Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Updates: 4035 (if approved)                                      A. Kato
Intended status: Informational Standards Track                               Keio/WIDE
Expires: December 29, 2016 February 3, 2017                                      W. Kumari
                                                                  Google
                                                           June 27,
                                                         August 02, 2016

                      Aggressive use of NSEC/NSEC3
                 draft-ietf-dnsop-nsec-aggressiveuse-00
                 draft-ietf-dnsop-nsec-aggressiveuse-01

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 generate negative answers within a
   range.  This increases resilience to DoS attacks, increases
   performance / decreases latency, decreases resource utilization on
   both authoritative and recursive servers, and also increases privacy.

   This document updates RFC4035 by allowing resolvers to generate
   negative answers based upon NSEC/NSEC3 records.

   [ 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.

   Known / open issues [To be moved to Github issue tracker]:

   1.  We say things like: "Currently the DNS does ..." - this will not
       be true after this is deployed, but I'm having a hard time
       rewording this.  "Without the techniques described in this
       document..." seems klunky.  Perhaps "historically?!"

   2.  We currently say this SHOULD be enabled by default.  Is that what
       the working group wants, or should this be an implementation
       choice?

   ]

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
   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 December 29, 2016. February 3, 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4   3
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Proposed Solution . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Aggressive Negative Caching . . . . . . . . . . . . . . .   5
     5.2.  NSEC  . . . . . . . . . . . . . . . . . . . . . . . . . .   6   5
     5.3.  NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Wildcard  . . . . . . . . . . . . . . . . . . . . . . . .   7   6
     5.5.  Consideration on TTL  . . . . . . . . . . . . . . . . . .   7
   6.  Effects . . . . . . . . . . . . . . . . .  Update to RFC 4035  . . . . . . . . . .   7
     6.1.  Decrease of root DNS server queries . . . . . . . . . . .   7
     6.2.  Mitigation of random subdomain attacks  . . . . . . . . .   8
   7.  Additional proposals  . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Partial implementation  . . . . . . . . . . . . . . . . .   8
     7.2.  Aggressive negative caching flag idea . . . . . . . . . .   9
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   9.   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10.   7
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   11.   8
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   12.   8
   11. Change History  . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.   8
     11.1.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . . .  10
     12.2.   9
     11.2.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . . .  10
     12.3.   9
     11.3.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . . .  11
   13.   9
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     13.1.   9
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     13.2.  10
     12.2.  Informative References . . . . . . . . . . . . . . . . .  12  10
   Appendix A.  Aggressive negative caching from RFC 5074  . . . . .  12
   Appendix B.  Detailed implementation idea . . . . . . . . . . . .  13  11
   Appendix B.  Side effect: mitigation of random subdomain attacks   11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15  12

1.  Introduction

   A DNS negative cache currently 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, which
   have negative implications for DoS survivability, 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 aggressively cache negative answers.
   This would allow such 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.

   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 current 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:

      apple.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

   If a recursive resolver gets a query for cat.example.com, it will
   query the example.com authoritative servers and will get back an NSEC
   (or NSEC3) record starting that there are no records between apple
   and elephant.  The recursive resolver then knows that cat.example.com
   does not exist; however, it (currently) 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
   recursive resolvers 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).

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 lion.example.com it would receive
   a (signed) NSEC/NSEC3 record stating that there are no labels between
   "elephant" and "zebra".  This is a signed, cryptographic proof that
   these names are the ones before and after the queried for label.  As
   lion.example.com falls within this range, the recursive server knows
   that lion.example.com really does not exist.  This document specifies
   that this NSEC/NSEC3 record should be used to generate negative
   answers for any queries that the recursive server receives that fall
   within the range covered by the record (for the TTL for the record).

   [RFC4035]; Section 4.5 states:

   For a zone signed with NSEC, it would be possible to use the
   information carried in NSEC resource records to indicate the non-
   existence of a range of names.  However, such use is discouraged by
   Section 4.5 of RFC4035.  It is recommended that readers read RFC4035
   in its entirety for a better understanding.  At the root of the
   concern is that new records could have been added to the zone during
   the TTL of the NSEC record, and that generating negative responses
   from the NSEC record would hide these.  We believe this
   recommendation can be relaxed because lookups for the specific name
   could have come in during the normal negative cache time and so
   operators should have no expectation that an added name would work
   immediately.  We think that the TTL of the NSEC record is the
   authoritive
   authoritative statement of how quickly a name can start working
   within a zone.

5.  Proposed Solution

5.1.  Aggressive Negative Caching

   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".

