Domain Name System Operations (dnsop) Working Group        S. Bortzmeyer
Internet-Draft                                                     AFNIC
Intended status: Experimental                              June 19, 2015
Expires: December 21, 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", where the DNS resolver no
   longer sends the full original QNAME to the upstream name server.

   REMOVE BEFORE PUBLICATION 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 December 21, 2015.

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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  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Possible issues . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Operational considerations  . . . . . . . . . . . . . . . . .   5
   6.  Performance considerations  . . . . . . . . . . . . . . . . .   5
   7.  Security considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Implementation status - REMOVE BEFORE PUBLICATION . . . . . .   7
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix A.  An algorithm to find the zone cut  . . . . . . . . .   9
   Appendix B.  Alternatives . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

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.

   Under current practice, 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 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.

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2.  QNAME minimisation

   The idea is to minimise the amount of data sent from the DNS resolver
   to the authoritative name server.  In the example in the previous
   section, sending "What are the NS records for .com?" would have been
   sufficient (since it will be the answer from the root anyway).  The
   rest of this section describes the recommended way to do QNAME
   minimisation, the one which maximimes privacy benefits (other
   alternatives are discussed in appendixes).

   A resolver which implements QNAME minimisation, and which does not
   have already the answer in its cache, instead of sending the full
   QNAME and the original QTYPE upstream, sends a request to the name
   server authoritative for the closest known parent of the original
   QNAME.  The request is done with:

      the QTYPE NS,

      the QNAME which is the original QNAME, stripped to just one label
      more than the zone for which the server is authoritative.

   For example, a resolver receives a request to resolve  Let's assume it already knows that
   ns1.nic.example is authoritative for .example and the resolver does
   not know a more specific authoritative name server.  It will send the
   query QTYPE=NS,QNAME=baz.example to ns1.nic.example.

   The minimising resolver works perfectly when it knows the zone cut
   [RFC2181] (section 6).  But zone cuts do not necessarily exist 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 where
   the zone cut will be.  To find it out, it will query the .example
   name servers for the NS records for bar.example.  It will get a
   NODATA response, indicating there is no zone cut at that point, so it
   has to to query the .example name servers again with one more label,
   and so on.  (Appendix A describes this algorithm in deeper details.)

   Since the information about the zone cuts will be stored in the
   resolver's cache, the performance cost is probably reasonable.
   Section 6 discusses this performance discrepancy further.

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

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3.  Possible issues

   QNAME minimisation is legal, since the original DNS RFC do not
   mandate sending the full QNAME.  So, in theory, it should work
   without any problems.  However, in practice, some problems may occur
   (see an analysis in [huque-qnamemin]).

   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 QTYPE=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 non-conformant 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

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

   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).

4.  Discussion

   QNAME minimisation 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 it is an
   unilateral change, resolver implementers may do QNAME minimisation in
   slightly different ways, see the appendices for examples.)

   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).  As said in
   Section 1, the current method, sending the full QNAME, is not
   mandated by the DNS protocol.

   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.

5.  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) [kaliski-minimum].

   DNS administrators are reminded that the data on DNS requests that
   they store may have legal consequences, depending on your
   jurisdiction (check with your local lawyer).

6.  Performance considerations

   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.example
   followed by B.example followed by C.example, the result will be three

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   NXDOMAINs, since .example does not exist in the root zone.  Under
   query name minimisation, the root name servers would hear only one
   question (for .example 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.example or C.example
   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 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.

   QNAME minimisation can decrease performance in some cases, for
   instance for a deep domain name (like where is hosted on's name
   servers).  Let's assume a resolver which knows only the name servers
   of .example.  Without QNAME minimisation, it would send these
   .example nameservers a query for and immediately get a specific
   referral or an answer, without the need for more queries to probe for
   the zone cut.  For such a name, a cold resolver with QNAME
   minimisation will, depending how QNAME minimisation is implemented,
   send more queries, one per label.  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.  Actual testing
   is described in [huque-qnamemin].  Such deep domains are specially
   common under

7.  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.  Also, the effect of QNAME minimisation on
   wire sniffers depend on whether the sniffer is, on the DNS path.)

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

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8.  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

   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 but
   it has been publically announced for the future Knot DNS resolver
   [1].  For Unbound, see ticket 648 [2] and for PowerDNS [3].

   The algorithm to find the zone cuts described in Appendix A is
   implemented with QNAME minimisation in the sample code zonecut.go
   [4].  It is also implemented, for a much longer time, in an option of
   dig, "dig +trace", but without QNAME minimisation.

   Another implementation was done by Shumon Huque for testing, and is
   described in [huque-qnamemin].

9.  Acknowledgments

   Thanks to Olaf Kolkman for the original idea although the concept is
   probably much older [5].  Thanks for Shumon Huque for implementation
   and testing.  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 and for discussion of the algorithm.
   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-

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10.  References

10.1.  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-06 (work in progress), June 2015.

10.2.  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

              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

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

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

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              Kaliski, B., "Minimum Disclosure: What Information Does a
              Name Server Need to Do Its Job?", March 2015,

              Huque, S., "Query name minimization and authoritative
              server behavior", May 2015, <https://indico.dns-

              Huque, S., "Qname Minimization @ DNS-OARC", May 2015,

10.3.  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.

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      (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.

Appendix B.  Alternatives

   Remember that QNAME minimisation is unilateral so a resolver is not
   forced to implement it exactly as described here.

   There are several ways to perform QNAME minimisation.  The one in
   Section 2 is the suggested one.  It can be called the aggressive
   algorithm, since 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.  Another possible algorithm, not fully studied at this
   time, could be to "piggyback" on the traditional resolution code.  At
   startup, 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 could require
   fewer changes in the existing resolver codebase.

   In the above specification, the original QTYPE is replaced by NS (or
   may be A, if too many servers react incorrectly to NS requests),
   which is the best approach to preserve privacy.  But this erases
   information about the relative use of the various QTYPEs, which may
   be interesting for researchers (for instance if they try to follow
   IPv6 deployment by counting the percentage of AAAA vs. A queries).  A
   variant of QNAME minimisation would be to keep the original QTYPE.

   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.)

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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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