenticated denial of existence allows a resolver to validate that
   a certain domain name does not exist.  It is also used to signal that
   a domain name exists, but does not have the specific RR type you were
   asking for.  This document attempts to answer two simple questions.

   When returning a negative DNSSEC response, a name server sometimes
   includes up to two NSEC records.  With NSEC3 the maximum amount is

   o  Why do you need up to two NSEC records?

   o  And why does NSEC3 sometimes require an extra record?

   The answer to the questions hinges on the concept of wildcards and
   the "closest encloser".  With NSEC, the name that is the "closest
   encloser" is implicitly given in the record that also denies the
   existence of the domain name.  With NSEC3, due to its hashing, this
   information has to be given explicitly to a resolver.  It needs one
   record to tell the resolver the closest encloser and then another to
   deny the existence of the domain name.  Both NSEC and NSEC3 may need
   yet another record to deny or assert a wildcard presence.  This
   results in a maximum of two NSEC and three NSEC3 records,

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on February 2, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3

   2.  Denial of Existence  . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  NXDOMAIN . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  NODATA . . . . . . . . . . . . . . . . . . . . . . . . . .  5

   3.  Secure Denial of Existence . . . . . . . . . . . . . . . . . .  5
     3.1.  NXT  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  NODATA Responses . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  NO, NSEC2 and DNSNR  . . . . . . . . . . . . . . . . . . . 10
     3.5.  NSEC3  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Slaving an NSEC3 Zone  . . . . . . . . . . . . . . . . . . 12
     3.7.  Wildcards in the DNS . . . . . . . . . . . . . . . . . . . 12
     3.8.  Returning Three NSEC3s . . . . . . . . . . . . . . . . . . 15

   4.  List of Hashed Owner Names . . . . . . . . . . . . . . . . . . 19

   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19

   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19

   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19

   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   DNSSEC can be somewhat of a complicated matter, and there are certain
   areas of the specification that are more difficult to comprehend than
   others.  One such area is "authenticated denial of existence".

   Authenticated denial of existence allows a DNSSEC enabled resolver to
   validate that a certain domain name does not exist.  It is also used
   to signal that a domain name exists, but does not have the specific
   RR type you were asking for.

   The first is referred to as an NXDOMAIN [RFC2308] (non-existent
   domain) and the latter a NODATA [RFC2308] response.

   In this document we will explain how authenticated denial of
   existence works.  We begin by explaining the current technique in the
   DNS and work our way up to DNSSEC.  We explain the first steps taken
   in DNSSEC and describe how NXT, NSEC and NSEC3 work.  NO, NSEC2 and
   DNSNR also briefly make their appearance, as they have paved the way
   for NSEC3.

   To complete the picture we also need to explain DNS wildcards as it
   complicates matters.

   Note: In this document domain names in zone file examples will have a
   trailing dot, in the running text they will not.  This text is
   written for people who have a fair understanding of DNSSEC.  This
   document does not explain NSEC3 opt-out and secure delegations.

   The following RFCs are not required reading, but they might help in
   understanding the problem space.

   o  RFC 5155 [RFC5155] - Hashed Authenticated Denial of Existence;

   o  RFC 4592 [RFC4592] - The Role of Wildcards in the DNS.

   And these provide some general DNSSEC information.

   o  RFC 4033, RFC 4034, RFC 4035 [RFC4033], [RFC4034], [RFC4035] -
      DNSSEC Spec;

   o  RFC 4956 [RFC4956] - DNS Security (DNSSEC) Opt-In.  This RFC has
      the status experimental, but is a good read.

   And these three drafts give some background information on the NSEC3

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   o  The NO RR [I-D.ietf-dnsext-not-existing-rr];

   o  The NSEC2 RR [I-D.laurie-dnsext-nsec2v2];

   o  The DNSNR RR [I-D.arends-dnsnr].

2.  Denial of Existence

   We start with the basics and take a look at NXDOMAIN handling in the
   DNS.  To make it more visible we are going to use a small DNS zone,
   with 3 names ("example.org", "a.example.org" and "d.example.org") and
   3 types (SOA, A and TXT).  For brevity the class is not shown
   (defaults to IN), the NS records are left out and the SOA and RRSIG
   records are shortened.  Resulting in the following unsigned zone

   example.org.        SOA ( ... )
   a.example.org.      A
                       TXT "a record"
   d.example.org.      A
                       TXT "d record"


   If a resolver asks for the TXT type belonging to "a.example.org" to
   the name server serving this zone, it sends the following question:
   "a.example.org TXT"

   The name server looks in its zone data and generates an answer.  In
   this case a positive one: "Yes it exists and this is the data",
   resulting in this reply:

   ;; status: NOERROR, id: 28203

   a.example.org.      TXT "a record"

   example.org.        NS a.example.org.

