QNAME Minimisation: DNS Query Name Minimisation to Improve Privacy

Internet Engineering Task Force (IETF)                     S. Bortzmeyer
Request for Comments: 9156                                         AFNIC
Obsoletes: 7816                                               R. Dolmans
Category: Standards Track                                     NLnet Labs
ISSN: 2070-1721                                               P. Hoffman
                                                                   ICANN
                                                           November 2021


             DNS Query Name Minimisation to Improve Privacy

Abstract

   This document describes a technique called "QNAME minimisation" to
   improve DNS privacy, where the DNS resolver no longer always sends
   the full original QNAME and original QTYPE to the upstream name
   server.  This document obsoletes RFC 7816.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9156.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction and Background
     1.1.  Experience from RFC 7816
     1.2.  Terminology
   2.  Description of QNAME Minimisation
     2.1.  QTYPE Selection
     2.2.  QNAME Selection
     2.3.  Limitation of the Number of Queries
     2.4.  Implementation by Stub and Forwarding Resolvers
   3.  Algorithm to Perform QNAME Minimisation
   4.  QNAME Minimisation Examples
   5.  Performance Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction and Background

   The problem statement for this document is described in [RFC9076].
   This specific solution is not intended to fully solve the DNS privacy
   problem; instead, it should be viewed as one tool amongst many.

   QNAME minimisation follows the principle explained in Section 6.1 of
   [RFC6973]: the less data you send out, the fewer privacy problems you
   have.

   Before QNAME minimisation, when a resolver received the query "What
   is the AAAA record for www.example.com?", it sent to the root
   (assuming a resolver, whose cache is empty) the very same question.
   Sending the full QNAME to the authoritative name server was a
   tradition, not a protocol requirement.  In a conversation with one of
   the authors in January 2015, Paul Mockapetris explained that this
   tradition comes from a desire to optimise the number of requests,
   when the same name server is authoritative for many zones in a given
   name (something that 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 that is strongly
   discouraged now).  Whatever the merits of this choice at this time,
   the DNS is quite different now.

   QNAME minimisation is compatible with the current DNS system and
   therefore can easily be deployed.  Because it is only a change to the
   way that the resolver operates, it does not change the DNS protocol
   itself.  The behaviour suggested here (minimising the amount of data
   sent in QNAMEs from the resolver) is allowed by Section 5.3.3 of
   [RFC1034] and Section 7.2 of [RFC1035].

1.1.  Experience from RFC 7816

   This document obsoletes [RFC7816].  [RFC7816] was categorised
   "Experimental", but ideas from it were widely deployed since its
   publication.  Many resolver implementations now support QNAME
   minimisation.  The lessons learned from implementing QNAME
   minimisation were used to create this new revision.

   Data from DNSThought [dnsthought-qnamemin], Verisign
   [verisign-qnamemin], and APNIC [apnic-qnamemin] shows that a large
   percentage of the resolvers deployed on the Internet already support
   QNAME minimisation in some way.

   Academic research has been performed on QNAME minimisation
   [devries-qnamemin].  This work shows that QNAME minimisation in
   relaxed mode causes almost no problems.  The paper recommends using
   the A QTYPE and limiting the number of queries in some way.  Some of
   the issues that the paper found are covered in Section 5.

1.2.  Terminology

   The terminology used in this document is defined in [RFC8499].

   In this document, a "cold" cache is one that is empty, having
   literally no entries in it.  A "warm" cache is one that has some
   entries in it.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Description of QNAME Minimisation

   The idea behind QNAME minimisation is to minimise the amount of
   privacy-sensitive data sent from the DNS resolver to the
   authoritative name server.  This section describes how to do QNAME
   minimisation.  The algorithm is summarised in Section 3.

   When a resolver is not able to answer a query from cache, it has to
   send a query to an authoritative name server.  Traditionally, these
   queries would contain the full QNAME and the original QTYPE as
   received in the client query.

