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rfc:rfc8198

Internet Engineering Task Force (IETF) K. Fujiwara Request for Comments: 8198 JPRS Updates: 4035 A. Kato Category: Standards Track Keio/WIDE ISSN: 2070-1721 W. Kumari

                                                                Google
                                                             July 2017
              Aggressive Use of DNSSEC-Validated Cache

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
 increases privacy.  Also, it may help increase resilience to certain
 DoS attacks in some circumstances.
 This document updates RFC 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 in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8198.

Fujiwara, et al. Standards Track [Page 1] RFC 8198 NSEC/NSEC3 Usage July 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 DNSSEC-Validated 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 . . . . . . . . . . . . . . . . . . . . .   9
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
 10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
   10.2.  Informative References . . . . . . . . . . . . . . . . .  10
 Appendix A.  Detailed Implementation Notes  . . . . . . . . . . .  11
 Appendix B.  Procedure for Determining ENT vs. NXDOMAIN with NSEC  11
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

Fujiwara, et al. Standards Track [Page 2] RFC 8198 NSEC/NSEC3 Usage July 2017

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 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 in Section 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 in BCP
 14 [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 192.0.2.3

Fujiwara, 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 the

Fujiwara, 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-validated

Fujiwara, 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 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
 that 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, and 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 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 the

Fujiwara, 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 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 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 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.         |
 +-----------------------------------------------------------------+

Fujiwara, et al. Standards Track [Page 8] RFC 8198 NSEC/NSEC3 Usage July 2017

8. 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. References

10.1. 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 in RFC
            2119 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 2017

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.
 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 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.

Fujiwara, et al. Standards Track [Page 12] RFC 8198 NSEC/NSEC3 Usage July 2017

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
 United States of America
 Email: warren@kumari.net

Fujiwara, et al. Standards Track [Page 13]

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