   To reduce non-existent queries sent to authoritative DNS servers,

   This document relaxes this restriction could be relaxed, this restriction, as follows:

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

   If the full-service resolver's cache have enough has sufficient information to
   validate the query, the full-service resolver MAY use NSEC/NSEC3/
   wildcard records aggressively.  Otherwise, the full-service resolver
   MUST fall back to send the query to the authoritative DNS servers.

   If the query name has the matching NSEC/NSEC3 RR and it proves proving the
   information requested does not exist, the full-service resolver may
   respond with a NODATA (empty) answer.

5.2.  NSEC

   If a full-service resolver implementation supports aggressive
   negative caching, then it SHOULD support aggressive use of NSEC and
   enable it by default.  It SHOULD provide a configuration switch to
   disable aggressive use of NSEC and allow it to be enabled or disabled
   for specific zones.

   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 the full-service resolver's cache contains an NSEC RR covering the
   source of synthesis and the covering NSEC RR of the query name, the
   full-service resolver may respond with NXDOMAIN error immediately.

5.3.  NSEC3

   NSEC3 aggressive negative caching is more difficult.  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 the full-service resolver's cache contains an NSEC3 RR matching
   the closest encloser, an NSEC3 RR covering the next closer name, and
   an NSEC3 RR covering the source of synthesis, it is possible for the
   full-service resolver to respond with NXDOMAIN immediately.

   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.

   A full-service resolver implementation MAY support aggressive use of
   NSEC3.  It  If it does aggressive use of NSEC3, it SHOULD provide a
   configuration switch to disable aggressive use of NSEC3 and allow it
   to be enabled or disabled for specific zones.

5.4.  Wildcard

   The last paragraph of RFC 4035 Section 4.5 discusses aggressive use
   of a cached deduced wildcard (as well as aggressive use of NSEC) and
   recommends that it is not relied upon.

   Just like the case for the aggressive use of NSEC discussed in this
   draft, we could revisit revise this recommendation.  As long as the full-
   service full-service
   resolver knows a name would not exist without the wildcard match, it could
   can answer a query for that name using the cached deduced wildcard,
   and it may be justified for performance and other benefits.  (Note
   that, so far, this is orthogonal to "when aggressive use (of NSEC) is
   enabled").

   Furthermore, when aggressive use of NSEC is enabled, the aggressive
   use of cached deduced wildcard will be more effective.

   A full-service resolver implementation MAY support aggressive use of
   wildcards.  It SHOULD provide a configuration switch to disable
   aggressive use of wildcards.

5.5.  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 RFC 2308 states that the
   maximum number of negative cache TTL value is 3 hours (10800).  So the full-
   service resolver SHOULD  It is
   RECOMMENDED that full-service resolvers limit the maximum effective
   TTL value of negative responses (NSEC/NSEC3 RRs) to 10800 (3 hours).  It is
   reasonably small but still effective for the purpose of this
   document, since it can eliminate significant amount of DNS attacks
   with randomly generated names. same value.

6.  Effects

6.1.  Decrease of root DNS server queries

   Aggressive use  Update to RFC 4035

   Section 4.5 of NSEC/NSEC3 resource records results in [RFC4035] shows that "In theory, a decrease
   of queries resolver could use
   wildcards or NSEC RRs to generate positive and negative responses
   (respectively) until the root - this decreases load TTL or signatures on the root servers (the
   majority of queries currently result in NXDOMAIN responses), and
   increases privacy.

   People may generate many typos in TLD, and they will result records in
   unnecessary DNS queries.  Some implementations leak non-existent TLD
   queries whose second level domain are different each other.  Well
   observed examples are ".local" and ".belkin".  With this proposal, question
   expire.  However, it
   is possible to return NXDOMAIN immediately to such queries without
   further DNS recursive resolution process.  It may reduces round trip
   time, as well as reduces the DNS queries to corresponding
   authoritative servers, including Root DNS servers.

6.2.  Mitigation of random subdomain attacks

   Random sub-domain attacks (referred to as "Water Torture" attacks or
   NXDomain attacks) send many queries seems prudent for non-existent information to
   full-service resolvers.  Their query names consist of random prefixes
   and a target domain name.  The negative cache does not work well, and
   thus targeted full-service resolvers end up sending queries to avoid blocking
   new authoritative DNS servers of the target domain name.

   When the number of queries is large, the full-service resolvers drop
   queries from both legitimate users and attackers as data or synthesizing new data on their outstanding
   queues are filled up.

   For example, BIND 9.10.2 [BIND9] full-service resolvers answer
   SERVFAIL and Unbound 1.5.2 full-service resolvers drop most of
   queries under 10,000 queries per second attack.

   The countermeasures implemented at own.
   Resolvers that follow this moment are rate limiting and
   disabling name resolution of target domain names in ad-hoc manner.

