Compact Denial of Existence in DNSSEC
draft-ietf-dnsop-compact-denial-of-existence-05

Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                Salesforce
Updates: 4034, 4035 (if approved)                             C. Elmerot
Intended status: Standards Track                              Cloudflare
Expires: 20 April 2025                                    O. Gudmundsson
                                                  Retired / Unaffiliated
                                                         17 October 2024

                 Compact Denial of Existence in DNSSEC
            draft-ietf-dnsop-compact-denial-of-existence-05

Abstract

   This document describes a technique to generate a signed DNS response
   on demand for a non-existent name by claiming that the name exists
   but doesn't have any data for the queried record type.  Such answers
   require only one minimal NSEC record, allow online signing servers to
   minimize signing operations and response sizes, and prevent zone
   content disclosure.

   This document updates RFC 4034 and 4035.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/shuque/id-dnssec-compact-lies.

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 https://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
   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 20 April 2025.

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Copyright Notice

   Copyright (c) 2024 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 (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
   2.  Distinguishing non-existent names . . . . . . . . . . . . . .   3
   3.  Generating Responses  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Responses for Non-Existent Names  . . . . . . . . . . . .   4
     3.2.  Responses for Non-Existent Types  . . . . . . . . . . . .   5
     3.3.  Responses for Wildcard Matches  . . . . . . . . . . . . .   5
     3.4.  Responses to explicit queries for NXNAME  . . . . . . . .   5
   4.  Response Code Restoration . . . . . . . . . . . . . . . . . .   6
     4.1.  Signaled Response Code Restoration  . . . . . . . . . . .   6
   5.  Operational Implications  . . . . . . . . . . . . . . . . . .   7
   6.  Updates to RFCs . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Updates to RFC 4034 . . . . . . . . . . . . . . . . . . .   8
     6.2.  Updates to RFC 4035 . . . . . . . . . . . . . . . . . . .   8
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Other Approaches . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction and Motivation

   One of the functions of the Domain Name System Security Extensions
   (DNSSEC) [RFC9364] is "Authenticated Denial of Existence", i.e.
   proving that a DNS name or record type does not exist.  Normally,
   this is done by means of signed NSEC or NSEC3 records.  In the
   precomputed signature model, these records chain together existing
   names, or cryptographic hashes of them in the zone.  In the online
   signing model, described in NSEC and NSEC3 "White Lies" [RFC4470]

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   [RFC7129], they are used to dynamically compute an epsilon function
   around the queried name.  A 'type bitmap' in the data field of the
   NSEC or NSEC3 record asserts which resource record types are present
   at the name.

   The response for a non-existent name requires up to 2 signed NSEC
   records or up to 3 signed NSEC3 records (and for online signers, the
   associated cryptographic computation), to prove that (1) the name did
   not explicitly exist in the zone, and (2) that it could not have been
   synthesized by a wildcard.

   This document describes an alternative technique, "Compact Denial of
   Existence" or "Compact Answers", to generate a signed DNS response on
   demand for a non-existent name by claiming that the name exists but
   has no resource records associated with the queried type, i.e. it
   returns a NODATA response rather than an NXDOMAIN response.  A NODATA
   response, which has a response code (RCODE) of NOERROR and an empty
   ANSWER section, requires only one NSEC record matching the queried
   name.  This has two advantages: the DNS response size is smaller, and
   it reduces the online cryptographic work involved in generating the
   response.

   The use of minimally covering NSEC records also prevents adversaries
   from enumerating the entire contents of DNS zones by walking NSEC
   chains.

2.  Distinguishing non-existent names

   Since NODATA responses are generated for non-existent names, and
   there are no defined record types for the name, the NSEC type bitmap
   in the response will only contain "NSEC" and "RRSIG".  Tools that
   need to accurately identify non-existent names in responses cannot
   rely on this specific type bitmap because Empty Non-Terminal (ENT)
   names (which positively exist) also have no record types at the name
   and will return exactly the same type bitmap.

   This specification defines the use of a synthetic Resource Record
   type to signal the presence of a non-existent name.  The mnemonic for
   this RR type is "NXNAME", chosen to clearly distinguish it from the
   response code mnemonic NXDOMAIN.

         Type    Value  Meaning
         NXNAME  128    NXDOMAIN indicator for Compact Denial of Existence

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   This RR type is added to the NSEC type bitmap for responses to non-
   existent names (in addition to the required RRSIG and NSEC types).
   It is a "Meta-Type", as defined in [RFC6895], stores no data in a DNS
   zone, and cannot be usefully queried.  Section 3.4 describes what a
   DNS resolver or authoritative server should do if it receives an
   explicit query for NXNAME.

