File: rfc9076.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 9076: DNS Privacy Considerations</title>
<meta content="Tim Wicinski" name="author">
<meta content="
       This document describes the privacy issues associated with the use of the DNS
  by Internet users. It provides general observations about typical current
  privacy practices. It is intended to be an analysis of the present situation
  and does not prescribe solutions. This document obsoletes RFC 7626.
 
    " name="description">
<meta content="xml2rfc 3.9.1" name="generator">
<meta content="DNS" name="keyword">
<meta content="9076" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.9.1
    Python 3.6.10
    appdirs 1.4.4
    ConfigArgParse 1.2.3
    google-i18n-address 2.3.5
    html5lib 1.0.1
    intervaltree 3.0.2
    Jinja2 2.11.2
    kitchen 1.2.6
    lxml 4.4.2
    pycairo 1.19.0
    pycountry 19.8.18
    pyflakes 2.1.1
    PyYAML 5.3.1
    requests 2.22.0
    setuptools 40.6.2
    six 1.14.0
    WeasyPrint 51
-->
<link href="rfc9076.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 {
  margin: 1em 0 0.5em;
  font-weight: bold;
  line-height: 1.3;
}
#title {
  clear: both;
  border-bottom: 1px solid #ddd;
  margin: 0 0 0.5em 0;
  padding: 1em 0 0.5em;
}
.author {
  padding-bottom: 4px;
}
h1 {
  font-size: 26px;
  margin: 1em 0;
}
h2 {
  font-size: 22px;
  margin-top: -20px;  /* provide offset for in-page anchors */
  padding-top: 33px;
}
h3 {
  font-size: 18px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h4 {
  font-size: 16px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h5, h6 {
  font-size: 14px;
}
#n-copyright-notice {
  border-bottom: 1px solid #ddd;
  padding-bottom: 1em;
  margin-bottom: 1em;
}
/* general structure */
p {
  padding: 0;
  margin: 0 0 1em 0;
  text-align: left;
}
div, span {
  position: relative;
}
div {
  margin: 0;
}
.alignRight.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignRight.art-text pre {
  padding: 0;
}
.alignRight {
  margin: 1em 0;
}
.alignRight > *:first-child {
  border: none;
  margin: 0;
  float: right;
  clear: both;
}
.alignRight > *:nth-child(2) {
  clear: both;
  display: block;
  border: none;
}
svg {
  display: block;
}
.alignCenter.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignCenter.art-text pre {
  padding: 0;
}
.alignCenter {
  margin: 1em 0;
}
.alignCenter > *:first-child {
  border: none;
  /* this isn't optimal, but it's an existence proof.  PrinceXML doesn't
     support flexbox yet.
  */
  display: table;
  margin: 0 auto;
}

/* lists */
ol, ul {
  padding: 0;
  margin: 0 0 1em 2em;
}
ol ol, ul ul, ol ul, ul ol {
  margin-left: 1em;
}
li {
  margin: 0 0 0.25em 0;
}
.ulCompact li {
  margin: 0;
}
ul.empty, .ulEmpty {
  list-style-type: none;
}
ul.empty li, .ulEmpty li {
  margin-top: 0.5em;
}
ul.ulBare, li.ulBare {
  margin-left: 0em !important;
}
ul.compact, .ulCompact,
ol.compact, .olCompact {
  line-height: 100%;
  margin: 0 0 0 2em;
}

/* definition lists */
dl {
}
dl > dt {
  float: left;
  margin-right: 1em;
}
/* 
dl.nohang > dt {
  float: none;
}
*/
dl > dd {
  margin-bottom: .8em;
  min-height: 1.3em;
}
dl.compact > dd, .dlCompact > dd {
  margin-bottom: 0em;
}
dl > dd > dl {
  margin-top: 0.5em;
  margin-bottom: 0em;
}

/* links */
a {
  text-decoration: none;
}
a[href] {
  color: #22e; /* Arlen: WCAG 2019 */
}
a[href]:hover {
  background-color: #f2f2f2;
}
figcaption a[href],
a[href].selfRef {
  color: #222;
}
/* XXX probably not this:
a.selfRef:hover {
  background-color: transparent;
  cursor: default;
} */

/* Figures */
tt, code, pre, code {
  background-color: #f9f9f9;
  font-family: 'Roboto Mono', monospace;
}
pre {
  border: 1px solid #eee;
  margin: 0;
  padding: 1em;
}
img {
  max-width: 100%;
}
figure {
  margin: 0;
}
figure blockquote {
  margin: 0.8em 0.4em 0.4em;
}
figcaption {
  font-style: italic;
  margin: 0 0 1em 0;
}
@media screen {
  pre {
    overflow-x: auto;
    max-width: 100%;
    max-width: calc(100% - 22px);
  }
}

/* aside, blockquote */
aside, blockquote {
  margin-left: 0;
  padding: 1.2em 2em;
}
blockquote {
  background-color: #f9f9f9;
  color: #111; /* Arlen: WCAG 2019 */
  border: 1px solid #ddd;
  border-radius: 3px;
  margin: 1em 0;
}
cite {
  display: block;
  text-align: right;
  font-style: italic;
}

/* tables */
table {
  width: 100%;
  margin: 0 0 1em;
  border-collapse: collapse;
  border: 1px solid #eee;
}
th, td {
  text-align: left;
  vertical-align: top;
  padding: 0.5em 0.75em;
}
th {
  text-align: left;
  background-color: #e9e9e9;
}
tr:nth-child(2n+1) > td {
  background-color: #f5f5f5;
}
table caption {
  font-style: italic;
  margin: 0;
  padding: 0;
  text-align: left;
}
table p {
  /* XXX to avoid bottom margin on table row signifiers. If paragraphs should
     be allowed within tables more generally, it would be far better to select on a class. */
  margin: 0;
}

/* pilcrow */
a.pilcrow {
  color: #666; /* Arlen: AHDJ 2019 */
  text-decoration: none;
  visibility: hidden;
  user-select: none;
  -ms-user-select: none;
  -o-user-select:none;
  -moz-user-select: none;
  -khtml-user-select: none;
  -webkit-user-select: none;
  -webkit-touch-callout: none;
}
@media screen {
  aside:hover > a.pilcrow,
  p:hover > a.pilcrow,
  blockquote:hover > a.pilcrow,
  div:hover > a.pilcrow,
  li:hover > a.pilcrow,
  pre:hover > a.pilcrow {
    visibility: visible;
  }
  a.pilcrow:hover {
    background-color: transparent;
  }
}

/* misc */
hr {
  border: 0;
  border-top: 1px solid #eee;
}
.bcp14 {
  font-variant: small-caps;
}

.role {
  font-variant: all-small-caps;
}

/* info block */
#identifiers {
  margin: 0;
  font-size: 0.9em;
}
#identifiers dt {
  width: 3em;
  clear: left;
}
#identifiers dd {
  float: left;
  margin-bottom: 0;
}
#identifiers .authors .author {
  display: inline-block;
  margin-right: 1.5em;
}
#identifiers .authors .org {
  font-style: italic;
}

/* The prepared/rendered info at the very bottom of the page */
.docInfo {
  color: #666; /* Arlen: WCAG 2019 */
  font-size: 0.9em;
  font-style: italic;
  margin-top: 2em;
}
.docInfo .prepared {
  float: left;
}
.docInfo .prepared {
  float: right;
}

/* table of contents */
#toc  {
  padding: 0.75em 0 2em 0;
  margin-bottom: 1em;
}
nav.toc ul {
  margin: 0 0.5em 0 0;
  padding: 0;
  list-style: none;
}
nav.toc li {
  line-height: 1.3em;
  margin: 0.75em 0;
  padding-left: 1.2em;
  text-indent: -1.2em;
}
/* references */
.references dt {
  text-align: right;
  font-weight: bold;
  min-width: 7em;
}
.references dd {
  margin-left: 8em;
  overflow: auto;
}

.refInstance {
  margin-bottom: 1.25em;
}

.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 {
  margin: 0.5em 0 1em;
}
.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;
  }
  .index ul ul {
    -moz-column-count: 1;
    -moz-column-gap: 0;
  }
}

/* authors */
address.vcard {
  font-style: normal;
  margin: 1em 0;
}

address.vcard .nameRole {
  font-weight: 700;
  margin-left: 0;
}
address.vcard .label {
  font-family: "Noto Sans",Arial,Helvetica,sans-serif;
  margin: 0.5em 0;
}
address.vcard .type {
  display: none;
}
.alternative-contact {
  margin: 1.5em 0 1em;
}
hr.addr {
  border-top: 1px dashed;
  margin: 0;
  color: #ddd;
  max-width: calc(100% - 16px);
}

/* temporary notes */
.rfcEditorRemove::before {
  position: absolute;
  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 */
  padding: 2px 4px;
}
.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;
  }
  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;
  }
}

/* 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;
    border-bottom: 1px solid #ddd;
    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%;
  }
  p {
    orphans: 3;
    widows: 3;
  }
  #n-copyright-notice {
    border-bottom: none;
  }
  #toc, #n-introduction {
    page-break-before: always;
  }
  #toc {
    border-top: none;
    padding-top: 0;
  }
  figure, pre {
    page-break-inside: avoid;
  }
  figure {
    overflow: scroll;
  }
  h1, h2, h3, h4, h5, h6 {
    page-break-after: avoid;
  }
  h2+*, h3+*, h4+*, h5+*, h6+* {
    page-break-before: avoid;
  }
  pre {
    white-space: pre-wrap;
    word-wrap: break-word;
    font-size: 10pt;
  }
  table {
    border: 1px solid #ddd;
  }
  td {
    border-top: 1px solid #ddd;
  }
}

/* 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 {
  string-set: ears-top-center content();
}

.ears thead .right {
  string-set: ears-top-right content();
}

.ears tfoot .left {
  string-set: ears-bottom-left content();
}

.ears tfoot .center {
  string-set: ears-bottom-center content();
}

.ears tfoot .right {
  string-set: ears-bottom-right content();
}
*/

@page :first {
  padding-top: 0;
  @top-left {
    content: normal;
    border: none;
  }
  @top-center {
    content: normal;
    border: none;
  }
  @top-right {
    content: normal;
    border: none;
  }
}

@page {
  size: A4;
  margin-bottom: 45mm;
  padding-top: 20px;
  /* 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;
    border-bottom: solid 1px #ccc;
  }
  @top-center {
    content: string(ears-top-center);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-right {
    content: string(ears-top-right);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @bottom-left {
    content: string(ears-bottom-left);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-center {
    content: string(ears-bottom-center);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-right {
      content: '[Page ' counter(page) ']';
      vertical-align: top;
      border-top: solid 1px #ccc;
  }
  */

}

/* 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 {
  padding-right: 0.5em;
}

/* prevent monospace from becoming overly large */
tt, code, pre, code {
  font-size: 95%;
}

/* Fix the height/width aspect for ascii art*/
pre.sourcecode,
.art-text pre {
  line-height: 1.12;
}