   The full QNAME and original QTYPE are only needed at the name server
   that is authoritative for the record requested by the client.  All
   other name servers queried while resolving the query only need to
   receive enough of the QNAME to be able to answer with a delegation.
   The QTYPE in these queries is not relevant, as the name server is not
   able to authoritatively answer the records the client is looking for.
   Sending the full QNAME and original QTYPE to these name servers
   therefore exposes more privacy-sensitive data than necessary to
   resolve the client's request.

   A resolver that implements QNAME minimisation obscures the QNAME and
   QTYPE in queries directed to an authoritative name server that is not
   known to be responsible for the original QNAME.  These queries
   contain:

   *  a QTYPE selected by the resolver to possibly obscure the original
      QTYPE

   *  the QNAME that is the original QNAME, stripped to just one label
      more than the longest matching domain name for which the name
      server is known to be authoritative

2.1.  QTYPE Selection

   Note that this document relaxes the recommendation in [RFC7816] to
   use the NS QTYPE to hide the original QTYPE.  Using the NS QTYPE is
   still allowed.  The authority of NS records lies at the child side.
   The parent side of the delegation will answer using a referral, like
   it will do for queries with other QTYPEs.  Using the NS QTYPE
   therefore has no added value over other QTYPEs.

   The QTYPE to use while minimising queries can be any possible data
   type (as defined in Section 3.1 of [RFC6895]) for which the authority
   always lies below the zone cut (i.e., not DS, NSEC, NSEC3, OPT, TSIG,
   TKEY, ANY, MAILA, MAILB, AXFR, and IXFR), as long as there is no
   relation between the incoming QTYPE and the selection of the QTYPE to
   use while minimising.  The A or AAAA QTYPEs are always good
   candidates to use because these are the least likely to raise issues
   in DNS software and middleboxes that do not properly support all
   QTYPEs.  QTYPE=A or QTYPE=AAAA queries will also blend into traffic
   from nonminimising resolvers, making it in some cases harder to
   observe that the resolver is using QNAME minimisation.  Using a QTYPE
   that occurs most in incoming queries will slightly reduce the number
   of queries, as there is no extra check needed for delegations on non-
   apex records.

2.2.  QNAME Selection

   The minimising resolver works perfectly when it knows the zone cut
   (zone cuts are described in Section 6 of [RFC2181]).  But zone cuts
   do not necessarily exist at every label boundary.  In the name
   www.foo.bar.example, it is possible that there is a zone cut between
   "foo" and "bar" but not between "bar" and "example".  So, assuming
   that 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 where the zone cut will be.  To find the zone
   cut, it will query the example name servers for a record for
   bar.example.  It will get a non-referral answer, so it has to query
   the example name servers again with one more label, and so on.
   (Section 3 describes this algorithm in deeper detail.)

2.3.  Limitation of the Number of Queries

   When using QNAME minimisation, the number of labels in the received
   QNAME can influence the number of queries sent from the resolver.
   This opens an attack vector and can decrease performance.  Resolvers
   supporting QNAME minimisation MUST implement a mechanism to limit the
   number of outgoing queries per user request.

   Take for example an incoming QNAME with many labels, like
   www.host.group.department.example.com, where
   host.group.department.example.com is hosted on example.com's name
   servers.  (Such deep domains are especially common under ip6.arpa.)
   Assume a resolver that knows only the name servers of example.com.
   Without QNAME minimisation, it would send these example.com name
   servers a query for www.host.group.department.example.com 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 send more queries, one per
   label.  Once the cache is warm, there will be less difference with a
   traditional resolver.  Testing of this is described in
   [Huque-QNAME-Min].

   The behaviour of sending multiple queries can be exploited by sending
   queries with a large number of labels in the QNAME that will be
   answered using a wildcard record.  Take for example a record for
   *.example.com, hosted on example.com's name servers.  An incoming
   query containing a QNAME with more than 100 labels, ending in
   example.com, will result in a query per label.  By using random
   labels, the attacker can bypass the cache and always require the
   resolver to send many queries upstream.  Note that [RFC8198] can
   limit this attack in some cases.