   No special handling of this RR type is required on the part of DNS
   resolvers.  However, resolvers may optionally implement the behavior
   described in Section 4.1 (Response Code Restoration) to better
   restore NXDOMAIN visibility to various applications.

3.  Generating Responses

   This section describes various types of answers generated by
   authoritative servers implementing Compact Denial of Existence.  At
   the current time, the compact denial scheme is only defined for NSEC.
   While it could support NSEC3 too, there is no benefit in introducing
   the additional complexity associated with it.

3.1.  Responses for Non-Existent Names

   When the authoritative server receives a query for a non-existent
   name in a zone that it serves, a NODATA response (response code
   NOERROR, empty Answer section) is generated with a dynamically
   constructed NSEC record with the owner name matching the queried name
   (QNAME).

   The Next Domain Name field SHOULD be set to the immediate
   lexicographic successor of the QNAME.  The Type Bit Maps field MUST
   only have the bits set for the following RR Types: RRSIG, NSEC, and
   NXNAME.  (The immedidate lexicographic successor is the typical case
   of the "DNS Name Successor" defined in [RFC4471]).

   For example, a request for the non-existing name a.example.com would
   cause the following NSEC record to be generated (in DNS presentation
   format):

          a.example.com. 3600 IN NSEC \000.a.example.com. RRSIG NSEC NXNAME

   The NSEC record MUST have corresponding RRSIGs generated.

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3.2.  Responses for Non-Existent Types

   When the authoritative server receives a query for a name that
   exists, but has no resource record sets associated with the queried
   type, it generates a NODATA response, with a dynamically constructed
   signed NSEC record in the Authority Section.  The owner name of the
   NSEC record matches the queried name.  The Next Domain Name field is
   set to the immediate lexicographic successor of the QNAME.  The Type
   Bitmaps field lists the available Resource Record types at the name.

   An Empty Non-Terminal is a special subset of this category, where the
   name has no resource record sets of any type (but has descendant
   names that do).  For a query for an Empty Non-Terminal, the NSEC type
   bitmap will only contain RRSIG and NSEC.  (Note that this is
   substantially different than the ENT response in precomputed NSEC,
   where the NSEC record has the same type bitmap, but "covers" rather
   than matches the ENT, and has the Next Domain Name field set to the
   next lexicographic descendent of the ENT in the zone.)

3.3.  Responses for Wildcard Matches

   For wildcard matches, the authoritative server will provide a
   dynamically signed response that claims that the queried name exists
   explicitly.  Specifically, the answer RR set will have an RRSIG
   record demonstrating an exact match (i.e. the label count in the
   RRSIG RDATA will be equal to the number of labels in the query name
   minus the root label).  This obviates the need to include an NSEC
   record in the Authority section of the response that shows that no
   closer match than the wildcard was possible.

   For a Wildcard NODATA match (where the queried name matches a
   wildcard but no data for the queried type exists), a response akin to
   a non-wildcard NODATA is returned.  The Answer section is empty, and
   the Authority section contains a single NSEC record that matches the
   query name with a type bitmap representing the list of types
   available at the wildcard.

3.4.  Responses to explicit queries for NXNAME

   NXNAME is a meta type which should not appear anywhere in a DNS
   message apart from the NSEC type bitmap of a Compact Answer response
   for a non-existent name.  If however a DNS server implementing this
   specification receives an explicit query for the NXNAME RR type, this
   section describes what the response should be.

   If an explicit query for the NXNAME RR type is received, the DNS
   server MUST return a Format Error (response code FORMERR).  A
   resolver should not forward these queries upstream or attempt

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   iterative resolution.  Many DNS server implementations already return
   errors for all types in the meta and q-type range except those types
   that are already defined to support queries.

   Optionally, a DNS server MAY also include the following [RFC8914]
   Extended DNS Error Code in the response:

            INFO-CODE  Purpose
            30         Invalid Query Type

   Note that this EDE code is generally applicable to any RR type that
   should not appear in DNS queries.

