wdiff rfc9156xml2.original.xml rfc9156.xml

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]>

<rfc xmlns:xi="http://www.w3.org/2001/XInclude" docName="draft-ietf-dnsop-rfc7816bis-11"
  category="std" number="9156" ipr="trust200902" obsoletes="7816" consensus="yes">

<?rfc toc="yes"?>
<?rfc sortrefs="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?> updates="" submissionType="IETF" category="std" consensus="true" xml:lang="en" tocInclude="true" sortRefs="true" symRefs="true" version="3">

  <!-- xml2rfc v2v3 conversion 3.9.1 -->
  <front>
    <title abbrev="QNAME Minimisation">DNS Query Name Minimisation to Improve Privacy</title>
    <seriesInfo name="RFC" value="9156"/>
    <author fullname="Stephane Bortzmeyer" initials="S." surname="Bortzmeyer">
      <organization>AFNIC</organization>
      <address>
        <postal>
          <street>1, rue Stephenson</street>
          <code>78180</code>
          <city>Montigny-le-Bretonneux</city>
          <country>France</country>
        </postal>
        <phone>+33 1 39 30 83 46</phone>
        <email>bortzmeyer+ietf@nic.fr</email>
        <uri>https://www.afnic.fr/</uri>
      </address>
    </author>
    <author fullname="Ralph Dolmans" initials="R." surname="Dolmans">
      <organization>NLnet Labs</organization>
      <address>
        <email>ralph@nlnetlabs.nl</email>
      </address>
    </author>
    <author fullname="Paul Hoffman" initials="P." surname="Hoffman">
      <organization>ICANN</organization>
      <address>
        <email>paul.hoffman@icann.org</email>
      </address>
    </author>
    <date /> year="2021" month="November"/>
    <keyword>QNAME</keyword>

    <abstract>
      <t>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.</t>
    </abstract>
  </front>
  <middle>
    <section anchor="intro" title="Introduction numbered="true" toc="default">
      <name>Introduction and Background"> Background</name>
      <t>The problem statement for this document is described in <xref target="RFC9076"/>. target="RFC9076" format="default"/>.
This specific solution is not
intended to fully solve the DNS the DNS privacy problem; instead, it
should be viewed as one tool amongst many.</t>
      <t>QNAME minimisation follows the principle explained in
Section 6.1 of
      <xref target="RFC6973"/>: target="RFC6973" sectionFormat="of" section="6.1"/>: the less data you send out,
      the fewer privacy problems you have.</t> you have.</t>
      <t>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 resolver, whose cache is empty) the very same question. Sending
the full QNAME to the authoritative name server name server was a tradition,
not a protocol requirement. In a conversation with one of the authors in
January 2015,
January 2015, Paul Mockapetris explained that this tradition comes
from a desire to optimise the number of requests, when the same
name server
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
name servers served .com and the root) or when the same
name server
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.</t>
      <t>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
      <xref target="RFC1034"/> target="RFC1034" sectionFormat="of" section="5.3.3"/> and
Section 7.2 of
      <xref target="RFC1035"/>.</t> target="RFC1035" sectionFormat="of" section="7.2"/>.</t>
      <section title="Experience From numbered="true" toc="default">
        <name>Experience from RFC 7816"> 7816</name>
        <t>This document obsoletes <xref target="RFC7816" />.
RFC 7816 format="default"/>.
	<xref target="RFC7816" format="default"/> was labelled "experimental", 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.</t>
        <t>Data from DNSThought <xref target="dnsthought-qnamemin" />, format="default"/>,
	Verisign <xref target="verisign-qnamemin" /> format="default"/>, and APNIC <xref target="apnic-qnamemin"/>
	target="apnic-qnamemin" format="default"/>
	shows that a large percentage of the resolvers deployed on the Internet already support
	QNAME minimisation in some way.</t>
        <t>Academic research has been performed on QNAME minimisation
	<xref target="devries-qnamemin"/>. target="devries-qnamemin" format="default"/>. This
	work shows that QNAME minimisation in relaxed mode causes almost no problems.
	The paper recommends using the A QTYPE, QTYPE and limiting the number of queries in
	some way. Some of the issues that the paper found are covered in <xref target="perf_cons"/>.</t>
	target="perf_cons" format="default"/>.</t>
      </section>
      <section title="Terminology"> numbered="true" toc="default">
        <name>Terminology</name>
        <t>The terminology used in this document is defined in  <xref target="RFC8499"/>.</t> target="RFC8499"
	format="default"/>.</t>
        <t>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.</t>