   If the full-service resolver supports aggressive negative caching and
   the target domain name is signed with NSEC/NSEC3 (without Opt-Out),
   it may be used as a possible countermeasure of random subdomain
   attacks.

   However, attackers can set the CD bit to their attack queries.  The
   CD bit disables signature validation and the aggressive negative
   caching recommendation will be of no use.

7.  Additional proposals

   There are additional proposals to the aggressive negative caching.

7.1.  Partial implementation

   It is possible to implement aggressive negative caching partially.

   DLV aggressive negative caching [RFC5074] is an implementation of
   NSEC aggressive negative caching which targets DLV domain names.

   NSEC3 is somewhat more complex to implement, and some implementations
   may choose to only implement aggressive negative caching for NSEC.

   Root only aggressive negative caching is also possible.  It uses NSEC
   and RRSIG resource records whose signer domain name is root.

   [I-D.wkumari-dnsop-cheese-shop] proposed root only aggressive
   negative caching in order to decrease defects and standardize
   quickly.  The root zone has certain properties that make it have a special
   case: It is DNSSEC signed and uses NSEC, the majority of the queries
   are "junk" queries, the rate of change is relatively slow, and there
   are no corner cases such as wildcards.  Because of these properties,
   we know that generated negative answers will work.

7.2.  Aggressive negative caching flag idea

   Authoritative DNS servers that dynamically generate NSEC records
   normally generate minimally covering NSEC Records [RFC4470].
   Aggressive negative caching does not work with minimally covering
   NSEC records.  Most more consistent
   view of DNS operators don't want zone enumeration and
   zone information leaks.  They prefer NSEC resource the namespace".

   (If approved, ) The paragraph is updated as follows:

   +--------------------------------------------------------------+
   |  Once the records with
   narrow ranges.  When a flag shows a are validated, DNSSEC enabled full-service resolver supporting
   the aggressive negative caching |
   |  resolvers MAY use wildcards and a query has the aggressive
   negative caching flag, authoritative DNS servers can generate NSEC NSEC/NSEC3 resource records with wider range under random subdomain attacks.
   However, anyone (including attackers) can always use the flag..

8. |
   |  to generate (positive and) negative responses until their   |
   |  effective TTLs or signatures for those records expire.      |
   +--------------------------------------------------------------+

7.  IANA Considerations

   This document has no IANA actions.

9.

8.  Security Considerations

   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.

   Aggressive use of NSEC / NSEC3 resource records without DNSSEC
   validation may cause security problems.  It is highly recommended to
   apply DNSSEC validation.

10.

9.  Implementation Status

   Unbound has aggressive negative caching code in its DLV validator.
   The author implemented NSEC aggressive caching using Unbound and its
   DLV validator code.

11.

10.  Acknowledgments

   The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
   and Unbound developers.  Olafur Gudmundsson and Pieter Lexis proposed
   aggressive negative caching flag idea.  Valuable comments were provided by Alexander
   Dupuy, Olafur Gudmundsson, Pieter Lexis, Bob Harold, Tatuya JINMEI,
   Shumon Huque, Mark Andrews, Casey Deccio, Bob Harold, Stephane
   Bortzmeyer and Matthijs Mekking.

12.

11.  Change History

   This

   RFC Editor: Please remove this section is used for tracking prior to publication.

   -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 update TTL section a bit.

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

   o  Moved "mitigaton of this document.  Will
   be removed after finalize. 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.

12.1.

11.1.  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.

12.2.

11.2.  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

12.3.

11.3.  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

13.

12.  References

13.1.
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>.

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

   [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>.

   [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>.

13.2.

12.2.  Informative References

   [BIND9]    Internet Systems Consortium, Inc., "Name Server Software",
              2000, <https://www.isc.org/downloads/bind/>.

   [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.

   [I-D.wkumari-dnsop-cheese-shop]
              Kumari, W. and G. Huston, "Believing NSEC records in the
              DNS root.", draft-wkumari-dnsop-cheese-shop-01 (work in
              progress), February 2016.

   [UNBOUND]  NLnet Labs, "Unbound DNS validating resolver", 2006,
              <http://www.unbound.net/>.

Appendix A.  Aggressive negative caching from RFC 5074

   Imported from Section 6 of [RFC5074].  Detailed implementation idea

   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 6.1 of [RFC5074].

   Implementing aggressive negative caching suggests that a validator
   will need to build an ordered data structure of NSEC records in order
   to efficiently find covering NSEC records.  Only NSEC records from
   DLV domains need to be included in this data structure.

Appendix B.  Detailed implementation idea the CD bit was set
      on the incoming query.  (Imported from Section 6.1 6 of [RFC5074] is expanded as follows. [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.

   The aggressive negative caching may be inserted at the cache lookup
   part of the full-service resolvers.

