File: rfc9156.html

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<!DOCTYPE html>
<html lang="en" class="RFC">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 9156: DNS Query Name Minimisation to Improve Privacy</title>
<meta content="Stephane Bortzmeyer" name="author">
<meta content="Ralph Dolmans" name="author">
<meta content="Paul Hoffman" name="author">
<meta content='
       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. 
    ' name="description">
<meta content="xml2rfc 3.11.1" name="generator">
<meta content="QNAME" name="keyword">
<meta content="9156" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.11.1
    Python 3.6.13
    appdirs 1.4.4
    ConfigArgParse 1.4.1
    google-i18n-address 2.4.0
    html5lib 1.0.1
    intervaltree 3.0.2
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    kitchen 1.2.6
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    pyflakes 2.1.1
    PyYAML 5.4.1
    requests 2.24.0
    setuptools 40.5.0
    six 1.14.0
    WeasyPrint 52.5
-->
<link href="rfc9156.xml" rel="alternate" type="application/rfc+xml">
<link href="#copyright" rel="license">
<style type="text/css">/*

  NOTE: Changes at the bottom of this file overrides some earlier settings.

  Once the style has stabilized and has been adopted as an official RFC style,
  this can be consolidated so that style settings occur only in one place, but
  for now the contents of this file consists first of the initial CSS work as
  provided to the RFC Formatter (xml2rfc) work, followed by itemized and
  commented changes found necssary during the development of the v3
  formatters.

*/

/* fonts */
@import url('https://fonts.googleapis.com/css?family=Noto+Sans'); /* Sans-serif */
@import url('https://fonts.googleapis.com/css?family=Noto+Serif'); /* Serif (print) */
@import url('https://fonts.googleapis.com/css?family=Roboto+Mono'); /* Monospace */

@viewport {
  zoom: 1.0;
  width: extend-to-zoom;
}
@-ms-viewport {
  width: extend-to-zoom;
  zoom: 1.0;
}
/* general and mobile first */
html {
}
body {
  max-width: 90%;
  margin: 1.5em auto;
  color: #222;
  background-color: #fff;
  font-size: 14px;
  font-family: 'Noto Sans', Arial, Helvetica, sans-serif;
  line-height: 1.6;
  scroll-behavior: smooth;
}
.ears {
  display: none;
}

/* headings */
#title, h1, h2, h3, h4, h5, h6 {
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  font-weight: bold;
  line-height: 1.3;
}
#title {
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h4 {
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svg {
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ul.ulBare, li.ulBare {
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ul.compact, .ulCompact,
ol.compact, .olCompact {
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/* definition lists */
dl {
}
dl > dt {
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/* 
dl.nohang > dt {
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}
*/
dl > dd {
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dl.compact > dd, .dlCompact > dd {
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dl > dd > dl {
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}

/* links */
a {
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}
a[href] {
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}
a[href]:hover {
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figcaption a[href],
a[href].selfRef {
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}
/* XXX probably not this:
a.selfRef:hover {
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} */

/* Figures */
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pre {
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/* aside, blockquote */
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blockquote {
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cite {
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table {
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th, td {
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th {
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table caption {
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table p {
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/* pilcrow */
a.pilcrow {
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@media screen {
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.role {
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/* info block */
#identifiers {
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#identifiers dt {
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/* Fix PDF info block run off issue */
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}
#identifiers .authors .author {
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}
#identifiers .authors .org {
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}

/* The prepared/rendered info at the very bottom of the page */
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}
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}
.docInfo .prepared {
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}

/* table of contents */
#toc  {
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}
nav.toc ul {
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  padding: 0;
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}
nav.toc li {
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  margin: 0.75em 0;
  padding-left: 1.2em;
  text-indent: -1.2em;
}
/* references */
.references dt {
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}
.references dd {
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}

.refInstance {
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}

.references .ascii {
  margin-bottom: 0.25em;
}

/* index */
.index ul {
  margin: 0 0 0 1em;
  padding: 0;
  list-style: none;
}
.index ul ul {
  margin: 0;
}
.index li {
  margin: 0;
  text-indent: -2em;
  padding-left: 2em;
  padding-bottom: 5px;
}
.indexIndex {
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}
.index a {
  font-weight: 700;
}
/* make the index two-column on all but the smallest screens */
@media (min-width: 600px) {
  .index ul {
    -moz-column-count: 2;
    -moz-column-gap: 20px;
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  .index ul ul {
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}

/* authors */
address.vcard {
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}

address.vcard .nameRole {
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}
address.vcard .label {
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}
address.vcard .type {
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}
.alternative-contact {
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}
hr.addr {
  border-top: 1px dashed;
  margin: 0;
  color: #ddd;
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}

/* temporary notes */
.rfcEditorRemove::before {
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  top: 0.2em;
  right: 0.2em;
  padding: 0.2em;
  content: "The RFC Editor will remove this note";
  color: #9e2a00; /* Arlen: WCAG 2019 */
  background-color: #ffd; /* Arlen: WCAG 2019 */
}
.rfcEditorRemove {
  position: relative;
  padding-top: 1.8em;
  background-color: #ffd; /* Arlen: WCAG 2019 */
  border-radius: 3px;
}
.cref {
  background-color: #ffd; /* Arlen: WCAG 2019 */
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}
.crefSource {
  font-style: italic;
}
/* alternative layout for smaller screens */
@media screen and (max-width: 1023px) {
  body {
    padding-top: 2em;
  }
  #title {
    padding: 1em 0;
  }
  h1 {
    font-size: 24px;
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  h2 {
    font-size: 20px;
    margin-top: -18px;  /* provide offset for in-page anchors */
    padding-top: 38px;
  }
  #identifiers dd {
    max-width: 60%;
  }
  #toc {
    position: fixed;
    z-index: 2;
    top: 0;
    right: 0;
    padding: 0;
    margin: 0;
    background-color: inherit;
    border-bottom: 1px solid #ccc;
  }
  #toc h2 {
    margin: -1px 0 0 0;
    padding: 4px 0 4px 6px;
    padding-right: 1em;
    min-width: 190px;
    font-size: 1.1em;
    text-align: right;
    background-color: #444;
    color: white;
    cursor: pointer;
  }
  #toc h2::before { /* css hamburger */
    float: right;
    position: relative;
    width: 1em;
    height: 1px;
    left: -164px;
    margin: 6px 0 0 0;
    background: white none repeat scroll 0 0;
    box-shadow: 0 4px 0 0 white, 0 8px 0 0 white;
    content: "";
  }
  #toc nav {
    display: none;
    padding: 0.5em 1em 1em;
    overflow: auto;
    height: calc(100vh - 48px);
    border-left: 1px solid #ddd;
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}