   One mechanism that MAY be used to reduce this attack vector is by
   appending more than one label per iteration for QNAMEs with a large
   number of labels.  To do this, a maximum number of QNAME minimisation
   iterations MUST be selected (MAX_MINIMISE_COUNT); a RECOMMENDED value
   is 10.  Optionally, a value for the number of queries that should
   only have one label appended MAY be selected (MINIMISE_ONE_LAB); a
   good value is 4.  The assumption here is that the number of labels on
   delegations higher in the hierarchy are rather small; therefore, not
   exposing too many labels early on has the most privacy benefit.

   Another potential, optional mechanism for limiting the number of
   queries is to assume that labels that begin with an underscore (_)
   character do not represent privacy-relevant administrative
   boundaries.  For example, if the QNAME is "_25._tcp.mail.example.org"
   and the algorithm has already searched for "mail.example.org", the
   next query can be for all the underscore-prefixed names together,
   namely "_25._tcp.mail.example.org".

   When a resolver needs to send out a query, it will look for the
   closest-known delegation point in its cache.  The number of not-yet-
   exposed labels is the difference between this closest name server and
   the incoming QNAME.  The first MINIMISE_ONE_LAB labels will be
   handled as described in Section 2.  The number of labels that are
   still not exposed now need to be divided proportionally over the
   remaining iterations (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB).  If the
   not-yet-exposed labels cannot be equally divided over the remaining
   iterations, the remainder of the division should be added to the last
   iterations.  For example, when resolving a QNAME with 18 labels with
   MAX_MINIMISE_COUNT set to 10 and MINIMISE_ONE_LAB set to 4, the
   number of labels added per iteration are: 1,1,1,1,2,2,2,2,3,3.

2.4.  Implementation by Stub and Forwarding Resolvers

   Stub and forwarding resolvers MAY implement QNAME minimisation.
   Minimising queries that will be sent to an upstream resolver does not
   help in hiding data from the upstream resolver because all
   information will end up there anyway.  It might however limit the
   data exposure between the upstream resolver and the authoritative
   name server in the situation where the upstream resolver does not
   support QNAME minimisation.  Using QNAME minimisation in a stub or
   forwarding resolver that does not have a mechanism to find and cache
   zone cuts will drastically increase the number of outgoing queries.

3.  Algorithm to Perform QNAME Minimisation

   This algorithm performs name resolution with QNAME minimisation in
   the presence of zone cuts that are not yet known.

   Although a validating resolver already has the logic to find the zone
   cuts, implementers of resolvers may want to use this algorithm to
   locate the zone cuts.

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

   (1)  Get the closest delegation point that can be used for the
        original QNAME from the cache.

        (1a)  For queries with a QTYPE for which the authority only lies
              at the parent side (like QTYPE=DS), this is the NS RRset
              with the owner matching the most labels with QNAME
              stripped by one label.  QNAME will be a subdomain of (but
              not equal to) this NS RRset.  Call this ANCESTOR.

        (1b)  For queries with other original QTYPEs, this is the NS
              RRset with the owner matching the most labels with QNAME.
              QNAME will be equal to or a subdomain of this NS RRset.
              Call this ANCESTOR.

   (2)  Initialise CHILD to the same as ANCESTOR.

   (3)  If CHILD is the same as QNAME, or if CHILD is one label shorter
        than QNAME and the original QTYPE can only be at the parent side
        (like QTYPE=DS), resolve the original query as normal, starting
        from ANCESTOR's name servers.  Start over from step 0 if new
        names need to be resolved as a result of this answer, for
        example, when the answer contains a CNAME or DNAME [RFC6672]
        record.

   (4)  Otherwise, update the value of CHILD by adding the next relevant
        label or labels from QNAME to the start of CHILD.  The number of
        labels to add is discussed in Section 2.3.

