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Versions: 00 01 02 03 draft-ietf-dnsop-nsec-aggressiveuse

Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Informational                                   A. Kato
Expires: January 7, 2016                                       Keio/WIDE
                                                           July 06, 2015


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

Abstract

   While DNS highly depends on cache, its cache usage of non-existence
   information was limited to exact matching.  This draft proposes the
   aggressive use of a NSEC/NSEC3 resource record, which is able to
   express non-existence of range of names authoritatively.  With this
   proposal, shorter latency to many of negative responses is expected
   as well as some level of mitigation of random sub-domain attacks
   (referred to as "Water Torture" attacks).  It is also expected that
   non-existent TLD queries to Root DNS servers will decrease.

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 January 7, 2016.

Copyright Notice

   Copyright (c) 2015 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Proposed Solution: Aggressive Negative Caching  . . . . . . .   3
   5.  Possible side effect  . . . . . . . . . . . . . . . . . . . .   5
   6.  Another option  . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Aggressive negative caching flag  . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. Implementation Considerations . . . . . . . . . . . . . . . .   6
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   12. Change History  . . . . . . . . . . . . . . . . . . . . . . .   7
     12.1.  Version 01 . . . . . . . . . . . . . . . . . . . . . . .   7
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     13.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Aggressive negative caching from RFC 5074  . . . . .   8
   Appendix B.  Detailed implementation idea . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   While negative (non-existence) information of DNS caching mechanism
   has been known as DNS negative cache [RFC2308], it requires exact
   matching in most cases.  Assume that "example.com" zone doesn't have
   names such as "a.example.com" and "b.example.com".  When a full-
   service resolver receives a query "a.example.com" , it performs a DNS
   resolution process, and eventually gets NXDOMAIN and stores it into
   its negative cache.  When the full-service resolver receives another
   query "b.example.com", it doesn't match with "a.example.com".  So it
   will send a query to one of the authoritative servers of
   "example.com".  This was because the NXDOMAIN response just says
   there is no such name "a.example.com" and it doesn't tell anything
   for "b.example.com".

   Recently, DNSSEC [RFC4035] [RFC5155] has been practically deployed.
   Two types of resource record (NSEC and NSEC3) are used for authentic
   non-existence.  For a zone signed with NSEC, it may be possible to
   use the information carried in NSEC resource records to indicate that
   the range of names where no valid name exists.  Such use is
   discouraged by Section 4.5 of RFC 4035, however.



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   This document proposes to make a minor change to RFC 4035 and the
   full-service resolver can use NSEC/NSEC3 resource records
   aggressively.

   Aggressive Negative Caching was first proposed in Section 6 of DNSSEC
   Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
   records efficiently.  Unbound [UNBOUND] has aggressive negative
   caching code in its DLV validator.  Unbound TODO file contains "NSEC/
   NSEC3 aggressive negative caching".

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 specification are defined
   in DNS Terminology [I-D.ietf-dnsop-dns-terminology].

3.  Problem Statement

   Random sub-domain attacks (referred to as "Water Torture" attacks)
   send many non-existent queries to full-service resolvers.  Their
   query names consist of random prefixes and a target domain name.  The
   negative cache does not work well and target full-service resolvers
   result in sending queries to authoritative DNS servers of the target
   domain name.

   When number of queries is large, the full-service resolvers drop
   queries from both legitimate users and attackers as 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 this moment are rate limiting and
   disabling name resolution of target domain names.

4.  Proposed Solution: Aggressive Negative Caching

   If the target domain names are DNSSEC signed, aggressive use of NSEC/
   NSEC3 resource records mitigates the problem.

   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



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   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, it
   is suggested to relax this restriction as follows:

   +--------------------------------------------------------------+
   |  DNSSEC enabled full-service resolvers MAY use               |
   |  NSEC/NSEC3 resource records to generate negative responses  |
   |  until their effective TTLs or signatures on the records     |
   |  in question expire.                                         |
   +--------------------------------------------------------------+

   If the full-service resolver's cache have enough 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.

   Necessary information to validate are wildcards which match the query
   name, covering NSEC/NSEC3 of the wildcards, and covering NSEC/NSEC3
   of (parts of) the query name.

   If the zone has a wildcard and it is in the full-service resolver's
   cache, the full-service resolver MAY generate positive responses
   based on the information associated with the wildcard in the cache.

   This approach is effective for DNSSEC signed zones with NSEC or
   NSEC3, except zones with NSEC3 Opt-Out.  To identify signing types of
   the zone, validating resolvers need to build special cache of NSEC
   and NSEC3 resource records for each signer domain name.  When a query
   name is not in the cache, find closest enclosing NS RRset in the
   cache.  The owner of this NS RRset may be the longest signer domain
   name of the query name.  If the NSEC/NSEC3 cache of the signer domain
   name is empty, the aggressive negative caching is not possible.
   Otherwise, there is at least one NSEC or NSEC3 resource records.  The
   record shows the signing type.  If the resource record is NSEC3 and
   with Opt-Out, the aggressive negative caching is not possible.

   When the query name has a matching NSEC resource records in the cache
   and there is no wildcard in the zone which the query name matches
   with, the full-service resolver is allowed to respond with NXDOMAIN
   error immediately.

   NSEC3 aggressive negative caching is more difficult.  If the zone is
   signed with NSEC3, the validating resolver need to check the
   existence of each label from the query name.  If a label is not exist
   in the zone, and there is no matching wildcard in the zone, the full-



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   service resolver is allowed to respond with NXDOMAIN error
   immediately.

