Aggressive Use of DNSSEC-Validated Cache
draft-ietf-dnsop-nsec-aggressiveuse-10

Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Updates: 4035 (if approved)                                      A. Kato
Intended status: Standards Track                               Keio/WIDE
Expires: April 10, 2017                                        W. Kumari
                                                                  Google
                                                         October 7, 2016


                      Aggressive use of NSEC/NSEC3
                 draft-ietf-dnsop-nsec-aggressiveuse-04

Abstract

   The DNS relies upon caching to scale; however, the cache lookup
   generally requires an exact match.  This document specifies the use
   of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers
   to generate negative answers within a range, and positive answers
   from wildcards.  This increases performance / decreases latency,
   decreases resource utilization on both authoritative and recursive
   servers, and also increases privacy.  It may also help increase
   resilience to certain DoS attacks in some circumstances.

   This document updates RFC4035 by allowing validating resolvers to
   generate negative based upon NSEC/NSEC3 records (and positive answers
   in the presence of wildcards).

   [ Ed note: Text inside square brackets ([]) is additional background
   information, answers to frequently asked questions, general musings,
   etc.  They will be removed before publication.This document is being
   collaborated on in Github at: https://github.com/wkumari/draft-ietf-
   dnsop-nsec-aggressiveuse.  The most recent version of the document,
   open issues, etc should all be available here.  The authors
   (gratefully) accept pull requests.]

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 April 10, 2017.

Copyright Notice

   Copyright (c) 2016 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Aggressive Caching  . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  NSEC  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Wildcards . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.4.  Consideration on TTL  . . . . . . . . . . . . . . . . . .   7
   6.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Update to RFC 4035  . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Change History . . . . . . . . . . . . . . . . . . . . .  10
       11.1.1.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 .  12
       11.1.2.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 .  12
       11.1.3.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 .  13
     11.2.  new section  . . . . . . . . . . . . . . . . . . . . . .  13
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     12.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Detailed implementation notes  . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15




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1.  Introduction

   A DNS negative cache exists, and is used to cache the fact that a
   name does not exist.  This method of negative caching requires exact
   matching; this leads to unnecessary additional lookups, increases
   latency, leads to extra resource utilization on both authoritative
   and recursive servers, and decreases privacy by leaking queries.

   This document updates RFC 4035 to allow recursive resolvers to use
   NSEC/NSEC3 resource records to synthesize negative answers from the
   information they have in the cache.  This allows validating resolvers
   to respond with NXDOMAIN immediately if the name in question falls
   into a range expressed by a NSEC/NSEC3 resource record already in the
   cache.  It also allows the synthesis of positive answers in the
   presence of wildcard records.

   Aggressive Negative Caching was first proposed in Section 6 of DNSSEC
   Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
   records efficiently.

   Section 3 of [I-D.vixie-dnsext-resimprove] "Stopping Downward Cache
   Search on NXDOMAIN" and [I-D.ietf-dnsop-nxdomain-cut] proposed
   another approach to use NXDOMAIN information effectively.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Many of the specialized terms used in this document are defined in
   DNS Terminology [RFC7719].

   The key words "Closest Encloser" and "Source of Synthesis" in this
   document are to be interpreted as described in [RFC4592].

   "Closest Encloser" is also defined in NSEC3 [RFC5155], as is "Next
   closer name".

3.  Problem Statement

   The DNS negative cache caches negative (non-existent) information,
   and requires an exact match in most instances [RFC2308].

   Assume that the (DNSSEC signed) "example.com" zone contains:






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   apple.example.com    IN A 192.0.2.1
   elephant.example.com IN A 192.0.2.2
   *.example.com        IN A 192.0.2.3
   zebra.example.com    IN A 192.0.2.4

   If a validating resolver receives a query for cat.example.com, it
   contacts its resolver (which may be itself) to query the example.com
   servers and will get back an NSEC record starting that there are no
   records (alphabetically) between apple and elephant, or an NSEC3
   record stating there is nothing between two hashed names.  The
   resolver then knows that cat.example.com does not exist; however, it
   does not use the fact that the proof covers a range (apple to
   elephant) to suppress queries for other labels that fall within this
   range.  This means that if the validating resolver gets a query for
   ball.example.com (or dog.example.com) it will once again go off and
   query the example.com servers for these names.

   Further, if a query is received for lion.example.com, it contacts its
   resolver (which may be itself) to query the example.com servers and
   will get back an NSEC record stating that there are no records
   (alphabetically) between elephant and zebra (or an NSEC3 record
   stating there is nothing between two hashed names), as well as an
   answer for lion.example.com, with the label count of the signature
   set to two (see [RFC7129], section 5.3 for more details).