<t>The
        <t>
    The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”,
“SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and “OPTIONAL” "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in
BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t> here.
        </t>
      </section>
    </section>
    <section anchor="qname-main" title="Description numbered="true" toc="default">
      <name>Description of QNAME Minimisation"> Minimisation</name>
      <t>The idea behind QNAME minimisation is to minimise the amount of
privacy-sensitive data sent from the DNS resolver to the authoritative
name server.
name server. This section describes how to do QNAME minimisation. The algorithm
is summarised in <xref target="zonecutalgo"/>.</t> target="zonecutalgo" format="default"/>.</t>
      <t>When a resolver is not able to answer a query from cache cache, it has
to send a query to an authoritative nameserver. Traditionally name server. Traditionally, these
queries would contain the full QNAME and the original QTYPE as received
in the client query.</t>
      <t>The full QNAME and original QTYPE are only needed at the nameserver name server that
is authoritative for the record requested by the client. All other nameservers 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 nameserver name server is not able to authoritatively answer the records
the client is looking for. Sending the full QNAME and original QTYPE to
these nameservers name servers therefore exposes more privacy-sensitive data than
necessary to resolve the client's request.</t>
      <t>A resolver that implements QNAME minimisation obscures the QNAME and QTYPE in
      queries directed to an authoritative nameserver name server that is not known to be
      responsible for the original QNAME. These queries contain:

<list style="symbols">

<t>a contain:</t>
      <ul spacing="normal">
        <li>a QTYPE selected by the resolver to possibly obscure the original QTYPE</t>

<t>the QTYPE</li>
        <li>the QNAME that is the original QNAME, stripped to just one
	label more than the longest matching domain name for which the
nameserver
	name server is known to be authoritative</t>

</list></t> authoritative</li>
      </ul>
      <section title="QTYPE Selection"> numbered="true" toc="default">
        <name>QTYPE Selection</name>
        <t>Note that this document relaxes the recommendation in RFC 7816 <xref target="RFC7816"
	format="default"/> 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.</t>
        <t>The QTYPE to use while minimising queries can be any possible data type
	(as defined in <xref target="RFC6895"/> Section 3.1) target="RFC6895" section="3.1" sectionFormat="of"
	format="default"/>) for which the authority always lies below the zone cut
(i.e.
	(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.
Good candidates
	The A or AAAA QTYPEs are to always good candidates to use the "A" or "AAAA"
QTYPE 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 non-minimising 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.
</t>
	records.</t>
      </section>
      <section title="QNAME Selection"> numbered="true" toc="default">
        <name>QNAME Selection</name>
        <t>The minimising resolver works perfectly when it knows the zone cut
	(zone cuts are described in Section 6 of
	<xref target="RFC2181"/>). target="RFC2181" sectionFormat="of" section="6"/>).  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 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,
	zone cut, it will query the example name servers name servers for a record for
	bar.example. It will get a non-referral answer, so it has to query the
	example name servers name servers again with one more label, and so on.
	(<xref target="zonecutalgo"/> target="zonecutalgo" format="default"/> describes this algorithm in deeper
	detail.)</t>
      </section>
      <section anchor="number-of-queries" title="Limit numbered="true" toc="default">
        <name>Limitation of the Number of Queries"> Queries</name>
        <t>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
<bcp14>MUST</bcp14> implement a mechanism to limit the number of outgoing queries per user
request.</t>
        <t>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.
name servers.
(Such deep domains are especially common under ip6.arpa.)
Assume a resolver that knows only the
name servers
name servers of example.com. Without QNAME minimisation, it would
send these example.com name servers 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 <xref target="Huque-QNAME-Min"/>. target="Huque-QNAME-Min" format="default"/>.
</t>
        <t>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 <xref target="RFC8198"/> target="RFC8198" format="default"/> can limit this attack in
some cases.</t>
        <t>One mechanism that MAY <bcp14>MAY</bcp14> 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 <bcp14>MUST</bcp14> be
	selected (MAX_MINIMISE_COUNT); a RECOMMENDED <bcp14>RECOMMENDED</bcp14> value is 10. Optionally, a
	value for the number of
	queries that should only have one label appended MAY <bcp14>MAY</bcp14> be selected
(MINIMISE_ONE_LAB),
	(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 small; therefore,
	not exposing too many labels early on has the most privacy benefit.</t>
        <t>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".</t>
        <t>When a resolver needs to send out a query, it will look for the closest known closest-known
	delegation point in its cache. The number of not yet exposed not-yet-exposed labels is
	the difference between this closest nameserver name server and the incoming QNAME. The
	first MINIMISE_ONE_LAB labels will be handled as described in
	<xref target="qname-main"/>. target="qname-main" format="default"/>. 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 not-yet-exposed labels can not 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.</t>
      </section>
      <section title="Stub numbered="true" toc="default">
        <name>Implementation by Stub and Forwarding Resolvers"> Resolvers</name>
        <t>Stub and forwarding resolvers MAY <bcp14>MAY</bcp14> 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, might however limit the data exposure between the upstream resolver
	and the authoritative nameserver name server in the situation where the upstream resolver does
	not support QNAME minimisation. Using QNAME minimisation in a stub or forwarding
resolvers
	resolver that does not have a mechanism to find and cache zone cuts will
	drastically increase the number of outgoing queries.</t>
      </section>
    </section>
    <section anchor="zonecutalgo" title="Algorithm numbered="true" toc="default">
      <name>Algorithm to Perform QNAME Minimisation"> Minimisation</name>
      <t>This algorithm performs name resolution with QNAME minimisation in
      the presence of zone cuts that are not yet known.</t>
      <t>Although a validating resolver already has the logic to find the
zone cuts,
      zone cuts, implementers of resolvers may want to use
      this algorithm to locate the zone cuts.

