Network Working Group                                        K. Fujiwara
Internet-Draft                                                      JPRS
Intended status: Informational                                   A. Kato
Expires: September 11, 2015                                    Keio/WIDE
                                                          March 10, 2015


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

Abstract

   DNS highly depends on cache, however, cache usage of non-existence
   information was limited to exact matching.  This draft proposes the
   aggressive use of 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 response is expected as
   well as some level of mitigation of random sub-domain attacks
   (referred to as "Water Torture" attacks).  And more, 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
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   This Internet-Draft will expire on September 11, 2015.

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Possible Solution . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Possible side effect  . . . . . . . . . . . . . . . . . . . .   4
   5.  Another option  . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

.  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
   resolver receives a query "a.example.com" , it performs a DNS
   resolution process, and eventually gets NXDOMAIN and cache it into
   its negative cache.  When the full 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 has no ability to tell that there is no
   such name "b.example.com".

   By the way, DNSSEC [RFC4035] [RFC5155] has been practically deployed
   recently.  Two resource record types (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.

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








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.  Problem Statement

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

   When number of queries is very large, the full resolver's outstanding
   queue will be full, and then, the full resolver will drop queries
   from both users and attackers.

   The countermeasures performed at present are rate limiting and
   disabling name resolution of target domain names.

.  Possible Solution

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

   DNSSEC defines NSEC resource record.  Section 4.5 of [RFC4035] shows
   that "In theory, a resolver could use wildcards or NSEC RRs to
   generate positive and negative responses (respectively) until the TTL
   or signatures on the records in question expire.  However, it seems
   prudent for resolvers to avoid blocking new authoritative data or
   synthesizing new data on their own.  Resolvers that follow this
   recommendation will have a more consistent view of the namespace".

   To reduce non-existent queries to authoritative DNS servers, the
   countermeasure is to relax this restriction.

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

   This technique is called as "NSEC/NSEC3 aggressive negative caching"
   in Unbound [Unbound] TODO file.  Unbound has aggressive negative
   caching code in its DLV validator.

   The full resolver need to check the existence of wildcards.  If the
   cache does not have an NSEC/NSEC3 resource record whose range
   includes a wildcard ('*') in a zone which the query name belongs to,
   wildcard may exist in the zone, then, the aggressive use of NSEC/
   NSEC3 cannot be applied and the full resolver need to send the query
   to authoritative DNS servers.





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   If the zone has a wildcard and it is in full resolver's cache, the
   full resolver may generate positive responses from the wildcard in
   the cache.

   This approach is effective for DNSSEC signed zones with NSEC or
   NSEC3, except zones with NSEC3 Opt-Out.

   NSEC/NSEC3 aggressive negative caching works as follows.  When the
   query name has a matching NSEC or NSEC3 resource records in the cache
   and there is no wildcard in the zone which the query name belongs to,
   a full resolver is allowed to respond with NXDOMAIN error
   immediately.

   The matching procedure may be applied to all ancestor domain names of
   the query name.

   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, and this value is reasonable small
   but still effective for the purpose of this document.

   It can eliminate significant amount of DNS queries when an attacker
   tries to send large number of DNS queries by using randomly generated
   names.

   The same discussion is applicable for wildcards.  If a query name is
   covered by NSEC or NSEC3 resource records in the cache and there is a
   covering wildcard, full resolvers can use wildcards to generate
   positive responses until wildcard and NSEC/NSEC3 resource records in
   the cache are effective.

   Aggressive use of wildcards requires aggressive use of negative
   information because there may be other domain names.

.  Possible side effect

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

   People may generate many typos and they tend to generate 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 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.




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.  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 resolvers without DNSSEC
   validation.  Full 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.

.  IANA Considerations

   This document has no effect on IANA registries.

.  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.

.  References

8.1.  Normative References

   [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.

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




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8.2.  Informative References

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

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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