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Authenticated Denial of Existence in the DNS

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7129.
Authors R. (Miek) Gieben , Matthijs Mekking
Last updated 2012-12-07 (Latest revision 2012-11-23)
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Network Working Group                                          R. Gieben
Internet-Draft                                                 SIDN Labs
Intended status: Informational                                W. Mekking
Expires: May 25, 2013                                         NLnet Labs
                                                       November 23, 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.  When returning a negative DNSSEC response, a name server
   usually includes up to two NSEC records.  With NSEC3 this amount is
   three.  This document provides extra documentation and context on the
   mechanisms behind NSEC and NSEC3

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 May 25, 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
   ( 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2

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   2.  Denial of Existence  . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  NXDOMAIN Responses . . . . . . . . . . . . . . . . . . . .  4
     2.2.  NODATA Responses . . . . . . . . . . . . . . . . . . . . .  4
   3.  Secure Denial of Existence . . . . . . . . . . . . . . . . . .  5
     3.1.  NXT  . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  NODATA Responses . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Drawbacks of NSEC  . . . . . . . . . . . . . . . . . . . .  9
     3.5.  NO, NSEC2 and DNSNR  . . . . . . . . . . . . . . . . . . .  9
     3.6.  NSEC3  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.7.  Loading an NSEC3 Zone  . . . . . . . . . . . . . . . . . . 11
     3.8.  Wildcards in the DNS . . . . . . . . . . . . . . . . . . . 12
     3.9.  CNAME Records  . . . . . . . . . . . . . . . . . . . . . . 14
     3.10. The Closest Encloser NSEC3 Record  . . . . . . . . . . . . 15
     3.11. Three To Tango . . . . . . . . . . . . . . . . . . . . . . 19
   4.  List of Hashed Owner Names . . . . . . . . . . . . . . . . . . 20
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Changelog . . . . . . . . . . . . . . . . . . . . . . 22
     A.1.  -00  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     A.2.  -01  . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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.  Both are also
   known as negative responses.

   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.  The NO, NSEC2
   and DNSNR records 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
   these complicate matters, especially combined with CNAME records.

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   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.  NSEC3
   opt-out and secure delegations are out of scope for this document.

   The following RFCs are not required reading, but they 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 Specification;

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

   These three drafts give some background information on the NSEC3

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

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

   o  The DNSNR record [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 ("", "" and "") 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 record is
   shortened, resulting in the following zone file:

                 SOA ( ... )
                                       TXT "a record"
                                       TXT "d record"

                     The unsigned "" zone.

                                Figure 1

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2.1.  NXDOMAIN Responses

   If a resolver asks for the TXT type belonging to "" to
   the name server serving this zone, it sends the following question:
   " 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
   ;; ANSWER SECTION:      TXT "a record"

   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 the names of the
   name servers that have information concerning the "" zone.
   Note that including this information is optional.

   If a resolver asks for " TXT" it gets an answer that
   this name does not exist:

   ;; status: NXDOMAIN, id: 7042
   ;; AUTHORITY SECTION:        SOA ( ... )

   In this case, we do not get an ANSWER section and the status is set
   to NXDOMAIN.  From this the resolver concludes that ""
   does not exist.  The AUTHORITY section holds the SOA record of
   "" that the resolver can use to cache the negative

2.2.  NODATA Responses

   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 "
   AAAA", and look at the answer:

   ;; status: NOERROR, id: 7944
   ;; AUTHORITY SECTION:        SOA ( ... )

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   The status is NOERROR meaning that the "" name exists,
   but the reply does not contain an ANSWER section.  This
   differentiates a NODATA response from an NXDOMAIN response, the rest
   of the packet is very similar.  The resolver has to put these pieces
   of information together and conclude that "" 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 did not 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 Authentic Data, see
       RFC 3655 [RFC3655]), which may give some food for thought;

   3.  DNS wildcards and CNAME records complicate matters significantly.
       More about this in later sections (Section 3.8 and Section 3.9).

   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.  A generic denial record which
   can be used in all denial of existence proofs is not an option: such
   a record is susceptible to replay attacks.  When you are querying a
   name server for a record that actually exists, a man-in-the-middle
   may replay that generic denial record and it would be impossible to
   tell whether the response was genuine or spoofed.

   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
      that 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.