/* 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 {
  background-color: #eee;
  padding: 0.5em;
  margin-bottom: 0.5em;
  display: none;
}
div#internal-metadata {
  padding: 0.5em;                       /* to match the external-metadata padding */
}
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<table class="ears">
<thead><tr>
<td class="left">RFC 9076</td>
<td class="center">DNS Privacy Considerations</td>
<td class="right">July 2021</td>
</tr></thead>
<tfoot><tr>
<td class="left">Wicinski</td>
<td class="center">Informational</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/rfc9076" class="eref">9076</a></dd>
<dt class="label-obsoletes">Obsoletes:</dt>
<dd class="obsoletes">
<a href="https://www.rfc-editor.org/rfc/rfc7626" class="eref">7626</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Informational</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2021-07" class="published">July 2021</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Author:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">T. Wicinski, <span class="editor">Ed.</span>
</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 9076</h1>
<h1 id="title">DNS Privacy Considerations</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">This document describes the privacy issues associated with the use of the DNS
  by Internet users. It provides general observations about typical current
  privacy practices. It is intended to be an analysis of the present situation
  and does not prescribe solutions. This document obsoletes RFC 7626.<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 document is not an Internet Standards Track specification; it is
            published for informational purposes.<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).  Not all documents
            approved by the IESG are candidates for any level of Internet
            Standard; see 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/rfc9076">https://www.rfc-editor.org/info/rfc9076</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 Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<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="ulEmpty compact ulBare toc">
<li class="ulEmpty compact ulBare toc" 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" class="xref">Introduction</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-scope" class="xref">Scope</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1" class="keepWithNext"><a href="#section-3" class="xref">3</a>.  <a href="#name-risks" class="xref">Risks</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" 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-risks-in-the-dns-data" class="xref">Risks in the DNS Data</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.4.2.1">
                <p id="section-toc.1-1.4.2.1.1"><a href="#section-4.1" class="xref">4.1</a>.  <a href="#name-the-public-nature-of-dns-da" class="xref">The Public Nature of DNS Data</a></p>
</li>
              <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.4.2.2">
                <p id="section-toc.1-1.4.2.2.1"><a href="#section-4.2" class="xref">4.2</a>.  <a href="#name-data-in-the-dns-request" class="xref">Data in the DNS Request</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.4.2.2.2.1">
                    <p id="section-toc.1-1.4.2.2.2.1.1"><a href="#section-4.2.1" class="xref">4.2.1</a>.  <a href="#name-data-in-the-dns-payload" class="xref">Data in the DNS Payload</a></p>
</li>
                </ul>
</li>
              <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.4.2.3">
                <p id="section-toc.1-1.4.2.3.1"><a href="#section-4.3" class="xref">4.3</a>.  <a href="#name-cache-snooping" class="xref">Cache Snooping</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact ulBare toc" 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-risks-on-the-wire" class="xref">Risks on the Wire</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.5.2.1">
                <p id="section-toc.1-1.5.2.1.1"><a href="#section-5.1" class="xref">5.1</a>.  <a href="#name-unencrypted-transports" class="xref">Unencrypted Transports</a></p>
</li>
              <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.5.2.2">
                <p id="section-toc.1-1.5.2.2.1"><a href="#section-5.2" class="xref">5.2</a>.  <a href="#name-encrypted-transports" class="xref">Encrypted Transports</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact ulBare toc" 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-risks-in-the-servers" class="xref">Risks in the Servers</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.1">
                <p id="section-toc.1-1.6.2.1.1"><a href="#section-6.1" class="xref">6.1</a>.  <a href="#name-in-the-recursive-resolvers" class="xref">In the Recursive Resolvers</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.1.2.1">
                    <p id="section-toc.1-1.6.2.1.2.1.1"><a href="#section-6.1.1" class="xref">6.1.1</a>.  <a href="#name-resolver-selection" class="xref">Resolver Selection</a></p>
</li>
                  <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.1.2.2">
                    <p id="section-toc.1-1.6.2.1.2.2.1"><a href="#section-6.1.2" class="xref">6.1.2</a>.  <a href="#name-active-attacks-on-resolver-" class="xref">Active Attacks on Resolver Configuration</a></p>
</li>
                  <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.1.2.3">
                    <p id="section-toc.1-1.6.2.1.2.3.1"><a href="#section-6.1.3" class="xref">6.1.3</a>.  <a href="#name-blocking-of-dns-resolution-" class="xref">Blocking of DNS Resolution Services</a></p>
</li>
                  <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.1.2.4">
                    <p id="section-toc.1-1.6.2.1.2.4.1"><a href="#section-6.1.4" class="xref">6.1.4</a>.  <a href="#name-encrypted-transports-and-re" class="xref">Encrypted Transports and Recursive Resolvers</a></p>
</li>
                </ul>
</li>
              <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.6.2.2">
                <p id="section-toc.1-1.6.2.2.1"><a href="#section-6.2" class="xref">6.2</a>.  <a href="#name-in-the-authoritative-name-s" class="xref">In the Authoritative Name Servers</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact ulBare toc" 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-other-risks" class="xref">Other Risks</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc 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-re-identification-and-other" class="xref">Re-identification and Other Inferences</a></p>
</li>
              <li class="ulBare compact toc 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-more-information" class="xref">More Information</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#section-8" class="xref">8</a>.  <a href="#name-actual-attacks" class="xref">Actual "Attacks"</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#section-9" class="xref">9</a>.  <a href="#name-legalities" class="xref">Legalities</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#section-10" class="xref">10</a>. <a href="#name-security-considerations" class="xref">Security Considerations</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#section-11" class="xref">11</a>. <a href="#name-iana-considerations" class="xref">IANA Considerations</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.12">
            <p id="section-toc.1-1.12.1"><a href="#section-12" class="xref">12</a>. <a href="#name-references" class="xref">References</a></p>
<ul class="ulBare compact toc ulEmpty">
<li class="ulBare compact toc ulEmpty" id="section-toc.1-1.12.2.1">
                <p id="section-toc.1-1.12.2.1.1"><a href="#section-12.1" class="xref">12.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a></p>
</li>
              <li class="ulBare compact toc ulEmpty" id="section-toc.1-1.12.2.2">
                <p id="section-toc.1-1.12.2.2.1"><a href="#section-12.2" class="xref">12.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
            </ul>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.13">
            <p id="section-toc.1-1.13.1"><a href="#appendix-A" class="xref">Appendix A</a>.  <a href="#name-updates-since-rfc-7626" class="xref">Updates since RFC 7626</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.14">
            <p id="section-toc.1-1.14.1"><a href="#appendix-B" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.15">
            <p id="section-toc.1-1.15.1"><a href="#appendix-C" class="xref"></a><a href="#name-contributions" class="xref">Contributions</a></p>
</li>
          <li class="ulEmpty compact ulBare toc" id="section-toc.1-1.16">
            <p id="section-toc.1-1.16.1"><a href="#appendix-D" class="xref"></a><a href="#name-authors-address" class="xref">Author's Address</a></p>
</li>
        </ul>
</nav>
</section>
</div>
<div id="introduction">
<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<p id="section-1-1">This document is an analysis of the DNS privacy issues, in the spirit
   of <span><a href="https://www.rfc-editor.org/rfc/rfc6973#section-8" class="relref">Section 8</a> of [<a href="#RFC6973" class="xref">RFC6973</a>]</span>.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">The Domain Name System (DNS) is specified in <span>[<a href="#RFC1034" class="xref">RFC1034</a>]</span>, <span>[<a href="#RFC1035" class="xref">RFC1035</a>]</span>, and
   many later RFCs, which have never been consolidated. It is one of the most
   important infrastructure components of the Internet and is often ignored or
   misunderstood by Internet users (and even by many professionals). Almost
   every activity on the Internet starts with a DNS query (and often several).
   Its use has many privacy implications, and this document is an attempt at a
   comprehensive and accurate list.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">Let us begin with a simplified reminder of how the DNS works (see also
   <span>[<a href="#RFC8499" class="xref">RFC8499</a>]</span>). A client, the stub resolver, issues a
   DNS query to a server called the recursive resolver (also called caching
   resolver, full resolver, or recursive name server). Let's use the query
   "What are the AAAA records for www.example.com?" as an example. AAAA is the
   QTYPE (Query Type), and www.example.com is the QNAME (Query Name). (The
   description that follows assumes a cold cache, for instance, because the
   server just started.) The recursive resolver will first query the root name
   servers. In most cases, the root name servers will send a referral. In this
   example, the referral will be to the .com name servers. The resolver repeats
   the query to one of the .com name servers. The .com name servers, in turn,
   will refer to the example.com name servers. The example.com name servers will
   then return the answers. The root name servers, the name servers of .com, and
   the name servers of example.com are called authoritative name servers. It is
   important, when analyzing the privacy issues, to remember that the question
   asked to all these name servers is always the original question, not a
   derived question. The question sent to the root name servers is "What are
   the AAAA records for www.example.com?", not "What are the name servers of
   .com?". By repeating the full question, instead of just the relevant part of
   the question to the next in line, the DNS provides more information than
   necessary to the name server. In this simplified description, recursive
   resolvers do not implement QNAME minimization as described in <span>[<a href="#RFC7816" class="xref">RFC7816</a>]</span>,
   which will only send the relevant part of the question to the upstream name
   server.<a href="#section-1-3" class="pilcrow">¶</a></p>
<p id="section-1-4">DNS relies heavily on caching, so the algorithm described
   above is actually a bit more complicated, and not all questions are
   sent to the authoritative name servers.  If the
   stub resolver asks the recursive resolver a few seconds later, "What are the SRV records
   of _xmpp-server._tcp.example.com?", the recursive resolver will
   remember that it knows the name servers of example.com and will just
   query them, bypassing the root and .com.  Because there is typically
   no caching in the stub resolver, the recursive resolver, unlike the
   authoritative servers, sees all the DNS traffic.  (Applications, like
   web browsers, may have some form of caching that does not follow DNS
   rules, for instance, because it may ignore the TTL.  So, the
   recursive resolver does not see all the name resolution activity.)<a href="#section-1-4" class="pilcrow">¶</a></p>
<p id="section-1-5">It should be noted that DNS recursive resolvers sometimes forward
   requests to other recursive resolvers, typically bigger machines,
   with a larger and more shared cache (and the query hierarchy can be
   even deeper, with more than two levels of recursive resolvers).  From
   the point of view of privacy, these forwarders are like resolvers
   except that they do not see all of the requests being made (due to
