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Internet Engineering Task Force (IETF)                       K. FujiwaraRequest for Comments: 8198                                          JPRSUpdates:4035                                                    A. KatoCategory: Standards Track                                      Keio/WIDEISSN: 2070-1721                                                W. Kumari                                                                  Google                                                               July 2017Aggressive Use of DNSSEC-Validated CacheAbstract   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   increases privacy.  Also, it may help increase resilience to certain   DoS attacks in some circumstances.   This document updatesRFC 4035 by allowing validating resolvers to   generate negative answers based upon NSEC/NSEC3 records and positive   answers in the presence of wildcards.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8198.Fujiwara, et al.             Standards Track                    [Page 1]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017Copyright Notice   Copyright (c) 2017 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .33.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .34.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .45.  Aggressive Use of DNSSEC-Validated Cache  . . . . . . . . . .65.1.  NSEC  . . . . . . . . . . . . . . . . . . . . . . . . . .65.2.  NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . .65.3.  Wildcards . . . . . . . . . . . . . . . . . . . . . . . .65.4.  Consideration on TTL  . . . . . . . . . . . . . . . . . .76.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .77.  Update toRFC 4035  . . . . . . . . . . . . . . . . . . . . .88.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .99.  Security Considerations . . . . . . . . . . . . . . . . . . .910. References  . . . . . . . . . . . . . . . . . . . . . . . . .910.1.  Normative References . . . . . . . . . . . . . . . . . .910.2.  Informative References . . . . . . . . . . . . . . . . .10Appendix A.  Detailed Implementation Notes  . . . . . . . . . . .11Appendix B.  Procedure for Determining ENT vs. NXDOMAIN with NSEC  11   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .12   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .13Fujiwara, et al.             Standards Track                    [Page 2]

RFC 8198                    NSEC/NSEC3 Usage                   July 20171.  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 updatesRFC 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 an 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 inSection 6 of DNSSEC   Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC   records efficiently.   [RFC8020] and [RES-IMPROVE] 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", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described inBCP14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.   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 192.0.2.3Fujiwara, et al.             Standards Track                    [Page 3]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   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, 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 the type does not exist.  Proof that a name does   not exist is accomplished by providing a (DNSSEC-secured) record   containing the names that appear alphabetically before and after theFujiwara, et al.             Standards Track                    [Page 4]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   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 that 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, earlier,Section 4.5 of [RFC4035] says:      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 that 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 use of DNSSEC-validatedFujiwara, et al.             Standards Track                    [Page 5]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   cache, 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.5.  Aggressive Use of DNSSEC-Validated Cache   This document relaxes the restriction given inSection 4.5 of   [RFC4035].  SeeSection 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 inSection 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   that derive from query names.   If denial of existence can be determined according to the rules set   out in [RFC5155], Sections8.4,8.5,8.6, and8.7, 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 an 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 theFujiwara, et al.             Standards Track                    [Page 6]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   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   rules set out inSection 5.3.4 of [RFC4035] (NSEC), or inSection 8.8   of [RFC5155], it SHOULD synthesize an answer (or NODATA response) for   that name using the cache-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.Fujiwara, et al.             Standards Track                    [Page 7]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   Decreased authoritative server load:  Because recursive servers can      answer queries without asking the authoritative server, the      authoritative servers receive fewer queries.  This decreases the      authoritative server bandwidth, queries per second, and CPU      utilization.   The scale of the benefit depends upon multiple factors, including the   query distribution.  For example, at the time of this writing, around   65% of queries to root name servers result in NXDOMAIN responses (see   statistics from [ROOT-SERVERS]); 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 subdomains 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).   The technique may help mitigate these attacks by allowing the   resolver to answer directly from the cache for any random queries   that 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 toRFC 4035Section 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.         |   +-----------------------------------------------------------------+Fujiwara, et al.             Standards Track                    [Page 8]

RFC 8198                    NSEC/NSEC3 Usage                   July 20178.  IANA Considerations   This document does not require any IANA actions.9.  Security Considerations   Use of NSEC/NSEC3 resource records without DNSSEC validation may   create serious security issues, and so this technique requires DNSSEC   validation.   Newly registered resource records may not be used immediately.   However, choosing a suitable TTL value and a 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   (3 hours), 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.  References10.1.  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>.Fujiwara, et al.             Standards Track                    [Page 9]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   [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>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174,              May 2017, <http://www.rfc-editor.org/info/rfc8174>.10.2.  Informative References   [RES-IMPROVE]              Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS              Resolvers for Resiliency, Robustness, and Responsiveness",              Work in Progress,draft-vixie-dnsext-resimprove-00, 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]              "Root Server Technical Operations Assn",              <http://www.root-servers.org/>.Fujiwara, et al.             Standards Track                   [Page 10]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017Appendix A.  Detailed Implementation Notes   o  Previously, cached negative responses were indexed by QNAME,      QCLASS, QTYPE, and the setting of the CD bit (seeRFC 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 fromSection 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 fromSection 6.1 of [RFC5074] and      expanded.)   o  The aggressive negative caching may be inserted at the cache      lookup part of the recursive resolvers.   o  If errors happen in an 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. NXDOMAIN 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.Fujiwara, et al.             Standards Track                   [Page 11]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017   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 an NSEC record that proves the NXDOMAIN or ENT.   If the next domain name is a subdomain of the given name, you have an   ENT.  Otherwise, you have an NXDOMAIN.Acknowledgments   The authors gratefully acknowledge DNSSEC Lookaside Validation (DLV)   [RFC5074] author Samuel Weiler and the Unbound developers.   Thanks to Mark Andrews for providing the helpful notes for   implementors provided inAppendix 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.Fujiwara, et al.             Standards Track                   [Page 12]

RFC 8198                    NSEC/NSEC3 Usage                   July 2017Authors' 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   United States of America   Email: warren@kumari.netFujiwara, et al.             Standards Track                   [Page 13]