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   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 on, NXT was superseded
   by the 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

   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 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.

   For this to work, we need our "" zone to be sorted in
   canonical order ([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, as depicted in Figure 2.

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

   2.  The second NSEC covers "" to "";

   3.  The third NSEC points back to "", and covers
       "" to "" (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.

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                                +-- ** <--+
                           (1) /  .    .   \ (3)
                              /  .      .   \
                             |  .        .  |
                             v .          . |
                             **    (2)     **
      ** ---------> **

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

                                Figure 2

   This signed zone is loaded into the name server.  It looks like this:        SOA ( ... )
                         DNSKEY ( ... )
                         NSEC SOA NSEC DNSKEY RRSIG
                         RRSIG(SOA) ( ... )
                         RRSIG(DNSKEY) ( ... )
                         RRSIG(NSEC) ( ... )      A
                         TXT "a record"
                         NSEC A TXT NSEC RRSIG
                         RRSIG(A) ( ... )
                         RRSIG(TXT) ( ... )
                         RRSIG(NSEC) ( ... )      A
                         TXT "d record"
                         NSEC A TXT NSEC RRSIG
                         RRSIG(A) ( ... )
                         RRSIG(TXT) ( ... )
                         RRSIG(NSEC) ( ... )

   The signed and sorted "" zone with the added NSEC records
    (and signatures).  For brevity, the class is not shown (defaults to
      IN), the NS records are left out and the SOA, DNSKEY and RRSIG
                          records are shortened.

                                Figure 3

   If a DNSSEC aware resolver asks for "", 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 a signed NSEC record which covers the
   name space where "b" lives.  The record:

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

   Only by making that calculation, is a resolver able to 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

   Note that a man-in-the-middle may still replay this NXDOMAIN response
   when you're querying for, say, "".  But it would not do
   any harm since it is provably the proper response to the query.  In
   the future, there may be data published for "".
   Therefore, the RRSIGs RDATA include a validity period (not visible in
   the zone above), so that an attacker cannot replay this NXDOMAIN
   response for "" forever.

3.3.  NODATA Responses

   NSEC records are also used in NODATA responses.  In that case we need
   to look more closely at the type bitmap.  The type bitmap in an NSEC
   record tells which types are defined for a name.  If we look at the
   NSEC record of "", we see the following types in the
   bitmap: 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 bitmap of the NSEC record, a resolver can establish
   that a name is there, but the type is not.  For example, if a
   resolver asks for " AAAA", the reply that comes back is:

   ;; status: NOERROR, id: 44638
   ;; AUTHORITY SECTION:        SOA ( ... )        RRSIG(SOA) ( ... )      NSEC A TXT NSEC RRSIG      RRSIG(NSEC) ( ... )

   The resolver should check the AUTHORITY section and conclude that:

   (1) "" exists (because of the NSEC with that owner name)

   (2) the type (AAAA) does not as it is not listed in the type bitmap.

   The techniques used by NSEC, form the basics of authenticated denial
   of existence in DNSSEC.

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3.4.  Drawbacks of NSEC

   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: "", points to ""
   which points to "" which points back to "".
   So we can reconstruct the entire "" 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 such zones who are deploying DNSSEC, face
   up front costs.  This could hinder DNSSEC adoption.

   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 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 was incorporated into RFC 5155 [RFC5155].

   But before we delve into NSEC3, let us first take a look at its
   predecessors: NO, NSEC2 and, DNSNR.

3.5.  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 that
   time (around 2000) zone walking was not considered important enough
   to warrant the new record.  People were also worried that DNSSEC
   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

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

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   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 did
   not make it because of issues with wildcards and the odd EXIST
   resource record.

   The DNSNR record 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.

3.6.  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
   denial of existence: NSEC and NSEC3.  In NSEC3 every name is hashed,
   including the owner name.  This means that NSEC3 chain is sorted in
   hash order, instead of canonical order.  Because the owner names are
   hashed, the next owner name for "" is unlikely to be
   "".  Because the next owner name is hashed, zone walking
   becomes more difficult.

   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 "" we have hashed the names
   thrice ([RFC5155], Section 5) and use the salt "DEAD".  Lets look at
   typical NSEC3 record: (

   On the first line we see the hashed owner name:
   "", this is the hashed
   name of the fully qualified domain name (FQDN) "".  Note
   that even though we hashed "", the zone's name is added to
   make it look like a domain name again.  In our zone, the basic format
   is "SHA1(FQDN)".