<list style="hanging" hangIndent="4">

<t hangText="(0)">If cuts.</t>
      <ol type="(%d)" start="0">
	<li>If the query can be answered from the cache, do so;
	otherwise, iterate as follows:</t>

<t hangText="(1)">Get follows:</li>
	<li>
           <t>Get the closest delegation point that can be used for
	the original QNAME from the cache.
<list style="hanging" hangIndent="5">

<t hangText="(1a)">
For cache.</t>
            <ol type="(1%c)">
	    <li>For queries with a QTYPE for which the authority only lies at the
	    parent side (like QTYPE=DS) 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.</t>

<t hangText="(1b)">
For ANCESTOR.</li>
	    <li>For queries with other original QTYPEs 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.</t>
</list></t>

<t hangText="(2)">Initialise ANCESTOR.</li>
            </ol>
        </li>
	<li>Initialise CHILD to the same as ANCESTOR.</t>

<t hangText="(3)">If ANCESTOR.</li>
	<li>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 normal, starting from ANCESTOR's name servers.
	name servers. Start over from step 0 if new names need to be resolved as a
	result of this answer, for example example, when the answer contains a CNAME or DNAME <xref target="RFC6672"/> record.</t>

<t hangText="(4)">Otherwise,
	target="RFC6672" format="default"/> record.</li>
	<li>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 <xref target="number-of-queries"/>.</t>

<t hangText="(5)">Look
	target="number-of-queries" format="default"/>.</li>
	<li>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
	<xref target="RFC8020"/>, target="RFC8020" format="default"/>,
	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
RFC 8020,
	<xref target="RFC8020" format="default"/>, go back to step 3.</t>

<t hangText="(6)">Query 3.</li>
	<li><t>Query for CHILD with the selected QTYPE using one of ANCESTOR's
name servers.
	name servers. The response can be:

<list style="hanging" hangIndent="5">

<t hangText="(6a)">A be:</t>
            <ol type="(6%c)">
	    <li>A referral. Cache the NS RRset from the authority
	    section, and go back to step 1.</t>

<t hangText="(6b)">A 1.</li>
	    <li>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.</t>

<t hangText="(6c)">All target.</li>
	    <li>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.</t>

<t hangText="(6d)">An QNAME.</li>
	    <li>An NXDOMAIN response.
	    If the resolver supports RFC8020, <xref target="RFC8020" format="default"/>, return an
	    NXDOMAIN response to the original query
	    query, and stop. If the resolver does not support RFC8020, <xref target="RFC8020"
	    format="default"/>, go to step 3.</t>

<t hangText="(6e)"> A 3.</li>
	    <li>A timeout or response with another RCODE. The implementation
	    may choose to retry step (6) 6 with a different ANCESTOR name server.</t>