4.  Response Code Restoration

   For non-existent names, implementations should try wherever possible,
   to preserve the response code value of 3 (NXDOMAIN).  This is
   generally possible for non-DNSSEC enabled queries, namely those which
   do not set the DNSSEC_OK EDNS flag (DO bit).  For such queries,
   authoritative servers implementing Compact Denial of Existence could
   return a normal NXDOMAIN response.  There may be limited benefit to
   doing this however, since most modern DNS resolvers are DNSSEC-aware,
   and by [RFC3225] Section 3, DNSSEC-aware recursive servers are
   required to set the DO bit on outbound queries, regardless of the
   status of the DO bit on incoming requests.

   A validating resolver that understands the NXNAME signal from an
   authoritative server could modify the response code from NOERROR to
   NXDOMAIN in responses they return to downstream queriers that have
   not set the DO bit in their requests.

4.1.  Signaled Response Code Restoration

   This section describes an optional but recommended scheme to permit
   signaled restoration of the NXDOMAIN RCODE for DNSSEC enabled
   responses.  A new EDNS0 [RFC6891] header flag is defined in the 2nd
   most significant bit of the flags field in the EDNS0 OPT header.
   This flag is referred to as the "Compact Answers OK (CO)" flag.

                   +0 (MSB)                +1 (LSB)
            +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         0: |   EXTENDED-RCODE      |       VERSION         |
            +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         2: |DO|CO|                 Z                       |
            +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

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   When this flag is sent in a query by a resolver, it indicates that
   the resolver will accept a signed NXNAME enhanced NODATA response for
   a non-existent name together with the response code field set to
   NXDOMAIN (3).

   In responses to such queries, a Compact Denial authoritative server
   implementing this signaling scheme, will set the Compact Answers OK
   EDNS header flag, and for non-existent names will additionally set
   the response code field to NXDOMAIN.

   EDNS is a hop by hop signal, so resolvers will need to record the
   presence of this flag in associated cache data and respond to
   downstream DNSSEC enabled queriers appropriately.  If the querier
   does not set the Compact Answers OK flag, the resolver will need to
   reset the response code back to NOERROR (0) for an NXNAME response.

5.  Operational Implications

   For DNSSEC enabled queries, a signed zone at an authoritative server
   implementing Compact Answers will never return a response with a
   response code of NXDOMAIN, unless they have implemented the optional
   response code restoration feature described in Section 4.1.
   Similarly, resolvers not implementing this feature will also not be
   able to return NXDOMAIN.  In the absence of this, tools that rely on
   accurately determining non-existent names will need to infer them
   from the presence of the NXNAME RR type in the type bitmap of the
   NSEC record in NODATA responses from these servers.

   Address lookup functions typically invoked by applications may need
   to do more work when dealing with implementations of Compact Answers.
   For example, a NODATA response to the lookup of an AAAA record for a
   non-existent name, can cause such functions to issue another query at
   the same name for an A record.  Whereas an NXDOMAIN response to the
   first query would suppress additional queries for other types at that
   name.  Address lookup functions could be enhanced to issue DNSSEC
   enabled queries and to examine the NSEC type bitmaps in responses to
   accurately determine non-existent names.

   Protocol optimizations that permit DNS resolvers to synthesize
   NXDOMAIN responses, like [RFC8020] and [RFC8198], cannot be realized
   with zones using Compact Denial of Existence.  In general, no online
   signing scheme (including this one) that employs minimally covering
   NSEC records permits RFC 8198 style NXDOMAIN synthesis.
   Additionally, this protocol also precludes RFC 8020 style NXDOMAIN
   synthesis for DNSSEC enabled responses.

6.  Updates to RFCs

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6.1.  Updates to RFC 4034

   [RFC4034] Section 4.1.2, The Type Bit Maps Field, states the
   following:

   *  Bits representing pseudo-types MUST be clear, as they do not
      appear in zone data.  If encountered, they MUST be ignored upon
      being read.

   This paragraph is updated to the following:

   *  Bits representing pseudo-types MUST be clear, as they do not
      appear in zone data.  If encountered, they MUST be ignored upon
      being read.  There is one exception to this rule for Compact
      Denial of Existence (RFC TBD), where the NXNAME pseudo-type is
      allowed to appear in responses to non-existent names.

   Note: as a practical matter, no known resolver insists that pseudo-
   types must be clear in the NSEC type bitmap, so this restriction
   (prior to its revision) has posed no problem for the deployment of
   this mechanism.