   The next hashed owner name "A6EDKB6V8VL5OL8JNQQLT74QMJ7HEB84" (line
   2) is the hashed version of "".  Note that
   "" is not added to the next hashed owner name, as this
   name always falls in the current zone.

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

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

   o  Opt Out = 0 (disabled);

   o  Hash Iterations = 2, this yields three iterations, as a zero value
      is already one iteration;

   o  Salt = "DEAD".

   At the end we see the type bitmap, which is identical to NSEC's
   bitmap, 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
   ("") does not have the NSEC3 type.  It only exists for the
   hashed name.

   Names like "" hash to one label in NSEC3,
   "" becomes:
   "" when used as an owner
   name.  This is an important observation.  By hashing the names you
   lose the depth of a zone - hashing introduces a flat space of names,
   as opposed to NSEC.

   The domain name used above ("") creates an empty non-
   terminal.  Empty non-terminals are domain names that have no RRs
   associated with them, and exist only because they have one or more
   subdomains that do ([RFC5155], Section 1.3).  The record:    TXT "1.h record"

   creates two names:

   1.  "" that has the type: TXT;

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

3.7.  Loading an NSEC3 Zone

   Whenever an authoritative server receives a query for a non-existing
   record, it has to hash the incoming query name to determine into
   which interval between two existing hashes it falls.  To do that it
   needs to know the zone's specific NSEC3 parameters (hash iterations

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   and salt).

   One way to learn them is to scan the zone during loading for NSEC3
   records and glean 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.  The 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 an
   authoritative name server can directly see the NSEC3 parameters used.
   If NSEC3 were designed in the early days of DNS (+/- 1984) this
   information would probably have been put in the SOA record.

3.8.  Wildcards in the DNS

   So far, we have only talked about denial of existence in negative
   responses.  However, denial of existence may also occur in positive
   responses, i.e., where the ANSWER section of the response is not
   empty.  This can happen because of 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
   "" zone we could for instance add a wildcard record:

   *      TXT "wildcard record"

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

             SOA ( ... )
               *      TXT "wildcard record"
                                   TXT "a record"
                                   TXT "d record"

               The zone with a wildcard record.

                                Figure 4

   If a resolver asks for " TXT", the name server will
   respond with an expanded wildcard, instead of an NXDOMAIN:

   ;; status: NOERROR, id: 13658
   ;; ANSWER SECTION:      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?

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   ;; status: NOERROR, id: 51790
   ;; ANSWER SECTION:      TXT "wildcard record"      RRSIG(TXT) ( ... )

   The RRSIG of the "" 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.

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

   The DNSSEC standard mandates that an NSEC (or NSEC3) is included in
   such responses.  If it wasn't, an attacker could mount a replay
   attack and poison the cache with false data: Suppose that the
   resolver has asked for " 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
   " TXT".

   The tweaking simply consists of adjusting the ANSWER section to:

   ;; status: NOERROR, id: 31827
   ;; ANSWER SECTION      TXT "wildcard record"      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 " 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 must
   be present.

   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
   look up as one of these steps (3c), so with DNSSEC this must be
   communicated to the resolver: hence the NSEC(3) record.

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3.9.  CNAME Records

   So far, the maximum number of NSEC records a response will have 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, this is three (as to why, we will explain in the next

   When we take CNAME wildcard records into account, we can have more
   NSEC(3) records.  For every wildcard expansion, we need to prove that
   the expansion was allowed.  Lets add some CNAME wildcard records to
   our zone:

             SOA ( ... )
               *      TXT "wildcard record"
                                   TXT "a record"
               *    CNAME w.b
               *    CNAME w.c
               *    A
                                   TXT "d record"
           CNAME w.a

          A wildcard CNAME chain added to the "" zone.

