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rfc:rfc7816

Internet Engineering Task Force (IETF) S. Bortzmeyer Request for Comments: 7816 AFNIC Category: Experimental March 2016 ISSN: 2070-1721

           DNS Query Name Minimisation to Improve Privacy

Abstract

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

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  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).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7816.

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Bortzmeyer Experimental [Page 1] RFC 7816 QNAME Minimisation March 2016

Table of Contents

 1. Introduction and Background .....................................2
 2. QNAME Minimisation ..............................................3
 3. Possible Issues .................................................4
 4. Protocol and Compatibility Discussion ...........................5
 5. Operational Considerations ......................................5
 6. Performance Considerations ......................................6
 7. On the Experimentation ..........................................6
 8. Security Considerations .........................................7
 9. References ......................................................7
    9.1. Normative References .......................................7
    9.2. Informative References .....................................8
 Appendix A. An Algorithm to Perform QNAME Minimisation .............9
 Appendix B. Alternatives  .........................................10
 Acknowledgments ...................................................11
 Author's Address ..................................................11

1. Introduction and Background

 The problem statement is described in [RFC7626].  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.
 QNAME minimisation follows the principle explained in Section 6.1 of
 [RFC6973]: the less data you send out, the fewer privacy problems
 you have.
 Currently, when a resolver receives the query "What is the AAAA
 record for www.example.com?", 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.  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.

Bortzmeyer Experimental [Page 2] RFC 7816 QNAME Minimisation March 2016

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 way that maximises privacy benefits (other
 alternatives are discussed in the appendices).
 Instead of sending the full QNAME and the original QTYPE upstream, a
 resolver that implements QNAME minimisation and does not already have
 the answer in its cache sends a request to the name server
 authoritative for the closest known ancestor of the original QNAME.
 The request is done with:
 o  the QTYPE NS
 o  the QNAME that 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
 foo.bar.baz.example.  Let's assume that 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
 (zone cuts are described in Section 6 of [RFC2181]).  But zone cuts
 do not necessarily exist at every label boundary.  If we take 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 the NS records for bar.example.  It will get a
 NODATA response, indicating that there is no zone cut at that point,
 so it has to query the .example name servers again with one more
 label, and so on.  (Appendix A describes this algorithm in deeper
 detail.)
 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.

Bortzmeyer Experimental [Page 3] RFC 7816 QNAME Minimisation March 2016

 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 is just above).

3. Possible Issues

 QNAME minimisation is legal, since the original DNS RFCs do not
 mandate sending the full QNAME.  So, in theory, it should work
 without any problems.  However, in practice, some problems may occur
 (see [Huque-QNAME-Min] for an analysis and [Huque-QNAME-storify] for
 an interesting discussion on this topic).
 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.
 This behaviour is a protocol violation, and there is no need to stop
 improving the DNS because of such behaviour.  However, QNAME
 minimisation may still work with such domains, since they are only
 leaf domains (no need to send them NS requests).  Such a 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.
 Another way to deal with such incorrect name servers would be to try
 with QTYPE=A requests (A being chosen because it is the most common
 and hence a QTYPE that will always be accepted, while a QTYPE NS may
 ruffle the feathers of some middleboxes).  Instead of querying
 name servers with a query "NS example.com", we could use
 "A _.example.com" and see if we get a referral.
 A problem can also appear when a name server does not react properly
 to ENTs (Empty Non-Terminals).  If ent.example.com has no resource
 records but foobar.ent.example.com does, then ent.example.com is an
 ENT.  Whatever the QTYPE, a query for ent.example.com must return
 NODATA (NOERROR / ANSWER: 0).  However, some name servers incorrectly
 return NXDOMAIN for ENTs.  If a resolver queries only
 foobar.ent.example.com, everything will be OK, but if it implements
 QNAME minimisation, it may query ent.example.com and get an NXDOMAIN.
 See also Section 3 of [DNS-Res-Improve] for the other bad
 consequences of this bad behaviour.
 A possible solution, currently implemented in Knot, is to retry with
 the full query when you receive an NXDOMAIN.  It works, but it is not
 ideal for privacy.
 Other practices that do not conform to the DNS protocol standards may
 pose a problem: there is a common DNS trick used by some web hosters
 that also do DNS hosting that exploits the fact that the DNS protocol

Bortzmeyer Experimental [Page 4] RFC 7816 QNAME Minimisation March 2016

 (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 192.0.2.6
 (They could just wildcard all of "*.", which would be sufficient.  We
 don't know why they don't do it.)
 This lets them have many web-hosting customers without having to
 configure thousands of individual zones on their name servers.  They
 just tell the prospective customer to point their NS records at the
 hoster's name servers, 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 not give the right result, which may
 endanger QNAME minimisation (it will be a problem for DNSSEC, too).

