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Versions: (draft-fujiwara-dnsop-nsec-aggressiveuse) 00 01 02 03 04 05 06 07 08 09 10 RFC 8198

Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Updates: 4035 (if approved)                                      A. Kato
Intended status: Standards Track                               Keio/WIDE
Expires: November 25, 2017                                     W. Kumari
                                                                  Google
                                                            May 24, 2017


                Aggressive use of DNSSEC-validated Cache
                 draft-ietf-dnsop-nsec-aggressiveuse-10

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 answers 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.  RFC Editor, please remove 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 November 25, 2017.

Copyright Notice

   Copyright (c) 2017 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 use of Cache . . . . . . . . . . . . . . . . . . .   6
     5.1.  NSEC  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Wildcards . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Consideration on TTL  . . . . . . . . . . . . . . . . . .   7
   6.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Update to RFC 4035  . . . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
     11.1.  Change History . . . . . . . . . . . . . . . . . . . . .  10
       11.1.1.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 .  13
       11.1.2.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 .  13
       11.1.3.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Detailed implementation notes  . . . . . . . . . . .  15
   Appendix B.  Procedure for determining ENT vs NXDOMAN with NSEC .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16




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1.  Introduction

   A DNS negative cache exists, and is used to cache the fact that an
   RRset 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 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 a negative answer 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.

   [RFC8020] and [I-D.vixie-dnsext-resimprove] propose steps to using
   NXDOMAIN information for more effective caching.  This document takes
   this technique further.

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 "Source of Synthesis" in this document are to be
   interpreted as described in [RFC4592].

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:

   albatross.example.com IN A 192.0.2.1
   elephant.example.com  IN A 192.0.2.2
   zebra.example.com     IN A 192.0.2.3





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   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 stating that there are no
   records (alphabetically) between albatross 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 (albatross 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.

   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).

   Another example: assume that the (DNSSEC signed) "example.org" zone
   contains:

   avocado.example.org   IN A 192.0.2.1
   *.example.org         IN A 192.0.2.2
   zucchini.example.org IN A 192.0.2.3

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

   If the validating resolver gets a query for banana.example.org it
   will once again go off and query the example.org servers for
   banana.example.org (even though it already has proof that there is a
   wildcard record) - just like above, this has privacy implications,
   wastes resources, can be used to contribute to a DoS, etc.

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 or type does not exist.  Proof that a name does not
   exist is accomplished by providing a (DNSSEC secured) record
   containing the names which appear alphabetically before and after the



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   queried for name.  In the first 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 "albatross" 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.  Proof that a type does
   not exist is accomplished by providing a (DNSSEC secured) record
   containing the queried for name, and a type bitmap which does not
   include the requested type.

   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 TTL).

   We believe this recommendation can be relaxed because, in the absence
   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/
   NSEC3 record and the SOA.MINIMUM field are the authoritative
   statement of how quickly a name can start working within a zone.






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5.  Aggressive use of Cache

   This document relaxes the restriction given in Section 4.5 of
   [RFC4035], see Section 7 for more detail.

   If the negative cache of the validating resolver has sufficient
   information to validate the query, the resolver SHOULD use NSEC,
   NSEC3 and wildcard records to synthesize answers as described in this
   document.  Otherwise, it MUST fall back to send the query to the
   authoritative DNS servers.

5.1.  NSEC

   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.

   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).

   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, determined according to the



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   rules set out in Section 5.3.4 of [RFC4035] (NSEC), or in Section 8.8
   of [RFC5155], it SHOULD synthesize an answer (or NODATA response) 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.

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] suggests a maximum default negative cache TTL
   value of 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 SOA.MINIMUM
   field in the authority section of a negative response, if SOA.MINIMUM
   is smaller.

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 queries from the cache
      by synthesizing answers, 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 authoritative server load:  Because recursive servers can
      answer queries without asking the authoritative server, the
      authoritative servers receive fewer queries.  This decreases the




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      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
   statistics from [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.

   As these benefits are only accrued by those using DNSSEC, it is hoped
   that these techniques will lead to more DNSSEC deployment.

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".