                                Figure 5

   A query for " A" will result in the following response:

   ;; status: NOERROR, id: 4307
   ;; ANSWER SECTION:      CNAME      RRSIG(CNAME) ( ... )    CNAME    RRSIG(CNAME) ( ... )    CNAME    RRSIG(CNAME) ( ... )    A    RRSIG(A) ( ... )
   *    RRSIG(NSEC) ( ... )
   *    RRSIG(NSEC) ( ... )

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   *    RRSIG(NSEC) ( ... )

   The NSEC record "*" proves that wildcard expansion to
   "" was appropriate: "w.a."  falls in the gap "*.a" to
   "*.b".  Similar, the NSEC record "*" proves that there
   was no direct match for "" and "*"
   denies the direct match for "".

3.10.  The Closest Encloser NSEC3 Record

   We can have one or more NSEC3 records that deny the existence of the
   requested name and one NSEC3 record that deny 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 plane.  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], "The closest encloser is
      the node in the zone's tree of existing domain names that has the
      most labels matching the query name (consecutively, counting from
      the root label downward)."  In our example, if the query name is
      "" then "" is the "closest encloser";

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

   An NSEC3 "closest encloser proof" consists of:

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

   These two records already deny the existence of the requested name,
   so we do not need an NSEC3 record that covers the actual queried
   name: By denying the existence of the next closer name, you also deny
   the existence of the queried name.

   For a given query name, there is one (and only one) place where
   wildcard expansion is possible.  This is the "source of synthesis",
   and is defined ([RFC4592], Section 2.1.1 and Section 3.3.1) as:

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   <asterisk label>.<closest encloser>

   In other words, to deny wildcard synthesis, the resolver needs to
   know the hash of the source of synthesis.  Since it does not know
   beforehand what the closest encloser of the query name is, it must be
   provided in the answer.

   Take the following example.  We take our zone, and put two TXT
   records to it.  The records added are "" and
   "".  It is signed with NSEC3, resulting in the
   following unsigned zone.

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

   The added TXT records in  These records create two non-
              terminals: `` and ``.

                                Figure 6

   The resolver asks the following: " 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 7 the numbers in that figure correspond with the
   following NSEC3 records: (
           RRSIG ) (

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

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                      +-- ** . . . . . . . . . . .
                 (1) /  . /\ .                    .
                    /  .  |   .                    .
                   |  .   |    .                    .
                   v .    |     .                    .
                   **     |      **                  -- ** ----+----> **    --
                   .     /   (3)  . |                .
                   .    /         . | (2)            .
                   .   /          . |                .
                   .  /           . v                . **            **                  --
                   ** <--------- **  -- does not exist.  The arrows represent the NSEC3
    records, the ones numbered (1), (2) and (3) are the NSEC3s returned
                              in our answer.

                                Figure 7

   The hash of "" is "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1".
   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.

   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 resolver keeps
   hashing increasingly shorter names from the query name until an owner
   name of an NSEC3 matches.  This owner name is the closest encloser.

   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 NSEC3 records.

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

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   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: "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1", last chopped
      label: "<empty>"; "7T70DRG4EKC28V93Q7GNBLEOPA7VLP6Q", last chopped
      label: "x"; "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 (unhashed: "").  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" is the last label chopped, when
   "" 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  "" exists;

   o  "" does not exist.

   And if "" does not exist, there is also no direct match
   for "".  The last step is to deny the existence of the
   source of synthesis, to prove that no wildcard expansion was

   The resolver hashes "*" to
   "22670TRPLHSR72PQQMEDLTG1KDQEOLB7" and checks that it is covered: in
   this case by the last NSEC3 (see Figure 7), the hash falls in the
   interval set by "1AVVQN74SG75UKFVF25DGCETHGQ638EK" and
   "75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ".  This means there is no wildcard
   record directly below the closest encloser and ""
   definitely does not exist.

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

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3.11.  Three To Tango

   One extra NSEC3 record plus additional signature may seem a lot just
   to deny the existence of the wildcard record, but we cannot leave it
   out.  If the standard would not mandate the closest encloser NSEC3
   record, but instead required two NSEC3 records: one to deny the query
   name and one to deny the wildcard record.  An attacker could fool the
   resolver that the source of synthesis does not exist, while it in
   fact does.