   The "status: NOERROR" signals that everything is OK, "id" is an
   integer used to match questions and answers.  In the ANSWER section
   we find our answer.  The AUTHORITY section holds information of the
   name servers that have information concerning the "example.org"

   If a resolver now asks for "b.example.org TXT" it gets an answer that
   this name does not exist:

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   ;; status: NXDOMAIN, id: 7042

   example.org.        SOA ( ... )

   In this case we do not get an ANSWER section and the status is set to
   NXDOMAIN.  From this the resolver concludes "b.example.org" does not

2.2.  NODATA

   It is important to realize, that NXDOMAIN is not the only type of
   does-not-exist.  A name may exist, but the type you are asking for
   may not.  This occurrence of non-existence is called a NODATA
   [RFC2308] response.  Let us ask our name server for "a.example.org
   AAAA", and look at the answer:

   ;; status: NOERROR, id: 7944

   example.org.        SOA ( ... )

   The status is NOERROR meaning that the "a.example.org" name exists.
   But the reply does not contain an ANSWER section.  Instead it has an
   AUTHORITY section which holds the SOA record of "example.org".  The
   resolver has to put these pieces of information together and conclude
   that "a.example.org" exists, but it does not have an "AAAA" record.

3.  Secure Denial of Existence

   The above has to be translated to the security aware world of DNSSEC.
   But there are a few requirements DNSSEC brings to the table:

   1.  There is no online signing defined in DNSSEC.  Although a name
       server is free to compute the answer and signature(s) on-the-fly,
       the protocol is written with a "first sign", "then load" attitude
       in mind.  It is rather asymmetrical, but a lot of the design in
       DNSSEC stems from fact that you need to accommodate authenticated
       denial of existence.  If the DNS didn't have NXDOMAIN, DNSSEC
       would be a lot simpler, but a lot less useful!

   2.  The DNS packet header is not signed.  This means that a "status:
       NXDOMAIN" can not be trusted.  In fact the entire header may be
       forged, including the AD bit (AD stands for Authenticated Data,
       see RFC 3655 [RFC3655]), which may give some food for thought;

   3.  DNS wildcards complicate matters significantly.  More about this
       in later sections.

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   The first requirement implies that all denial of existence answers
   need to be pre-computed, but it is impossible to precompute (all
   conceivable) non-existence answers.  In the example above, you need a
   way to tell somebody who is asking for "b.example.org" that it does
   not exists without using the name "b.example.org" in the answer.
   This has been solved by introducing a record that defines an interval
   between two existing names.  Or to put it another way: it defines the
   holes (non-existing names) in the zone.  This record can be signed
   beforehand and given to the resolver.

      Given all these troubles, why didn't the designers of DNSSEC go
      for the (easy) route and allowed for online signing?  Well, at the
      time (pre 2000), online signing was not feasible with the current
      hardware.  Keep in mind that the larger servers get between 2000
      and 6000 queries per second (qps), with peaks up to 20,000 qps or
      more.  Scaling signature generation to these kind of levels is
      always a challenge.  Another issue was (and is) key management,
      for online signing to work you need access to the private key(s).
      This is considered a security risk.

   The road to the current solution (NSEC/NSEC3) was long.  It started
   with the NXT (next) record.  The NO (not existing) record was
   introduced, but never made it to RFC.  Later NXT was superseded by
   NSEC (next secure) record.  From there it went through NSEC2/DNSNR to
   finally reach NSEC3 (next secure, version 3) in RFC 5155.

3.1.  NXT

   The first attempt to specify authenticated denial of existence was
   NXT (RFC 2535 [RFC2535]).  Section 5.1 of that RFC introduces the

      The NXT resource record is used to securely indicate that RRs with
      an owner name in a certain name interval do not exist in a zone
      and to indicate what RR types are present for an existing name.

   By specifying what you do have, you implicitly tell what you don't
   have.  NXT is superseded by NSEC.  In the next section we explain how
   NSEC (and thus NXT) works.