   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 full-resolver resolve the query in Recursive-
   mode as if the full-service resolver does not implement aggressive
   negative caching.

   To implement aggressive negative caching, resolver algorithm near
   cache lookup will be changed as follows:

   QNAME = the query name;
   QTYPE = the query type;
   if ({QNAME,QTYPE} entry exists in the cache) {
       // the resolver responds the RRSet from the cache
       resolve the query as usual;
   }

   // if NSEC* exists, QTYPE existence is proved by type bitmap
   if (matching NSEC/NSEC3 of QNAME exists in the cache) {
       if (QTYPE exists in type bitmap of NSEC/NSEC3 of QNAME) {
           // the entry exists, however, it is not in the cache.
           // need to iterate QNAME/QTYPE.
           resolve the query as usual;
       } else {
           // QNAME exists, QTYPE does not exist.
           the resolver can generate NODATA response;

       }
   }

   // Find closest enclosing NS RRset in the cache.
   // The owner of this NS RRset will be a suffix of the QNAME
   //    - the longest suffix of any NS RRset in the cache.
   SIGNER = closest enclosing NS RRSet of QNAME in the cache;

   // Check the NS RR of the SIGNER
   if (NS RR of SIGNER and its RRSIG RR do not exist in the cache
       or SIGNER zone is not signed or not validated) {
      Resolve the query as usual;
   }

   if (SIGNER zone does not have NSEC_TABLE) {
       Resolve the query as usual;
   }

   if (SIGNER zone is signed with NSEC) { // NSEC mode

       // Check the non-existence of QNAME
       CoveringNSEC = Find the covering NSEC of QNAME from NSEC_TABLE;
       if (Covering NSEC doesn't exist in the cache and NSEC_TABLE) {
           Resolve the query as usual.
       }

       // Select the longest existing name of QNAME from covering NSEC
       ClosestEncloser = common part of both owner name and
                           next domain name of CoveringNSEC;

       if (*.LongestExistName entry exists in the cache) {
           the resolver can generate positive response
           // synthesize the wildcard *.TEST
       }
       if covering NSEC RR
      table as NSEC_TABLE.  (Imported from Section 6.1 of "*.LongestExistName" [RFC5074] and
      expanded.)

   o  The aggressive negative caching may be inserted at SIGNER zone exists
            in the cache {
      lookup part of the resolver can generate negative response;
       }
       //*.LongestExistName may exist. cannot generate full-service resolvers.

   o  If errors happen in aggressive negative response
       Resolve caching algorithm,
      resolvers MUST fall back to resolve the query as usual.

   } else
   if (SIGNER zone is signed with NSEC3) {
       // NSEC3 mode

       ClosestEncloser = Find  "Resolve
      the closest encloser of QNAME
                                             from query as usual" means that the cache
       // to prove full-resolver resolve the non-existence of QNAME,
       // closest encloser of QNAME must be query
      in Recursive-mode as if the cache

       NextCloserName = the next closer name full-service resolver does not
      implement aggressive negative caching.

Appendix B.  Side effect: mitigation of QNAME
       SourceOfSyhthesis = *.ClosestEncloser

       if (matching NSEC3 random subdomain attacks

   Random sub-domain attacks (referred to as "Water Torture" attacks or
   NXDomain attacks) send many queries for non-existent information to
   full-service resolvers.  Their query names consist of ClosestEncloser exists in the random prefixes
   and a target domain name.  The negative cache does not work well, and
           covering NSEC3
   thus targeted full-service resolvers end up sending queries to
   authoritative DNS servers of NextCloserName exists in the cache
           and covering NSEC3 target domain name.

   The aggressive negative caching is not Opt-Out flag set) {

           // ClosestEncloser exists, and NextCloserName does not exist
           // then we need one of possible countermeasures to check *.ClosestEncloser

           if (*.ClosestEncloser entry exists in
   random subdomain attacks.  If the cache) {
               if (*.ClosestEncloser/QTYPE entry exists in full-service resolver supports
   aggressive negative caching and the cache) { target domain name is signed with
   NSEC/NSEC3 (without Opt-Out), the resolver can generate positive response
               } else {
                   // lack aggressive negative caching is one
   of *.ClosestEncloser/QTYPE information
                   Resolve the query as usual
               }
           } else
           if (covering NSEC3 countermeasures of *.ClosestEncloser exists random subdomain attacks.

   However, attackers can set the CD bit to their attack queries.  The
   CD bit disables signature validation and covering NSEC3 is not Opt-Out flag set) { the resolver can generate aggressive negative response;
           }
       }
       // no matching/covering NSEC3
   caching will be of QNAME information
       Resolve the query as usual
   } no use.

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