DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-02

Domain Name System Operations (dnsop) Working Group        S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Experimental                              March 4, 2015
Expires: September 5, 2015

             DNS query name minimisation to improve privacy
                 draft-ietf-dnsop-qname-minimisation-02

Abstract

   This document describes one of the techniques that could be used to
   improve DNS privacy (see [I-D.ietf-dprive-problem-statement]), a
   technique called "qname minimisation".

   REMOVE BEFORE PUBLICATION Discussions of the document should take
   place on the DNSOP working group mailing list [dnsop].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 5, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
   2.  Qname minimisation  . . . . . . . . . . . . . . . . . . . . .   2
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   3
   4.  Performance implications  . . . . . . . . . . . . . . . . . .   5
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Implementation status - REMOVE BEFORE PUBLICATION . . . . . .   6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     8.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  An algorithm to find the zone cut  . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction and background

   The problem statement is exposed in
   [I-D.ietf-dprive-problem-statement] TODO: add a reference to the
   specific section when ietf-dprive-problem-statement will be published
   as RFC.  The terminology ("qname", "resolver", etc) is also defined
   in this companion document.  This specific solution is not intended
   to fully solve the DNS privacy problem; instead, it should be viewed
   as one tool amongst many.

   It follows the principle explained in section 6.1 of [RFC6973]: the
   less data you send out, the fewer privacy problems you'll get.

2.  Qname minimisation

   The idea is to minimise the amount of data sent from the DNS
   resolver.  Under current practice, when a resolver receives the query
   "What is the AAAA record for www.example.com?", it sends to the root
   (assuming a cold resolver, whose cache is empty) the very same
   question.  Sending "What are the NS records for .com?" would be
   sufficient (since it will be the answer from the root anyway).  This
   is compatible with the current DNS system and therefore can easily be
   deployed; since it is a unilateral change to the resolver, it does
   not change the protocol.  Because of that, resolver implementers may
   do qname minmisation in slightly different ways.

   To do such minimisation, the resolver needs to know the zone cut
   [RFC2181].  Zone cuts do not necessarily exist at every label
   boundary.  If we take the name www.foo.bar.example, it is possible

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   that there is a zone cut between "foo" and "bar" but not between
   "bar" and "example".  So, assuming the resolver already knows the
   name servers of .example, when it receives the query "What is the
   AAAA record of www.foo.bar.example", it does not always know whether
   the request should be sent to the name servers of bar.example or to
   those of example.  [RFC2181] suggests a method to find the zone cut
   (section 6), so resolvers may try it.

   Note that DNSSEC-validating resolvers already have access to this
   information, since they have to find the zone cut (the DNSKEY record
   set is just below, the DS record set just above).

   One should note that the behaviour suggested here (minimising the
   amount of data sent in qnames from the resolver) is NOT forbidden by
   the [RFC1034] (section 5.3.3) or [RFC1035] (section 7.2).  Sending
   the full qname to the authoritative name server is a tradition, not a
   protocol requirement.  This tradition comes[mockapetris-history] from
   a desire to optimize the number of requests, when the same name
   server is authoritative for many zones in a given name (something
   which was more common in the old days, where the same name servers
   served .com and the root) or when the same name server is both
   recursive and authoritative (something which is strongly discouraged
   now).  Whatever the merits of this choice at this time, the DNS is
   quite different now.

   It may be noticed that many documents explaining the DNS and intended
   for a wide audience, incorrectly describe the resolution process as
   using qname minimisation, for instance by showing a request going to
   the root, with just the TLD in the query.  As a result, these
   documents may confuse the privacy analysis of the users who see them.

   As mentioned before, there are several ways to implement qname
   minimisation.  Two main strategies are the aggressive one and the
   lazy one.  In the aggressive one, the resolver only sends NS queries
   as long as it does not know the zone cuts.  This is the safest, from
   a privacy point of view.  The lazy way "piggybacks" on the
   traditional resolution code.  It sends traditional full qnames and
   learns the zone cuts from the referrals received, then switches to NS
   queries asking only for the minimum domain name.  This leaks more
   data but probably requires fewer changes in the existing resolver
   codebase.

3.  Operational considerations

   The administrators of the forwarders, and of the authoritative name
   servers, will get less data, which will reduce the utility of the
   statistics they can produce (such as the percentage of the various
   qtypes).  On the other hand, it may decrease their legal

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   responsibility in some jurisdictions.  (TODO: do we keep any mention
   of legal issues?  We're not lawyers.)