   caching in the first resolver).<a href="#section-1-5" class="pilcrow">¶</a></p>
<p id="section-1-6">At the time of writing, almost all this DNS traffic is currently
  sent unencrypted. However, there is increasing deployment
  of DNS over TLS (DoT) <span>[<a href="#RFC7858" class="xref">RFC7858</a>]</span> and DNS over HTTPS (DoH)
  <span>[<a href="#RFC8484" class="xref">RFC8484</a>]</span>, particularly in mobile devices, browsers, and by
  providers of anycast recursive DNS resolution services. There are a
  few cases where there is some alternative channel encryption, for
  instance, in an IPsec VPN tunnel, at least between the stub resolver and
  the resolver.   Some recent analysis on the service quality of encrypted DNS
  traffic can be found in <span>[<a href="#dns-over-encryption" class="xref">dns-over-encryption</a>]</span>.<a href="#section-1-6" class="pilcrow">¶</a></p>
<p id="section-1-7">Today, almost all DNS queries are sent over UDP <span>[<a href="#thomas-ditl-tcp" class="xref">thomas-ditl-tcp</a>]</span>. This has
   practical consequences when considering encryption of the traffic as a
   possible privacy technique. Some encryption solutions are only designed for
   TCP, not UDP, although new solutions are still emerging <span>[<a href="#RFC9000" class="xref">RFC9000</a>]</span>
        <span>[<a href="#I-D.ietf-dprive-dnsoquic" class="xref">DPRIVE-DNSOQUIC</a>]</span>.<a href="#section-1-7" class="pilcrow">¶</a></p>
<p id="section-1-8">Another important point to keep in mind when analyzing the privacy
   issues of DNS is the fact that DNS requests received by a server are
   triggered for different reasons.  Let's assume an eavesdropper wants
   to know which web page is viewed by a user.  For a typical web page,
   there are three sorts of DNS requests being issued:<a href="#section-1-8" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlNewline" id="section-1-9">
        <dt id="section-1-9.1">Primary request:</dt>
        <dd style="margin-left: 1.5em" id="section-1-9.2"> This is the domain name in the URL that the user
typed, selected from a bookmark, or chose by clicking on a
hyperlink.  Presumably, this is what is of interest for the
eavesdropper.<a href="#section-1-9.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-1-9.3">Secondary requests:</dt>
        <dd style="margin-left: 1.5em" id="section-1-9.4">These are the additional requests performed by
the user agent (here, the web browser) without any direct
involvement or knowledge of the user.  For the Web, they are
triggered by embedded content, Cascading Style Sheets (CSS),
JavaScript code, embedded images, etc.  In some cases, there can
be dozens of domain names in different contexts on a single web
page.<a href="#section-1-9.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-1-9.5">Tertiary requests:</dt>
        <dd style="margin-left: 1.5em" id="section-1-9.6"> These are the additional requests performed by
the DNS service itself.  For instance, if the answer to a query is
a referral to a set of name servers and the glue records are not
returned, the resolver will have to send additional requests to turn
the name servers' names into IP addresses.  Similarly, even if
glue records are returned, a careful recursive server will send
tertiary requests to verify the IP addresses of those records.<a href="#section-1-9.6" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
<p id="section-1-10">It can also be noted that, in the case of a typical web browser, more
   DNS requests than strictly necessary are sent, for instance, to
   prefetch resources that the user may query later or when
   autocompleting the URL in the address bar.  Both are a significant privacy
   concern since they may leak information even about non-explicit
   actions.  For instance, just reading a local HTML page, even without
   selecting the hyperlinks, may trigger DNS requests.<a href="#section-1-10" class="pilcrow">¶</a></p>
<p id="section-1-11">Privacy-related terminology is from
   <span>[<a href="#RFC6973" class="xref">RFC6973</a>]</span>. This document obsoletes <span>[<a href="#RFC7626" class="xref">RFC7626</a>]</span>.<a href="#section-1-11" class="pilcrow">¶</a></p>
</section>
</div>
<div id="scope">
<section id="section-2">
      <h2 id="name-scope">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-scope" class="section-name selfRef">Scope</a>
      </h2>
<p id="section-2-1">This document focuses mostly on the study of privacy risks for the
   end user (the one performing DNS requests).  The risks of
   pervasive surveillance <span>[<a href="#RFC7258" class="xref">RFC7258</a>]</span> are considered as well as risks coming from a more
   focused surveillance.  In this document, the term "end user" is used
   as defined in <span>[<a href="#RFC8890" class="xref">RFC8890</a>]</span>.<a href="#section-2-1" class="pilcrow">¶</a></p>
<p id="section-2-2">This document does not attempt a comparison of specific privacy protections
   provided by individual networks or organizations; it makes only general
   observations about typical current practices.<a href="#section-2-2" class="pilcrow">¶</a></p>
<p id="section-2-3">Privacy risks for the holder of a zone (the risk that someone gets the data)
   are discussed in <span>[<a href="#RFC5155" class="xref">RFC5155</a>]</span> and <span>[<a href="#RFC5936" class="xref">RFC5936</a>]</span>.<a href="#section-2-3" class="pilcrow">¶</a></p>
<p id="section-2-4">Privacy risks for recursive operators (including access providers and
   operators in enterprise networks) such as leakage of private namespaces or
   blocklists are out of scope for this document.<a href="#section-2-4" class="pilcrow">¶</a></p>
<p id="section-2-5">Non-privacy risks (e.g., security-related considerations such as cache poisoning) are
   also out of scope.<a href="#section-2-5" class="pilcrow">¶</a></p>
<p id="section-2-6">The privacy risks associated with the use of other protocols that make use of
   DNS information are not considered here.<a href="#section-2-6" class="pilcrow">¶</a></p>
</section>
</div>
<div id="risks">
<section id="section-3">
      <h2 id="name-risks">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-risks" class="section-name selfRef">Risks</a>
      </h2>
<p id="section-3-1">The following four sections outline the privacy considerations associated with
different aspects of the DNS for the end user. When reading these sections, it
needs to be kept in mind that many of the considerations (for example, recursive
resolver and transport protocol) can be specific to the network context that a
device is using at a given point in time. A user may have many devices, and each
device might utilize many different networks (e.g., home, work, public, or
cellular) over a period of time or even concurrently. An exhaustive analysis of
the privacy considerations for an individual user would need to take into
account the set of devices used and the multiple dynamic contexts of each
device. This document does not attempt such a complex analysis; instead, it
presents an overview of the various considerations that could form the basis of
such an analysis.<a href="#section-3-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="risks-in-the-dns-data">
<section id="section-4">
      <h2 id="name-risks-in-the-dns-data">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-risks-in-the-dns-data" class="section-name selfRef">Risks in the DNS Data</a>
      </h2>
<div id="the-public-nature-of-dns-data">
<section id="section-4.1">
        <h3 id="name-the-public-nature-of-dns-da">
<a href="#section-4.1" class="section-number selfRef">4.1. </a><a href="#name-the-public-nature-of-dns-da" class="section-name selfRef">The Public Nature of DNS Data</a>
        </h3>
<p id="section-4.1-1">It has been stated that "the data in the DNS is public".  This sentence
   makes sense for an Internet-wide lookup system,  and there
   are multiple facets to the data and metadata involved that deserve a
   more detailed look.  First, access control lists (ACLs) and private
   namespaces notwithstanding, the DNS operates under the assumption
   that public-facing authoritative name servers will respond to "usual"
   DNS queries for any zone they are authoritative for, without further
   authentication or authorization of the client (resolver).  Due to the
   lack of search capabilities, only a given QNAME will reveal the
   resource records associated with that name (or that name's nonexistence).  In other words: one needs to know what to ask for in
   order to receive a response. There are many ways in which supposedly "private"
   resources currently leak. A few examples are DNSSEC NSEC zone walking <span>[<a href="#RFC4470" class="xref">RFC4470</a>]</span>,
   passive DNS services <span>[<a href="#passive-dns" class="xref">passive-dns</a>]</span>, etc. The zone transfer QTYPE <span>[<a href="#RFC5936" class="xref">RFC5936</a>]</span> is
   often blocked or restricted to authenticated/authorized access to
   enforce this difference (and maybe for other reasons).<a href="#section-4.1-1" class="pilcrow">¶</a></p>
<p id="section-4.1-2">Another difference between the DNS data and a particular DNS
  transaction (i.e., a DNS name lookup): DNS data and the results of a
  DNS query are public, within the boundaries described above, and may
  not have any confidentiality requirements.  However, the same is not
  true of a single transaction or a sequence of transactions; those
  transactions are not / should not be public.  A single transaction
  reveals both the originator of the query and the query contents; this
  potentially leaks sensitive information about a specific user. A
   typical example from outside the DNS world is that the website of Alcoholics Anonymous is public but the fact that you visit it should not be. Furthermore,
   the ability to link queries reveals information about individual use
   patterns.<a href="#section-4.1-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="data-in-the-dns-request">
<section id="section-4.2">
        <h3 id="name-data-in-the-dns-request">
<a href="#section-4.2" class="section-number selfRef">4.2. </a><a href="#name-data-in-the-dns-request" class="section-name selfRef">Data in the DNS Request</a>
        </h3>
<p id="section-4.2-1">The DNS request includes many fields, but two of them seem particularly
   relevant for the privacy issues: the QNAME and the source IP address.
   "Source IP address" is used in a loose sense of "source IP address + maybe
   source
   port number", because the port number is also in the request and can be used to
   differentiate between several users sharing an IP address (behind a
   Carrier-Grade NAT (CGN), for instance <span>[<a href="#RFC6269" class="xref">RFC6269</a>]</span>).<a href="#section-4.2-1" class="pilcrow">¶</a></p>
<p id="section-4.2-2">The QNAME is the full name sent by the user.  It gives information
   about what the user does ("What are the MX records of example.net?"
   means they probably want to send email to someone at example.net,
   which may be a domain used by only a few persons and is therefore
   very revealing about communication relationships).  Some QNAMEs are
   more sensitive than others.  For instance, querying the A record of a
   well-known web statistics domain reveals very little (everybody
   visits websites that use this analytics service), but querying the A
   record of www.verybad.example where verybad.example is the domain of
   an organization that some people find offensive or objectionable may
   create more problems for the user.  Also, sometimes, the QNAME embeds
   the software one uses, which could be a privacy issue (for instance,
   _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
   There are also some BitTorrent clients that query an SRV record for
   _bittorrent-tracker._tcp.domain.example.<a href="#section-4.2-2" class="pilcrow">¶</a></p>
<p id="section-4.2-3">Another important thing about the privacy of the QNAME is future
   usages.  Today, the lack of privacy is an obstacle to putting
   potentially sensitive or personally identifiable data in the DNS.  At
   the moment, your DNS traffic might reveal that you are exchanging emails but not with whom.  If your Mail User Agent (MUA) starts looking up
   Pretty Good Privacy (PGP) keys in the DNS <span>[<a href="#RFC7929" class="xref">RFC7929</a>]</span>, then
   privacy becomes a lot more important.  And email is just an example;
   there would be other really interesting uses for a more privacy-friendly DNS.<a href="#section-4.2-3" class="pilcrow">¶</a></p>
<p id="section-4.2-4">For the communication between the stub resolver and the recursive resolver,
   the source IP address is the address of the user's machine. Therefore, all
   the issues and warnings about collection of IP addresses apply here. For the communication between the recursive resolver and the authoritative name
   servers, the source IP address has a different meaning; it does not have the
   same status as the source address in an HTTP connection. It is typically the
   IP address of the recursive resolver that, in a way, "hides" the real user.

