Network Working Group                                      S. Bortzmeyer
Internet-Draft                                                     AFNIC
Obsoletes: 7816 (if approved)                                 R. Dolmans
Intended status: Standards Track                              NLnet Labs
Expires: September 7, 2020                                    P. Hoffman
                                                           March 6, 2020

             DNS Query Name Minimisation to Improve Privacy


   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 <>

   NOTE FOR THE DNSOP WORKING GROUP: There is still much work to be done
   in this draft.  Future versions of this draft will contain
   descriptions of different minimisation implementation choices that
   have been made since the RFC 7816 first came out, as well as
   deployment experience.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 7, 2020.

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Copyright Notice

   Copyright (c) 2020 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
   ( 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.

Table of Contents

   1.  Introduction and Background . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Description of QNAME Minimisation . . . . . . . . . . . . . .   3
     2.1.  Algorithm to Perform Aggressive Method QNAME Minimisation   5
   3.  QNAME Minimisation Examples . . . . . . . . . . . . . . . . .   5
   4.  Limit number of queries . . . . . . . . . . . . . . . . . . .   6
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   6.  Performance Considerations  . . . . . . . . . . . . . . . . .   9
   7.  Alternative Methods for QNAME Minimisation  . . . . . . . . .  10
   8.  Results of the Experimentation  . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   10. Implementation Status . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     11.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Changes from RFC 7816 . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction and Background

   The problem statement for this document and its predecessor [RFC7816]
   is described in [I-D.bortzmeyer-dprive-rfc7626-bis].  The terminology
   ("QNAME", "resolver", etc.) is defined in
   [I-D.ietf-dnsop-terminology-bis].  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.

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   Before QNAME minimisation, when a resolver received the query "What
   is the AAAA record for", 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.

   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 protocol.  The
   behaviour suggested here (minimising the amount of data sent in
   QNAMEs from the resolver) is allowed by Section 5.3.3 of [RFC1034] or
   Section 7.2 of [RFC1035].

1.1.  Terminology

   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",
   "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 summarized in Section 2.1.

   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

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   relevant, as the nameserver is not authoritative to answer with 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 authoritative nameserver that are not known to be
   responsible for the original QNAME.  These request are done with:

   o  a QTYPE selected by the resolver to hide the original QTYPE

   o  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.  Other methods are described in
   Section 7.

   The QTYPE to use while minimising queries can be any possible data
   TYPE RRTYPE (rfc6895 #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 as this is
   the least likely to give issues at 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 resolving has QNAME minimisation

   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, 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",
   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 2.1 describes this algorithm in deeper detail.)

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2.1.  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 QNAME from
       the cache.  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, 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 negative cache entry for the NS RRset at CHILD.  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/NODATA answer 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.

3.  QNAME Minimisation Examples

   For example, a resolver receives a request to resolve  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 QTYPE=NS, QNAME=baz.example 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 A record of

   QTYPE   QNAME           TARGET                 NOTE
   A root nameserver
   A org nameserver
   A nameserver

   Cold cache, aggressive QNAME minimisation method, request for A
   record of, using NS QTYPE to hide the original QTYPE:

   QTYPE   QNAME           TARGET                 NOTE
   NS      org             root nameserver
   NS     org nameserver
   NS nameserver
   NS nameserver "a" may be delegated
   A nameserver

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

   QTYPE   QNAME           TARGET                 NOTE
   NS     org nameserver
   NS nameserver
   NS nameserver "a" may be delegated
   A nameserver

4.  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 should implement a mechanism to limit
   the number of outgoing queries per user request.

   Take for example an incoming QNAME with many labels, like, where is hosted on's
   name servers).  Assume a resolver that knows only the name servers of  Without QNAME minimisation, it would send these name servers 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

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   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

   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 *, hosted on's name servers.  An incoming query containing a QNAME
   with more than 100 labels, ending in, will result in a
   query per label.  By using random labels the attacker can bypass the
   caching 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 sending 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.

5.  Operational Considerations

   TODO may be remove the whole section now that it is no longer

   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-Discuss] for
   an interesting discussion on this topic).

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   Note that the aggressive method described in this document prevents
   authoritative servers other than the server for a full name from
   seeing information about the relative use of the various QTYPEs.
   That information may be interesting for researchers (for instance, if
   they try to follow IPv6 deployment by counting the percentage of AAAA
   vs. A queries).