   This function needs care on the TTL value of negative information
   because newly added domain names cannot be used while the negative
   information is effective.  RFC 2308 states the maximum number of
   negative cache TTL value is 10800 (3 hours).  So the full-service
   resolver SHOULD 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 as it can
   eliminate significant amount of DNS attacks with randomly generated
   names.

   The same discussion is also applicable to wildcards.  If a query name
   is covered by a NSEC or a NSEC3 resource record in the cache and
   there is a covering wildcard, the full-service resolver MAY use
   wildcards to generate positive responses while wildcard and NSEC/
   NSEC3 resource records in the cache are effective.

5.  Possible side effect

   Aggressive use of NSEC/NSEC3 resource records may decrease queries to
   Root DNS servers.

   People may generate many typos in TLD, and they will result in
   unnecessary DNS queries.  Some implementations leak non-existent TLD
   queries whose second level domain are different each other.  Well
   observed TLDs are ".local" and ".belkin".  With this proposal, 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.  Another option

   The proposed technique is applicable to zones where there is a NSEC
   record to each owner name in the zone even without DNSSEC signed.
   And it is also applicable to full-service resolvers without DNSSEC
   validation.  Full-service resolvers can set DNSSEC OK bit in query
   packets and they will cache NSEC/NSEC3 resource records.  They can
   apply aggressive use of NSEC/NSEC3 resource records without DNSSEC
   validation.

   It is highly recommended to sign the zone, of course, and it is
   recommended to apply DNSSEC validation of NSEC record prior to cache
   it in the negative cache.





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7.  Aggressive negative caching flag

   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.  DNS operators don't want zone walking and zone
   information leaks.  They prefer NSEC resource records with narrow
   ranges.  When a query have the aggressive negative caching flag (AN
   flag), they can generate NSEC resource records with wider range under
   random subdomain attacks.

   A full-service resolver which supports aggressive negative caching
   SHOULD set the AN flag when sending queries to authoritative DNS
   servers.

8.  IANA Considerations

   This document reserves one of these bits as the AN flag (Aggressive
   Negative Caching flag) in EDNS Header Flags defined in EDNS0
   [RFC6891].

9.  Security Considerations

   Newly registered resource records may not be used immediately.
   However, choosing suitable TTL value will mitigate the problem and it
   is not a security problem.

   It is also suggested to limit the maximum TTL value of NSEC resource
   records in the negative cache to, for example, 10800 seconds (3hrs),
   to mitigate the issue.  Implementations which comply with this
   proposal is suggested 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.

10.  Implementation Considerations

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

11.  Acknowledgments

   The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
   and Unbound developers.  Olafur Gudmundsson and Pieter Lexis proposed
   aggressive negative caching flag idea.




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12.  Change History

   This section is used for tracking the update of this document.  Will
   be removed after finalize.

12.1.  Version 01

   o  Added reference to DLV [RFC5074] and imported some sentences.

   o  Added Aggressive Negative Caching Flag idea.

   o  Added detailed algorithms.

13.  References

13.1.  Normative References

   [I-D.ietf-dnsop-dns-terminology]
              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-dns-terminology-03 (work in
              progress), June 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4470]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
              and DNSSEC On-line Signing", RFC 4470, April 2006.

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

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.







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13.2.  Informative References

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

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

Appendix A.  Aggressive negative caching from RFC 5074

   Imported from RFC 5074.

   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.

   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

   Implementing aggressive negative caching suggests that a validator
   need to build an ordered data structure of NSEC/NSEC3 records for
   each signer domain name in order to efficiently find covering NSEC/
   NSEC3 records.

   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.

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

   If the query name entry exists in the cache {
       resolve the query as usual.



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       // if RRSet (query name and query type) exists in the cache,
       //     the resolver responds the RRSet from the cache
       // Otherwise, the resolver needs to iterate the query.
   }

   // 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 the QNAME in the cache.

   If SIGNER zone does not have a special NSEC/NSEC3 data structure {
       Resolve the query as usual.
   }

   if SIGNER zone is not signed or not validated {
      Resolve the query as usual
   }

   If SIGNER zone is signed with NSEC {
       // NSEC mode
       If covering NSEC RR of the query name at SIGNER zone
          doesn't exist in the cache {
           Resolve the query as usual.
       }

       TEST = Find the longest existing domain name of the query name
              from the covering NSEC RR

       if "*.TEST" exists in the cache {
           the resolver can generate positive response
           or resolve the query as usual.
       }

       if covering NSEC RR of "*.TEST" at SIGNER zone exists
            in the cache {
           the resolver can generate negative response.
       }

       // Lack of information, need to resolve the query as usual

   } else
   if SIGNER zone is signed with NSEC3 and does not use Opt-Out {
       // NSEC3 mode

       QNAME = the query name
       TEST = SIGNER
       do {
           UPPER = TEST



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           add a label from the QNAME to the start of TEST
           If the covering NSEC3 of TEST exist in the cache {
               // non-terminal name TEST does not exist
               if *.UPPER or NSEC3 of *.UPPPER exist in the cache {
                   the resolver can generate negative response.
               } else {
               if the covering NSEC3 of *.UPPER exist in the cache {
                   the resolver can generate positive response.
               }
               // lack of information
               break
           } else
           if the NSEC3 of TEST does not exist {
               // lack of information
               break
           }
           // TEST label exist, then need to check the rest of the QNAME
       } while(TEST != QNAME)
       // lack of information
   }
   Resolve the query as usual

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









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