   However, hiding does not always work. The edns-client-subnet (ECS) EDNS0 option <span>[<a href="#RFC7871" class="xref">RFC7871</a>]</span> is sometimes used (see one privacy analysis in <span>[<a href="#denis-edns-client-subnet" class="xref">denis-edns-client-subnet</a>]</span>).

   Sometimes the end user has a personal recursive resolver on their machine.
   In both cases, the IP address originating queries to the authoritative server
   is as sensitive as it is for HTTP <span>[<a href="#sidn-entrada" class="xref">sidn-entrada</a>]</span>.<a href="#section-4.2-4" class="pilcrow">¶</a></p>
<p id="section-4.2-5">A note about IP addresses: there is currently no IETF document that describes
   in detail all the privacy issues around IP addressing in general, although
   <span>[<a href="#RFC7721" class="xref">RFC7721</a>]</span> does discuss privacy considerations for IPv6 address generation
   mechanisms. In the meantime, the discussion here is intended to include both
   IPv4 and IPv6 source addresses. For a number of reasons, their assignment and
   utilization characteristics are different, which may have implications for
   details of information leakage associated with the collection of source
   addresses. (For example, a specific IPv6 source address seen on the public
   Internet is less likely than an IPv4 address to originate behind an address-sharing scheme.) However, for both IPv4 and IPv6 addresses, it is
   important to note that source addresses are propagated with queries
via the ECS option and comprise metadata about the host, user,
or application that originated them.<a href="#section-4.2-5" class="pilcrow">¶</a></p>
<div id="data-in-the-dns-payload">
<section id="section-4.2.1">
          <h4 id="name-data-in-the-dns-payload">
<a href="#section-4.2.1" class="section-number selfRef">4.2.1. </a><a href="#name-data-in-the-dns-payload" class="section-name selfRef">Data in the DNS Payload</a>
          </h4>
<p id="section-4.2.1-1">At the time of writing, there are no standardized client identifiers contained in
the DNS payload itself (ECS, as described in <span>[<a href="#RFC7871" class="xref">RFC7871</a>]</span>, is widely used; however, <span>[<a href="#RFC7871" class="xref">RFC7871</a>]</span> is only an Informational RFC).<a href="#section-4.2.1-1" class="pilcrow">¶</a></p>
<p id="section-4.2.1-2">DNS Cookies <span>[<a href="#RFC7873" class="xref">RFC7873</a>]</span> are a lightweight DNS transaction security mechanism that
provides limited protection against a variety of increasingly common
denial-of-service and amplification/forgery or cache poisoning attacks by
off-path attackers. It is noted, however, that they are designed to just verify
IP addresses (and should change once a client's IP address changes), but they are
not designed to actively track users (like HTTP cookies).<a href="#section-4.2.1-2" class="pilcrow">¶</a></p>
<p id="section-4.2.1-3">There are anecdotal accounts of <a href="https://lists.dns-oarc.net/pipermail/dns-operations/2016-January/014143.html">Media Access Control (MAC) addresses</a>
and even user names being inserted in nonstandard EDNS(0) options <span>[<a href="#RFC6891" class="xref">RFC6891</a>]</span>
for stub-to-resolver communications to support proprietary functionality
implemented at the resolver (e.g., parental filtering).<a href="#section-4.2.1-3" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="cache-snooping">
<section id="section-4.3">
        <h3 id="name-cache-snooping">
<a href="#section-4.3" class="section-number selfRef">4.3. </a><a href="#name-cache-snooping" class="section-name selfRef">Cache Snooping</a>
        </h3>
<p id="section-4.3-1">The content of recursive resolvers' caches can reveal data about the
   clients using it (the privacy risks depend on the number of clients).
   This information can sometimes be examined by sending DNS queries
   with RD=0 to inspect cache content, particularly looking at the DNS
   TTLs <span>[<a href="#grangeia.snooping" class="xref">grangeia.snooping</a>]</span>.  Since this also is a reconnaissance
   technique for subsequent cache poisoning attacks, some countermeasures have already been developed and deployed <span>[<a href="#cache-snooping-defence" class="xref">cache-snooping-defence</a>]</span>.<a href="#section-4.3-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="risks-on-the-wire">
<section id="section-5">
      <h2 id="name-risks-on-the-wire">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-risks-on-the-wire" class="section-name selfRef">Risks on the Wire</a>
      </h2>
<div id="unencrypted-transports">
<section id="section-5.1">
        <h3 id="name-unencrypted-transports">
<a href="#section-5.1" class="section-number selfRef">5.1. </a><a href="#name-unencrypted-transports" class="section-name selfRef">Unencrypted Transports</a>
        </h3>
<p id="section-5.1-1">For unencrypted transports, DNS traffic can be seen by an eavesdropper like
   any other traffic. (DNSSEC, specified in <span>[<a href="#RFC4033" class="xref">RFC4033</a>]</span>, explicitly excludes
   confidentiality from its goals.) So, if an initiator starts an HTTPS
   communication with a recipient, the HTTP traffic will be encrypted, but the
   DNS exchange prior to it will not be. When other protocols become more
   and more privacy aware and secured against surveillance (e.g., <span>[<a href="#RFC8446" class="xref">RFC8446</a>]</span>,
   <span>[<a href="#RFC9000" class="xref">RFC9000</a>]</span>), the use of unencrypted transports for DNS may
   become "the weakest link" in privacy. It is noted that, at the time of writing,
   there is ongoing work attempting to encrypt the Server Name Identification (SNI) in the TLS handshake
   <span>[<a href="#RFC8744" class="xref">RFC8744</a>]</span>, which is one of the
   last remaining non-DNS cleartext identifiers of a connection target.<a href="#section-5.1-1" class="pilcrow">¶</a></p>
<p id="section-5.1-2">An important specificity of the DNS traffic is that it may take a
   different path than the communication between the initiator and the
   recipient.  For instance, an eavesdropper may be unable to tap the
   wire between the initiator and the recipient but may have access to
   the wire going to the recursive resolver or to the authoritative
   name servers.<a href="#section-5.1-2" class="pilcrow">¶</a></p>
<p id="section-5.1-3">The best place to tap, from an eavesdropper's point of view, is
   clearly between the stub resolvers and the recursive resolvers,
   because traffic is not limited by DNS caching.<a href="#section-5.1-3" class="pilcrow">¶</a></p>
<p id="section-5.1-4">The attack surface between the stub resolver and the rest of the
   world can vary widely depending upon how the end user's device is
   configured.  By order of increasing attack surface:<a href="#section-5.1-4" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-5.1-5.1">The recursive resolver can be on the end user's device.  In (currently) a small number of cases, individuals may choose to
operate their own DNS resolver on their local machine.  In this
case, the attack surface for the connection between the stub
resolver and the caching resolver is limited to that single
machine. The recursive resolver will expose data to authoritative
resolvers as discussed in <a href="#in-the-authoritative-name-servers" class="xref">Section 6.2</a>.<a href="#section-5.1-5.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.1-5.2">The recursive resolver may be at the local network edge.  For
many/most enterprise networks and for some residential networks, the
caching resolver may exist on a server at the edge of the local
network.  In this case, the attack surface is the local network.
Note that in large enterprise networks, the DNS resolver may not
be located at the edge of the local network but rather at the edge
of the overall enterprise network.  In this case, the enterprise
network could be thought of as similar to the Internet Access
Provider (IAP) network referenced below.<a href="#section-5.1-5.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.1-5.3">The recursive resolver can be in the IAP network. For most residential
networks and potentially other networks, the typical case is for the
user's device to be configured (typically automatically through DHCP or
relay agent options) with the addresses of the DNS proxy in the Customer
Premises Equipment (CPE), which in turn
points to the DNS recursive resolvers at the IAP. The attack surface for
on-the-wire attacks is therefore from the end user system across the
local network and across the IAP network to the IAP's recursive resolvers.<a href="#section-5.1-5.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.1-5.4">The recursive resolver can be a public DNS service (or a privately run DNS
resolver hosted on the public Internet).  Some machines
may be configured to use public DNS resolvers such as those
operated by Google Public DNS or OpenDNS.  The user may
have configured their machine to use these DNS recursive resolvers
themselves -- or their IAP may have chosen to use the public DNS
resolvers rather than operating their own resolvers.  In this
case, the attack surface is the entire public Internet between the
user's connection and the public DNS service. It can be noted that if the
user selects a single resolver with a small client population (even when using
an encrypted transport), it can actually serve to aid tracking of that user as
they move across network environments.<a href="#section-5.1-5.4" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-5.1-6">It is also noted that, typically, a device connected <em>only</em> to a modern cellular
  network is<a href="#section-5.1-6" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-5.1-7.1">directly configured with only the recursive resolvers of the IAP and<a href="#section-5.1-7.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.1-7.2">
            <p id="section-5.1-7.2.1">afforded some level of protection against some types of eavesdropping
for all traffic (including DNS traffic) due to the cellular network
link-layer encryption.<a href="#section-5.1-7.2.1" class="pilcrow">¶</a></p>
</li>
        </ul>
<p id="section-5.1-8">The attack surface for this specific scenario is not considered here.<a href="#section-5.1-8" class="pilcrow">¶</a></p>
</section>
</div>
<div id="encrypted-transports">
<section id="section-5.2">
        <h3 id="name-encrypted-transports">
<a href="#section-5.2" class="section-number selfRef">5.2. </a><a href="#name-encrypted-transports" class="section-name selfRef">Encrypted Transports</a>
        </h3>
<p id="section-5.2-1">The use of encrypted transports directly mitigates passive surveillance of the
DNS payload; however, some privacy attacks are still possible. This section
enumerates the residual privacy risks to an end user when an attacker can
passively monitor encrypted DNS traffic flows on the wire.<a href="#section-5.2-1" class="pilcrow">¶</a></p>
<p id="section-5.2-2">These are cases where user identification, fingerprinting, or correlations may be
possible due to the use of certain transport layers or cleartext/observable
features. These issues are not specific to DNS, but DNS traffic is susceptible
to these attacks when using specific transports.<a href="#section-5.2-2" class="pilcrow">¶</a></p>
<p id="section-5.2-3">Some general examples exist; for example, certain studies highlight
that the <a href="http://netres.ec/?b=11B99BD">OS fingerprint values</a> of IPv4 TTL, IPv6 Hop Limit, or TCP Window size can be used to fingerprint client OSes or that various techniques can be
used to de-NAT DNS queries <span>[<a href="#dns-de-nat" class="xref">dns-de-nat</a>]</span>.<a href="#section-5.2-3" class="pilcrow">¶</a></p>
<p id="section-5.2-4">Note that even when using encrypted transports, the use of cleartext transport
options to decrease latency can provide correlation of a user's connections,
e.g., using TCP Fast Open <span>[<a href="#RFC7413" class="xref">RFC7413</a>]</span>.<a href="#section-5.2-4" class="pilcrow">¶</a></p>
<p id="section-5.2-5">Implementations that support encrypted transports also commonly reuse
connections for multiple DNS queries to optimize performance (e.g., via DNS
pipelining or HTTPS multiplexing). Default configuration options for encrypted
transports could, in principle, fingerprint a specific client application. 
For
example:<a href="#section-5.2-5" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-5.2-6.1">TLS version or cipher suite selection<a href="#section-5.2-6.1" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.2-6.2">session resumption<a href="#section-5.2-6.2" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.2-6.3">the maximum number of messages to send and<a href="#section-5.2-6.3" class="pilcrow">¶</a>
</li>
          <li class="normal" id="section-5.2-6.4">a maximum connection time before closing a connections and reopening.<a href="#section-5.2-6.4" class="pilcrow">¶</a>
</li>
        </ul>
<p id="section-5.2-7">If libraries or applications offer user configuration of such options (e.g.,
<span>[<a href="#getdns" class="xref">getdns</a>]</span>), then they could, in principle, help to identify a specific user. Users
may want to use only the defaults to avoid this issue.<a href="#section-5.2-7" class="pilcrow">¶</a></p>
<p id="section-5.2-8">While there are known attacks on older versions of TLS, the most recent
recommendations <span>[<a href="#RFC7525" class="xref">RFC7525</a>]</span> and the development of TLS 1.3 <span>[<a href="#RFC8446" class="xref">RFC8446</a>]</span> largely
mitigate those.<a href="#section-5.2-8" class="pilcrow">¶</a></p>
<p id="section-5.2-9">Traffic analysis of unpadded encrypted traffic is also possible
<span>[<a href="#pitfalls-of-dns-encryption" class="xref">pitfalls-of-dns-encryption</a>]</span> because the sizes and timing of encrypted DNS
requests and responses can be correlated to unencrypted DNS requests upstream
of a recursive resolver.<a href="#section-5.2-9" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="risks-in-the-servers">
<section id="section-6">
      <h2 id="name-risks-in-the-servers">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-risks-in-the-servers" class="section-name selfRef">Risks in the Servers</a>
      </h2>
<p id="section-6-1">Using the terminology of <span>[<a href="#RFC6973" class="xref">RFC6973</a>]</span>, the DNS servers (recursive
   resolvers and authoritative servers) are enablers: "they facilitate
   communication between an initiator and a recipient without being
   directly in the communications path".  As a result, they are often
   forgotten in risk analysis.  But, to quote <span>[<a href="#RFC6973" class="xref">RFC6973</a>]</span> again, "Although
   [...] enablers may not generally be considered as attackers, they may
   all pose privacy threats (depending on the context) because they are
   able to observe, collect, process, and transfer privacy-relevant
   data".  In <span>[<a href="#RFC6973" class="xref">RFC6973</a>]</span> parlance, enablers become observers when they
   start collecting data.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">Many programs exist to collect and analyze DNS data at the servers -- from
   the "query log" of some programs like BIND to tcpdump and more sophisticated
   programs like PacketQ <span>[<a href="#packetq" class="xref">packetq</a>]</span> and DNSmezzo <span>[<a href="#dnsmezzo" class="xref">dnsmezzo</a>]</span>. The
   organization managing the DNS server can use this data itself, or it can be
   part of a surveillance program like PRISM <span>[<a href="#prism" class="xref">prism</a>]</span> and pass data to an
   outside observer.<a href="#section-6-2" class="pilcrow">¶</a></p>
<p id="section-6-3">Sometimes this data is kept for a long time and/or distributed to
   third parties for research purposes <span>[<a href="#ditl" class="xref">ditl</a>]</span> <span>[<a href="#day-at-root" class="xref">day-at-root</a>]</span>, security
   analysis, or surveillance tasks.  These uses are sometimes under some
   sort of contract, with various limitations, for instance, on
   redistribution, given the sensitive nature of the data.  Also, there
   are observation points in the network that gather DNS data and then
   make it accessible to third parties for research or security purposes
   ("passive DNS" <span>[<a href="#passive-dns" class="xref">passive-dns</a>]</span>).<a href="#section-6-3" class="pilcrow">¶</a></p>
<div id="in-the-recursive-resolvers">
<section id="section-6.1">
        <h3 id="name-in-the-recursive-resolvers">
<a href="#section-6.1" class="section-number selfRef">6.1. </a><a href="#name-in-the-recursive-resolvers" class="section-name selfRef">In the Recursive Resolvers</a>
        </h3>
<p id="section-6.1-1">Recursive resolvers see all the traffic since there is typically no
   caching before them.  To summarize: your recursive resolver knows a
   lot about you.  The resolver of a large IAP, or a large public
   resolver, can collect data from many users.<a href="#section-6.1-1" class="pilcrow">¶</a></p>
<div id="resolver-selection">
<section id="section-6.1.1">
          <h4 id="name-resolver-selection">
<a href="#section-6.1.1" class="section-number selfRef">6.1.1. </a><a href="#name-resolver-selection" class="section-name selfRef">Resolver Selection</a>
          </h4>
<p id="section-6.1.1-1">Given all the above considerations, the choice of recursive resolver has
  direct privacy considerations for end users. Historically, end user devices
  have used the DHCP-provided local network recursive resolver. The choice by a
  user to join a particular network (e.g., by physically plugging in a cable or
  selecting a network in an OS dialogue) typically updates a number of system
  resources -- these can include IP addresses, the availability of IPv4/IPv6, DHCP
  server, and DNS resolver. These individual changes, including the change in
  DNS resolver, are not normally communicated directly to the user by the OS
  when the network is joined. The choice of network has historically determined
  the default system DNS resolver selection; the two are directly coupled in
  this model.<a href="#section-6.1.1-1" class="pilcrow">¶</a></p>
<p id="section-6.1.1-2">The vast majority of users do not change their default system DNS settings
  and so implicitly accept the network settings for the DNS. The network resolvers
  have therefore historically been the sole destination for all of the DNS
  queries from a device. These resolvers may have varied
  privacy policies depending on the network. Privacy policies for these servers
  may or may not be available, and users need to be aware that privacy
  guarantees will vary with the network.<a href="#section-6.1.1-2" class="pilcrow">¶</a></p>
<p id="section-6.1.1-3">All major OSes expose the system DNS settings and allow users to manually
  override them if desired.<a href="#section-6.1.1-3" class="pilcrow">¶</a></p>
<p id="section-6.1.1-4">More recently, some networks and users have actively chosen
   to use a large public resolver, e.g., <a href="https://developers.google.com/speed/public-dns">Google Public
   DNS</a>,
   <a href="https://developers.cloudflare.com/1.1.1.1/setting-up-1.1.1.1/">Cloudflare</a>,
   or <a href="https://www.quad9.net">Quad9</a>. There can be many reasons: cost
   considerations for network operators, better reliability, or anti-censorship
   considerations are just a few. Such services typically do provide a privacy
   policy, and the user can get an idea of the data collected by such
   operators by reading one, e.g., <a href="https://developers.google.com/speed/public-dns/privacy">Google Public DNS - Your
   Privacy</a>.<a href="#section-6.1.1-4" class="pilcrow">¶</a></p>
<p id="section-6.1.1-5">In general, as with many other protocols, issues around centralization also
   arise with DNS. 