   (5)  Look for a cache entry for the RRset at CHILD with the original
        QTYPE.  If the cached response code is NXDOMAIN and the resolver
        has support for [RFC8020], the NXDOMAIN can be used in response
        to the original query, and stop.  If the cached response code is
        NOERROR (including NODATA), go back to step 3.  If the cached
        response code is NXDOMAIN and the resolver does not support
        [RFC8020], go back to step 3.

   (6)  Query for CHILD with the selected QTYPE using one of ANCESTOR'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)  A DNAME response.  Proceed as if a DNAME is received for
              the original query.  Start over from step 0 to resolve the
              new name based on the DNAME target.

        (6c)  All other NOERROR answers (including NODATA).  Cache this
              answer.  Regardless of the answered RRset type, including
              CNAMEs, continue with the algorithm from step 3 by
              building the original QNAME.

        (6d)  An NXDOMAIN response.  If the resolver supports [RFC8020],
              return an NXDOMAIN response to the original query, and
              stop.  If the resolver does not support [RFC8020], go to
              step 3.

        (6e)  A timeout or response with another RCODE.  The
              implementation may choose to retry step 6 with a different
              ANCESTOR name server.

4.  QNAME Minimisation Examples

   As a first example, assume that a resolver receives a request to
   resolve foo.bar.baz.example.  Assume that the resolver already knows
   that ns1.nic.example is authoritative for .example and that the
   resolver does not know a more specific authoritative name server.  It
   will send the query with QNAME=baz.example and the QTYPE selected to
   hide the original QTYPE to ns1.nic.example.

   +=======+=================+=========================+======+
   | QTYPE | QNAME           | TARGET                  | NOTE |
   +=======+=================+=========================+======+
   | MX    | a.b.example.org | root name server        |      |
   +-------+-----------------+-------------------------+------+
   | MX    | a.b.example.org | org name server         |      |
   +-------+-----------------+-------------------------+------+
   | MX    | a.b.example.org | example.org name server |      |
   +-------+-----------------+-------------------------+------+

      Table 1: Cold Cache, Traditional Resolution Algorithm
       without QNAME Minimisation, Request for MX Record of
                         a.b.example.org

   The following are more detailed examples of requests for an MX record
   of a.b.example.org with QNAME minimisation, using A QTYPE to hide the
   original QTYPE and using other names and authoritative servers:

   +=======+=================+=========================+============+
   | QTYPE | QNAME           | TARGET                  | NOTE       |
   +=======+=================+=========================+============+
   | A     | org             | root name server        |            |
   +-------+-----------------+-------------------------+------------+
   | A     | example.org     | org name server         |            |
   +-------+-----------------+-------------------------+------------+
   | A     | b.example.org   | example.org name server |            |
   +-------+-----------------+-------------------------+------------+
   | A     | a.b.example.org | example.org name server | "a" may be |
   |       |                 |                         | delegated  |
   +-------+-----------------+-------------------------+------------+
   | MX    | a.b.example.org | example.org name server |            |
   +-------+-----------------+-------------------------+------------+

              Table 2: Cold Cache with QNAME Minimisation

   Note that, in the above example, one query would have been saved if
   the incoming QTYPE was the same as the QTYPE selected by the resolver
   to hide the original QTYPE.  Only one query for a.b.example.org would
   have been needed if the original QTYPE would have been A.  Using the
   most-used QTYPE to hide the original QTYPE therefore slightly reduces
   the number of outgoing queries compared to using any other QTYPE to
   hide the original QTYPE.