</list></t>

</list></t> server.</li>
          </ol>
	</li>
      </ol>
    </section>
    <section title="QNAME numbered="true" toc="default">
      <name>QNAME Minimisation Examples"> Examples</name>
      <t>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, .example and that the resolver does
not know a more specific authoritative name server. name server. It will send the
query with QNAME=baz.example and the QTYPE selected to hide the original
QTYPE to ns1.nic.example.</t>
<table align="left">
  <name>Cold Cache, Traditional Resolution Algorithm without QNAME Minimisation,
      Request for MX Record of a.b.example.org</name>
  <thead>
    <tr>
      <th>QTYPE</th>
      <th>QNAME</th>
      <th>TARGET</th>
      <th>NOTE</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>MX</td>
      <td>a.b.example.org</td>
      <td>root name server</td>
      <td></td>
    </tr>
    <tr>
      <td>MX</td>
      <td>a.b.example.org</td>
      <td>org name server</td>
      <td></td>
    </tr>
    <tr>
      <td>MX</td>
      <td>a.b.example.org</td>
      <td>example.org name server</td>
      <td></td>
    </tr>
  </tbody>
</table>
<t>The following are more detailed examples of other queries with QNAME
minimisation, using other names and authoritative servers:</t>

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

<figure><artwork>
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
</artwork></figure></t>

<t>Cold cache, with QNAME minimisation, request for MX record of
a.b.example.org, using the A QTYPE to hide the original QTYPE:

<figure><artwork>
QTYPE and using other names and authoritative servers:</t>
<table align="left">
  <name>Cold Cache with 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" Minimisation</name>
  <thead>
    <tr>
      <th>QTYPE</th>
      <th>QNAME</th>
      <th>TARGET</th>
      <th>NOTE</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>A</td>
      <td>org</td>
      <td>root name server</td>
      <td></td>
    </tr>
    <tr>
      <td>A</td>
      <td>example.org</td>
      <td>org name server</td>
      <td></td>
    </tr>
    <tr>
      <td>A</td>
      <td>b.example.org</td>
      <td>example.org name server</td>
      <td></td>
    </tr>
    <tr>
      <td>A</td>
      <td>a.b.example.org</td>
      <td>example.org name server</td>
      <td>"a" may be delegated
MX      a.b.example.org example.org nameserver
</artwork></figure>

Note that delegated</td>
    </tr>
    <tr>
      <td>MX</td>
      <td>a.b.example.org</td>
      <td>example.org name server</td>
      <td></td>
    </tr>
  </tbody>
</table>
      <t>Note that, in the above example 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 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.
</t>

<t>
Warm cache QTYPE.</t>
<table align="left">
  <name>Warm Cache with only org delegation known, (example.org's NS RRset is
not known), request for MX record of
a.b.example.org, using A QTYPE to hide the original QTYPE:

<figure><artwork>
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" Minimisation</name>
  <thead>
    <tr>
      <th>QTYPE</th>
      <th>QNAME</th>
      <th>TARGET</th>
      <th>NOTE</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>A</td>
      <td>example.org</td>
      <td>org name server</td>
      <td></td>
    </tr>
    <tr>
      <td>A</td>
      <td>b.example.org</td>
      <td>example.org name server</td>
      <td></td>
    </tr>
    <tr>
      <td>A</td>
      <td>a.b.example.org</td>
      <td>example.org name server</td>
      <td>"a" may be delegated
MX      a.b.example.org example.org nameserver
</artwork></figure></t> delegated</td>
    </tr>
    <tr>
      <td>MX</td>
      <td>a.b.example.org</td>
      <td>example.org name server</td>
      <td></td>
    </tr>
  </tbody>
</table>
    </section>
    <section title="Performance Considerations" anchor="perf_cons"> anchor="perf_cons" numbered="true" toc="default">
      <name>Performance Considerations</name>
      <t>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 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 (<xref target="RFC8020"/>). <xref target="RFC8020" format="default"/>. 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 counterintuitively less
      than the number of queries under the traditional iteration (as
      described in the DNS standard).
</t> standard).</t>
      <t>QNAME minimisation can increase the number of queries based on the incoming
      QNAME. This is described in <xref target="number-of-queries"/>. target="number-of-queries" format="default"/>.
      As described in <xref target="devries-qnamemin"/>, target="devries-qnamemin" format="default"/>, 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.</t>
    </section>
    <section title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>QNAME minimisation's benefits are clear in the case where you want
      to decrease exposure of the queried name to the authoritative name server. 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 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.)</t> path.</t>
      <t>QNAME minimisation offers no protection against the recursive
      resolver, which still sees the full request coming from the stub
      resolver.</t>
      <t>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
<xref target="number-of-queries"/>. target="number-of-queries" format="default"/>. That section proposes methods to
significantly dampen the effects of such attacks.</t>
    </section>
  </middle>
  <back>