The picture is fluid with several competing factors
   contributing, where these factors can also vary by geographic region. These include:<a href="#section-6.1.1-5" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-6.1.1-6.1">ISP outsourcing, including to third-party and public resolvers<a href="#section-6.1.1-6.1" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-6.1.1-6.2">regional market domination by one or only a few ISPs<a href="#section-6.1.1-6.2" class="pilcrow">¶</a>
</li>
            <li class="normal" id="section-6.1.1-6.3">applications directing DNS traffic by default to a limited subset of resolvers (see <a href="#applicationspecific-resolver-selection" class="xref">Section 6.1.1.2</a>)<a href="#section-6.1.1-6.3" class="pilcrow">¶</a>
</li>
          </ul>
<p id="section-6.1.1-7">An increased proportion of the global DNS resolution traffic being served by
  only a few entities means that the privacy considerations for users are
  highly dependent on the privacy policies and practices of those
  entities. Many of the issues around centralization are discussed in
  <span>[<a href="#centralisation-and-data-sovereignty" class="xref">centralisation-and-data-sovereignty</a>]</span>.<a href="#section-6.1.1-7" class="pilcrow">¶</a></p>
<div id="dynamic-discovery-of-doh-and-strict-dot">
<section id="section-6.1.1.1">
            <h5 id="name-dynamic-discovery-of-doh-an">
<a href="#section-6.1.1.1" class="section-number selfRef">6.1.1.1. </a><a href="#name-dynamic-discovery-of-doh-an" class="section-name selfRef">Dynamic Discovery of DoH and Strict DoT</a>
            </h5>
<p id="section-6.1.1.1-1">While support for opportunistic DoT can be determined by probing a resolver on
port 853, there is currently no standardized discovery mechanism for DoH and
Strict DoT servers.<a href="#section-6.1.1.1-1" class="pilcrow">¶</a></p>
<p id="section-6.1.1.1-2">This means that clients that might want to dynamically discover such encrypted
services, and where users are willing to trust such services, are not able to do
so. At the time of writing, efforts to provide standardized signaling mechanisms
to discover the services offered by local resolvers are in progress
<span>[<a href="#I-D.ietf-dnsop-resolver-information" class="xref">DNSOP-RESOLVER</a>]</span>. Note that an increasing number of ISPs
are deploying encrypted DNS; for example, see the Encrypted DNS Deployment
Initiative <span>[<a href="#EDDI" class="xref">EDDI</a>]</span>.<a href="#section-6.1.1.1-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="applicationspecific-resolver-selection">
<section id="section-6.1.1.2">
            <h5 id="name-application-specific-resolv">
<a href="#section-6.1.1.2" class="section-number selfRef">6.1.1.2. </a><a href="#name-application-specific-resolv" class="section-name selfRef">Application-Specific Resolver Selection</a>
            </h5>
<p id="section-6.1.1.2-1">An increasing number of applications are offering application-specific encrypted DNS resolution settings, rather than defaulting to
  using only the system resolver.  A variety of heuristics and
  resolvers are available in different applications, including hard-coded lists of recognized DoH/DoT servers.<a href="#section-6.1.1.2-1" class="pilcrow">¶</a></p>
<p id="section-6.1.1.2-2">Generally, users are not aware of application-specific DNS settings and may
  not have control over those settings. To address these limitations, users
  will only be aware of and have the ability to control such settings if
  applications provide the following functions:<a href="#section-6.1.1.2-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-6.1.1.2-3.1">communicate the change clearly to users when the default application
     resolver changes away from the system resolver<a href="#section-6.1.1.2-3.1" class="pilcrow">¶</a>
</li>
              <li class="normal" id="section-6.1.1.2-3.2">provide configuration options to change the default
  application resolver, including a choice to always use the system resolver<a href="#section-6.1.1.2-3.2" class="pilcrow">¶</a>
</li>
              <li class="normal" id="section-6.1.1.2-3.3">provide mechanisms for users to locally inspect, selectively forward,
    and filter queries (either via the application itself or use of the
    system resolver)<a href="#section-6.1.1.2-3.3" class="pilcrow">¶</a>
</li>
            </ul>
<p id="section-6.1.1.2-4">Application-specific changes to default destinations for users' DNS
  queries might increase or decrease user privacy; it is highly
  dependent on the network context and the application-specific
  default.  This is an area of active debate, and the IETF is working on
  a number of issues related to application-specific DNS settings.<a href="#section-6.1.1.2-4" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="active-attacks-on-resolver-configuration">
<section id="section-6.1.2">
          <h4 id="name-active-attacks-on-resolver-">
<a href="#section-6.1.2" class="section-number selfRef">6.1.2. </a><a href="#name-active-attacks-on-resolver-" class="section-name selfRef">Active Attacks on Resolver Configuration</a>
          </h4>
<p id="section-6.1.2-1">The previous section discussed DNS privacy, assuming that all the traffic
  was directed to the intended servers (i.e., those that would be used in the
  absence of an active attack) and that the potential attacker was purely
  passive. But, in reality, there can be active attackers in the network.<a href="#section-6.1.2-1" class="pilcrow">¶</a></p>
<p id="section-6.1.2-2">The Internet Threat model, as described in <span>[<a href="#RFC3552" class="xref">RFC3552</a>]</span>, assumes that the attacker
  controls the network. Such an attacker can completely control any insecure DNS
  resolution, both passively monitoring the queries and responses and substituting
  their own responses. Even if encrypted DNS such as DoH or DoT is used, unless
  the client has been configured in a secure way with the server identity, an active attacker can impersonate the server. This implies that opportunistic
  modes of DoH/DoT as well as modes where the client learns of the DoH/DoT server
  via in-network mechanisms such as DHCP are vulnerable to attack. In addition, if
  the client is compromised, the attacker can replace the DNS configuration with
  one of its own choosing.<a href="#section-6.1.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="blocking-of-dns-resolution-services">
<section id="section-6.1.3">
          <h4 id="name-blocking-of-dns-resolution-">
<a href="#section-6.1.3" class="section-number selfRef">6.1.3. </a><a href="#name-blocking-of-dns-resolution-" class="section-name selfRef">Blocking of DNS Resolution Services</a>
          </h4>
<p id="section-6.1.3-1">User privacy can also be at risk if there is blocking
   of access to remote recursive servers
  that offer encrypted transports, e.g., when the local resolver does not offer
  encryption and/or has very poor privacy policies. For example, active blocking
  of port 853 for DoT or blocking of specific IP addresses could restrict the resolvers
  available to the user. The extent of the risk to user privacy is highly
  dependent on the specific network and user context; a user on a network that
  is known to perform surveillance would be compromised if they could not access
  such services, whereas a user on a trusted network might have no privacy
  motivation to do so.<a href="#section-6.1.3-1" class="pilcrow">¶</a></p>
<p id="section-6.1.3-2">As a matter of policy, some recursive resolvers use their position in the query
  path to selectively block access to certain DNS records. This is a form of
  rendezvous-based blocking as described in <span><a href="https://www.rfc-editor.org/rfc/rfc7754#section-4.3" class="relref">Section 4.3</a> of [<a href="#RFC7754" class="xref">RFC7754</a>]</span>. Such
  blocklists often include servers known to be used for malware, bots, or other
  security risks. In order to prevent circumvention of their blocking policies,
  some networks also block access to resolvers with incompatible policies.<a href="#section-6.1.3-2" class="pilcrow">¶</a></p>
<p id="section-6.1.3-3">It is also noted that attacks on remote resolver services, e.g., DDoS, could
  force users to switch to other services that do not offer encrypted transports
  for DNS.<a href="#section-6.1.3-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="encrypted-transports-and-recursive-resolvers">
<section id="section-6.1.4">
          <h4 id="name-encrypted-transports-and-re">
<a href="#section-6.1.4" class="section-number selfRef">6.1.4. </a><a href="#name-encrypted-transports-and-re" class="section-name selfRef">Encrypted Transports and Recursive Resolvers</a>
          </h4>
<div id="dot-and-doh">
<section id="section-6.1.4.1">
            <h5 id="name-dot-and-doh">
<a href="#section-6.1.4.1" class="section-number selfRef">6.1.4.1. </a><a href="#name-dot-and-doh" class="section-name selfRef">DoT and DoH</a>
            </h5>
<p id="section-6.1.4.1-1">Use of encrypted transports does not reduce the data available in the recursive
resolver and ironically can actually expose more information about users to
operators. As described in <a href="#encrypted-transports" class="xref">Section 5.2</a>, use of session-based encrypted
transports (TCP/TLS) can expose correlation data about users.<a href="#section-6.1.4.1-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="doh-specific-considerations">
<section id="section-6.1.4.2">
            <h5 id="name-doh-specific-considerations">
<a href="#section-6.1.4.2" class="section-number selfRef">6.1.4.2. </a><a href="#name-doh-specific-considerations" class="section-name selfRef">DoH-Specific Considerations</a>
            </h5>
<p id="section-6.1.4.2-1">DoH inherits the full privacy properties of the HTTPS stack and as a consequence
introduces new privacy considerations when compared with DNS over UDP, TCP, or
TLS <span>[<a href="#RFC7858" class="xref">RFC7858</a>]</span>. <span><a href="https://www.rfc-editor.org/rfc/rfc8484#section-8.2" class="relref">Section 8.2</a> of [<a href="#RFC8484" class="xref">RFC8484</a>]</span> describes the privacy considerations in
the server of the DoH protocol.<a href="#section-6.1.4.2-1" class="pilcrow">¶</a></p>
<p id="section-6.1.4.2-2">A brief summary of some of the issues includes the following:<a href="#section-6.1.4.2-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="section-6.1.4.2-3.1">HTTPS presents new considerations for correlation, such as explicit HTTP
cookies and implicit fingerprinting of the unique set and ordering of HTTP
request header fields.<a href="#section-6.1.4.2-3.1" class="pilcrow">¶</a>
</li>
              <li class="normal" id="section-6.1.4.2-3.2">The User-Agent and Accept-Language request header fields often convey specific
information about the client version or locale.<a href="#section-6.1.4.2-3.2" class="pilcrow">¶</a>
</li>
              <li class="normal" id="section-6.1.4.2-3.3">Utilizing the full set of HTTP features enables DoH to be more than an HTTP
tunnel, but it is at the cost of opening up implementations to the full set of
privacy considerations of HTTP.<a href="#section-6.1.4.2-3.3" class="pilcrow">¶</a>
</li>
              <li class="normal" id="section-6.1.4.2-3.4">Implementations are advised to expose the minimal set of data needed to
achieve the desired feature set.<a href="#section-6.1.4.2-3.4" class="pilcrow">¶</a>
</li>
            </ul>
