Domain Name System Operations (dnsop) Working Group        S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Experimental                          February 15, 2015
Expires: August 19, 2015

             DNS query name minimisation to improve privacy


   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".

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

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   This Internet-Draft will expire on August 19, 2015.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction and background . . . . . . . . . . . . . . . . .   2
   2.  Qname minimisation  . . . . . . . . . . . . . . . . . . . . .   2
   3.  Operational considerations  . . . . . . . . . . . . . . . . .   3
   4.  Performance implications  . . . . . . . . . . . . . . . . . .   5
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
     7.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  An algorithm to find the zone cut  . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

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 less privacy problems you'll get.

2.  Qname minimisation

   The idea is to minimise the amount of data sent from the DNS
   resolver.  When a resolver receives the query "What is the AAAA
   record for", 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).  To do so would be
   compatible with the current DNS system and therefore could be easily
   deployable, since it is an unilateral change to the resolvers, it
   does not change the protocol.  Because of that, resolver implementers
   may do qname minmisation in slightly different ways.

   If "minimisation" is too long, you can write it "m10n".

   To do such minimisation, the resolver needs to know the zone cut
   [RFC2181].  Zone cuts do not necessarily exist at every label

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   boundary.  If we take the name, it is possible
   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", it does not always know if 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).

   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

   One should note that the behaviour suggested here (minimising the
   amount of data sent in qnames) 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
   requirment.  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.

   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
   learn the zone cuts from the referrals received, then switching to NS
   queries.  This leaks more data but probably requires less 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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   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 (not  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 has no resource
   records but does, then is an
   ENT.  A query, whatever the qtype, for must return
   NODATA (NOERROR / ANSWER: 0).  However, some broken name servers
   As an example of today, look at or with
   its delegations to Akamai.  If a resolver queries only, everything will be OK but, if it implements
   qname minimisation, it may query 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", we could use "A"
   and see if we get a referral.

   Other strange and illegal practices may pose a problem: for instance,
   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 like:

   *.example.          60  IN  A

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

   This lets them turn up tons of 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

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   their nameservers, and the Web hoster doesn't have to provision
   anything in order to make the customer's domain resolve.

   Qname minimisation can decrease performance in some cases, for
   instance for a deep domain name (like where is hosted on's name
   servers).  For such a name, a cold resolver will, depending how qname
   minimisation is implemented, send more queries.  Once 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 less than the number of
   queries under the traditional iteration (as described in the DNS

   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.

5.  Security considerations

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

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6.  Acknowledgments

   Thanks to Olaf Kolkman for the original idea although the concept is
   probably much older [1].  Thanks to Mark Andrews and Francis Dupont
   for the interesting discussions.  Thanks to Brian Dickson, Warren
   Kumari 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 m10n.
   Thanks to Phillip Hallam-Baker for the fallback on A queries, to deal
   with broken servers.  Thanks to Robert Edmonds for an interesting

7.  References

7.1.  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

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

7.2.  Informative References

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

              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.

              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

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   [dnsop]    IETF, , "The DNSOP working group of IETF", March 2014,

              Mockapetris, P., "Private discussion", January 2015.

7.3.  URIs


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.

      (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.

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Author's Address

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

   Phone: +33 1 39 30 83 46

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