   Apart from wasting bandwidth, this also wastes resources on the
   recursive server (it needs to keep state for outstanding queries),
   wastes resources on the authoritative server (it has to answer
   additional questions), increases latency (the end user has to wait
   longer than necessary to get back an NXDOMAIN answer), can be used by
   attackers to cause a DoS (see additional resources), and also has
   privacy implications (e.g: typos leak out further than necessary).

4.  Background

   DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of
   existence"; this is a cryptographic proof that the queried for name
   does not exist, accomplished by providing a (DNSSEC secured) record
   containing the names which appear alphabetically before and after the
   queried for name.  In the example above, if the (DNSSEC validating)
   recursive server were to query for dog.example.com it would receive a
   (signed) NSEC record stating that there are no labels between "apple"
   and "elephant" (or, for NSEC3, a similar pair of hashed names).  This
   is a signed, cryptographic proof that these names are the ones before
   and after the queried for label.  As dog.example.com falls within
   this range, the recursive server knows that dog.example.com really
   does not exist.




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   This document specifies that this NSEC/NSEC3 record should be used to
   generate negative answers for any queries that the validating server
   receives that fall within the range covered by the record (for the
   TTL for the record).  This document also specifies that a positive
   answer should be generated for any queries that the validating server
   receives that are proven to be covered by a wildcard record.

   Section 4.5 of [RFC4035] says:

   "In theory, a resolver could use wildcards or NSEC RRs to generate
   positive and negative responses (respectively) until the TTL or
   signatures on the records in question expire.  However, it seems
   prudent for resolvers to avoid blocking new authoritative data or
   synthesizing new data on their own.  Resolvers that follow this
   recommendation will have a more consistent view of the namespace."
   and "The reason for these recommendations is that, between the
   initial query and the expiration of the data from the cache, the
   authoritative data might have been changed (for example, via dynamic
   update).".  In other words, if a resolver generates negative answers
   from an NSEC record, it will not send any queries for names within
   that NSEC range (for the TTL).  If a new name is added to the zone
   during this interval the resolver will not know this.  Similarly, if
   the resolver is generating responses from a wildcard record, it will
   continue to do so (for the

   We believe this recommendation can be relaxed because, in the absense
   of this technique, a lookup for the exact name could have come in
   during this interval, and so a negative answer could already be
   cached (see [RFC2308] for more background).  This means that zone
   operators should have no expectation that an added name would work
   immediately.  With DNSSEC and Aggressive NSEC, the TTL of the NSEC
   record is the authoritative statement of how quickly a name can start
   working within a zone.

5.  Aggressive Caching

   Section 4.5 of [RFC4035] says that "In theory, a resolver could use
   wildcards or NSEC RRs to generate positive and negative responses
   (respectively) until the TTL or signatures on the records in question
   expire.  However, it seems prudent for resolvers to avoid blocking
   new authoritative data or synthesizing new data on their own.
   Resolvers that follow this recommendation will have a more consistent
   view of the namespace".

   This document relaxes this this restriction, as follows:






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   +--------------------------------------------------------------+
   |  Once the records are validated, DNSSEC enabled validating   |
   |  resolvers MAY use wildcards and NSEC/NSEC3 resource records |
   |  to generate positive and negative responses until the       |
   |  effective TTLs or signatures for those records expire.      |
   +--------------------------------------------------------------+

   If the validating resolver's cache has sufficient information to
   validate the query, the resolver SHOULD use NSEC/NSEC3/wildcard
   records aggressively.  Otherwise, it MUST fall back to send the query
   to the authoritative DNS servers.

5.1.  NSEC

   Implementations which support aggressive use of NSEC SHOULD enable
   this by default.  Implementations MAY provide a configuration switch
   to disable aggressive use of NSEC and allow it to be enabled or
   disabled per domain.

   The validating resolver needs to check the existence of an NSEC RR
   matching/covering the source of synthesis and an NSEC RR covering the
   query name.

   If denial of existence can be determined according to the rules set
   out in Section 5.4 of [RFC4035], using NSEC records in the cache,
   then the resolver can immediately return an NXDOMAIN or NODATA (as
   appropriate) response.

5.2.  NSEC3

   NSEC3 aggressive negative caching is more difficult than NSEC
   aggressive caching.  If the zone is signed with NSEC3, the validating
   resolver needs to check the existence of non-terminals and wildcards
   which derive from query names.