3.2.  NSEC

   In RFC 3755 [RFC3755] all the DNSSEC types were given new names, SIG
   was renamed RRSIG, KEY became DNSKEY and NXT was simply renamed to
   NSEC and a few, minor issues were fixed in the process.

   Just as NXT, NSEC is used to describe an interval between names: it
   indirectly tells a resolver which names do not exist in a zone.

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   For this to work, we need our "example.org" zone to be sorted in
   canonical ordering ([RFC4034], Section 6.1), and then create the
   NSECs.  We add three NSEC records, one for each name, and each one
   "covers" a certain interval.  The last NSEC record points back to the
   first as required by the RFC.  Also see Figure 1.

   1.  The first NSEC covers the interval between "example.org" and

   2.  The second NSEC covers: "a.example.org" to "d.example.org";

   3.  The third NSEC points back to "example.org", and covers
       "d.example.org" to "example.org" (i.e. the end of the zone).

   As we have defined the intervals and put those in resource records,
   we now have something that can be signed.

                                  +-- ** <--+
                                 /  .    .   \
                                /  .      .   \
                               |  .        .  |
                               v .          . |
                               **            **
                 a.example.org ** ---------> ** d.example.org

   The NSEC records of example.org.  The arrows represent NSEC records,
                          starting from the apex.

                                 Figure 1

   This signed zone is loaded into the name server.  It looks like this:

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   example.org.        SOA ( ... )
                       DNSKEY ( ... )
                       NSEC a.example.org. SOA NSEC DNSKEY RRSIG
                       RRSIG(SOA) ( ... )
                       RRSIG(DNSKEY) ( ... )
                       RRSIG(NSEC) ( ... )
   a.example.org.      A
                       TXT "a record"
                       NSEC d.example.org. A TXT NSEC RRSIG
                       RRSIG(A) ( ... )
                       RRSIG(TXT) ( ... )
                       RRSIG(NSEC) ( ... )
   d.example.org.      A
                       TXT "d record"
                       NSEC example.org. A TXT NSEC RRSIG
                       RRSIG(A) ( ... )
                       RRSIG(TXT) ( ... )
                       RRSIG(NSEC) ( ... )

   If a DNSSEC aware resolver asks for "b.example.org", it gets back a
   "status: NXDOMAIN" packet, which by itself is meaningless as the
   header can be forged.  To be able to securely detect that "b" does
   not exist, there must also be an NSEC record which _covers_ the name
   space where "b" lives:

   a.example.org.      NSEC d.example.org.

   does just do that: "b" should come after "a", but the next owner name
   is "d.example.org", so "b" does not exist.

   Only by making that calculation, can a resolver conclude that the
   name "b" does not exist.  If the signature of the NSEC record is
   valid, "b" is proven not to exist.  We have: _authenticated denial of

3.3.  NODATA Responses

   NSEC records are also used in NODATA responses.  In that case we need
   to look more closely at the type bit map.  The type bit map in an
   NSEC record tells which types are defined for a name.  If we look at
   the NSEC record of "a.example.org" (see the reply below for an
   example of the record) we see the following types in the bit map: A,
   TXT, NSEC and RRSIG.  So for the name "a" this indicates we must have
   an A, TXT, NSEC and RRSIG record in the zone.

   With the type bit map of the NSEC record a resolver can establish
   that a name is there, but the type is not.  A resolver asks for
   "a.example.org AAAA".  This is the reply that comes back:

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   ;; status: NOERROR, id: 44638

   example.org.        SOA ( ... )
   example.org.        RRSIG(SOA) ( ... )
   a.example.org.      NSEC d.example.org. A TXT NSEC RRSIG
   a.example.org.      RRSIG(NSEC) ( ... )

   Now the resolver should check the AUTHORITY section and conclude

   1.  "a.example.org" exists (because of the NSEC with that owner name)

   2.  that the type (AAAA) does not as it is _not_ listed in the type
       bit map.

   By understanding NSEC records, you have mastered the basics of
   authenticated denial of existence.

   But there were two issues with NSEC (and NXT).  The first is that it
   allows for zone walking.  NSEC records point from one name to
   another, in our example: "example.org", points to "a.example.org"
   which points to "d.example.org" which points back to "example.org".
   So we can reconstruct the entire "example.org" zone even when zone
   transfers (AXFR) on the server are denied.