<p id="section-6.1.4.2-4"><span>[<a href="#RFC8484" class="xref">RFC8484</a>]</span> specifically makes selection of HTTPS functionality vs. privacy an
implementation choice. At the extremes, there may be implementations that
attempt to achieve parity with DoT from a privacy perspective at the cost of
using no identifiable HTTP headers, and there might be others that provide feature-rich data flows where the low-level origin of the DNS query is easily
identifiable. Some implementations have, in fact, chosen to restrict the use of the User-Agent header so that resolver operators cannot identify the specific
application that is originating the DNS queries.<a href="#section-6.1.4.2-4" class="pilcrow">¶</a></p>
<p id="section-6.1.4.2-5">Privacy-focused users should be aware of the potential for additional client
identifiers in DoH compared to DoT and may want to only use DoH client
implementations that provide clear guidance on what identifiers they add.<a href="#section-6.1.4.2-5" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
</section>
</div>
<div id="in-the-authoritative-name-servers">
<section id="section-6.2">
        <h3 id="name-in-the-authoritative-name-s">
<a href="#section-6.2" class="section-number selfRef">6.2. </a><a href="#name-in-the-authoritative-name-s" class="section-name selfRef">In the Authoritative Name Servers</a>
        </h3>
<p id="section-6.2-1">Unlike what happens for recursive resolvers, the observation capabilities of
   authoritative name servers are limited by caching; they see only the requests
   for which the answer was not in the cache. For aggregated statistics ("What
   is the percentage of LOC queries?"), this is sufficient, but it prevents an
   observer from seeing everything. Similarly, the increasing deployment of QNAME
   minimization <span>[<a href="#ripe-qname-measurements" class="xref">ripe-qname-measurements</a>]</span> reduces the data visible at the
   authoritative name server. Still, the authoritative name servers see a part
   of the traffic, and this subset may be sufficient to violate some privacy
   expectations.<a href="#section-6.2-1" class="pilcrow">¶</a></p>
<p id="section-6.2-2">Also, the user often has some legal/contractual link with the
   recursive resolver (they have chosen the IAP, or they have chosen to use a
   given public resolver) while having no control and perhaps no
   awareness of the role of the authoritative name servers and their
   observation abilities.<a href="#section-6.2-2" class="pilcrow">¶</a></p>
<p id="section-6.2-3">As noted before, using a local resolver or a resolver close to the
   machine decreases the attack surface for an on-the-wire eavesdropper.
   But it may decrease privacy against an observer located on an
   authoritative name server.  This authoritative name server will see
   the IP address of the end client instead of the address of a big
   recursive resolver shared by many users.<a href="#section-6.2-3" class="pilcrow">¶</a></p>
<p id="section-6.2-4">This "protection", when using a large resolver with many clients, is
   no longer present if ECS <span>[<a href="#RFC7871" class="xref">RFC7871</a>]</span> is used because, in this case,
   the authoritative name server sees the original IP address (or
   prefix, depending on the setup).<a href="#section-6.2-4" class="pilcrow">¶</a></p>
<p id="section-6.2-5">As of today, all the instances of one root name server, L-root,
   receive together around 50,000 queries per second.  While most of it
   is "junk" (errors on the Top-Level Domain (TLD) name), it gives an
   idea of the amount of big data that pours into name servers.  (And
   even "junk" can leak information; for instance, if there is a typing
   error in the TLD, the user will send data to a TLD that is not the
   usual one.)<a href="#section-6.2-5" class="pilcrow">¶</a></p>
<p id="section-6.2-6">Many domains, including TLDs, are partially hosted by third-party
   servers, sometimes in a different country.  The contracts between the
   domain manager and these servers may or may not take privacy into
   account.  Whatever the contract, the third-party hoster may or may not be honest; in any case, it will have to follow its local laws.  For
   example,
   requests to a given ccTLD may go to servers managed by organizations
   outside of the ccTLD's country.  Users may not anticipate that
   when doing a security analysis.<a href="#section-6.2-6" class="pilcrow">¶</a></p>
<p id="section-6.2-7">Also, it seems (see the survey described in <span>[<a href="#aeris-dns" class="xref">aeris-dns</a>]</span>) that there is a
   strong concentration of authoritative name servers among "popular" domains
   (such as the Alexa Top N list). For instance, among the <a href="https://www.alexa.com/topsites">Alexa Top
   100K</a>, one DNS provider hosts 10% of
   the domains today. The ten most important DNS providers together host one-third of
   all domains. With the control (or the ability to sniff the traffic) of a few
   name servers, you can gather a lot of information.<a href="#section-6.2-7" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="other-risks">
<section id="section-7">
      <h2 id="name-other-risks">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-other-risks" class="section-name selfRef">Other Risks</a>
      </h2>
<div id="reidentification-and-other-inferences">
<section id="section-7.1">
        <h3 id="name-re-identification-and-other">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-re-identification-and-other" class="section-name selfRef">Re-identification and Other Inferences</a>
        </h3>
<p id="section-7.1-1">An observer has access not only to the data they directly collect but also
   to the results of various inferences about this data. The term "observer" here is used very generally; for example, the observer might 
   passively observe cleartext DNS traffic or be in the network 
   that is actively attacking the user by redirecting DNS resolution, or it might be a 
   local or remote resolver operator.<a href="#section-7.1-1" class="pilcrow">¶</a></p>
<p id="section-7.1-2">For instance, a user can be re-identified via DNS queries.  If the
   adversary knows a user's identity and can watch their DNS queries for
   a period, then that same adversary may be able to re-identify the
   user solely based on their pattern of DNS queries later on regardless
   of the location from which the user makes those queries.  For
   example, one study <span>[<a href="#herrmann-reidentification" class="xref">herrmann-reidentification</a>]</span> found that such re-identification is possible so that "73.1% of all day-to-day links
   were correctly established, i.e. user u was either re-identified
   unambiguously (1) or the classifier correctly reported that u was not
   present on day t + 1 any more (2)".  While that study related to web
   browsing behavior, equally characteristic patterns may be produced
   even in machine-to-machine communications or without a user taking
   specific actions, e.g., at reboot time if a characteristic set of
   services are accessed by the device.<a href="#section-7.1-2" class="pilcrow">¶</a></p>
<p id="section-7.1-3">For instance, one could imagine that an intelligence agency
   identifies people going to a site by putting in a very long DNS name
   and looking for queries of a specific length.  Such traffic analysis
   could weaken some privacy solutions.<a href="#section-7.1-3" class="pilcrow">¶</a></p>
<p id="section-7.1-4">The IAB Privacy and Security Program also has a document
   <span>[<a href="#RFC7624" class="xref">RFC7624</a>]</span> that considers such inference-based attacks in a more
   general framework.<a href="#section-7.1-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="more-information">
<section id="section-7.2">
        <h3 id="name-more-information">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-more-information" class="section-name selfRef">More Information</a>
        </h3>
<p id="section-7.2-1">Useful background information can also be found in <span>[<a href="#tor-leak" class="xref">tor-leak</a>]</span> (regarding the risk of privacy leaks through DNS) and in a few academic papers:
   <span>[<a href="#yanbin-tsudik" class="xref">yanbin-tsudik</a>]</span>, <span>[<a href="#castillo-garcia" class="xref">castillo-garcia</a>]</span>, <span>[<a href="#fangming-hori-sakurai" class="xref">fangming-hori-sakurai</a>]</span>, and
   <span>[<a href="#federrath-fuchs-herrmann-piosecny" class="xref">federrath-fuchs-herrmann-piosecny</a>]</span>.<a href="#section-7.2-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="actual-attacks">
<section id="section-8">
      <h2 id="name-actual-attacks">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-actual-attacks" class="section-name selfRef">Actual "Attacks"</a>
      </h2>
<p id="section-8-1">A very quick examination of DNS traffic may lead to the false conclusion that
   extracting the needle from the haystack is difficult. "Interesting" primary
   DNS requests are mixed with useless (for the eavesdropper) secondary and
   tertiary requests (see the terminology in <a href="#introduction" class="xref">Section 1</a>). But, in
   this time of "big data" processing, powerful techniques now exist to get from
   the raw data to what the eavesdropper is actually interested in.<a href="#section-8-1" class="pilcrow">¶</a></p>
<p id="section-8-2">Many research papers about malware detection use DNS traffic to
   detect "abnormal" behavior that can be traced back to the activity of
   malware on infected machines.  
Yes, this research was done for the greater good, but technically it is a privacy attack and it demonstrates the
   power of the observation of DNS traffic.  See <span>[<a href="#dns-footprint" class="xref">dns-footprint</a>]</span>,
   <span>[<a href="#dagon-malware" class="xref">dagon-malware</a>]</span>, and <span>[<a href="#darkreading-dns" class="xref">darkreading-dns</a>]</span>.<a href="#section-8-2" class="pilcrow">¶</a></p>
<p id="section-8-3">Passive DNS services <span>[<a href="#passive-dns" class="xref">passive-dns</a>]</span> allow reconstruction of the data of sometimes an entire zone. Well-known passive DNS services keep only the DNS
   responses and not the source IP address of the client, precisely for
   privacy reasons.  Other passive DNS services may not be so careful.
   And there are still potential problems with revealing QNAMEs.<a href="#section-8-3" class="pilcrow">¶</a></p>
<p id="section-8-4">The revelations from the Edward Snowden documents, which were leaked from the
   National Security Agency (NSA), provide evidence of the use of the DNS in mass
   surveillance operations <span>[<a href="#morecowbell" class="xref">morecowbell</a>]</span>. For example, the MORECOWBELL
   surveillance program uses a dedicated covert monitoring infrastructure
   to actively query DNS servers and perform HTTP requests to obtain meta-information about services and to check their availability. Also, the
   <a href="https://theintercept.com/document/2014/03/12/nsa-gchqs-quantumtheory-hacking-tactics/">QUANTUMTHEORY</a>
   project, which includes detecting lookups for certain addresses and injecting
   bogus replies, is another good example showing that the lack of privacy
   protections in the DNS is actively exploited.<a href="#section-8-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="legalities">
<section id="section-9">
      <h2 id="name-legalities">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-legalities" class="section-name selfRef">Legalities</a>