   A validating resolver implementation MAY support aggressive use of
   NSEC3.  If it does support aggressive use of NSEC3, it SHOULD enable
   this by default.  It MAY provide a configuration switch to disable
   aggressive use of NSEC3 and allow it to be enabled or disabled for
   specific zones.

   If denial of existence can be determined according to the rules set
   out in [RFC5155] sections 8.4, 8.5, 8.6, 8.7,using NSEC3 records in
   the cache, then the resolver can immediately return an NXDOMAIN or
   NODATA response (as appropriate).






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   If a covering NSEC3 RR has Opt-Out flag, the covering NSEC3 RR does
   not prove the non-existence of the domain name and the aggressive
   negative caching is not possible for the domain name.

5.3.  Wildcards

   The last paragraph of [RFC4035] Section 4.5 also discusses the use of
   wildcards and NSEC RRs to generate positive responses and recommends
   that it not be relied upon.  Just like the case for the aggressive
   use of NSEC/NSEC3 for negative answers, we revise this
   recommendation.

   As long as the validating resolver can determine that a name would
   not exist without the wildcard match, it MAY synthesize an answer for
   that name using the cached deduced wildcard.  If the corresponding
   wildcard record is not in the cache, it MUST fall back to send the
   query to the authoritative DNS servers.

   An implementation MAY support aggressive use of wildcards.  It SHOULD
   provide a configuration switch to disable aggressive use of
   wildcards.

5.4.  Consideration on TTL

   The TTL value of negative information is especially important,
   because newly added domain names cannot be used while the negative
   information is effective.

   Section 5 of [RFC2308] states that the maximum number of negative
   cache TTL value is 3 hours (10800).  It is RECOMMENDED that
   validating resolvers limit the maximum effective TTL value of
   negative responses (NSEC/NSEC3 RRs) to this same value.

   Section 5 of [RFC2308]also states that a negative cache entry TTL is
   taken from the minimum of the SOA.MINIMUM field and SOA's TTL.  This
   can be less than the TTL of an NSEC or NSEC3 record, since their TTL
   is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and
   [RFC5155] section 3.)

   A resolver that supports aggressive use of NSEC and NSEC3 should
   reduce the TTL of NSEC and NSEC3 records to match the TTL of the SOA
   record in the authority section of a negative response, if the SOA
   TTL is smaller.








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6.  Benefits

   The techniques described in this document provide a number of
   benefits, including (in no specific order):

   Reduced latency:  By answering directly from cache, validating
      resolvers can immediately inform clients that the name they are
      looking for does not exist, improving the user experience.

   Decreased recursive server load:  By answering negative queries from
      the cache, validating servers avoid having to send a query and
      wait for a response.  In addition to decreasing the bandwidth
      used, it also means that the server does not need to allocate and
      maintain state, thereby decreasing memory and CPU load.

   Decreased authorative server load:  Because recursive servers can
      answer (negative) queries without asking the authoritative server,
      the authoritative servers receive fewer queries.  This decreases
      the authoritative server bandwidth, queries per second and CPU
      utilization.

   The scale of the benefit depends upon multiple factors, including the
   query distribution.  For example, at the time of this writing, around
   65% of queries to Root Name servers result in NXDOMAIN responses (see
   statis [root-servers.org]); this technique will eliminate a sizable
   quantity of these.

   The technique described in this document may also mitigate so-called
   "random QNAME attacks", in which attackers send many queries for
   random sub-domains to resolvers.  As the resolver will not have the
   answers cached, it has to ask external servers for each random query,
   leading to a DoS on the authoritative servers (and often resolvers).
   Aggressive NSEC may help mitigate these attacks by allowing the
   resolver to answer directly from cache for any random queries which
   fall within already requested ranges.  It will not always work as an
   effective defense, not least because not many zones are DNSSEC signed
   at all -- but it will still provide an additional layer of defense.

7.  Update to RFC 4035

   Section 4.5 of [RFC4035] shows that "In theory, a resolver could use
   wildcards or NSEC RRs to generate positive and negative responses
   (respectively) until the TTL or signatures on the records in question
   expire.  However, it seems prudent for resolvers to avoid blocking
   new authoritative data or synthesizing new data on their own.
   Resolvers that follow this recommendation will have a more consistent
   view of the namespace".




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   The paragraph is updated as follows:

   +--------------------------------------------------------------+
   |  Once the records are validated, DNSSEC enabled validating   |
   |  resolvers MAY use wildcards and NSEC/NSEC3 resource records |
   |  to generate positive and negative responses until the       |
   |  effective TTLs or signatures for those records expire.      |
   +--------------------------------------------------------------+

8.  IANA Considerations

   This document has no IANA actions.