   The second issue is that when a large, delegation heavy, zone deploys
   DNSSEC, every name in the zone gets an NSEC plus RRSIG.  This leads
   to a huge increase in the zone size (when signed).  This would in
   turn mean that operators of large zones (or with a lot of zones) who
   are deploying DNSSEC, face up front costs.  This could hinder DNSSEC

   These two issues eventually lead to NSEC3 which:

   o  Adds a way to garble the next owner name, thus thwarting zone-

   o  Makes it possible to skip names for the next owner name.  This
      feature is called opt-out.  It means not all names in your zone
      get an NSEC3 plus ditto signature, making it possible to "grow
      into" your DNSSEC deployment.  Describing opt-out is (currently)
      out of scope for this document.  For those interested, opt-out is
      explained in RFC 4956 [RFC4956], which is curiously titled
      "(DNSSEC) Opt-In".  Later this is incorporated into RFC 5155

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   But before we delve in to NSEC3 lets first take a look at its
   predecessors, NO, NSEC2 and DNSNR.

3.4.  NO, NSEC2 and DNSNR

   The NO record was the first to introduce the idea of hashed owner
   names.  It also fixed other shortcomings of the NXT record.  At the
   time (around 2000) zone walking was not considered important enough
   to warrant the new record.  People were also worried that deployment
   would be hindered by developing an alternate means of denial of
   existence.  Thus the effort was shelved and NXT remained.  When the
   new DNSSEC specification was written, NSEC saw the light and
   inherited the two issues from NXT.

   Several years after that NSEC2 was introduced as a way to solve the
   two issues of NSEC.  The NSEC2 draft contains the following

      This document proposes an alternate scheme which hides owner names
      while permitting authenticated denial of existence of non-existent
      names.  The scheme uses two new RR types: NSEC2 and EXIST.

   When an authenticated denial of existence scheme starts to talk about
   EXIST records it is worth paying extra attention.

   NSEC2 solved the zone walking issue, by hashing (with SHA1 and a
   salt) the "next owner name" in the record, thereby making it useless
   for zone walking.

   But it did not have opt-out.  Although promising, the proposal didn't
   make it because of issues with wildcards and the odd EXISTS resource

   The DNSNR RR was another attempt that used hashed names to foil zone
   walking and it also introduced the concept of opting out (called
   "Authoritative Only Flag") which limited the use of DNSNR in
   delegation heavy zones.  This proposal didn't make it either, but it
   provided valuable insights into the problem.

3.5.  NSEC3

   From the experience gained with NSEC2 and DNSNR, NSEC3 was forged.
   It incorporates both opt-out and the hashing of names.  NSEC3 solves
   any issues people might have with NSEC, but it introduces some
   additional complexity.

   NSEC3 did not supersede NSEC, they are both defined for DNSSEC.  So
   DNSSEC is blessed with two different means to perform authenticated

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   denial of existence: NSEC and NSEC3.  In NSEC3 every name is hashed,
   including the owner name.

   SHA1 is always used for the hashing.  To make it even more difficult
   to retrieve the original names, the hashing can be repeated several
   times each time taking the previous hash as input.  To thwart rainbow
   table attacks, a custom salt is also added.  In the NSEC3 for
   "example.org" we have hashed the names twice and use the salt "DEAD".
   Lets look at typical NSEC3 record:

   15BG9L6359F5CH23E34DDUA6N1RIHL9H.example.org. (

   On the first line we see the hashed owner name:
   "15BG9L6359F5CH23E34DDUA6N1RIHL9H.example.org", this is the hashed
   name of "example.org".  Note that even though we hashed
   "example.org", the zone's name is added to make it look like a domain
   name again.  So un-hashed it sort of looks like:

   The next owner name "a.example.org" (line 2) is hashed to:
   "04SKNAPCA5AL7QOS3KM2L9TL3P5OKQ4C".  Note that ".example.org" is not
   added to the next owner name, as this name always falls in the
   current zone.

   The "1 0 2 DEAD" section of the NSEC3 states:

   o  Hash Algorithm = 1 (SHA1, this is the default, no other hash
      algorithms are defined for use in NSEC3);

   o  Opt Out = 0 (disabled);

   o  Hash Iterations = 2;

   o  Salt = "DEAD".