   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", a resolver could use
   "A" and see if it gets a referral.  TODO this is what
   Unbound does

   A problem can also appear when a name server does not react properly
   to ENTs (Empty Non-Terminals).  If has no resource
   records but does, then is an
   ENT.  Whatever the QTYPE, a query for must return
   NODATA (NOERROR / ANSWER: 0).  However, some name servers incorrectly
   return NXDOMAIN for ENTs.  If a resolver queries only, everything will be OK, but if it implements
   QNAME minimisation, it may query 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 or Unbound, 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
   (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:

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   *.example.          60  IN  A

   (They could just wildcard all of "*.", which would be sufficient.  It
   is impossible to tell 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).

   TODO report by Akamai about why they return erroneous responses

   TODO what to do if the resolver forwards?  Unbound disables QNAME
   minimisation in that case, since the forwarder will see everything,
   anyway.  What should a minimising resolver do when forwading the
   request to a forwarder, not to an authoritative name server?  Send
   the full qname?  Minimises?  (But how since the resolver does not
   know the zone cut?)

   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).

   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 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, 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 could be counterintuitively less than the number of
   queries under the traditional iteration (as described in the DNS

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   standard).  TODO mention [RFC8020]?  And [RFC8198], the latter
   depending on DNSSEC?

   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 4.

7.  Alternative Methods for QNAME Minimisation

   One useful optimisation may be, in the spirit of the HAMMER idea
   [HAMMER], The resolver can probe in advance for the introduction of
   zone cuts where none previously existed to confirm their continued
   absence or to discover them.

   To reduce the number of queries (an issue described in Section 6), a
   resolver could always use full name queries when the cache is cold
   and then to move to the aggressive method of QNAME minimisation when
   the cache is warm.  (Precisely defining what is "warm" or "cold" is
   left to the implementer).  This will decrease the privacy for initial
   queries but will guarantee no degradation of performance.

   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.

8.  Results of the Experimentation

   TODO various experiences from actual deployments, problems heard.
   TODO the Knot bug #339
   issues/339?  TODO Problems with AWS

9.  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

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   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

   All the alternatives mentioned in Section 7 decrease privacy in the
   hope of improving performance.  They must not be used if you want
   maximum privacy.

10.  Implementation Status

   \[\[ Note to RFC Editor: Remove this entire section, and the
   reference to RFC 7942, before publication. \]\]

   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 [RFC7942].
   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 [RFC7942], "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".

   Unbound has had a QNAME minimisation feature since version 1.5.7,
   December 2015, (see [Dolmans-Unbound]) and it has had QNAME
   minimisation turned default since version 1.7.2, June 2018.  It has
   two modes set by the "qname-minimisation-strict" configuration
   option.  In strict mode (option set to "yes"), there is no workaround
   for broken authoritative name servers.  In lax mode, Unbound retries
   when there is a NXDOMAIN response from the minimized query.  Since
   November 2016, Unbound uses only queries for the A RRtype and not the
   NS RRtype.

   Knot Resolver has had a QNAME minimisation feature since version
   1.0.0, May 2016, and it is activated by default.

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   BIND has had a QNAME minimisation feature since unstable development
   version 9.13.2, July 2018.  It currently has several modes, with or
   without workarounds for broken authoritative name servers.

   The Cloudflare's public resolver at IP address has QNAME
   minimisation.  (It currently uses Knot.)

   Testing with one thousand RIPE Atlas probes [atlas-qname-min], one
   can see that QNAME minimisation is now common:

% blaeu-resolve --requested 1000 --type TXT
["no - qname minimisation is not enabled on your resolver :("] : 888 occurrences
["hooray - qname minimisation is enabled on your resolver :)!"] : 105 occurrences
[ERROR: SERVFAIL] : 3 occurrences
Test #16113243 done at 2018-09-14T13:01:47Z

   10 % of the probes have a resolver with QNAME minimisation (it is not
   possible to infer the percentage of users having QNAME minimisation).

11.  References

11.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

11.2.  Informative References

              Bortzmeyer, S., "DNS resolution of
     on RIPE Atlas probes",
              September 2018,

              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

              Dolmans, R., "Unbound QNAME minimisation @ DNS-OARC",
              March 2016, <https://indico.dns-

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

              Huque, S., "Query name minimization and authoritative
              server behavior", May 2015,

              Bortzmeyer, S. and S. Dickinson, "DNS Privacy
              Considerations", draft-bortzmeyer-dprive-rfc7626-bis-02
              (work in progress), January 2019.

              Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", draft-ietf-dnsop-terminology-bis-14 (work in
              progress), September 2018.

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   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <>.

   [RFC8198]  Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
              DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
              July 2017, <>.


   TODO (refer to 7816)

Changes from RFC 7816

   o  Made changes to deal with errata #4644

   o  Changed status to be on standards track

   o  Major reorganization

Authors' Addresses

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

   Phone: +33 1 39 30 83 46

   Ralph Dolmans
   NLnet Labs


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   Paul Hoffman


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