/* alternative layout for wide screens */
@media screen and (min-width: 1024px) {
  body {
    max-width: 724px;
    margin: 42px auto;
    padding-left: 1.5em;
    padding-right: 29em;
  }
  #toc {
    position: fixed;
    top: 42px;
    right: 42px;
    width: 25%;
    margin: 0;
    padding: 0 1em;
    z-index: 1;
  }
  #toc h2 {
    border-top: none;
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    font-size: 1em;
    font-weight: normal;
    margin: 0;
    padding: 0.25em 1em 1em 0;
  }
  #toc nav {
    display: block;
    height: calc(90vh - 84px);
    bottom: 0;
    padding: 0.5em 0 0;
    overflow: auto;
  }
  img { /* future proofing */
    max-width: 100%;
    height: auto;
  }
}

/* pagination */
@media print {
  body {

    width: 100%;
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    widows: 3;
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  #toc, #n-introduction {
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  }
  #toc {
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    padding-top: 0;
  }
  figure, pre {
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  }
  figure {
    overflow: scroll;
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  h1, h2, h3, h4, h5, h6 {
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  h2+*, h3+*, h4+*, h5+*, h6+* {
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  pre {
    white-space: pre-wrap;
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    font-size: 10pt;
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  table {
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  td {
    border-top: 1px solid #ddd;
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}

/* This is commented out here, as the string-set: doesn't
   pass W3C validation currently */
/*
.ears thead .left {
  string-set: ears-top-left content();
}

.ears thead .center {
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.ears thead .right {
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.ears tfoot .left {
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.ears tfoot .center {
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}

.ears tfoot .right {
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}
*/

@page :first {
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    content: normal;
    border: none;
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}

@page {
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  /* The follwing is commented out here, but set appropriately by in code, as
     the content depends on the document */
  /*
  @top-left {
    content: 'Internet-Draft';
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-left {
    content: string(ears-top-left);
    vertical-align: bottom;
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  @top-center {
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  */

}

/* Changes introduced to fix issues found during implementation */
/* Make sure links are clickable even if overlapped by following H* */
a {
  z-index: 2;
}
/* Separate body from document info even without intervening H1 */
section {
  clear: both;
}


/* Top align author divs, to avoid names without organization dropping level with org names */
.author {
  vertical-align: top;
}

/* Leave room in document info to show Internet-Draft on one line */
#identifiers dt {
  width: 8em;
}

/* Don't waste quite as much whitespace between label and value in doc info */
#identifiers dd {
  margin-left: 1em;
}

/* Give floating toc a background color (needed when it's a div inside section */
#toc {
  background-color: white;
}

/* Make the collapsed ToC header render white on gray also when it's a link */
@media screen and (max-width: 1023px) {
  #toc h2 a,
  #toc h2 a:link,
  #toc h2 a:focus,
  #toc h2 a:hover,
  #toc a.toplink,
  #toc a.toplink:hover {
    color: white;
    background-color: #444;
    text-decoration: none;
  }
}

/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
  #toc {
    padding: 0 0 1em 1em;
  }
}

/* Style section numbers with more space between number and title */
.section-number {
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}

/* prevent monospace from becoming overly large */
tt, code, pre, code {
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}

/* Fix the height/width aspect for ascii art*/
pre.sourcecode,
.art-text pre {
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}


/* Add styling for a link in the ToC that points to the top of the document */
a.toplink {
  float: right;
  margin-right: 0.5em;
}

/* Fix the dl styling to match the RFC 7992 attributes */
dl > dt,
dl.dlParallel > dt {
  float: left;
  margin-right: 1em;
}
dl.dlNewline > dt {
  float: none;
}

/* Provide styling for table cell text alignment */
table td.text-left,
table th.text-left {
  text-align: left;
}
table td.text-center,
table th.text-center {
  text-align: center;
}
table td.text-right,
table th.text-right {
  text-align: right;
}

/* Make the alternative author contact informatio look less like just another
   author, and group it closer with the primary author contact information */
.alternative-contact {
  margin: 0.5em 0 0.25em 0;
}
address .non-ascii {
  margin: 0 0 0 2em;
}

/* With it being possible to set tables with alignment
  left, center, and right, { width: 100%; } does not make sense */
table {
  width: auto;
}

/* Avoid reference text that sits in a block with very wide left margin,
   because of a long floating dt label.*/
.references dd {
  overflow: visible;
}

/* Control caption placement */
caption {
  caption-side: bottom;
}

/* Limit the width of the author address vcard, so names in right-to-left
   script don't end up on the other side of the page. */

address.vcard {
  max-width: 30em;
  margin-right: auto;
}

/* For address alignment dependent on LTR or RTL scripts */
address div.left {
  text-align: left;
}
address div.right {
  text-align: right;
}

/* Provide table alignment support.  We can't use the alignX classes above
   since they do unwanted things with caption and other styling. */
table.right {
 margin-left: auto;
 margin-right: 0;
}
table.center {
 margin-left: auto;
 margin-right: auto;
}
table.left {
 margin-left: 0;
 margin-right: auto;
}

/* Give the table caption label the same styling as the figcaption */
caption a[href] {
  color: #222;
}

@media print {
  .toplink {
    display: none;
  }

  /* avoid overwriting the top border line with the ToC header */
  #toc {
    padding-top: 1px;
  }

  /* Avoid page breaks inside dl and author address entries */
  .vcard {
    page-break-inside: avoid;
  }