   The paragraph is updated as follows:

   +-----------------------------------------------------------------+
   |  Once the records are validated, DNSSEC enabled validating      |
   |  resolvers SHOULD 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.






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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.

   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

   [ Editor note: RFC Editor, please remove this entire section.
   RFC6982 says: "Since this information is necessarily time dependent,
   it is inappropriate for inclusion in a published RFC." ]

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

11.  Acknowledgments

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

   Thanks to Mark Andrews for providing the helpful notes for
   implementors provided in Appendix B.

   The authors would like to specifically thank Stephane Bortzmeyer (for
   standing next to and helping edit), Ralph Dolmans, 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, Matthijs Mekking (who even sent
   pull requests!) and Ondrej Sury.




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11.1.  Change History

   RFC Editor: Please remove this section prior to publication.

   -09 to -10:

   o  Addressed IESG comments at https://datatracker.ietf.org/doc/draft-
      ietf-dnsop-nsec-aggressiveuse/ballot/

   o  Main change "the resolver SHOULD use NSEC, NSEC3 and wildcard
      records aggressively." -> "HOULD use NSEC, NSEC3 and wildcard
      records to synthesize answers as described in this document"
      (Mirja) - aggressively wasn't really described...

   -08 to -09:

   o  Made RFC5074 Informative (after discussions with chairs.

   o  Addressed SecDir comments.

   o  Addressed OpsDir comments.

   -06 to -08:

   o  Largely editorial, but please see the diffs (editors forgot to
      update change log when editing, backfilling change log.)

   o  Changed "replacement" text to be "DNSSEC enabled validating
      resolvers SHOULD use wildcards ..." to align with text in doc.

   o  "A resolver that supports aggressive use of NSEC and NSEC3 SHOULD"
      (should -> SHOULD) - to align with rest of text.

   -05 to -06:

   o  Moved some dangling text around - when the examples were added
      some text added in the wrong place.

   o  There were some bits which mentioned "negative" in the title.

   o  We had the cut-and-paste of what changed in 4035 twice.

   o  Clarified that this also allows NODATA responses to be
      synthesized.

   -04 to -05:





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   o  Bob pointed out that I did a stupid - when I added the wildcard to
      'example.com' I made the example wrong / confusing.  I have
      attempted to fix this by adding a second example zone
      (example.org) with the wildcard instead.

   o  More helpful changes (in a pull request, thanks!) from Matthijs

   o  Included Mark Andrew's useful explanation of how to tell ENT from
      NXD as an Appendix.

   -03 to -04:

   o  Working group does want the "positive" answers, not just negative
      ones.  This requires reading 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, references 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.

   -02 to -03:

   o  Integrated a bunch of comments from Matthijs Mekking - details in:
      https://github.com/wkumari/draft-ietf-dnsop-nsec-aggressiveuse/



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      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

   o  Cleaned up the TTL section a bit.





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   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

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>.

   [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>.

   [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>.





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12.2.  Informative References

   [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.

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

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

   [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 (or NODATA response)
      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.

Appendix B.  Procedure for determining ENT vs NXDOMAN with NSEC

   This procedure outlines how to determine if a given name does not
   exist, or is an ENT (Empty Non-Terminal, see [RFC5155] Section 1.3)
   with NSEC.

   If the NSEC record has not been verified as secure discard it.

   If the given name sorts before or matches the NSEC owner name discard
   it as it does not prove the NXDOMAIN or ENT.

   If the given name is a subdomain of the NSEC owner name and the NS
   bit is present and the SOA bit is absent then discard the NSEC as it
   is from a parent zone.

   If the next domain name sorts after the NSEC owner name and the given
   name sorts after or matches next domain name then discard the NSEC
   record as it does not prove the NXDOMAIN or ENT.

   If the next domain name sorts before or matches the NSEC owner name
   and the given name is not a subdomain of the next domain name then
   discard the NSEC as it does not prove the NXDOMAIN or ENT.

   You now have a NSEC record that proves the NXDOMAIN or ENT.

   If the next domain name is a subdomain of the given name you have a
   ENT otherwise you have a NXDOMAIN.

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








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   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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