DNS Query Name Minimisation to Improve Privacy
draft-ietf-dnsop-rfc7816bis-06

Network Working Group                                      S. Bortzmeyer
Internet-Draft                                                     AFNIC
Obsoletes: 7816 (if approved)                                 R. Dolmans
Intended status: Standards Track                              NLnet Labs
Expires: 1 April 2021                                         P. Hoffman
                                                                   ICANN
                                                       28 September 2020

             DNS Query Name Minimisation to Improve Privacy
                draft-ietf-dnsop-rfc7816bis-06

Abstract

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

   This document is part of the IETF DNSOP (DNS Operations) Working
   Group.  The source of the document, as well as a list of open issues,
   is at <https://framagit.org/bortzmeyer/rfc7816-bis>

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 1 April 2021.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction and Background . . . . . . . . . . . . . . . . .   2
     1.1.  Experience From RFC 7816  . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Description of QNAME Minimisation . . . . . . . . . . . . . .   3
   3.  Algorithm to Perform Aggressive Method QNAME Minimisation . .   5
   4.  QNAME Minimisation Examples . . . . . . . . . . . . . . . . .   6
   5.  Limit Number of Queries . . . . . . . . . . . . . . . . . . .   7
   6.  Performance Considerations  . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Changes from RFC 7816 . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction and Background

   The problem statement for this document is described in [RFC7626].
   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 the
   author 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.

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   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].  RFC 7816 was labelled
   "experimental", but ideas from it were widely deployed since its
   publicaiton.  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] and Verisign
   [verisign-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.

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 the RECOMMENDED
   way to do QNAME minimisation -- the way that maximises privacy
   benefits.  That algorithm is summarised in Section 3.

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   When a resolver is not able to answer a query from cache it has to
   send a query to an authoritative nameserver.  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 nameserver
   that is authoritative for the record requested by the client.  All
   other nameservers 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 nameserver is not
   able to authoritatively answer the records the client is looking for.
   Sending the full QNAME and original QTYPE to these nameservers
   therefore exposes more privacy sensitive data than necessary to
   resolve the client's request.

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

   *  a QTYPE selected by the resolver to hide 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
      nameserver is known to be authoritative

   This method is called the "aggressive method" in this document
   because the resolver won't expose the original QTYPE to nameservers
   that are not known to be responsible for the desired name.  This
   method is the safest from a privacy point of view, and is thus the
   RECOMMENDED method for this document.

   Note that this document relaxes the recommendation in RFC 7816 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 [RFC6895] Section 3.1) 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.  A good candidate is to always use the "A"
   QTYPE because this is the least likely to raise issues in DNS
   software and middleboxes that do not properly support all QTYPEs.
   The QTYPE=A queries will also blend into traffic from non-minimising
   resolvers, making it in some cases harder to observe that the

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   resolver is using QNAME minimisation.  Using the 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.  Another potential benefit of using QTYPE=A is that
   [RFC8305] clients that need answers for both the A and AAAA types
   will send the AAAA query first.  When minimising using QTYPE=A the
   minimised query might be useful, and now already in the cache, for
   the happy eyeballs query for the A QTYPE.

   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, it has to query the
   example name servers again with one more label, and so on.
   (Section 3 describes this algorithm in deeper detail.)

   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
   nameserver in the situation where the upstream resolver does not
   support QNAME minimisation.  Using QNAME minimisation in a stub or
   forwarding resolvers that does not have a mechanism to find and cache
   zone cuts will drastically increase the number of outgoing queries.

3.  Algorithm to Perform Aggressive Method QNAME Minimisation

   This algorithm performs name resolution with aggressive method 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 other 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/QTYPE combination from the cache.

       (1a)  For queries with QTYPE=DS this is the NS RRset with the

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            owner matching the most labels with the QNAME stripped by
            one label.  The 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 the QNAME.  The
            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 the QNAME, or if the CHILD is one label
       shorter than the QNAME and the original QTYPE is DS, resolve the
       original query using ANCESTOR's name servers, and finish.

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

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

   (6)  Query for CHILD with the minimised QTYPE using 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 NOERROR answer.  Cache this answer, and go back to step
            3.

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

       (6d)  An answer with another RCODE, or no answer.  Try another
            name server at the same delegation point.  Stop if none of
            them are able to return a valid answer.