}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* Make the placement of figcaption match that of a table's caption
   by removing the figure's added bottom margin */
.alignLeft.art-text,
.alignCenter.art-text,
.alignRight.art-text {
   margin-bottom: 0;
}
.alignLeft,
.alignCenter,
.alignRight {
  margin: 1em 0 0 0;
}
/* In print, the pilcrow won't show on hover, so prevent it from taking up space,
   possibly even requiring a new line */
@media print {
  a.pilcrow {
    display: none;
  }
}
/* Styling for the external metadata */
div#external-metadata {
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<table class="ears">
<thead><tr>
<td class="left">RFC 9156</td>
<td class="center">QNAME Minimisation</td>
<td class="right">November 2021</td>
</tr></thead>
<tfoot><tr>
<td class="left">Bortzmeyer, et al.</td>
<td class="center">Standards Track</td>
<td class="right">[Page]</td>
</tr></tfoot>
</table>
<div id="external-metadata" class="document-information"></div>
<div id="internal-metadata" class="document-information">
<dl id="identifiers">
<dt class="label-stream">Stream:</dt>
<dd class="stream">Internet Engineering Task Force (IETF)</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc9156" class="eref">9156</a></dd>
<dt class="label-obsoletes">Obsoletes:</dt>
<dd class="obsoletes">
<a href="https://www.rfc-editor.org/rfc/rfc7816" class="eref">7816</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2021-11" class="published">November 2021</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">S. Bortzmeyer</div>
<div class="org">AFNIC</div>
</div>
<div class="author">
      <div class="author-name">R. Dolmans</div>
<div class="org">NLnet Labs</div>
</div>
<div class="author">
      <div class="author-name">P. Hoffman</div>
<div class="org">ICANN</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 9156</h1>
<h1 id="title">DNS Query Name Minimisation to Improve Privacy</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">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.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
        <h2 id="name-status-of-this-memo">
<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This is an Internet Standards Track document.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <span><a href="https://www.rfc-editor.org/info/rfc9156">https://www.rfc-editor.org/info/rfc9156</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2021 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) 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 Revised BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Revised BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction-and-background" class="xref">Introduction and Background</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.1">
                <p id="section-toc.1-1.1.2.1.1" class="keepWithNext"><a href="#section-1.1" class="xref">1.1</a>.  <a href="#name-experience-from-rfc-7816" class="xref">Experience from RFC 7816</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.2">
                <p id="section-toc.1-1.1.2.2.1" class="keepWithNext"><a href="#section-1.2" class="xref">1.2</a>.  <a href="#name-terminology" class="xref">Terminology</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1"><a href="#section-2" class="xref">2</a>.  <a href="#name-description-of-qname-minimi" class="xref">Description of QNAME Minimisation</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.1">
                <p id="section-toc.1-1.2.2.1.1"><a href="#section-2.1" class="xref">2.1</a>.  <a href="#name-qtype-selection" class="xref">QTYPE Selection</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.2">
                <p id="section-toc.1-1.2.2.2.1"><a href="#section-2.2" class="xref">2.2</a>.  <a href="#name-qname-selection" class="xref">QNAME Selection</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.3">
                <p id="section-toc.1-1.2.2.3.1"><a href="#section-2.3" class="xref">2.3</a>.  <a href="#name-limitation-of-the-number-of" class="xref">Limitation of the Number of Queries</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.4">
                <p id="section-toc.1-1.2.2.4.1"><a href="#section-2.4" class="xref">2.4</a>.  <a href="#name-implementation-by-stub-and-" class="xref">Implementation by Stub and Forwarding Resolvers</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1"><a href="#section-3" class="xref">3</a>.  <a href="#name-algorithm-to-perform-qname-" class="xref">Algorithm to Perform QNAME Minimisation</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-qname-minimisation-examples" class="xref">QNAME Minimisation Examples</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-performance-considerations" class="xref">Performance Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-references" class="xref">References</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1">
                <p id="section-toc.1-1.7.2.1.1"><a href="#section-7.1" class="xref">7.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.2">
                <p id="section-toc.1-1.7.2.2.1"><a href="#section-7.2" class="xref">7.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#appendix-A" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#appendix-B" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a></p>
</li>
        </ul>
</nav>
</section>
</div>
<div id="intro">
<section id="section-1">
      <h2 id="name-introduction-and-background">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction-and-background" class="section-name selfRef">Introduction and Background</a>
      </h2>
<p id="section-1-1">The problem statement for this document is described in <span>[<a href="#RFC9076" class="xref">RFC9076</a>]</span>.
This specific solution is not
intended to fully solve the DNS privacy problem; instead, it
should be viewed as one tool amongst many.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">QNAME minimisation follows the principle explained in
      <span><a href="https://www.rfc-editor.org/rfc/rfc6973#section-6.1" class="relref">Section 6.1</a> of [<a href="#RFC6973" class="xref">RFC6973</a>]</span>: the less data you send out,
      the fewer privacy problems you have.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">Before QNAME minimisation, when a resolver received the query "What is
the AAAA record for www.example.com?", it sent to the root (assuming
a resolver, whose cache is empty) the very same question. Sending
the full QNAME to the authoritative name server was a tradition,
not a protocol requirement. In a conversation with one of the authors in
January 2015, Paul Mockapetris explained that this tradition comes
from a desire to optimise the number of requests, when the same
name server is authoritative for many zones in a given name
(something that was more common in the old days, where the same
name servers served .com and the root) or when the same
name server is both recursive and authoritative (something that is
strongly discouraged now). Whatever the merits of this choice at this
time, the DNS is quite different now.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">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