   At the end we see the type bit map, which is identical to NSEC's bit
   map, that lists the types present at the original owner name.  Note
   that the type NSEC3 is absent from the list in the example above.
   This is due to the fact that the original owner name ("example.org")
   does not have the NSEC3 type.  It only exists for the hashed name.

   Names like "1.h.example.org" hash to one label in NSEC3,
   "1.h.example.org" becomes:
   "117GERCPRCJGG8J04EV1NDRK8D1JT14K.example.org" when used as a owner
   name.  This is an important observation.  By hashing the names you
   loose the depth of a zone - hashing introduces a flat space of names,

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   as opposed to NSEC.

   In fact the domain name used above: "1.h.example.org" creates an
   empty non-terminal.  Empty non-terminals are domain names that exist
   but have no RR types associated with them but has one or more
   subdomains that do ([RFC5155], Section 1.3).

       1.h.example.org.    TXT "1.h record"

   Creates 2 names:

   1.  "1.h.example.org" that has the type: TXT;

   2.  "h.example.org" which has no types.  This is the empty non-
       terminal.  An empty non-terminal will get an NSEC3 records, but
       not an NSEC record.

3.6.  Slaving an NSEC3 Zone

   A secondary server slaving a zone with NSEC3 records needs to find
   out the specifics (hash iterations and salt) to be able to hash
   incoming query names.

   To do this, it could scan the zone during the AXFR for NSEC3 records
   and glance the NSEC3 parameters from them.  However, it would need to
   make sure that there is at least one complete set of NSEC3 records
   for the zone using the same parameters.  Therefore, it would need to
   inspect all NSEC3 records.

   A more graceful solution was designed.  This solution was to create a
   new record, NSEC3PARAM, which must be placed at the apex of the zone.
   Its sole role is to provide a single, fixed place where a secondary
   name server can directly see the NSEC3 parameters used.  If NSEC3
   were designed in the early days of DNS (+/- 1985) this information
   was probably put in the SOA record.

3.7.  Wildcards in the DNS

   In the above sections we haven't revealed the entire story.  There is
   a complication: wildcards.  Wildcards have been part of the DNS since
   the first DNS RFCs.  They allow to define _all_ names for a certain
   type in one go.  In our "example.org" zone we could for instance add
   a wildcard record:

   *.example.org.      TXT "wildcard record"

   For completeness our (unsigned) zone now looks like this:

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   example.org.        SOA ( ... )
   *.example.org.      TXT "wildcard record"
   a.example.org.      A
                       TXT "a record"
   d.example.org.      A
                       TXT "d record"

   If a resolver asks for "z.example.org TXT", the name server will
   respond with an _expanded wildcard_, instead of an NXDOMAIN:

   ;; status: NOERROR, id: 13658

   z.example.org.      TXT "wildcard record"

   Note however that the resolver can not detect that this answer came
   from a wildcard.  It just sees the answer as-is.  How will this
   answer look with DNSSEC?

   ;; status: NOERROR, id: 51790

   z.example.org.      TXT "wildcard record"
   z.example.org.      RRSIG(TXT) ( ... )

   d.example.org.      NSEC example.org. TXT RRSIG NSEC
   d.example.org.      RRSIG(NSEC) ( ... )

   The RRSIG of the "z.example.org" TXT record indicates there is a
   wildcard configured.  The RDATA of the signature lists a label count
   [RFC4034], Section 3.1.3., of two (not visible in the answer above),
   but the owner name of the signature has three labels.  This mismatch
   indicates there is a wildcard configured for the name
   "*.example.org".  The AUTHORITY section holds an NSEC.  This NSEC
   proves that the queried name "z.example.org" does not exist, and
   wildcard name expansion was indeed allowed.

      An astute reader may notice that it appears as if a
      "z.example.org" RRSIG(TXT) is created out of thin air.  This is
      not the case.  The signature for "z.example.org" does not exist.
      The signature you are seeing is the one for "*.example.org" which
      does exist, only the owner name is switched to "z.example.org".
      So even with wildcards, no signatures are created on the fly.

   One thing you may notice is that this reply has an NSEC record in it
   _even_ though it is not an NXDOMAIN nor NODATA reply.  In this case
   it is there to tell the resolver this answer was synthesized from a

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   In the reply above we see that "z.example.org" was generated via
   wildcard expansion.  The DNSSEC standard mandates that an NSEC (or
   NSEC3) is included in such responses.  If it didn't, an attacker
   could poison the cache with false data.