6.2.  Updates to RFC 4035

   [RFC4035] Section 2.3, Including NSEC RRs in a Zone, states the
   following:

   *  An NSEC record (and its associated RRSIG RRset) MUST NOT be the
      only RRset at any particular owner name.  That is, the signing
      process MUST NOT create NSEC or RRSIG RRs for owner name nodes
      that were not the owner name of any RRset before the zone was
      signed.  The main reasons for this are a desire for namespace
      consistency between signed and unsigned versions of the same zone
      and a desire to reduce the risk of response inconsistency in
      security oblivious recursive name servers.

   This paragraph is updated to the following::

   *  An NSEC record (and its associated RRSIG RRset) MUST NOT be the
      only RRset at any particular owner name.  That is, the signing
      process MUST NOT create NSEC or RRSIG RRs for owner name nodes
      that were not the owner name of any RRset before the zone was
      signed.  The main reasons for this are a desire for namespace
      consistency between signed and unsigned versions of the same zone
      and a desire to reduce the risk of response inconsistency in
      security oblivious recursive name servers.  This concern only
      applies to implementations of DNSSEC that employ pre-computed
      signatures.  There is an exception to this rule for online signing

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      implementations of DNSSEC (e.g Minimally Covering NSEC, and
      Compact Denial of Existence (RFC TBD), where dynamically generated
      NSEC records can be produced for owner names that don't exist or
      are empty non-terminals.

7.  Implementation Status

   Cloudflare, NS1, and Amazon Route53 currently implement the Compact
   Denial of Existence method.  From early 2021 until November 2023, NS1
   had deployed the Empty Non-Terminal distinguisher [ENT-SENTINEL]
   using the private RR type code 65281.  A version of the NXNAME
   distinguisher using the private RR type code 65238 was deployed by
   both Cloudflare (from July 2023) and NS1 (from November 2023) until
   roughly September 2024.  Since September 2024 both Cloudflare and NS1
   have deployed NXNAME using the officially allocated code point of
   128.  At the current time, there are only prototype implementations
   of the signaled rcode restoration scheme.

8.  Security Considerations

   Online signing of DNS records requires authoritative servers for the
   DNS zone to have access to the private signing keys.  Exposing
   signing keys on Internet reachable servers makes them more vulnerable
   to attack.

   Additionally, generating signatures on-demand is more computationally
   intensive than returning pre-computed signatures.  Although the
   Compact Answers scheme reduces the number of online signing
   operations compared to previous techniques like White Lies, it still
   may make authoritative servers more vulnerable to computational
   denial of service attacks than pre-computed signatures.  The use of
   signature algorithms (like those based on Elliptic Curves) that have
   a comparatively low cost for signing is recommended.

   Some security tools rely on detection of non-existent domain names by
   examining the response code field of DNS response messages.  A
   NOERROR code in that field rather than NXDOMAIN will impact such
   tools.  Implementation of the optional response code restoration
   scheme will help recover NXDOMAIN visibility for these tools.  Note
   that the DNS header is not cryptographically protected, so the
   response code field cannot be authenticated.  Thus inferring the
   status of a response from signed data in the body of the DNS message
   is actually more secure.  These tools could be enhanced to recognize
   the (signed) NXNAME signal.

   Because this method does not allow for aggressive negative caching
   among resolvers, it allows for certain types of denial-of-service
   attacks to be more effective.  This includes so-called Water Torture

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   attacks, where random names are queried, either directly via botnets
   or across a wide range of public resolver services, in order to
   intentionally generate non-existence responses from the authoritative
   servers for a domain.  If the resolver cannot synthesize NXDOMAIN
   responses from NSEC records, it must pass all queries on to the
   domain's authority servers, making resource exhaustion more likely at
   those latter servers, if those servers do not have the capacity to
   absorb those additional queries.

   If the motivating aspects of this specification (minimizing response
   size and computational costs) are not a concern, DNSSEC deployments
   can opt for a different method (e.g.  traditional online signing or
   pre-computed signatures), and avoid imposing the challenges of
   NXDOMAIN visibility.

9.  Acknowledgements

   The Compact Answers technique (then called "Black Lies") was
   originally proposed in [BLACK-LIES] by Filippo Valsorda and Olafur
   Gudmundsson, and implemented by Cloudflare.  The Empty Non-Terminal
   distinguisher RR type was originally proposed in [ENT-SENTINEL] by
   Shumon Huque, and deployed by NS1.  The NXNAME type is based on the
   FDOM type proposed in [NXDOMAIN-TYPE] by Gudmundsson and Valsorda.