      <span><a href="https://www.rfc-editor.org/rfc/rfc1034#section-5.3.3" class="relref">Section 5.3.3</a> of [<a href="#RFC1034" class="xref">RFC1034</a>]</span> and
      <span><a href="https://www.rfc-editor.org/rfc/rfc1035#section-7.2" class="relref">Section 7.2</a> of [<a href="#RFC1035" class="xref">RFC1035</a>]</span>.<a href="#section-1-4" class="pilcrow">¶</a></p>
<section id="section-1.1">
        <h3 id="name-experience-from-rfc-7816">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-experience-from-rfc-7816" class="section-name selfRef">Experience from RFC 7816</a>
        </h3>
<p id="section-1.1-1">This document obsoletes <span>[<a href="#RFC7816" class="xref">RFC7816</a>]</span>.
 <span>[<a href="#RFC7816" class="xref">RFC7816</a>]</span> was categorised "Experimental", but ideas
 from it were widely deployed since its publication.
 Many resolver implementations now support QNAME minimisation. The lessons 
 learned from implementing QNAME minimisation were used to create this new
 revision.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
<p id="section-1.1-2">Data from DNSThought <span>[<a href="#dnsthought-qnamemin" class="xref">dnsthought-qnamemin</a>]</span>,
 Verisign <span>[<a href="#verisign-qnamemin" class="xref">verisign-qnamemin</a>]</span>, and APNIC <span>[<a href="#apnic-qnamemin" class="xref">apnic-qnamemin</a>]</span>
 shows that a large percentage of the resolvers deployed on the Internet already support
 QNAME minimisation in some way.<a href="#section-1.1-2" class="pilcrow">¶</a></p>
<p id="section-1.1-3">Academic research has been performed on QNAME minimisation
 <span>[<a href="#devries-qnamemin" class="xref">devries-qnamemin</a>]</span>. This
 work shows that QNAME minimisation in relaxed mode causes almost no problems.
 The paper recommends using the A QTYPE and limiting the number of queries in
 some way. Some of the issues that the paper found are covered in <a href="#perf_cons" class="xref">Section 5</a>.<a href="#section-1.1-3" class="pilcrow">¶</a></p>
</section>
<section id="section-1.2">
        <h3 id="name-terminology">
<a href="#section-1.2" class="section-number selfRef">1.2. </a><a href="#name-terminology" class="section-name selfRef">Terminology</a>
        </h3>
<p id="section-1.2-1">The terminology used in this document is defined in  <span>[<a href="#RFC8499" class="xref">RFC8499</a>]</span>.<a href="#section-1.2-1" class="pilcrow">¶</a></p>
<p id="section-1.2-2">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.<a href="#section-1.2-2" class="pilcrow">¶</a></p>
<p id="section-1.2-3">
    The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>", "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>", "<span class="bcp14">SHOULD NOT</span>", "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>",
    "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document are to be interpreted as
    described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span> 
    when, and only when, they appear in all capitals, as shown here.<a href="#section-1.2-3" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<div id="qname-main">
<section id="section-2">
      <h2 id="name-description-of-qname-minimi">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-description-of-qname-minimi" class="section-name selfRef">Description of QNAME Minimisation</a>
      </h2>
<p id="section-2-1">The idea behind QNAME minimisation is to minimise the amount of
privacy-sensitive data sent from the DNS resolver to the authoritative
name server. This section describes how to do QNAME minimisation. The algorithm
is summarised in <a href="#zonecutalgo" class="xref">Section 3</a>.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">When a resolver is not able to answer a query from cache, it has
to send a query to an authoritative name server. Traditionally, these
queries would contain the full QNAME and the original QTYPE as received
in the client query.<a href="#section-2-2" class="pilcrow">¶</a></p>
<p id="section-2-3">The full QNAME and original QTYPE are only needed at the name server that
is authoritative for the record requested by the client. All other name servers
queried while resolving the query only need to receive enough of the QNAME to
be able to answer with a delegation. The QTYPE in these queries is not
relevant, as the name server is not able to authoritatively answer the records
the client is looking for. Sending the full QNAME and original QTYPE to
these name servers therefore exposes more privacy-sensitive data than
necessary to resolve the client's request.<a href="#section-2-3" class="pilcrow">¶</a></p>
<p id="section-2-4">A resolver that implements QNAME minimisation obscures the QNAME and QTYPE in
      queries directed to an authoritative name server that is not known to be
      responsible for the original QNAME. These queries contain:<a href="#section-2-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-2-5.1">a QTYPE selected by the resolver to possibly obscure the original QTYPE<a href="#section-2-5.1" class="pilcrow">¶</a>
</li>
        <li class="normal" id="section-2-5.2">the QNAME that is the original QNAME, stripped to just one
 label more than the longest matching domain name for which the
 name server is known to be authoritative<a href="#section-2-5.2" class="pilcrow">¶</a>
</li>
      </ul>
<section id="section-2.1">
        <h3 id="name-qtype-selection">
<a href="#section-2.1" class="section-number selfRef">2.1. </a><a href="#name-qtype-selection" class="section-name selfRef">QTYPE Selection</a>
        </h3>
<p id="section-2.1-1">Note that this document relaxes the recommendation in <span>[<a href="#RFC7816" class="xref">RFC7816</a>]</span> 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.<a href="#section-2.1-1" class="pilcrow">¶</a></p>
<p id="section-2.1-2">The QTYPE to use while minimising queries can be any possible data type
 (as defined in <span><a href="https://www.rfc-editor.org/rfc/rfc6895#section-3.1" class="relref">Section 3.1</a> of [<a href="#RFC6895" class="xref">RFC6895</a>]</span>) for which the authority always lies below the zone cut
 (i.e., not DS, NSEC, NSEC3, OPT, TSIG, TKEY, ANY, MAILA, MAILB,
 AXFR, and IXFR), as long as there is no relation between the incoming QTYPE and
 the selection of the QTYPE to use while minimising.
 The A or AAAA QTYPEs are always good candidates to use because these 
 are the least likely to raise issues in DNS software and middleboxes 
 that do not properly support all QTYPEs.
 QTYPE=A or QTYPE=AAAA queries will also blend into traffic from nonminimising
 resolvers, making it in some cases harder to observe that the
 resolver is using QNAME minimisation.  Using a QTYPE that occurs
 most in incoming queries will slightly reduce the number of queries,
 as there is no extra check needed for delegations on non-apex
 records.<a href="#section-2.1-2" class="pilcrow">¶</a></p>
</section>
<section id="section-2.2">
        <h3 id="name-qname-selection">