   Suppose that the resolver would have asked for "a.example.org TXT",
   an attacker could modify the packet in such way that it looks like
   the response was generated through wildcard expansion, even though
   there exists a record for "a.example.org TXT": "a.example.org.  TXT
   "a record""

   The tweaking simply consists of adjusting the ANSWER section to:

   ;; status: NOERROR, id: 31827

   a.example.org.      TXT "wildcard record"
   a.example.org.      RRSIG(TXT) ( ... )

   Which would be a perfectly valid answer if we would not require the
   inclusion of an NSEC or NSEC3 record in the wildcard answer response.
   The resolver believes that "a.example.org TXT" is a wildcard record,
   and the real record is obscured.  This is bad and defeats all the
   security DNSSEC can deliver.  Because of this, the NSEC or NSEC3
   should be present.

   Thus a resolver can detect such a spoofing attempt:

   1.  If the NSEC(3) is not present, assume the answer is spoofed;

   2.  If the NSEC(3) is there, check it.  If the signature is not
       correct, assume a spoofed answer.

   Another way of putting this is that DNSSEC is there to ensure the
   name server has followed the steps as outlined in [RFC1034], Section
   4.3.2 for looking up names in the zone.  It explicitly lists wildcard
   lookup as one of these steps (3c), so with DNSSEC this must be
   communicated to the resolver: hence the NSEC(3) record.

   With NSEC the maximum number of NSEC records a resolver can get back
   is two: one for the denial of existence and another for the wildcard.
   We say maximum, because sometimes a single NSEC can prove both.  With
   NSEC3 it is three, as to why, we will explain in the next section.

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3.8.  Returning Three NSEC3s

   With NSEC3 matters are even more complicated.  So we have an NSEC3
   that denies the existence of the requested name and an NSEC3 that
   denies wildcard synthesis.  What do we miss?

   The short answer is that due to the hashing in NSEC3 you loose the
   depth of your zone: everything is hashed into a flat plain.  To make
   up for this loss of information you need an extra record.  The more
   detailed explanation is quite a bit longer...

   To understand NSEC3 we will need two definitions:

   Closest encloser:  Introduced in [RFC4592], this is the first
      _existing_ name (this may be an empty non-terminal) in the zone
      that is an ancestor of the name used in the query.  Suppose the
      query name is "x.2.example.org" then "example.org" is the "closest
      encloser" in our example;

   Next closer name:  Introduced in the NSEC3 RFC, this is the closest
      encloser with one more label added to the left.  So if
      "example.org" is the closest encloser for the query name
      "x.2.example.org", "2.example.org" is the "next closer name".

   An NSEC3 "closest encloser proof" consists of:

   1.  An NSEC3 RR that *matches* the "closest encloser".  This means
       the un-hashed owner name of the record is the closest encloser.
       This bit of information tells a resolver: "The name you are
       asking for does not exist, the closest I have is this".

   2.  An NSEC3 RR that *covers* the "next closer name".  This means it
       defines an interval in which the "next closer name" falls.  This
       tells the resolver: "The name in your question falls in this
       interval, and therefor the name in your question does not exist.
       In fact, the closest encloser is indeed the closest I have".

   Take the following example.  We take our zone, but now with the
   following two records and it is signed with NSEC3.  As said these
   records create two non-terminals: "h.example.org" and
   "3.example.org", but that is irrelevant for the theory here.

   1.h.example.org.    TXT "1.h record"
   3.3.example.org.    TXT "3.3 record"

   The complete unsigned zone now looks like this.

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   example.org.        SOA ( ... )
   1.h.example.org.    TXT "1.h record"
   3.3.example.org.    TXT "3.3 record"

   The resolver asks the following: "x.2.example.org TXT".  This leads
   to an NXDOMAIN response from the server, which contains three NSEC3
   records.  A list of hashed owner names can be found in Section 4.
   Also see Figure 2 the numbers in the figure correspond with the
   following NSEC3 records:

   15BG9L6359F5CH23E34DDUA6N1RIHL9H.example.org. (

   75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ.example.org. (
           RRSIG )

   1AVVQN74SG75UKFVF25DGCETHGQ638EK.example.org. (

   If we would follow the NSEC approach, the resolver is only interested
   in one thing.  Does the hash of "x.2.example.org" fall in any of the
   intervals of the NSEC3 records it got?