4. Protocol and Compatibility 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 a
 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
 Section 5.3.3 of [RFC1034] or Section 7.2 of [RFC1035].  As stated in
 Section 1, the current method, sending the full QNAME, is not
 mandated by the DNS protocol.
 One may notice that many documents that explain the DNS and that are
 intended for a wide audience incorrectly describe the resolution
 process as using QNAME minimisation (e.g., by showing a request going
 to the root, with just the TLD in the query).  As a result, these
 documents may confuse readers that use them for privacy analysis.

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).

Bortzmeyer Experimental [Page 5] RFC 7816 QNAME Minimisation March 2016

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
 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 nor C.example
 could exist.  Thus, in this common case the total number of upstream
 queries under QNAME minimisation would be counterintuitively 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 typical TLD, delegation-only, and with delegations
 just under the TLD, a two-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
 www.host.group.department.example.com, where
 host.group.department.example.com is hosted on example.com's
 name servers).  Let's assume a resolver that knows only the
 name servers of .example.  Without QNAME minimisation, it would send
 these .example 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.

7. On the Experimentation

 This document has status "Experimental".  Since the beginning of time
 (or DNS), the fully qualified host name was always sent to the
 authoritative name servers.  There was a concern that changing this
 behaviour may engage the Law of Unintended Consequences -- hence this
 status.
 The idea behind the experiment is to observe QNAME minimisation in
 action with multiple resolvers, various authoritative name servers,
 etc.

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8. 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 zero protection against the recursive
 resolver, which still sees the full request coming from the stub
 resolver.
 All the alternatives mentioned in Appendix B decrease privacy in the
 hope of improving performance.  They must not be used if you want
 maximum privacy.

9. References

9.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
            <http://www.rfc-editor.org/info/rfc1034>.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [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,
            <http://www.rfc-editor.org/info/rfc6973>.
 [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
            DOI 10.17487/RFC7626, August 2015,
            <http://www.rfc-editor.org/info/rfc7626>.

Bortzmeyer Experimental [Page 7] RFC 7816 QNAME Minimisation March 2016

9.2. Informative References

 [DNS-Res-Improve]
            Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
            Resolvers for Resiliency, Robustness, and Responsiveness",
            Work in Progress, draft-vixie-dnsext-resimprove-00,
            June 2010.
 [HAMMER]   Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly
            Automated Method for Maintaining Expiring Records", Work
            in Progress, draft-wkumari-dnsop-hammer-01, July 2014.
 [Huque-QNAME-Min]
            Huque, S., "Query name minimization and authoritative
            server behavior", May 2015,
            <https://indico.dns-oarc.net/event/21/contribution/9>.
 [Huque-QNAME-storify]
            Huque, S., "Qname Minimization @ DNS-OARC", May 2015,
            <https://storify.com/shuque/qname-minimization-dns-oarc>.
 [Kaliski-Minimum]
            Kaliski, B., "Minimum Disclosure: What Information Does a
            Name Server Need to Do Its Job?", March 2015,
            <http://blogs.verisigninc.com/blog/entry/
            minimum_disclosure_what_information_does>.
 [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
            Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
            <http://www.rfc-editor.org/info/rfc2181>.

Bortzmeyer Experimental [Page 8] RFC 7816 QNAME Minimisation March 2016

Appendix A. An 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 other resolvers may want to use this
 algorithm to locate the cuts.  This is just a possible aid for
 implementers; it is not intended to be normative:
 (0) If the query can be answered from the cache, do so; otherwise,
     iterate as follows:
 (1) Find the 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 ANCESTOR.
 (2) Initialise CHILD to the same as ANCESTOR.
 (3) If CHILD is the same as the QNAME, 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) 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 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) 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.

Bortzmeyer Experimental [Page 9] RFC 7816 QNAME Minimisation March 2016

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.  See Section 2
 for the suggested way.  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); this
 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
 [HAMMER], to probe in advance for the introduction of zone cuts where
 none previously existed (i.e., confirm their continued absence, or
 discover them).
 To address the "number of queries" issue described in Section 6, a
 possible solution is to always use the traditional algorithm when the
 cache is cold and then to move to QNAME minimisation (precisely
 defining what is "hot" or "cold" is left to the implementer).  This
 will decrease the privacy but will guarantee no degradation of
 performance.

Bortzmeyer Experimental [Page 10] RFC 7816 QNAME Minimisation March 2016

Acknowledgments

 Thanks to Olaf Kolkman for the original idea during a KLM flight from
 Amsterdam to Vancouver, although the concept is probably much older
 (e.g., <https://lists.dns-oarc.net/pipermail/dns-operations/
 2010-February/005003.html>).  Thanks to Shumon Huque and Marek
 Vavrusa 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-pattern.

Author's Address

 Stephane Bortzmeyer
 AFNIC
 1, rue Stephenson
 Montigny-le-Bretonneux  78180
 France
 Phone: +33 1 39 30 83 46
 Email: bortzmeyer+ietf@nic.fr
 URI:   http://www.afnic.fr/

Bortzmeyer Experimental [Page 11]

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