      </h2>
<p id="section-9-1">To our knowledge, there are no specific privacy laws for DNS data in any
   country. Interpreting general privacy laws, like the European Union's <span>[<a href="#data-protection-directive" class="xref">data-protection-directive</a>]</span>
   or <a href="https://gdpr.eu/tag/gdpr/">GDPR</a>, in the context of DNS traffic data is not an easy task, and
   there is no known court precedent. See an interesting analysis in
   <span>[<a href="#sidn-entrada" class="xref">sidn-entrada</a>]</span>.<a href="#section-9-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="security-considerations">
<section id="section-10">
      <h2 id="name-security-considerations">
<a href="#section-10" class="section-number selfRef">10. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-10-1">This document is entirely about security -- more precisely, privacy. It just
   lays out the problem; it does not try to set requirements (with the choices
   and compromises they imply), much less define solutions. Possible solutions
   to the issues described here are discussed in other documents (currently too
   many to all be mentioned); see, for instance, "Recommendations for DNS
   Privacy Operators" <span>[<a href="#RFC8932" class="xref">RFC8932</a>]</span>.<a href="#section-10-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="iana-considerations">
<section id="section-11">
      <h2 id="name-iana-considerations">
<a href="#section-11" class="section-number selfRef">11. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-11-1">This document has no IANA actions.<a href="#section-11-1" class="pilcrow">¶</a></p>
</section>
</div>
<section id="section-12">
      <h2 id="name-references">
<a href="#section-12" class="section-number selfRef">12. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-12.1">
        <h3 id="name-normative-references">
<a href="#section-12.1" class="section-number selfRef">12.1. </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="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="RFC7258">[RFC7258]</dt>
      <dd>
<span class="refAuthor">Farrell, S.</span> and <span class="refAuthor">H. Tschofenig</span>, <span class="refTitle">"Pervasive Monitoring Is an Attack"</span>, <span class="seriesInfo">BCP 188</span>, <span class="seriesInfo">RFC 7258</span>, <span class="seriesInfo">DOI 10.17487/RFC7258</span>, <time datetime="2014-05" class="refDate">May 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7258">https://www.rfc-editor.org/info/rfc7258</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-12.2">
        <h3 id="name-informative-references">
<a href="#section-12.2" class="section-number selfRef">12.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="aeris-dns">[aeris-dns]</dt>
        <dd>
<span class="refAuthor">Vinot, N.</span>, <span class="refTitle">"Vie privée: et le DNS alors? [Privacy: what about DNS?]"</span>, <time datetime="2015-02" class="refDate">February 2015</time>, <span>&lt;<a href="https://blog.imirhil.fr/vie-privee-et-le-dns-alors.html">https://blog.imirhil.fr/vie-privee-et-le-dns-alors.html</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="cache-snooping-defence">[cache-snooping-defence]</dt>
        <dd>
<span class="refAuthor">ISC</span>, <span class="refTitle">"DNS Cache snooping - should I be concerned?"</span>, <time datetime="2018-10" class="refDate">October 2018</time>, <span>&lt;<a href="https://kb.isc.org/docs/aa-00482">https://kb.isc.org/docs/aa-00482</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="castillo-garcia">[castillo-garcia]</dt>
        <dd>
<span class="refAuthor">Castillo-Perez, S.</span> and <span class="refAuthor">J. Garcia-Alfaro</span>, <span class="refTitle">"Anonymous Resolution of DNS Queries"</span>, <span class="refContent">Lecture Notes in Computer Science, Vol. 5332</span>, <span class="seriesInfo">DOI 10.1007/978-3-540-88873-4_5</span>, <time datetime="2008" class="refDate">2008</time>, <span>&lt;<a href="https://dl.acm.org/doi/10.1007/978-3-540-88873-4_5">https://dl.acm.org/doi/10.1007/978-3-540-88873-4_5</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="centralisation-and-data-sovereignty">[centralisation-and-data-sovereignty]</dt>
        <dd>
<span class="refAuthor">De Filippi, P.</span> and <span class="refAuthor">S. McCarthy</span>, <span class="refTitle">"Cloud Computing: Centralization and Data Sovereignty"</span>, <span class="refContent">European Journal of Law and Technology, Vol. 3, No. 2</span>, <time datetime="2012-10" class="refDate">October 2012</time>, <span>&lt;<a href="https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2167372">https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2167372</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dagon-malware">[dagon-malware]</dt>
        <dd>
<span class="refAuthor">Dagon, D.</span>, <span class="refTitle">"Corrupted DNS Resolution Paths: The Rise of a Malicious Resolution Authority"</span>, <span class="refContent">ISC/OARC Workshop</span>, <time datetime="2007" class="refDate">2007</time>, <span>&lt;<a href="https://www.dns-oarc.net/files/workshop-2007/Dagon-Resolution-corruption.pdf">https://www.dns-oarc.net/files/workshop-2007/Dagon-Resolution-corruption.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="darkreading-dns">[darkreading-dns]</dt>
        <dd>
<span class="refAuthor">Lemos, R.</span>, <span class="refTitle">"Got Malware? Three Signs Revealed In DNS Traffic"</span>, <time datetime="2013-05" class="refDate">May 2013</time>, <span>&lt;<a href="https://www.darkreading.com/analytics/security-monitoring/got-malware-three-signs-revealed-in-dns-traffic/d/d-id/1139680">https://www.darkreading.com/analytics/security-monitoring/got-malware-three-signs-revealed-in-dns-traffic/d/d-id/1139680</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="data-protection-directive">[data-protection-directive]</dt>
        <dd>
<span class="refAuthor">European Parliament</span>, <span class="refTitle">"Directive 95/46/EC of the European Parliament and of the Council of 24 October 1995 on the protection of individuals with regard to the processing of personal data and on the free movement of such data"</span>, <span class="refContent">Official Journal L 281, pp. 31-50</span>, <time datetime="1995-11" class="refDate">November 1995</time>, <span>&lt;<a href="https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31995L0046:EN:HTML">https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:31995L0046:EN:HTML</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="day-at-root">[day-at-root]</dt>
        <dd>
<span class="refAuthor">Castro, S.</span>, <span class="refAuthor">Wessels, D.</span>, <span class="refAuthor">Fomenkov, M.</span>, and <span class="refAuthor">K. Claffy</span>, <span class="refTitle">"A Day at the Root of the Internet"</span>, <span class="refContent">ACM SIGCOMM Computer Communication Review, Vol. 38, No. 5</span>, <span class="seriesInfo">DOI 10.1145/1452335.1452341</span>, <time datetime="2008-10" class="refDate">October 2008</time>, <span>&lt;<a href="https://www.sigcomm.org/sites/default/files/ccr/papers/2008/October/1452335-1452341.pdf">https://www.sigcomm.org/sites/default/files/ccr/papers/2008/October/1452335-1452341.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="denis-edns-client-subnet">[denis-edns-client-subnet]</dt>
        <dd>
<span class="refAuthor">Denis, F.</span>, <span class="refTitle">"Security and privacy issues of edns-client-subnet"</span>, <time datetime="2013-08" class="refDate">August 2013</time>, <span>&lt;<a href="https://00f.net/2013/08/07/edns-client-subnet/">https://00f.net/2013/08/07/edns-client-subnet/</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="ditl">[ditl]</dt>
        <dd>
<span class="refAuthor">CAIDA</span>, <span class="refTitle">"A Day in the Life of the Internet (DITL)"</span>, <span>&lt;<a href="https://www.caida.org/projects/ditl/">https://www.caida.org/projects/ditl/</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dns-de-nat">[dns-de-nat]</dt>
        <dd>
<span class="refAuthor">Orevi, L.</span>, <span class="refAuthor">Herzberg, A.</span>, <span class="refAuthor">Zlatokrilov, H.</span>, and <span class="refAuthor">D. Sigron</span>, <span class="refTitle">"DNS-DNS: DNS-based De-NAT Scheme"</span>, <time datetime="2017-01" class="refDate">January 2017</time>, <span>&lt;<a href="https://www.researchgate.net/publication/320322146_DNS-DNS_DNS-based_De-NAT_Scheme">https://www.researchgate.net/publication/320322146_DNS-DNS_DNS-based_De-NAT_Scheme</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dns-footprint">[dns-footprint]</dt>
        <dd>
<span class="refAuthor">Stoner, E.</span>, <span class="refTitle">"DNS Footprint of Malware"</span>, <span class="refContent">OARC Workshop</span>, <time datetime="2010-10" class="refDate">October 2010</time>, <span>&lt;<a href="https://www.dns-oarc.net/files/workshop-201010/OARC-ers-20101012.pdf">https://www.dns-oarc.net/files/workshop-201010/OARC-ers-20101012.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dns-over-encryption">[dns-over-encryption]</dt>
        <dd>
<span class="refAuthor">Lu, C.</span>, <span class="refAuthor">Liu, B.</span>, <span class="refAuthor">Li, Z.</span>, <span class="refAuthor">Hao, S.</span>, <span class="refAuthor">Duan, H.</span>, <span class="refAuthor">Zhang, M.</span>, <span class="refAuthor">Leng, C.</span>, <span class="refAuthor">Liu, Y.</span>, <span class="refAuthor">Zhang, Z.</span>, and <span class="refAuthor">J. Wu</span>, <span class="refTitle">"An End-to-End, Large-Scale Measurement of DNS-over-Encryption: How Far Have We Come?"</span>, <span class="refContent">IMC '19: Proceedings of the Internet Measurement Conference, pp. 22-35</span>, <span class="seriesInfo">DOI 10.1145/3355369.3355580</span>, <time datetime="2019-10" class="refDate">October 2019</time>, <span>&lt;<a href="https://dl.acm.org/citation.cfm?id=3355369.3355580">https://dl.acm.org/citation.cfm?id=3355369.3355580</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="dnsmezzo">[dnsmezzo]</dt>
        <dd>
<span class="refAuthor">Bortzmeyer, S.</span>, <span class="refTitle">"DNSmezzo"</span>, <span>&lt;<a href="http://www.dnsmezzo.net/">http://www.dnsmezzo.net/</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-dnsop-resolver-information">[DNSOP-RESOLVER]</dt>
        <dd>
<span class="refAuthor">Sood, P.</span>, <span class="refAuthor">Arends, R.</span>, and <span class="refAuthor">P. Hoffman</span>, <span class="refTitle">"DNS Resolver Information Self-publication"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-dnsop-resolver-information-01</span>, <time datetime="2020-02-11" class="refDate">11 February 2020</time>, <span>&lt;<a href="https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-resolver-information-01">https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-resolver-information-01</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="I-D.ietf-dprive-dnsoquic">[DPRIVE-DNSOQUIC]</dt>
        <dd>
<span class="refAuthor">Huitema, C.</span>, <span class="refAuthor">Dickinson, S.</span>, and <span class="refAuthor">A. Mankin</span>, <span class="refTitle">"Specification of DNS over Dedicated QUIC Connections"</span>, <span class="refContent">Work in Progress</span>, <span class="seriesInfo">Internet-Draft, draft-ietf-dprive-dnsoquic-03</span>, <time datetime="2021-07-12" class="refDate">12 July 2021</time>, <span>&lt;<a href="https://datatracker.ietf.org/doc/html/draft-ietf-dprive-dnsoquic-03">https://datatracker.ietf.org/doc/html/draft-ietf-dprive-dnsoquic-03</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="EDDI">[EDDI]</dt>