<a href="#section-2.2" class="section-number selfRef">2.2. </a><a href="#name-qname-selection" class="section-name selfRef">QNAME Selection</a>
        </h3>
<p id="section-2.2-1">The minimising resolver works perfectly when it knows the zone cut
 (zone cuts are described in 
 <span><a href="https://www.rfc-editor.org/rfc/rfc2181#section-6" class="relref">Section 6</a> of [<a href="#RFC2181" class="xref">RFC2181</a>]</span>).  But zone cuts do not
 necessarily exist at every label boundary.  In the name
 www.foo.bar.example, it is possible that there is a zone cut between
 "foo" and "bar" but not between "bar" and "example". So, assuming
 that the resolver already knows the name servers of example, when it
 receives the query "What is the AAAA record of www.foo.bar.example?",
 it does not always know where the zone cut will be.  To find the
 zone cut, it will query the example name servers for a record for
 bar.example. It will get a non-referral answer, so it has to query the
 example name servers again with one more label, and so on.
 (<a href="#zonecutalgo" class="xref">Section 3</a> describes this algorithm in deeper
 detail.)<a href="#section-2.2-1" class="pilcrow">¶</a></p>
</section>
<div id="number-of-queries">
<section id="section-2.3">
        <h3 id="name-limitation-of-the-number-of">
<a href="#section-2.3" class="section-number selfRef">2.3. </a><a href="#name-limitation-of-the-number-of" class="section-name selfRef">Limitation of the Number of Queries</a>
        </h3>
<p id="section-2.3-1">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
<span class="bcp14">MUST</span> implement a mechanism to limit the number of outgoing queries per user
request.<a href="#section-2.3-1" class="pilcrow">¶</a></p>
<p id="section-2.3-2">Take for example an incoming QNAME with many labels, like
www.host.group.department.example.com, where
host.group.department.example.com is hosted on example.com's
name servers.
(Such deep domains are especially common under ip6.arpa.)
Assume a resolver that knows only the
name servers of example.com. Without QNAME minimisation, it would
send these example.com name servers a query for
www.host.group.department.example.com and immediately get a
specific referral or an answer, without the need for more queries
to probe for the zone cut. For such a name, a cold resolver with
QNAME minimisation will send more queries, one per label. Once the cache is
warm, there will be less difference with a traditional resolver.
Testing of this is described in <span>[<a href="#Huque-QNAME-Min" class="xref">Huque-QNAME-Min</a>]</span>.<a href="#section-2.3-2" class="pilcrow">¶</a></p>
<p id="section-2.3-3">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 <span>[<a href="#RFC8198" class="xref">RFC8198</a>]</span> can limit this attack in
some cases.<a href="#section-2.3-3" class="pilcrow">¶</a></p>
<p id="section-2.3-4">One mechanism that <span class="bcp14">MAY</span> 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 <span class="bcp14">MUST</span> be
 selected (MAX_MINIMISE_COUNT); a <span class="bcp14">RECOMMENDED</span> value is 10. Optionally, a
 value for the number of
 queries that should only have one label appended <span class="bcp14">MAY</span> be selected
 (MINIMISE_ONE_LAB); a good value is 4. The assumption here is that the number
 of labels on delegations higher in the hierarchy are rather small; therefore,
 not exposing too many labels early on has the most privacy benefit.<a href="#section-2.3-4" class="pilcrow">¶</a></p>
<p id="section-2.3-5">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".<a href="#section-2.3-5" class="pilcrow">¶</a></p>
<p id="section-2.3-6">When a resolver needs to send out a query, it will look for the closest-known
 delegation point in its cache. The number of not-yet-exposed labels is
 the difference between this closest name server and the incoming QNAME. The
 first MINIMISE_ONE_LAB labels will be handled as described in
 <a href="#qname-main" class="xref">Section 2</a>. The number of labels that are still not
 exposed now need to be divided proportionally over the remaining iterations
 (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB). If the not-yet-exposed labels cannot be
 equally divided over the remaining iterations, the remainder of the division should 
 be added to the last iterations. For example, when resolving a QNAME with 18
 labels with MAX_MINIMISE_COUNT set to 10 and MINIMISE_ONE_LAB set to 4, the number of
 labels added per iteration are: 1,1,1,1,2,2,2,2,3,3.<a href="#section-2.3-6" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-2.4">
        <h3 id="name-implementation-by-stub-and-">
<a href="#section-2.4" class="section-number selfRef">2.4. </a><a href="#name-implementation-by-stub-and-" class="section-name selfRef">Implementation by Stub and Forwarding Resolvers</a>
        </h3>
<p id="section-2.4-1">Stub and forwarding resolvers <span class="bcp14">MAY</span> implement QNAME minimisation.
 Minimising queries that will be sent to an upstream resolver does not help in
 hiding data from the upstream resolver because all information will end up there
 anyway. It might however limit the data exposure between the upstream resolver
 and the authoritative name server in the situation where the upstream resolver does
 not support QNAME minimisation. Using QNAME minimisation in a stub or forwarding
 resolver that does not have a mechanism to find and cache zone cuts will
 drastically increase the number of outgoing queries.<a href="#section-2.4-1" class="pilcrow">¶</a></p>
</section>
</section>
</div>
<div id="zonecutalgo">
<section id="section-3">
      <h2 id="name-algorithm-to-perform-qname-">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-algorithm-to-perform-qname-" class="section-name selfRef">Algorithm to Perform QNAME Minimisation</a>
      </h2>
<p id="section-3-1">This algorithm performs name resolution with QNAME minimisation in
      the presence of zone cuts that are not yet known.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">Although a validating resolver already has the logic to find the
      zone cuts, implementers of resolvers may want to use
      this algorithm to locate the zone cuts.<a href="#section-3-2" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="olPercent" id="section-3-3">
 <dt>(0)</dt>
<dd id="section-3-3.1">If the query can be answered from the cache, do so;
 otherwise, iterate as follows:<a href="#section-3-3.1" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>(1)</dt>
<dd id="section-3-3.2">
          <p id="section-3-3.2.1">Get the closest delegation point that can be used for
 the original QNAME from the cache.<a href="#section-3-3.2.1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="olPercent" id="section-3-3.2.2">
     <dt>(1a)</dt>
<dd id="section-3-3.2.2.1">For queries with a QTYPE for which the authority only lies at the
     parent side (like QTYPE=DS), this is the NS RRset with the owner matching the
     most labels with QNAME stripped by one label. QNAME will be a subdomain of
     (but not equal to) this NS RRset.  Call this ANCESTOR.<a href="#section-3-3.2.2.1" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt>(1b)</dt>