   Some broken name servers do not react properly to qtype=NS requests.
   For instance, some authoritative name servers embedded in load
   balancers reply properly to A queries but send REFUSED to NS queries.
   REMOVE THIS SENTENCE BEFORE PUBLICATION: As an example of today, look
   at www.ratp.fr (not ratp.fr).  This behaviour is a gross protocol
   violation, and there is no need to stop improving the DNS because of
   such brokenness.  However, qname minimisation may still work with
   such domains since they are only leaf domains (no need to send them
   NS requests).  Such setup breaks more than just qname minimisation.
   It breaks negative answers, since the servers don't return the
   correct SOA, and it also breaks anything dependent upon NS and SOA
   records existing at the top of the zone.

   A problem can also appear when a name server does not react properly
   to ENT (Empty Non-Terminals).  If ent.example.com has no resource
   records but foobar.ent.example.com does, then ent.example.com is an
   ENT.  A query, whatever the qtype, for ent.example.com must return
   NODATA (NOERROR / ANSWER: 0).  However, some broken name servers
   return NXDOMAIN for ENTs.  REMOVE THIS SENTENCE BEFORE PUBLICATION:
   As an example of today, look at com.akadns.net or www.upenn.edu with
   its delegations to Akamai.  If a resolver queries only
   foobar.ent.example.com, everything will be OK but, if it implements
   qname minimisation, it may query ent.example.com and get a NXDOMAIN.
   See also section 3 of [I-D.vixie-dnsext-resimprove] for the other bad
   consequences of this brokenness.

   Another way to deal with such broken name servers would be to try
   with A requests (A being chosen because it is the most common and
   hence a qtype which will be always accepted, while a qtype NS may
   ruffle the feathers of some middleboxes).  Instead of querying name
   servers with a query "NS example.com", we could use "A _.example.com"
   and see if we get a referral.

   Other strange and illegal practices may pose a problem: there is a
   common DNS anti-pattern used by low-end web hosters that also do DNS
   hosting that exploits the fact that the DNS protocol (pre-DNSSEC)
   allows certain serious misconfigurations, such as parent and child
   zones disagreeing on the location of a zone cut.  Basically, they
   have a single zone with wildcards for each TLD like:

   *.example.          60  IN  A   192.0.2.6

   (It is not known why they don't just wildcard all of "*." and be done
   with it.)

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   This lets them turn up many web hosting customers without having to
   configure thousands of individual zones on their nameservers.  They
   just tell the prospective customer to point their NS records at the
   hoster's nameservers, and the Web hoster doesn't have to provision
   anything in order to make the customer's domain resolve.  NS queries
   to the hoster will therefore do not give the right result, which may
   endanger qname minimisation (it will be a problem for DNSSEC, too).

   Qname minimisation can decrease performance in some cases, for
   instance for a deep domain name (like
   www.host.group.department.example.com where
   host.group.department.example.com is hosted on example.com's name
   servers).  For such a name, a cold resolver will, depending how qname
   minimisation is implemented, send more queries.  Once the cache is
   warm, there will be no difference with a traditional resolver.  A
   possible solution is to always use the traditional algorithm when the
   cache is cold and then to move to qname minimisation.  This will
   decrease the privacy a bit but will guarantee no degradation of
   performance.

   Another useful optimisation may be, in the spirit of the HAMMER idea
   [I-D.wkumari-dnsop-hammer] to probe in advance for the introduction
   of zone cuts where none previously existed (i.e. confirm their
   continued absence, or discover them.)

4.  Performance implications

   The main goal of qname minimisation is to improve privacy by sending
   less data.  However, it may have other advantages.  For instance, if
   a root name server receives a query from some resolver for A.CORP
   followed by B.CORP followed by C.CORP, the result will be three
   NXDOMAINs, since .CORP does not exist in the root zone.  Under query
   name minimisation, the root name servers would hear only one question
   (for .CORP itself) to which they could answer NXDOMAIN, thus opening
   up a negative caching opportunity in which the full resolver could
   know a priori that neither B.CORP or C.CORP could exist.  Thus in
   this common case the total number of upstream queries under qname
   minimisation would be counter-intuitively inferior to the number of
   queries under the traditional iteration (as described in the DNS
   standard).

   Qname minimisation may also improve look-up performance for TLD
   operators.  For a typical TLD, delegation-only, and with delegations
   just under the TLD, a 2-label QNAME query is optimal for finding the
   delegation owner name.