9.  Security Considerations

   Use of NSEC / NSEC3 resource records without DNSSEC validation may
   create serious security issues, and so this technique requires DNSSEC
   validation.

   Newly registered resource records may not be used immediately.
   However, choosing suitable TTL value and negative cache TTL value
   (SOA MINIMUM field) will mitigate the delay concern, and it is not a
   security problem.

   It is also suggested to limit the maximum TTL value of NSEC / NSEC3
   resource records in the negative cache to, for example, 10800 seconds
   (3hrs), to mitigate this issue.  Implementations which comply with
   this proposal are recommended to have a configurable maximum value of
   NSEC RRs in the negative cache.

   Although the TTL of NSEC/NSEC3 records is typically fairly short
   (minutes or hours), their RRSIG expiration time can be much further
   in the future (weeks).  An attacker who is able to successfully spoof
   responses might poison a cache with old NSEC/NSEC3 records.  If the
   resolver is NOT making aggressive use of NSEC/NSEC3, the attacker has
   to repeat the attack for every query.  If the resolver IS making
   aggressive use of NSEC/NSEC3, one successful attack would be able to
   suppress many queries for new names, up to the negative TTL.

10.  Implementation Status

   Unbound currenty implements aggressive negative caching, as does
   Google Public DNS.








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11.  Acknowledgments

   The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
   and the Unbound developers.

   The authors would like to specifically thank Stephane Bortzmeyer,
   Tony Finch, Tatuya JINMEI for extensive review and comments, and also
   Mark Andrews, Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob
   Harold, Shumon Huque, John Levine, Pieter Lexis and Matthijs Mekking
   (who even sent pull requests!).

11.1.  Change History

   RFC Editor: Please remove this section prior to publication.

   -03 to -04:

   o  Working group does want the "positive" answers, not just negative
      ones.  This requires readding what used to be Section 7, and a
      bunch of cleanup, including:

      *  Additional text in the Problem Statement

      *  Added a wildcard record to the zone.

      *  Added "or positive answers from wildcards" type text (where
         appropriate) to explain that this isn't just for negative
         answers.

      *  Reworded much of the Wildcard text.

   o  Incorporated pull request from Tony Finch (thanks!):
      https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/
      pull/1

   o  More fixups from Tony (including text): https://www.ietf.org/mail-
      archive/web/dnsop/current/msg18271.html.  This included much
      clearer text on TTL, refernces to the NSEC / NSEC3 RFCs (instead
      of my clumsy summary), good text on replays, etc.

   o  Converted the "zone file" to a figure to make it more readable.

   o  Text from Tim W: "If a validating resolver receives a query for
      cat.example.com, it contacts its resolver (which may be itself) to
      query..." - which satisfies Jinmei's concern (which I was too
      dense to grock).

   o  Fixup of the "validation required" in security considerations.



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   -02 to -03:

   o  Integrated a bunch of comments from Matthijs Mekking - details in:
      https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/
      pull/1.  I decided to keep "Aggressive Negative Caching" instead
      of "Aggressive USE OF Negative Caching" for readability.

   o  Attempted to address Bob Harold's comment on the readability
      issues with "But, it will be more effective when both are
      enabled..." in Section 5.4 - https://www.ietf.org/mail-
      archive/web/dnsop/current/msg17997.html

   o  MAYs and SHOULD drifted in the text block.  Fixed - thanks to
      https://mailarchive.ietf.org/arch/msg/
      dnsop/2ljmmzxtIMCFMLOZmWcSbTYVOy4

   o  A number of good edits from Stephane in: https://www.ietf.org/
      mail-archive/web/dnsop/current/msg18109.html

   o  A bunch more edits from Jinmei, as in: https://www.ietf.org/mail-
      archive/web/dnsop/current/msg18206.html

   -01 to -02:

   o  Added Section 6 - Benefits (as suggested by Jinmei).

   o  Removed Appendix B (Jinmei)

   o  Replaced "full-service" with "validating" (where applicable)

   o  Integrated other comments from Jinmei from https://www.ietf.org/
      mail-archive/web/dnsop/current/msg17875.html

   o  Integrated comment from co-authors, including re-adding parts of
      Appendix B, terminology, typos.

   o  Tried to explain under what conditions this may actually mitigate
      attacks.

