Network Working Group                                      S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Informational                          October 22, 2014
Expires: April 25, 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.bortzmeyer-dnsop-dns-privacy]), a
   technique called "qname minimisation".

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

Status of This Memo

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   This Internet-Draft will expire on April 25, 2015.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

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

1.  Introduction and background

   The problem statement is exposed in
   [I-D.bortzmeyer-dnsop-dns-privacy].  The terminology ("qname",
   "resolver", etc) is also defined in this companion document.  This
   specific solution is not intended to completely solve the problem,
   far from it.  It is better to see it as one tool among a toolbox.

   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.

   If "minimisation" is too long, you can write it "m12n".

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   To do such minimisation, the resolver needs to know the zone cut
   [RFC2181].  There is not a zone cut at every label 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).

   It can be noted that minimising the amount of data sent also
   partially addresses the case of a wire sniffer, not just the case of
   privacy invasion by the servers.

   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

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 will decrease their legal
   responsability, in many cases.

   Some broken name servers do not react properly to qtype=NS requests.
   As an example of today, look at (not, which is
   delegated to two name servers that reply properly to "A"
   queries but send REFUSED to queries "NS".  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).  Anyway, such setup breaks many
   things (besides qname minimisation), it breaks negative answers as
   the servers don't return the correct SOA.  It also breaks anything
   that depends on NS and SOA records existing at the top of the zone.

   Another way to deal with such broken name servers would be to try
   with A requests (A being choosen because it is the most common and
   hence the least revealing qtype).  Instead of querying name servers

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   with a query "NS", we could use "A" and see
   if we get a referral.

   Other strange and illegal practice 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:

   *.com.          60  IN  A

   ; and:

   *.uk.           60  IN  A

   ; etc.

   (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
   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.

4.  Other advantages

   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

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   minimization, 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 query
   minimisation would be counter-intuitively less than the number of
   queries under the traditional iteration (as described in the DNS

5.  Security considerations

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

6.  Acknowledgments

   Thanks to Olaf Kolkman for the original idea.  Thanks to Mark Andrews
   and Francis Dupont for the interesting discussions.  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 m12n.
   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.

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

   [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-
              bortzmeyer-dnsop-dns-privacy-02 (work in progress), April

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7.2.  Informative References

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

   [dprive]   IETF, , "The DPRIVE working group of IETF", March 2014,

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

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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