<references title='Normative References'>
&rfc1034;
&rfc1035;
&rfc2119;
&rfc6973;
&rfc8174;
&rfc8499;
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6973.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8499.xml"/>
      </references>

<references title='Informative References'>
&rfc2181;
&rfc6672;
&rfc6895;
&rfc7816;
&rfc8020;
&rfc8198;
&rfc9076;
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6672.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6895.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7816.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8020.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8198.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.9076.xml"/>

        <reference anchor="Huque-QNAME-Min" target="https://indico.dns-oarc.net/event/21/contribution/9">
          <front>
            <title>Query name minimization and authoritative server behavior</title>
            <author fullname="Shumon Huque" initials="S." surname="Huque"/>
            <date month="May" year="2015"/>
          </front>
        </reference>
        <reference anchor="dnsthought-qnamemin" target="https://dnsthought.nlnetlabs.nl/#qnamemin">
          <front>
    <title>DNSThought QNAME minimisation results. Using Atlas probes</title>
            <title>Qname Minimisation</title>
            <author fullname="" initials="" surname=""/>
            <date month="March" year="2020"/> month="October" year="2021"/>
          </front>
        </reference>

        <reference anchor="devries-qnamemin" target="https://nlnetlabs.nl/downloads/publications/devries2019.pdf">
          <front>
            <title>A First Look at QNAME Minimization in the Domain Name System</title>
            <author fullname="de Vries et al." initials=""
        surname=""/> fullname="Wouter B. de Vries" initials="W" surname="de Vries"/>
	    <author fullname="Quirin Scheitle" initials="Q" surname="Scheitle"/>
	    <author fullname="Moritz Müller" initials="M" surname="Müller"/>
	    <author fullname="Willem Toorop" initials="W" surname="Toorop"/>
	    <author fullname="Ralph Dolmans" initials="R" surname="Dolmans"/>
	    <author fullname="Roland van Rijswijk-Deij" initials="R" surname="van
									       Rijswijk-Deij"/>
            <date month="March" year="2019"/>
          </front>
        </reference>

        <reference anchor="verisign-qnamemin" target="https://blog.verisign.com/security/maximizing-qname-minimization-a-new-chapter-in-dns-protocol-evolution/">
          <front>
            <title>Maximizing Qname Minimization: A New Chapter in DNS Protocol Evolution</title>
            <author fullname="Matt Thomas" initials="M." surname="Thomas"/>
            <date month="September" year="2020"/>
          </front>
        </reference>

        <reference anchor="apnic-qnamemin" target="https://indico.dns-oarc.net/event/34/contributions/787/attachments/777/1326/2020-09-28-oarc33-qname-minimisation.pdf">
          <front>
            <title>Measuring Query Name Minimization</title>
            <author fullname="Geoff Huston" initials="G." surname="Huston"/>
            <author fullname="Joao Damas" initials="J." surname="Damas"/>
            <date month="September" year="2020"/>
          </front>
        </reference>
      </references>
    </references>
    <section title="Acknowledgments" numbered="no"> numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>The acknowledgements 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.
</t>
</section>

<section title="Changes from RFC 7816" numbered="no">

<t>Changed in -07<list style="symbols">
<t>Stopped using the term "aggressive" for the method described</t>
<t>Clarified some terminology</t>
<t>More reorganization</t>
</list></t>

<t>Changed in -06<list style="symbols">
<t>Removed lots of text from when this was experimental</t>
<t>Lots of reorganization</t>
</list></t>

<t>Changed in -04<list style="symbols">
<t>Start structure for implementation section</t>
<t>Add clarification why the used QTYPE does not matter</t>
<t>Make algorithm DS QTYPE compatible</t>
</list></t>

<t>Changed in -03<list style="symbols">

<t>Drop recommendation to use the NS QTYPE to hide the incoming QTYPE</t>

<t>Describe DoS attach vector for QNAME with large number of labels, and propose
a mitigation.</t>

<t>Simplify examples and change qname to a.b.example.com to show the change in
number of queries.</t>

</list></t>

<t>Changed in -00, -01, and -02<list style="symbols">
<t>Made changes to deal with errata #4644</t>

<t>Changed status to be on standards track</t>

<t>Major reorganization</t>

</list></t> help.</t>
    </section>
  </back>
</rfc>