   Especially detailed discussions on many technical aspects of this
   document were conducted with Paul Vixie, Jan Vcelak, Viktor Dukhovni,
   and Ed Lewis.  The authors would also like to thank the many other
   members of the IETF DNS Operations working group for helpful comments
   and discussions.

10.  IANA Considerations

   IANA has done or is requested to do the following:

   Done: Allocate a new DNS Resource Record type code for NXNAME in the
   DNS parameters registry, from the meta type range.  Specifically, the
   lowest available number (currently 128) in the meta range is
   requested to be allocated.  A lower number lowers the size of the
   type bitmap, which reduces the size of the DNS response message.

         Type    Value  Meaning
         NXNAME  128  NXDOMAIN indicator for Compact Denial of Existence

   Allocate the "Compact Answers OK" flag in the EDNS header, as
   described in Section 4.1.

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   Done: Allocate a code point for the "Invalid Query Type" Extended DNS
   Error in the DNS parameters registry, as described in Section 3.4.
   Specifically code point 30 has been allocated.

11.  References

11.1.  Normative References

   [RFC3225]  Conrad, D., "Indicating Resolver Support of DNSSEC",
              RFC 3225, DOI 10.17487/RFC3225, December 2001,
              <https://www.rfc-editor.org/info/rfc3225>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
              and DNSSEC On-line Signing", RFC 4470,
              DOI 10.17487/RFC4470, April 2006,
              <https://www.rfc-editor.org/info/rfc4470>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC7129]  Gieben, R. and W. Mekking, "Authenticated Denial of
              Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
              February 2014, <https://www.rfc-editor.org/info/rfc7129>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,
              <https://www.rfc-editor.org/info/rfc8914>.

   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/info/rfc9364>.

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11.2.  Informative References

   [BLACK-LIES]
              Valsorda, F. and O. Gudmundsson, "Compact DNSSEC Denial of
              Existence or Black Lies", <https://tools.ietf.org/html/
              draft-valsorda-dnsop-black-lies>.

   [ENT-SENTINEL]
              Huque, S., "Empty Non-Terminal Sentinel for Black Lies",
              <https://www.ietf.org/archive/id/draft-huque-dnsop-
              blacklies-ent-01.html>.

   [NXDOMAIN-TYPE]
              Gudmundsson, O. and F. Valsorda, "Signaling NSEC record
              owner name nonexistence", <https://tools.ietf.org/html/
              draft-ogud-fake-nxdomain-type/>.

   [RFC4471]  Sisson, G. and B. Laurie, "Derivation of DNS Name
              Predecessor and Successor", RFC 4471,
              DOI 10.17487/RFC4471, September 2006,
              <https://www.rfc-editor.org/info/rfc4471>.

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <https://www.rfc-editor.org/info/rfc8020>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

Appendix A.  Other Approaches

   In the past, some implementations of Compact Denial of Existence have
   used other means to differentiate NXDOMAIN from Empty Non-Terminals.

   One method employed by both Cloudflare and Amazon Route53 for a
   period of time was the following: for responses to Empty Non-
   Terminals, they synthesized the NSEC type bitmap to include all
   record types supported except for the queried type.  This method has
   the undesirable effect of no longer being able to reliably determine
   the existence of ENTs, and of making the Type Bitmaps field larger
   than it needs to be.  It also has the potential to confuse validators
   and others tools that infer type existence from the NSEC record.

   Another way to distinguish NXDOMAIN from ENT is to define the
   synthetic Resource Record type for ENTs instead, as specified in
   [ENT-SENTINEL].  This method was successfully deployed in the field
   by NS1 for a period of time.  This typically imposes less work on the

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   server since NXDOMAIN responses are a lot more common than ENTs.  At
   the time it was deployed, it also allowed a common bitmap pattern
   ("NSEC RRSIG") to identify NXDOMAIN across this and other
   implementations that returned a broad bitmap pattern for Empty Non-
   Terminals.  However, the advantage of the NXNAME RR type is that it
   explicitly identifies NXDOMAIN responses, and allows them to be
   distinguished conclusively from potential ENT responses in other
   online signing NSEC implementations.

Authors' Addresses

   Shumon Huque
   Salesforce
   415 Mission Street, 3rd Floor
   San Francisco, CA 94105
   United States of America
   Email: shuque@gmail.com

   Christian Elmerot
   Cloudflare
   101 Townsend St.
   San Francisco, CA 94107
   United States of America
   Email: elmerot@cloudflare.com

   Olafur Gudmundsson
   Retired / Unaffiliated
   Email: ogud@ogud.com

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