   Suppose the wildcard record does exist, so our unsigned zone looks
   like this:        SOA ( ... )
   *      TXT "wildcard record"    TXT "1.h record"    TXT "3.3 record"

   The query " TXT" should now be answered with:    TXT "wildcard record"

   An attacker can deny this wildcard expansion by calculating the hash
   for the wildcard name "*" and searching for an NSEC3
   record that covers that hash.  The hash of "*" is
   "FBQ73BFKJLRKDOQS27K5QF81AQQD7HHO".  Looking through the NSEC3
   records in our zone we see that the NSEC3 record of "3.3" covers this
   hash: (
       NSEC3 1 0 2 DEAD 15BG9L6359F5CH23E34DDUA6N1RIHL9H TXT RRSIG )

   This record also covers the query name ""

   Now an attacker adds this NSEC3 record to the AUTHORITY section of
   the reply to deny both "" and any wildcard expansion.
   The net result is that the resolver determines that ""
   does not exist, while in fact it should have been synthesized via
   wildcard expansion.  With the NSEC3 matching the closest encloser
   "", the resolver can be sure that the wildcard expansion
   should occur at "*" and nowhere else.

   Coming back to the original question: why do we need up to three
   NSEC3 records to deny a requested name?  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
   whether wildcard expansion was possible.  That makes three to tango.

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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 "".

        | Original Name  | Hashed Name                         |
        | "a"            | "04SKNAPCA5AL7QOS3KM2L9TL3P5OKQ4C"  |
        | "1.h"          | "117GERCPRCJGG8J04EV1NDRK8D1JT14K"  |
        | "@"            | "15BG9L6359F5CH23E34DDUA6N1RIHL9H"  |
        | "h"            | "1AVVQN74SG75UKFVF25DGCETHGQ638EK"  |
        | "*"            | "22670TRPLHSR72PQQMEDLTG1KDQEOLB7"  |
        | "3"            | "75B9ID679QQOV6LDFHD8OCSHSSSB6JVQ"  |
        | "2"            | "7T70DRG4EKC28V93Q7GNBLEOPA7VLP6Q"  |
        | "3.3"          | "8555T7QEGAU7PJTKSNBCHG4TD2M0JNPJ"  |
        | "d"            | "A6EDKB6V8VL5OL8JNQQLT74QMJ7HEB84"  |
        | "*.2"          | "FBQ73BFKJLRKDOQS27K5QF81AQQD7HHO"  |
        | "b"            | "IUU8L5LMT76JELTP0BIR3TMG4U3UU8E7"  |
        | "x.2"          | "NDTU6DSTE50PR4A1F2QVR1V31G00I2I1"  |

             Hashed owner names for in hash order.

                                Table 1

5.  Security Considerations

   DNSSEC does not protect against denial of service attacks, nor does
   it provide confidentiality.  For more general security considerations
   related to DNSSEC, please see RFC 4033, RFC 4034, RFC 4035 and RFC
   5155 ([RFC4033], [RFC4034], [RFC4035] and [RFC5155]).

   These RFCs are concise about why certain design choices have been
   made in the area of authenticated denial of existence.
   Implementations that do not correctly handle this aspect of DNSSEC,
   create a severe hole in the security DNSSEC adds.  This is
   specifically troublesome for secure delegations: If an attacker is
   able to deny the existence of a DS record, the resolver cannot
   establish a chain of trust, and the resolver has to fall back to
   insecure DNS for the remainder of the query resolution.

   This document aims to fill this "documentation gap" and provide
   would-be implementors and other interested parties with enough
   background knowledge to better understand authenticated denial of

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6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgments

   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, Antoin Verschuren
   and Lukas Wunner.  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.

   [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

              Arends, R., "DNSSEC Non-Repudiation Resource Record",
              Internet-Draft draft-arends-dnsnr-00, July 2004.

              Josefsson, S., "Authenticating denial of existence in DNS
              with minimum disclosure", Internet-Draft draft-ietf-
              dnsext-not-existing-rr-01, November 2000.

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              Laurie, B., "DNSSEC NSEC2 Owner and RDATA Format",
              Internet-Draft draft-laurie-dnsext-nsec2v2-00, December

   [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.

Appendix A.  Changelog

   [This section should be removed by the RFC editor before publishing]

A.1.  -00

   1.  Initial document.

A.2.  -01

   1.  Style and language changes;

   2.  Figure captions;

   3.  Security considerations added;

   4.  Fix erroneous NSEC3 RR;

   5.  Section on CNAMEs added;

   6.  More detailed text on closest encloser proof.

Authors' Addresses

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

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

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