   -00 to -01:

   o  Comments from DNSOP meeting in Berlin.

   o  Changed intended status to Standards Track (updates RFC 4035)

   o  Added a section "Updates to RFC 4035"

   o  Some language clarification / typo / cleanup



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   o  Cleaned up the TTL section a bit.

   o  Removed Effects section, Additional proposal section, and pseudo
      code.

   o  Moved "mitigation of random subdomain attacks" to Appendix.

   From draft-fujiwara-dnsop-nsec-aggressiveuse-03 -> draft-ietf-dnsop-
   nsec-aggressiveuse

   o  Document adopted by DNSOP WG.

   o  Adoption comments

   o  Changed main purpose to performance

   o  Use NSEC3/Wildcard keywords

   o  Improved wordings (from good comments)

   o  Simplified pseudo code for NSEC3

   o  Added Warren as co-author.

   o  Reworded much of the problem statement

   o  Reworked examples to better explain the problem / solution.

11.1.1.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-01

   o  Added reference to DLV [RFC5074] and imported some sentences.

   o  Added Aggressive Negative Caching Flag idea.

   o  Added detailed algorithms.

11.1.2.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-02

   o  Added reference to [I-D.vixie-dnsext-resimprove]

   o  Added considerations for the CD bit

   o  Updated detailed algorithms.

   o  Moved Aggressive Negative Caching Flag idea into Additional
      Proposals





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11.1.3.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-03

   o  Added "Partial implementation"

   o  Section 4,5,6 reorganized for better representation

   o  Added NODATA answer in Section 4

   o  Trivial updates

   o  Updated pseudo code

11.2.  new section

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <http://www.rfc-editor.org/info/rfc2308>.

   [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,
              <http://www.rfc-editor.org/info/rfc4035>.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
              <http://www.rfc-editor.org/info/rfc4592>.

   [RFC5074]  Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
              DOI 10.17487/RFC5074, November 2007,
              <http://www.rfc-editor.org/info/rfc5074>.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <http://www.rfc-editor.org/info/rfc5155>.

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



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   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <http://www.rfc-editor.org/info/rfc7719>.

12.2.  Informative References

   [I-D.ietf-dnsop-nxdomain-cut]
              Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there
              is nothing underneath", draft-ietf-dnsop-nxdomain-cut-03
              (work in progress), May 2016.

   [I-D.vixie-dnsext-resimprove]
              Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
              Resolvers for Resiliency, Robustness, and Responsiveness",
              draft-vixie-dnsext-resimprove-00 (work in progress), June
              2010.

   [root-servers.org]
              IANA, "Root Server Technical Operations Assn",
              <http://www.root-servers.org/>.

Appendix A.  Detailed implementation notes

   o  Previously, cached negative responses were indexed by QNAME,
      QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035,
      Section 4.7), and only queries matching the index key would be
      answered from the cache.  With aggressive negative caching, the
      validator, in addition to checking to see if the answer is in its
      cache before sending a query, checks to see whether any cached and
      validated NSEC record denies the existence of the sought
      record(s).  Using aggressive negative caching, a validator will
      not make queries for any name covered by a cached and validated
      NSEC record.  Furthermore, a validator answering queries from
      clients will synthesize a negative answer whenever it has an
      applicable validated NSEC in its cache unless the CD bit was set
      on the incoming query.  (Imported from Section 6 of [RFC5074]).

   o  Implementing aggressive negative caching suggests that a validator
      will need to build an ordered data structure of NSEC and NSEC3
      records for each signer domain name of NSEC / NSEC3 records in
      order to efficiently find covering NSEC / NSEC3 records.  Call the
      table as NSEC_TABLE.  (Imported from Section 6.1 of [RFC5074] and
      expanded.)

   o  The aggressive negative caching may be inserted at the cache
      lookup part of the recursive resolvers.





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   o  If errors happen in aggressive negative caching algorithm,
      resolvers MUST fall back to resolve the query as usual.  "Resolve
      the query as usual" means that the resolver must process the query
      as though it does not implement aggressive negative caching.

Authors' Addresses

   Kazunori Fujiwara
   Japan Registry Services Co., Ltd.
   Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
   Chiyoda-ku, Tokyo  101-0065
   Japan

   Phone: +81 3 5215 8451
   Email: fujiwara@jprs.co.jp


   Akira Kato
   Keio University/WIDE Project
   Graduate School of Media Design, 4-1-1 Hiyoshi
   Kohoku, Yokohama  223-8526
   Japan

   Phone: +81 45 564 2490
   Email: kato@wide.ad.jp


   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: warren@kumari.net

















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