                       +-- ** .....................
                  [1] /  . /\ .                    .
                     /  .  |   .                    .
                    |  .   |    .                    .
                    v .    |     .                    .
                    **     |      **                  ++
      h.example.org ** ----+----> ** 3.example.org    ++ 2.example.org
                    .     /  [3]   . |                .
                    .    /         . | [2]            .
                    .   /          . |                .
                    .  /           . v                .
    1.h.example.org **            **                  ++
                    ** <--------- ** 3.3.example.org  ++ x.2.example.org

      x.2.example.org does not exist.  The arrows represent the NSEC3
    records, the ones numbered [1], [2] and [3] are the NSEC3s returned
                              in our answer.

                                 Figure 2

   The hash of "x.2.example.org" is "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1".

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   Checking this hash on the first NSEC3 yields that it does not fall in
   between the interval: "15BG9L6359F5CH23E34DDUA6N1RIHL9H" and
   "1AVVQN74SG75UKFVF25DGCETHGQ638EK".  For the second NSEC3 the answer
   is also negative: the hash sorts outside the interval described by
   "8555T7QEGAU7PJTKSNBCHG4TD2M0JNPJ".  And the last NSEC3 also isn't of
   any help.  What is a resolver to do?  It has been given the maximum
   amount of NSEC3s and they all seem useless.

   A question that you might have at this point is why doesn't the
   server send an NSEC3 that covers the hash of "x.2.example.org", so
   the resolver can validate in one step?  While this indeed denies the
   existence of "x.2.example.org" it is only half the answer.  As
   explained, a denial of existence answer needs to say something about
   whether or not a wildcard should have been expanded.  And to
   communicate which wildcard that could have been, you need to tell the
   resolver what the closest encloser is.

   So this is where the closest encloser proof comes into play.  And for
   the proof to work, the resolver needs to know what the "closest
   encloser" is.  There must be an existing ancestor in the zone: a name
   must exist that is shorter than the query name.  The resolvers keeps
   hashing, increasingly shorter names from the query name until an
   owner name of an NSEC3 matches.  This owner name is the "closest

   When the resolver has found the closest encloser, the next step is to
   construct the "next closer name".  This is the closest encloser with
   the last chopped label from query name prepended to it: "<last
   chopped label>.<closest encloser>".  The hash of this name should be
   covered by the interval set in any of the other NSEC3 records.

   Then the resolver needs to check the presence of a wildcard.  It
   creates the wildcard name by prepending the wildcard label to the
   closest encloser: "*.<closest encloser>", and use the hash of that.

   Going back to our example the resolver must first detect the NSEC3
   that matches the closest encloser.  It does this by chopping up the
   query name, hashing each instance (with the same number of iterations
   and hash as the zone it is querying) and comparing that to the
   answers given.  So it has the following hashes to work with:

   x.2.example.org:  "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1", last chopped
      label: "<empty>";

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   2.example.org:  "7T70DRG4EKC28V93Q7GNBLEOPA7VLP6Q", last chopped
      label: "x";

   example.org:  "15BG9L6359F5CH23E34DDUA6N1RIHL9H", last chopped label:

   Of these hashes only one matches the owner name of one of the NSEC3
   records: "15BG9L6359F5CH23E34DDUA6N1RIHL9H".  This must be the
   closest encloser (un-hashed: "example.org").  That's the main purpose
   of that NSEC3 record: tell the resolver what the closest encloser is.

   From that knowledge the resolver constructs the next closer, which in
   this case is: "2.example.org"; "2" is the last label chopped, when
   "example.org" is the closest encloser.  The hash of this name should
   be covered in any of the other NSEC3s.  And it is,
   "7T70DRG4EKC28V93Q7GNBLEOPA7VLP6Q" falls in the interval set by:
   "8555T7QEGAU7PJTKSNBCHG4TD2M0JNPJ" (this is our second NSEC3).

   So what does the resolver learn from this?

   o  "example.org" exists;

   o  "2.example.org" does not exist.

   And if "2.example.org" does not exist, "x.2.example.org" also does
   not exist.  But only if there was no wildcard configured.  So this is
   the last step: check if there is a wildcard configured at the closest

   The resolver hashes "*.example.org" to
   "22670TRPLHSR72PQQMEDLTG1KDQEOLB7".  Only the last NSEC3 covers this
   hash.  The hash falls in the interval set by
   "75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ" (this is our third NSEC3).  This
   means there is no wildcard at the closest encloser and
   "x.2.example.org" definitely does not exist.