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5.  Security considerations

   Qname minimisation's benefits are clear in the case where you want to
   decrease exposure to the authoritative name server.  But minimising
   the amount of data sent also, in part, addresses the case of a wire
   sniffer as well the case of privacy invasion by the servers.
   (Encryption is of course a better defense against wire sniffers but,
   unlike qname minimisation, it changes the protocol and cannot be
   deployed unilaterally.)

   Qname minimisation offers zero protection against the recursive
   resolver, which still sees the full request coming from the stub
   resolver.

   At this stage, this document does not recommend one of the two qname
   minimisation approaches (aggressive or lazy) against the other.

   No security consequence (besides privacy improvment) is known at this
   time.

6.  Implementation status - REMOVE BEFORE PUBLICATION

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   As of today, no production resolver implements qname minimisation.
   For Unbound, see ticket 648 [1].

   The algorithm to find the zone cuts described in Appendix A is
   implemented with qname minimisation in the sample code zonecut.go

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   [2].  It is also implemented, for a much longer time, in an option of
   dig, "dig +trace", but without qname minimisation.

7.  Acknowledgments

   Thanks to Olaf Kolkman for the original idea although the concept is
   probably much older [3].  Thanks to Mark Andrews and Francis Dupont
   for the interesting discussions.  Thanks to Brian Dickson, Warren
   Kumari, Evan Hunt and David Conrad for remarks and suggestions.
   Thanks to Mohsen Souissi for proofreading.  Thanks to Tony Finch for
   the zone cut algorithm in Appendix A.  Thanks to Paul Vixie for
   pointing out that there are practical advantages (besides privacy) to
   qname minimisation.  Thanks to Phillip Hallam-Baker for the fallback
   on A queries, to deal with broken servers.  Thanks to Robert Edmonds
   for an interesting anti-pattern.

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973, July
              2013.

   [I-D.ietf-dprive-problem-statement]
              Bortzmeyer, S., "DNS privacy considerations", draft-ietf-
              dprive-problem-statement-01 (work in progress), January
              2015.

8.2.  Informative References

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982, July
              2013.

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   [I-D.wkumari-dnsop-hammer]
              Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly
              Automated Method for Maintaining Expiring Records", draft-
              wkumari-dnsop-hammer-01 (work in progress), July 2014.

   [I-D.vixie-dnsext-resimprove]
              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
              2010.

   [dnsop]    IETF, , "The DNSOP working group of IETF", March 2014,
              <https://datatracker.ietf.org/wg/dnsop/charter/>.

   [mockapetris-history]
              Mockapetris, P., "Private discussion", January 2015.

   [kaliski-minimum]
              Kaliski, B., "Minimum Disclosure: What Information Does a
              Name Server Need to Do Its Job?", March 2015,
              <http://blogs.verisigninc.com/blog/entry/
              minimum_disclosure_what_information_does>.

8.3.  URIs

   [1] https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=648

   [2] https://github.com/bortzmeyer/my-IETF-work/blob/master/draft-
       ietf-dnsop-qname-minimisation/zonecut.go

   [3] https://lists.dns-oarc.net/pipermail/dns-
       operations/2010-February/005003.html

Appendix A.  An algorithm to find the zone cut

   Although a validating resolver already has the logic to find the zone
   cut, other resolvers may be interested by this algorithm to follow in
   order to locate this cut:

      (0) If the query can be answered from the cache, do so, otherwise
      iterate as follows:

      (1) Find closest enclosing NS RRset in your cache.  The owner of
      this NS RRset will be a suffix of the QNAME - the longest suffix
      of any NS RRset in the cache.  Call this PARENT.

      (2) Initialize CHILD to the same as PARENT.

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      (3) If CHILD is the same as the QNAME, resolve the original query
      using PARENT's name servers, and finish.

      (4) Otherwise, add a label from the QNAME to the start of CHILD.

      (5) If you have a negative cache entry for the NS RRset at CHILD,
      go back to step 3.

      (6) Query for CHILD IN NS using PARENT's name servers.  The
      response can be:

         (6a) A referral.  Cache the NS RRset from the authority section
         and go back to step 1.

         (6b) An authoritative answer.  Cache the NS RRset from the
         answer section and go back to step 1.

         (6c) An NXDOMAIN answer.  Return an NXDOMAIN answer in response
         to the original query and stop.

         (6d) A NOERROR/NODATA answer.  Cache this negative answer and
         go back to step 3.

Author's Address

   Stephane Bortzmeyer
   AFNIC
   1, rue Stephenson
   Montigny-le-Bretonneux  78180
   France

   Phone: +33 1 39 30 83 46
   Email: bortzmeyer+ietf@nic.fr
   URI:   http://www.afnic.fr/

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