Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Updates: 4035 (if approved)                                      A. Kato
Intended status: Standards Track                               Keio/WIDE
Expires: February 3, March 17, 2017                                        W. Kumari
                                                         August 02,
                                                      September 13, 2016

                      Aggressive use of NSEC/NSEC3


   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.  It may also help increase resilience to
   certain DoS attacks in some circumstances.

   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:
   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?!"


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
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   Internet-Drafts are draft documents valid for a maximum of six months
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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 February 3, March 17, 2017.

Copyright Notice

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   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3   4
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Proposed Solution . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Aggressive Negative Caching . . . . . . . . . . . . . . .   5
     5.2.  NSEC  . . . . . . . . . . . . . . . . . . . . . . . . . .   5   6
     5.3.  NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.4.  Wildcard  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.5.  Consideration on TTL  . . . . . . . . . . . . . . . . . .   7
   6.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Update to RFC 4035  . . . . . . . . . . . . . . . . . . . . .   7
   7.   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.   9
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   10.   9
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   11.   9
   12. Change History  . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.   9
     12.1.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . . .   9
     11.2.  11
     12.2.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . . .   9
     11.3.  11
     12.3.  Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . . .   9
   12.  11
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     12.1.  11
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     12.2.  11
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10  12
   Appendix A.  Detailed implementation idea . notes  . . . . . . . . . . .  11
   Appendix B.  Side effect: mitigation of random subdomain attacks   11  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12  13

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

   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",
   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].  In this document we are using the terms
   "recursive resolver" or "recursive server" as a more readable
   alternative to the more formal[RFC7719] "full-service resolver"

   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) "" zone contains: IN A IN A IN A

   If a recursive resolver gets a query for, it will
   query the 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
   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 (or it will once again go off and query the
   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 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 falls within this range, the recursive server knows
   that 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
   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".

   This document relaxes this this restriction, as follows:

   |  Once the records are validated, DNSSEC enabled full-service validating   |
   |  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 validating resolver's cache has sufficient information to
   validate the query, the full-service resolver MAY SHOULD use NSEC/NSEC3/
   wildcard NSEC/NSEC3/wildcard
   records aggressively.  Otherwise, the full-service resolver it MUST fall back to send the query
   to the authoritative DNS servers.

   If the query name has the matching NSEC/NSEC3 RR 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

   Implementations SHOULD support enable aggressive use of NSEC and
   enable it by default.  It
   Implementations SHOULD provide a configuration switch to disable
   aggressive use of NSEC and allow it to be enabled or disabled
   for specific zones. per

   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 validating resolver's cache contains an NSEC RR covering the
   source of synthesis and the covering NSEC RR of the query name, the
   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

   If the full-service validating 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
   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 validating resolver implementation MAY support aggressive use of
   NSEC3.  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 revise this recommendation.  As long as the full-service resolver knows
   a name would not exist without the wildcard match, it 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

   Such aggressive use (of NSEC) is
   enabled"). of cached deduced wildcard can be employed
   independently from aggressive use of NSEC.  But, it will be more
   effective when both are enabled since the resolver can determine the
   name subject to wildcard would not otherwise exist more efficiently.

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

   A full-service resolver

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

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).  It is
   RECOMMENDED that full-service resolvers limit the maximum effective TTL value of
   negative responses (NSEC/NSEC3 RRs) to this same value.

6.  Update to RFC 4035

   Section 4.5  Benefits

   The techniques described in this document provide a number of [RFC4035] shows
   benefits, including (in no specific order):

   Latency  By answering directly from cache, recursive resolvers can
      immediately inform clients 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 name they are looking for resolvers to avoid blocking
   new authoritative data does
      not exist, improving the user experience.

   Decreased recursive server load  By answering negative queries from
      the cache, recursive servers avoid having 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 less queries.  This decreases
      the authoritative server bandwidth, queries per second and CPU

   The scale of the benefit depends upon multiple factors, including the
   query distribution.  For example, currently around 65% of queries to
   Root Name servers result in NXDOMAIN responses; this technique will
   eliminate a sizable quantity of these.

   [ Editor note: There has been some discussion on if this document
   should discuss this attack and mitigation.  The authors think that
   this is useful / important, but some participants feel that it
   oversells the DoS mitigation benefit.  Please let us know if the
   below is helpful.  Also, the below description is not as clear as it
   could be - it's been tricky to balance readability, correctness and
   conciseness.  Text gratefully accepted... ]

   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 recursive resolvers.  As the recursive server
   will not have the answers cached it has to ask the authoritative
   servers for each random query, leading to a DoS on the authoritative
   (and often recursive) servers.  Aggressive NSEC may help mitigate
   these attacks by allowing the recursive to answer directly from cache
   for any random queries which fall within already requested ranges.
   The effectiveness of this depends upon a number of factors, including
   if the attacker is making his queries through recursive resolvers
   (e.g to hide his source), the number of entries in the zone, the TTL,
   if the zone is using NSEC, if the attacker is setting the CD bit,
   etc.  In the ideal case, authoritative servers under attack will need
   to answer somewhere between number_of_entries_in_zone queries and 2 *
   number_of_entries_in_zone queries from each recursive server.  This
   is because there are as many "holes" between labels as there are
   labels in a zone.  If the random query falls in range for which
   recursive server does not have an NSEC record cached, it will send a
   query to the authoritative server, and so it will send approximately
   the same number of queries as there are "holes" between entries.  If
   the random queries happen to be for names which exist in the zone,
   the recursive will send those as well.

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".

   (If approved, )

   The paragraph is updated as follows:

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


8.  IANA Considerations

   This document has no IANA actions.


9.  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.  Implementation Status

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

10. caching.

11.  Acknowledgments

   The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
   and the Unbound developers.  Valuable comments were provided by

   The authors would like to specifically thank Tatuya JINMEI for
   extensive review and comments, and also Mark Andrews, Stephane
   Bortzmeyer, Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Pieter Lexis, Bob
   Harold, Tatuya JINMEI, Shumon Huque, Mark Andrews, Casey Deccio, Bob Harold, Stephane
   Bortzmeyer Pieter Lexis and Matthijs Mekking.


12.  Change History

   RFC Editor: Please remove this section prior to publication.

   -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

   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

   -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.

   o  Removed Effects section, Additional proposal section, and pseudo

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

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

   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.  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.  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


12.3.  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.  References

13.1.  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,

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [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,

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, DOI 10.17487/RFC4592, July 2006,

   [RFC5074]  Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
              DOI 10.17487/RFC5074, November 2007,

   [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,

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <>.


13.2.  Informative References

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

              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

Appendix A.  Detailed implementation idea 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

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

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

Appendix B.  Side effect: mitigation of 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 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
   authoritative DNS servers of the target domain name.

   The aggressive negative caching is one of possible countermeasures to
   random subdomain attacks.  If the full-service resolver supports
   aggressive negative caching and the target domain name is signed with
   NSEC/NSEC3 (without Opt-Out), the aggressive negative caching is one
   of countermeasures 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 will be of 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

   Phone: +81 3 5215 8451

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

   Phone: +81 45 564 2490

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043