   When we have validated the signatures, we reached our goal:
   authenticated denial of existence.

   Coming back to the original question: why do we need (up to) three
   NSEC3 records?  The resolver needs to be explicitly told what the
   "closest encloser" is and this takes up a full NSEC3 record.  Then
   the next closer name needs to be covered in an NSEC3 record, and
   finally an NSEC3 must say something about the wildcard.  That makes
   three records.

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4.  List of Hashed Owner Names

   The following owner names are used in this document.  The origin for
   these names is "example.org".

          | Original Name | Hashed Name                        |
          | "@"           | "15BG9L6359F5CH23E34DDUA6N1RIHL9H" |
          | "*"           | "22670TRPLHSR72PQQMEDLTG1KDQEOLB7" |
          | "1.h"         | "117GERCPRCJGG8J04EV1NDRK8D1JT14K" |
          | "2"           | "7T70DRG4EKC28V93Q7GNBLEOPA7VLP6Q" |
          | "3.3"         | "8555T7QEGAU7PJTKSNBCHG4TD2M0JNPJ" |
          | "3"           | "75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ" |
          | "a"           | "04SKNAPCA5AL7QOS3KM2L9TL3P5OKQ4C" |
          | "b"           | "IUU8L5LMT76JELTP0BIR3TMG4U3UU8E7" |
          | "h"           | "1AVVQN74SG75UKFVF25DGCETHGQ638EK" |
          | "x.2"         | "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1" |

               Hashed owner names for the example.org zone.

                                  Table 1

5.  Security Considerations

   This document raises no security issues.

6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgements

   This document would not be possible without the help of Ed Lewis, Roy
   Arends, Wouter Wijngaards, Olaf Kolkman, Carsten Strotmann, Jan-Piet
   Mens, Peter van Dijk, Marco Davids, Esther Makaay and Antoin
   Verschuren.  Also valuable, was the source code of Unbound
   ("validator/val_nsec3.c").  Extensive feedback was received from
   Karst Koymans.

8.  References

8.1.  Normative References

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

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

   [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.

   [RFC4592]                          Lewis, E., "The Role of Wildcards
                                      in the Domain Name System",
                                      RFC 4592, July 2006.

   [RFC5155]                          Laurie, B., Sisson, G., Arends,
                                      R., and D. Blacka, "DNS Security
                                      (DNSSEC) Hashed Authenticated
                                      Denial of Existence", RFC 5155,
                                      March 2008.

8.2.  Informative References

   [I-D.arends-dnsnr]                 Arends, R., "DNSSEC Non-
                                      Repudiation Resource Record",
                                      draft-arends-dnsnr-00 (work in
                                      progress), July 2004.

   [I-D.ietf-dnsext-not-existing-rr]  Josefsson, S., "Authenticating
                                      denial of existence in DNS with
                                      minimum disclosure", draft-ietf-
                                      dnsext-not-existing-rr-01 (work in
                                      progress), November 2000.

   [I-D.laurie-dnsext-nsec2v2]        Laurie, B., "DNSSEC NSEC2 Owner
                                      and RDATA Format",

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                                      (work in progress), December 2004.

   [RFC2535]                          Eastlake, D., "Domain Name System
                                      Security Extensions", RFC 2535,
                                      March 1999.

   [RFC3655]                          Wellington, B. and O. Gudmundsson,
                                      "Redefinition of DNS Authenticated
                                      Data (AD) bit", RFC 3655,
                                      November 2003.

   [RFC3755]                          Weiler, S., "Legacy Resolver
                                      Compatibility for Delegation
                                      Signer (DS)", RFC 3755, May 2004.

   [RFC4956]                          Arends, R., Kosters, M., and D.
                                      Blacka, "DNS Security (DNSSEC)
                                      Opt-In", RFC 4956, July 2007.

Authors' Addresses

   R. (Miek) Gieben
   SIDN Labs
   Meander 501
   Arnhem,   6825 MD

   EMail: miek.gieben@sidn.nl
   URI:   https://sidn.nl/

   W. (Matthijs) Mekking
   NLnet Labs
   Science Park 400
   Amsterdam,   1098 XH

   EMail: matthijs@nlnetlabs.nl
   URI:   http://www.nlnetlabs.nl/

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