<dd id="section-3-3.2.2.2">For queries with other original QTYPEs, this is the NS RRset with the
     owner matching the most labels with QNAME. QNAME will be equal to or a
     subdomain of this NS RRset.  Call this ANCESTOR.<a href="#section-3-3.2.2.2" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
</dl>
</dd>
        <dd class="break"></dd>
<dt>(2)</dt>
<dd id="section-3-3.3">Initialise CHILD to the same as ANCESTOR.<a href="#section-3-3.3" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>(3)</dt>
<dd id="section-3-3.4">If CHILD is the same as QNAME, or if CHILD is one
 label shorter than QNAME and the original QTYPE can only be at the parent side
 (like QTYPE=DS), resolve the original query as normal, starting from ANCESTOR's
 name servers. Start over from step 0 if new names need to be resolved as a
 result of this answer, for example, when the answer contains a CNAME or DNAME <span>[<a href="#RFC6672" class="xref">RFC6672</a>]</span> record.<a href="#section-3-3.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>(4)</dt>
<dd id="section-3-3.5">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 <a href="#number-of-queries" class="xref">Section 2.3</a>.<a href="#section-3-3.5" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>(5)</dt>
<dd id="section-3-3.6">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
 <span>[<a href="#RFC8020" class="xref">RFC8020</a>]</span>,
 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
 <span>[<a href="#RFC8020" class="xref">RFC8020</a>]</span>, go back to step 3.<a href="#section-3-3.6" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt>(6)</dt>
<dd id="section-3-3.7">
          <p id="section-3-3.7.1">Query for CHILD with the selected QTYPE using one of ANCESTOR's
 name servers. The response can be:<a href="#section-3-3.7.1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="olPercent" id="section-3-3.7.2">
     <dt>(6a)</dt>
<dd id="section-3-3.7.2.1">A referral. Cache the NS RRset from the authority
     section, and go back to step 1.<a href="#section-3-3.7.2.1" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt>(6b)</dt>
<dd id="section-3-3.7.2.2">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.<a href="#section-3-3.7.2.2" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt>(6c)</dt>
<dd id="section-3-3.7.2.3">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.<a href="#section-3-3.7.2.3" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt>(6d)</dt>
<dd id="section-3-3.7.2.4">An NXDOMAIN response.
     If the resolver supports <span>[<a href="#RFC8020" class="xref">RFC8020</a>]</span>, return an
     NXDOMAIN response to the original
     query, and stop. If the resolver does not support <span>[<a href="#RFC8020" class="xref">RFC8020</a>]</span>, go to step 3.<a href="#section-3-3.7.2.4" class="pilcrow">¶</a>
</dd>
            <dd class="break"></dd>
<dt>(6e)</dt>
<dd id="section-3-3.7.2.5">A timeout or response with another RCODE. The implementation
     may choose to retry step 6 with a different ANCESTOR name server.<a href="#section-3-3.7.2.5" class="pilcrow">¶</a>
</dd>
          <dd class="break"></dd>
</dl>
</dd>
      <dd class="break"></dd>
</dl>
</section>
</div>
<section id="section-4">
      <h2 id="name-qname-minimisation-examples">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-qname-minimisation-examples" class="section-name selfRef">QNAME Minimisation Examples</a>
      </h2>
<p id="section-4-1">As a first example, assume that a resolver receives a request to resolve
foo.bar.baz.example. Assume that the resolver already knows that
ns1.nic.example is authoritative for .example and that the resolver does
not know a more specific authoritative name server. It will send the
query with QNAME=baz.example and the QTYPE selected to hide the original
QTYPE to ns1.nic.example.<a href="#section-4-1" class="pilcrow">¶</a></p>
<span id="name-cold-cache-traditional-reso"></span><table class="left" id="table-1">
        <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-cold-cache-traditional-reso" class="selfRef">Cold Cache, Traditional Resolution Algorithm without QNAME Minimisation,
      Request for MX Record of a.b.example.org</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">QTYPE</th>
            <th class="text-left" rowspan="1" colspan="1">QNAME</th>
            <th class="text-left" rowspan="1" colspan="1">TARGET</th>
            <th class="text-left" rowspan="1" colspan="1">NOTE</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">MX</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">root name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">MX</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">MX</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
        </tbody>
      </table>
<p id="section-4-3">The following are more detailed examples of requests for an MX record of
a.b.example.org with QNAME minimisation, using A QTYPE to hide the original
QTYPE and using other names and authoritative servers:<a href="#section-4-3" class="pilcrow">¶</a></p>
<span id="name-cold-cache-with-qname-minim"></span><table class="left" id="table-2">
        <caption>
<a href="#table-2" class="selfRef">Table 2</a>:
<a href="#name-cold-cache-with-qname-minim" class="selfRef">Cold Cache with QNAME Minimisation</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">QTYPE</th>
            <th class="text-left" rowspan="1" colspan="1">QNAME</th>
            <th class="text-left" rowspan="1" colspan="1">TARGET</th>
            <th class="text-left" rowspan="1" colspan="1">NOTE</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">org</td>
            <td class="text-left" rowspan="1" colspan="1">root name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">example.org</td>
            <td class="text-left" rowspan="1" colspan="1">org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1">"a" may be delegated</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">MX</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
        </tbody>
      </table>
<p id="section-4-5">Note that, in the above example, one query would have been saved if the incoming QTYPE
      was the same as the QTYPE selected by the resolver to hide the
      original QTYPE. Only one query for a.b.example.org would have been needed if
      the original QTYPE would have been A. Using the most-used QTYPE to hide the
      original QTYPE therefore slightly reduces the number of outgoing queries compared to
      using any other QTYPE to hide the original QTYPE.<a href="#section-4-5" class="pilcrow">¶</a></p>
<span id="name-warm-cache-with-qname-minim"></span><table class="left" id="table-3">
        <caption>
<a href="#table-3" class="selfRef">Table 3</a>:
<a href="#name-warm-cache-with-qname-minim" class="selfRef">Warm Cache with QNAME Minimisation</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">QTYPE</th>
            <th class="text-left" rowspan="1" colspan="1">QNAME</th>
            <th class="text-left" rowspan="1" colspan="1">TARGET</th>