4.  QNAME Minimisation Examples

   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.

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   Here are more detailed examples of queries with the aggressive method
   of QNAME minimisation:

   Cold cache, traditional resolution algorithm without QNAME
   minimisation, request for MX record of a.b.example.org:

   QTYPE   QNAME           TARGET                 NOTE
   MX      a.b.example.org root nameserver
   MX      a.b.example.org org nameserver
   MX      a.b.example.org example.org nameserver

   Cold cache, aggressive QNAME minimisation method, request for MX
   record of a.b.example.org, using the A QTYPE to hide the original
   QTYPE:

   QTYPE   QNAME           TARGET                 NOTE
   A       org             root nameserver
   A       example.org     org nameserver
   A       b.example.org   example.org nameserver
   A       a.b.example.org example.org nameserver "a" may be delegated
   MX      a.b.example.org example.org nameserver

   Note that in above example one query would have been saved if the
   incoming QTYPE would have been the same as the QTYPE selected by the
   resolver to hide the original QTYPE.  Only one query needed with as
   QTYPE 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.

   Warm cache with only org delegation known, (example.org's NS RRset is
   not known), aggressive QNAME minimisation method, request for MX
   record of a.b.example.org, using A QTYPE to hide the original QTYPE:

   QTYPE   QNAME           TARGET                 NOTE
   A       example.org     org nameserver
   A       b.example.org   example.org nameserver
   A       a.b.example.org example.org nameserver "a" may be delegated
   MX      a.b.example.org example.org nameserver

5.  Limit 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.

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   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.  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, depending on 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.
   Actual testing is described in [Huque-QNAME-Min].  Such deep domains
   are especially common under ip6.arpa.

   This behaviour 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 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 has to be
   selected (MAX_MINIMISE_COUNT), a good value is 10.  Optionally a
   value for the number of queries that should only have one label
   appended can 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 may labels
   early on has the most privacy benefit.

   When a resolver needs to send out a query if will look for the
   closest known delegation point in its cache.  The number of QNAME
   minimisation iterations is the difference between this closest
   nameserver and the incoming QNAME.  The first MINIMISE_ONE_LAB
   iterations will be handles as described in Section 2.  The number of
   labels that are not exposed yet now need to be divided over the
   iterations that are left (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB).
   The remainder of the division should be added to the last iterations.
   For example, when resolving a QNAME with 18 labels, the number of
   labels added per iteration are: 1,1,1,1,2,2,2,2,3,3.

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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 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 as 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 counterintuitively be
   less than the number of queries under the traditional iteration (as
   described in the DNS standard).

   QNAME minimisation may also improve lookup performance for TLD
   operators.  For a TLD that is delegation-only, a two-label QNAME
   query may be optimal for finding the delegation owner name, depending
   on the way domain matching is implemented.

   QNAME minimisation can increase the number of queries based on the
   incoming QNAME.  This is described in Section 5.

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 as 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 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.

8.  References

8.1.  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>.

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   [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>.

   [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>.

   [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>.

8.2.  Informative References

   [devries-qnamemin]
              "A First Look at QNAME Minimization in the Domain Name
              System", March 2019,
              <https://nlnetlabs.nl/downloads/publications/
              devries2019.pdf>.

   [dnsthought-qnamemin]
              "DNSThought QNAME minimisation results. Using Atlas
              probes", March 2020,
              <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>.

   [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>.

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   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/info/rfc7626>.

   [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>.

   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [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>.

   [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

   TODO (refer to 7816)

Changes from RFC 7816

   Changed in -06

   *  Removed lots of text from when this was experimental

   *  Lots of reorganization

   Changed in -04

   *  Start structure for implementation section

   *  Add clarification why the used QTYPE does not matter

   *  Make algorithm DS QTYPE compatible

   Changed in -03

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   *  Drop recommendation to use the NS QTYPE to hide the incoming QTYPE

   *  Describe DoS attach vector for QNAME with large number of labels,
      and propose a mitigation.

   *  Simplify examples and change qname to a.b.example.com to show the
      change in number of queries.

   Changed in -00, -01, and -02

   *  Made changes to deal with errata #4644

   *  Changed status to be on standards track

   *  Major reorganization

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

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