   +=======+=================+=========================+============+
   | QTYPE | QNAME           | TARGET                  | NOTE       |
   +=======+=================+=========================+============+
   | A     | example.org     | org name server         |            |
   +-------+-----------------+-------------------------+------------+
   | A     | b.example.org   | example.org name server |            |
   +-------+-----------------+-------------------------+------------+
   | A     | a.b.example.org | example.org name server | "a" may be |
   |       |                 |                         | delegated  |
   +-------+-----------------+-------------------------+------------+
   | MX    | a.b.example.org | example.org name server |            |
   +-------+-----------------+-------------------------+------------+

              Table 3: Warm Cache with QNAME Minimisation

5.  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 resolver sends a root name server queries for A.example followed by
   B.example followed by C.example, the result will be three NXDOMAINs,
   since .example does not exist in the root zone.  When using QNAME
   minimisation, the resolver would send only one question (for .example
   itself) to which they could answer NXDOMAIN.  The resolver can cache
   this answer and use it to prove that nothing below .example exists
   [RFC8020].  A resolver now knows a priori that neither B.example nor
   C.example exist.  Thus, in this common case, the total number of
   upstream queries under QNAME minimisation could be counterintuitively
   less than the number of queries under the traditional iteration (as
   described in the DNS standard).

   QNAME minimisation can increase the number of queries based on the
   incoming QNAME.  This is described in Section 2.3.  As described in
   [devries-qnamemin], QNAME minimisation both increases the number of
   DNS lookups by up to 26% and leads to up to 5% more failed lookups.
   Filling the cache in a production resolver will soften that overhead.

6.  Security Considerations

   QNAME minimisation's benefits are clear in the case where you want to
   decrease exposure of the queried name to the authoritative name
   server.  But minimising the amount of data sent also, in part,
   addresses the case of a wire sniffer as well as the case of privacy
   invasion by the authoritative name 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 depends on whether the sniffer is on the DNS path.

   QNAME minimisation offers no protection against the recursive
   resolver, which still sees the full request coming from the stub
   resolver.

   A resolver using QNAME minimisation can possibly be used to cause a
   query storm to be sent to servers when resolving queries containing a
   QNAME with a large number of labels, as described in Section 2.3.
   That section proposes methods to significantly dampen the effects of
   such attacks.

7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

7.2.  Informative References

   [apnic-qnamemin]
              Huston, G. and J. Damas, "Measuring Query Name
              Minimization", September 2020, <https://indico.dns-
              oarc.net/event/34/contributions/787/
              attachments/777/1326/2020-09-28-oarc33-qname-
              minimisation.pdf>.

   [devries-qnamemin]
              de Vries, W., Scheitle, Q., Müller, M., Toorop, W.,
              Dolmans, R., and R. van Rijswijk-Deij, "A First Look at
              QNAME Minimization in the Domain Name System", March 2019,
              <https://nlnetlabs.nl/downloads/publications/
              devries2019.pdf>.

   [dnsthought-qnamemin]
              "Qname Minimisation", October 2021,
              <https://dnsthought.nlnetlabs.nl/#qnamemin>.

   [Huque-QNAME-Min]
              Huque, S., "Query name minimization and authoritative
              server behavior", May 2015,
              <https://indico.dns-oarc.net/event/21/contribution/9>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
              <https://www.rfc-editor.org/info/rfc6672>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <https://www.rfc-editor.org/info/rfc6895>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
              <https://www.rfc-editor.org/info/rfc7816>.

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <https://www.rfc-editor.org/info/rfc8020>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <https://www.rfc-editor.org/info/rfc8198>.

   [RFC9076]  Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
              DOI 10.17487/RFC9076, July 2021,
              <https://www.rfc-editor.org/info/rfc9076>.

   [verisign-qnamemin]
              Thomas, M., "Maximizing Qname Minimization: A New Chapter
              in DNS Protocol Evolution", September 2020,
              <https://blog.verisign.com/security/maximizing-qname-
              minimization-a-new-chapter-in-dns-protocol-evolution/>.

Acknowledgments

   The acknowledgments from RFC 7816 apply here.  In addition, many
   participants from the DNSOP Working Group helped with proposals for
   simplification, clarification, and general editorial help.

Authors' Addresses

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

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


   Ralph Dolmans
   NLnet Labs

   Email: ralph@nlnetlabs.nl


   Paul Hoffman
   ICANN

   Email: paul.hoffman@icann.org