            <th class="text-left" rowspan="1" colspan="1">NOTE</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">example.org</td>
            <td class="text-left" rowspan="1" colspan="1">org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">A</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1">"a" may be delegated</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">MX</td>
            <td class="text-left" rowspan="1" colspan="1">a.b.example.org</td>
            <td class="text-left" rowspan="1" colspan="1">example.org name server</td>
            <td class="text-left" rowspan="1" colspan="1"></td>
          </tr>
        </tbody>
      </table>
</section>
<div id="perf_cons">
<section id="section-5">
      <h2 id="name-performance-considerations">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-performance-considerations" class="section-name selfRef">Performance Considerations</a>
      </h2>
<p id="section-5-1">The main goal of QNAME minimisation is to improve privacy by
      sending less data. However, it may have other advantages. For
      instance, if a resolver sends a root name server queries
      for A.example followed by B.example followed by C.example, the result
      will be three NXDOMAINs, since .example does not exist in the root
      zone. When using QNAME minimisation, the resolver would send
      only one question (for .example itself) to which they could answer
      NXDOMAIN. The resolver can cache this answer and use it to prove that
      nothing below .example exists <span>[<a href="#RFC8020" class="xref">RFC8020</a>]</span>. A resolver now
      knows a priori that neither B.example nor C.example exist. Thus, in this common case,
      the total number of upstream
      queries under QNAME minimisation could be counterintuitively less
      than the number of queries under the traditional iteration (as
      described in the DNS standard).<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">QNAME minimisation can increase the number of queries based on the incoming
      QNAME. This is described in <a href="#number-of-queries" class="xref">Section 2.3</a>.
      As described in <span>[<a href="#devries-qnamemin" class="xref">devries-qnamemin</a>]</span>, 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.<a href="#section-5-2" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-6">
      <h2 id="name-security-considerations">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-6-1">QNAME minimisation's benefits are clear in the case where you want
      to decrease exposure of the queried name to the authoritative name server. But
      minimising the amount of data sent also, in part, addresses the case of a wire
      sniffer as well as the case of privacy invasion by the authoritative name
      servers. Encryption is of course a better defense against wire
      sniffers, but, unlike QNAME minimisation, it changes the protocol and
      cannot be deployed unilaterally. Also, the effect of QNAME
      minimisation on wire sniffers depends on whether the sniffer is on
      the DNS path.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">QNAME minimisation offers no protection against the recursive
      resolver, which still sees the full request coming from the stub
      resolver.<a href="#section-6-2" class="pilcrow">¶</a></p>
<p id="section-6-3">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
<a href="#number-of-queries" class="xref">Section 2.3</a>. That section proposes methods to
significantly dampen the effects of such attacks.<a href="#section-6-3" class="pilcrow">¶</a></p>
</section>
<section id="section-7">
      <h2 id="name-references">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-7.1">
        <h3 id="name-normative-references">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC1034">[RFC1034]</dt>
        <dd>
<span class="refAuthor">Mockapetris, P.</span>, <span class="refTitle">"Domain names - concepts and facilities"</span>, <span class="seriesInfo">STD 13</span>, <span class="seriesInfo">RFC 1034</span>, <span class="seriesInfo">DOI 10.17487/RFC1034</span>, <time datetime="1987-11" class="refDate">November 1987</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1034">https://www.rfc-editor.org/info/rfc1034</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC1035">[RFC1035]</dt>
        <dd>
<span class="refAuthor">Mockapetris, P.</span>, <span class="refTitle">"Domain names - implementation and specification"</span>, <span class="seriesInfo">STD 13</span>, <span class="seriesInfo">RFC 1035</span>, <span class="seriesInfo">DOI 10.17487/RFC1035</span>, <time datetime="1987-11" class="refDate">November 1987</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc1035">https://www.rfc-editor.org/info/rfc1035</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC2119">[RFC2119]</dt>
        <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03" class="refDate">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6973">[RFC6973]</dt>
        <dd>
<span class="refAuthor">Cooper, A.</span>, <span class="refAuthor">Tschofenig, H.</span>, <span class="refAuthor">Aboba, B.</span>, <span class="refAuthor">Peterson, J.</span>, <span class="refAuthor">Morris, J.</span>, <span class="refAuthor">Hansen, M.</span>, and <span class="refAuthor">R. Smith</span>, <span class="refTitle">"Privacy Considerations for Internet Protocols"</span>, <span class="seriesInfo">RFC 6973</span>, <span class="seriesInfo">DOI 10.17487/RFC6973</span>, <time datetime="2013-07" class="refDate">July 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6973">https://www.rfc-editor.org/info/rfc6973</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8174">[RFC8174]</dt>
        <dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05" class="refDate">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8499">[RFC8499]</dt>
      <dd>
<span class="refAuthor">Hoffman, P.</span>, <span class="refAuthor">Sullivan, A.</span>, and <span class="refAuthor">K. Fujiwara</span>, <span class="refTitle">"DNS Terminology"</span>, <span class="seriesInfo">BCP 219</span>, <span class="seriesInfo">RFC 8499</span>, <span class="seriesInfo">DOI 10.17487/RFC8499</span>, <time datetime="2019-01" class="refDate">January 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8499">https://www.rfc-editor.org/info/rfc8499</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-7.2">
        <h3 id="name-informative-references">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="apnic-qnamemin">[apnic-qnamemin]</dt>
        <dd>
<span class="refAuthor">Huston, G.</span> and <span class="refAuthor">J. Damas</span>, <span class="refTitle">"Measuring Query Name Minimization"</span>, <time datetime="2020-09" class="refDate">September 2020</time>, <span>&lt;<a href="https://indico.dns-oarc.net/event/34/contributions/787/attachments/777/1326/2020-09-28-oarc33-qname-minimisation.pdf">https://indico.dns-oarc.net/event/34/contributions/787/attachments/777/1326/2020-09-28-oarc33-qname-minimisation.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="devries-qnamemin">[devries-qnamemin]</dt>
        <dd>
<span class="refAuthor">de Vries, W.</span>, <span class="refAuthor">Scheitle, Q.</span>, <span class="refAuthor">Müller, M.</span>, <span class="refAuthor">Toorop, W.</span>, <span class="refAuthor">Dolmans, R.</span>, and <span class="refAuthor">R. van                 Rijswijk-Deij</span>, <span class="refTitle">"A First Look at QNAME Minimization in the Domain Name System"</span>, <time datetime="2019-03" class="refDate">March 2019</time>, <span>&lt;<a href="https://nlnetlabs.nl/downloads/publications/devries2019.pdf">https://nlnetlabs.nl/downloads/publications/devries2019.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dnsthought-qnamemin">[dnsthought-qnamemin]</dt>
        <dd>
<span class="refTitle">"Qname Minimisation"</span>, <time datetime="2021-10" class="refDate">October 2021</time>, <span>&lt;<a href="https://dnsthought.nlnetlabs.nl/#qnamemin">https://dnsthought.nlnetlabs.nl/#qnamemin</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="Huque-QNAME-Min">[Huque-QNAME-Min]</dt>
        <dd>
<span class="refAuthor">Huque, S.</span>, <span class="refTitle">"Query name minimization and authoritative server behavior"</span>, <time datetime="2015-05" class="refDate">May 2015</time>, <span>&lt;<a href="https://indico.dns-oarc.net/event/21/contribution/9">https://indico.dns-oarc.net/event/21/contribution/9</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC2181">[RFC2181]</dt>
        <dd>
<span class="refAuthor">Elz, R.</span> and <span class="refAuthor">R. Bush</span>, <span class="refTitle">"Clarifications to the DNS Specification"</span>, <span class="seriesInfo">RFC 2181</span>, <span class="seriesInfo">DOI 10.17487/RFC2181</span>, <time datetime="1997-07" class="refDate">July 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2181">https://www.rfc-editor.org/info/rfc2181</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6672">[RFC6672]</dt>
        <dd>
<span class="refAuthor">Rose, S.</span> and <span class="refAuthor">W. Wijngaards</span>, <span class="refTitle">"DNAME Redirection in the DNS"</span>, <span class="seriesInfo">RFC 6672</span>, <span class="seriesInfo">DOI 10.17487/RFC6672</span>, <time datetime="2012-06" class="refDate">June 2012</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6672">https://www.rfc-editor.org/info/rfc6672</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6895">[RFC6895]</dt>
        <dd>
<span class="refAuthor">Eastlake 3rd, D.</span>, <span class="refTitle">"Domain Name System (DNS) IANA Considerations"</span>, <span class="seriesInfo">BCP 42</span>, <span class="seriesInfo">RFC 6895</span>, <span class="seriesInfo">DOI 10.17487/RFC6895</span>, <time datetime="2013-04" class="refDate">April 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6895">https://www.rfc-editor.org/info/rfc6895</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7816">[RFC7816]</dt>
        <dd>
<span class="refAuthor">Bortzmeyer, S.</span>, <span class="refTitle">"DNS Query Name Minimisation to Improve Privacy"</span>, <span class="seriesInfo">RFC 7816</span>, <span class="seriesInfo">DOI 10.17487/RFC7816</span>, <time datetime="2016-03" class="refDate">March 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7816">https://www.rfc-editor.org/info/rfc7816</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8020">[RFC8020]</dt>
        <dd>
<span class="refAuthor">Bortzmeyer, S.</span> and <span class="refAuthor">S. Huque</span>, <span class="refTitle">"NXDOMAIN: There Really Is Nothing Underneath"</span>, <span class="seriesInfo">RFC 8020</span>, <span class="seriesInfo">DOI 10.17487/RFC8020</span>, <time datetime="2016-11" class="refDate">November 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8020">https://www.rfc-editor.org/info/rfc8020</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8198">[RFC8198]</dt>
        <dd>
<span class="refAuthor">Fujiwara, K.</span>, <span class="refAuthor">Kato, A.</span>, and <span class="refAuthor">W. Kumari</span>, <span class="refTitle">"Aggressive Use of DNSSEC-Validated Cache"</span>, <span class="seriesInfo">RFC 8198</span>, <span class="seriesInfo">DOI 10.17487/RFC8198</span>, <time datetime="2017-07" class="refDate">July 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8198">https://www.rfc-editor.org/info/rfc8198</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC9076">[RFC9076]</dt>
        <dd>
<span class="refAuthor">Wicinski, T., Ed.</span>, <span class="refTitle">"DNS Privacy Considerations"</span>, <span class="seriesInfo">RFC 9076</span>, <span class="seriesInfo">DOI 10.17487/RFC9076</span>, <time datetime="2021-07" class="refDate">July 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc9076">https://www.rfc-editor.org/info/rfc9076</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="verisign-qnamemin">[verisign-qnamemin]</dt>
      <dd>
<span class="refAuthor">Thomas, M.</span>, <span class="refTitle">"Maximizing Qname Minimization: A New Chapter in DNS Protocol Evolution"</span>, <time datetime="2020-09" class="refDate">September 2020</time>, <span>&lt;<a href="https://blog.verisign.com/security/maximizing-qname-minimization-a-new-chapter-in-dns-protocol-evolution/">https://blog.verisign.com/security/maximizing-qname-minimization-a-new-chapter-in-dns-protocol-evolution/</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<section id="appendix-A">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="appendix-A-1">The acknowledgments from  RFC 7816 apply here.
      In addition, many participants from the DNSOP Working Group helped
      with proposals for simplification, clarification, and general
      editorial help.<a href="#appendix-A-1" class="pilcrow">¶</a></p>
</section>
<div id="authors-addresses">
<section id="appendix-B">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Stephane Bortzmeyer</span></div>
<div dir="auto" class="left"><span class="org">AFNIC</span></div>
<div dir="auto" class="left"><span class="street-address">1, rue Stephenson</span></div>
<div dir="auto" class="left">
<span class="postal-code">78180</span> <span class="locality">Montigny-le-Bretonneux</span>
</div>
<div dir="auto" class="left"><span class="country-name">France</span></div>
<div class="tel">
<span>Phone:</span>
<a href="tel:+33%201%2039%2030%2083%2046" class="tel">+33 1 39 30 83 46</a>
</div>
<div class="email">
<span>Email:</span>
<a href="mailto:bortzmeyer+ietf@nic.fr" class="email">bortzmeyer+ietf@nic.fr</a>
</div>
<div class="url">
<span>URI:</span>
<a href="https://www.afnic.fr/" class="url">https://www.afnic.fr/</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Ralph Dolmans</span></div>
<div dir="auto" class="left"><span class="org">NLnet Labs</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:ralph@nlnetlabs.nl" class="email">ralph@nlnetlabs.nl</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Paul Hoffman</span></div>
<div dir="auto" class="left"><span class="org">ICANN</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:paul.hoffman@icann.org" class="email">paul.hoffman@icann.org</a>
</div>
</address>
</section>
</div>
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