        <dd>
<span class="refAuthor">EDDI</span>, <span class="refTitle">"Encrypted DNS Deployment Initiative"</span>, <span>&lt;<a href="https://www.encrypted-dns.org">https://www.encrypted-dns.org</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="fangming-hori-sakurai">[fangming-hori-sakurai]</dt>
        <dd>
<span class="refAuthor">Zhao, F.</span>, <span class="refAuthor">Hori, Y.</span>, and <span class="refAuthor">K. Sakurai</span>, <span class="refTitle">"Analysis of Privacy Disclosure in DNS Query"</span>, <span class="refContent">MUE '07: Proceedings of the 2007 International Conference on Multimedia and Ubiquitous Engineering</span>, <span class="refContent">pp. 952-957</span>, <span class="seriesInfo">DOI 10.1109/MUE.2007.84</span>, <span class="seriesInfo">ISBN 0-7695-2777-9</span>, <time datetime="2007-04" class="refDate">April 2007</time>, <span>&lt;<a href="https://dl.acm.org/citation.cfm?id=1262690.1262986">https://dl.acm.org/citation.cfm?id=1262690.1262986</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="federrath-fuchs-herrmann-piosecny">[federrath-fuchs-herrmann-piosecny]</dt>
        <dd>
<span class="refAuthor">Federrath, H.</span>, <span class="refAuthor">Fuchs, K.-P.</span>, <span class="refAuthor">Herrmann, D.</span>, and <span class="refAuthor">C. Piosecny</span>, <span class="refTitle">"Privacy-Preserving DNS: Analysis of Broadcast, Range Queries and Mix-based Protection Methods"</span>, <span class="refContent">ESORICS 2011, pp. 665-683</span>, <span class="seriesInfo">DOI 10.1007/978-3-642-23822-2_36</span>, <span class="seriesInfo">ISBN 978-3-642-23822-2</span>, <time datetime="2011" class="refDate">2011</time>, <span>&lt;<a href="https://svs.informatik.uni-hamburg.de/publications/2011/2011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf">https://svs.informatik.uni-hamburg.de/publications/2011/2011-09-14_FFHP_PrivacyPreservingDNS_ESORICS2011.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="getdns">[getdns]</dt>
        <dd>
<span class="refTitle">"getdns"</span>, <span>&lt;<a href="https://getdnsapi.net">https://getdnsapi.net</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="grangeia.snooping">[grangeia.snooping]</dt>
        <dd>
<span class="refAuthor">Grangeia, L.</span>, <span class="refTitle">"Cache Snooping or Snooping the Cache for Fun and Profit"</span>, <time datetime="2005" class="refDate">2005</time>, <span>&lt;<a href="https://www.semanticscholar.org/paper/Cache-Snooping-or-Snooping-the-Cache-for-Fun-and-1-Grangeia/9b22f606e10b3609eafbdcbfc9090b63be8778c3">https://www.semanticscholar.org/paper/Cache-Snooping-or-Snooping-the-Cache-for-Fun-and-1-Grangeia/9b22f606e10b3609eafbdcbfc9090b63be8778c3</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="herrmann-reidentification">[herrmann-reidentification]</dt>
        <dd>
<span class="refAuthor">Herrmann, D.</span>, <span class="refAuthor">Gerber, C.</span>, <span class="refAuthor">Banse, C.</span>, and <span class="refAuthor">H. Federrath</span>, <span class="refTitle">"Analyzing Characteristic Host Access Patterns for Re-Identification of Web User Sessions"</span>, <span class="refContent">Lecture Notes in Computer Science, Vol. 7127</span>, <span class="seriesInfo">DOI 10.1007/978-3-642-27937-9_10</span>, <time datetime="2012" class="refDate">2012</time>, <span>&lt;<a href="https://epub.uni-regensburg.de/21103/1/Paper_PUL_nordsec_published.pdf">https://epub.uni-regensburg.de/21103/1/Paper_PUL_nordsec_published.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="morecowbell">[morecowbell]</dt>
        <dd>
<span class="refAuthor">Grothoff, C.</span>, <span class="refAuthor">Wachs, M.</span>, <span class="refAuthor">Ermert, M.</span>, and <span class="refAuthor">J. Appelbaum</span>, <span class="refTitle">"NSA's MORECOWBELL: Knell for DNS"</span>, <time datetime="2015-01" class="refDate">January 2015</time>, <span>&lt;<a href="https://pdfs.semanticscholar.org/2610/2b99bdd6a258a98740af8217ba8da8a1e4fa.pdf">https://pdfs.semanticscholar.org/2610/2b99bdd6a258a98740af8217ba8da8a1e4fa.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="packetq">[packetq]</dt>
        <dd>
<span class="refAuthor">DNS-OARC</span>, <span class="refTitle">"A tool that provides a basic SQL-frontend to PCAP-files"</span>, <span class="refContent">Release 1.4.3</span>, <span class="refContent">commit 29a8288</span>, <time datetime="2020-10" class="refDate">October 2020</time>, <span>&lt;<a href="https://github.com/DNS-OARC/PacketQ">https://github.com/DNS-OARC/PacketQ</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="passive-dns">[passive-dns]</dt>
        <dd>
<span class="refAuthor">Weimer, F.</span>, <span class="refTitle">"Passive DNS Replication"</span>, <span class="refContent">17th Annual FIRST Conference</span>, <time datetime="2005-04" class="refDate">April 2005</time>, <span>&lt;<a href="https://www.first.org/conference/2005/papers/florian-weimer-slides-1.pdf">https://www.first.org/conference/2005/papers/florian-weimer-slides-1.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="pitfalls-of-dns-encryption">[pitfalls-of-dns-encryption]</dt>
        <dd>
<span class="refAuthor">Shulman, H.</span>, <span class="refTitle">"Pretty Bad Privacy: Pitfalls of DNS Encryption"</span>, <span class="refContent">WPES '14: Proceedings of the 13th Workshop on Privacy in the Electronic Society, pp. 191-200</span>, <span class="seriesInfo">DOI 10.1145/2665943.2665959</span>, <time datetime="2014-11" class="refDate">November 2014</time>, <span>&lt;<a href="https://dl.acm.org/citation.cfm?id=2665959">https://dl.acm.org/citation.cfm?id=2665959</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="prism">[prism]</dt>
        <dd>
<span class="refAuthor">Wikipedia</span>, <span class="refTitle">"PRISM (surveillance program)"</span>, <time datetime="2015-07" class="refDate">July 2015</time>, <span>&lt;<a href="https://en.wikipedia.org/w/index.php?title=PRISM_(surveillance_program)&amp;oldid=673789455">https://en.wikipedia.org/w/index.php?title=PRISM_(surveillance_program)&amp;oldid=673789455</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC3552">[RFC3552]</dt>
        <dd>
<span class="refAuthor">Rescorla, E.</span> and <span class="refAuthor">B. Korver</span>, <span class="refTitle">"Guidelines for Writing RFC Text on Security Considerations"</span>, <span class="seriesInfo">BCP 72</span>, <span class="seriesInfo">RFC 3552</span>, <span class="seriesInfo">DOI 10.17487/RFC3552</span>, <time datetime="2003-07" class="refDate">July 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3552">https://www.rfc-editor.org/info/rfc3552</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4033">[RFC4033]</dt>
        <dd>
<span class="refAuthor">Arends, R.</span>, <span class="refAuthor">Austein, R.</span>, <span class="refAuthor">Larson, M.</span>, <span class="refAuthor">Massey, D.</span>, and <span class="refAuthor">S. Rose</span>, <span class="refTitle">"DNS Security Introduction and Requirements"</span>, <span class="seriesInfo">RFC 4033</span>, <span class="seriesInfo">DOI 10.17487/RFC4033</span>, <time datetime="2005-03" class="refDate">March 2005</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4033">https://www.rfc-editor.org/info/rfc4033</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4470">[RFC4470]</dt>
        <dd>
<span class="refAuthor">Weiler, S.</span> and <span class="refAuthor">J. Ihren</span>, <span class="refTitle">"Minimally Covering NSEC Records and DNSSEC On-line Signing"</span>, <span class="seriesInfo">RFC 4470</span>, <span class="seriesInfo">DOI 10.17487/RFC4470</span>, <time datetime="2006-04" class="refDate">April 2006</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4470">https://www.rfc-editor.org/info/rfc4470</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5155">[RFC5155]</dt>
        <dd>
<span class="refAuthor">Laurie, B.</span>, <span class="refAuthor">Sisson, G.</span>, <span class="refAuthor">Arends, R.</span>, and <span class="refAuthor">D. Blacka</span>, <span class="refTitle">"DNS Security (DNSSEC) Hashed Authenticated Denial of Existence"</span>, <span class="seriesInfo">RFC 5155</span>, <span class="seriesInfo">DOI 10.17487/RFC5155</span>, <time datetime="2008-03" class="refDate">March 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5155">https://www.rfc-editor.org/info/rfc5155</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC5936">[RFC5936]</dt>
        <dd>
<span class="refAuthor">Lewis, E.</span> and <span class="refAuthor">A. Hoenes, Ed.</span>, <span class="refTitle">"DNS Zone Transfer Protocol (AXFR)"</span>, <span class="seriesInfo">RFC 5936</span>, <span class="seriesInfo">DOI 10.17487/RFC5936</span>, <time datetime="2010-06" class="refDate">June 2010</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5936">https://www.rfc-editor.org/info/rfc5936</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6269">[RFC6269]</dt>
        <dd>
<span class="refAuthor">Ford, M., Ed.</span>, <span class="refAuthor">Boucadair, M.</span>, <span class="refAuthor">Durand, A.</span>, <span class="refAuthor">Levis, P.</span>, and <span class="refAuthor">P. Roberts</span>, <span class="refTitle">"Issues with IP Address Sharing"</span>, <span class="seriesInfo">RFC 6269</span>, <span class="seriesInfo">DOI 10.17487/RFC6269</span>, <time datetime="2011-06" class="refDate">June 2011</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6269">https://www.rfc-editor.org/info/rfc6269</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC6891">[RFC6891]</dt>
        <dd>
<span class="refAuthor">Damas, J.</span>, <span class="refAuthor">Graff, M.</span>, and <span class="refAuthor">P. Vixie</span>, <span class="refTitle">"Extension Mechanisms for DNS (EDNS(0))"</span>, <span class="seriesInfo">STD 75</span>, <span class="seriesInfo">RFC 6891</span>, <span class="seriesInfo">DOI 10.17487/RFC6891</span>, <time datetime="2013-04" class="refDate">April 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6891">https://www.rfc-editor.org/info/rfc6891</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7413">[RFC7413]</dt>
        <dd>
<span class="refAuthor">Cheng, Y.</span>, <span class="refAuthor">Chu, J.</span>, <span class="refAuthor">Radhakrishnan, S.</span>, and <span class="refAuthor">A. Jain</span>, <span class="refTitle">"TCP Fast Open"</span>, <span class="seriesInfo">RFC 7413</span>, <span class="seriesInfo">DOI 10.17487/RFC7413</span>, <time datetime="2014-12" class="refDate">December 2014</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7413">https://www.rfc-editor.org/info/rfc7413</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7525">[RFC7525]</dt>
        <dd>
<span class="refAuthor">Sheffer, Y.</span>, <span class="refAuthor">Holz, R.</span>, and <span class="refAuthor">P. Saint-Andre</span>, <span class="refTitle">"Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)"</span>, <span class="seriesInfo">BCP 195</span>, <span class="seriesInfo">RFC 7525</span>, <span class="seriesInfo">DOI 10.17487/RFC7525</span>, <time datetime="2015-05" class="refDate">May 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7525">https://www.rfc-editor.org/info/rfc7525</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7624">[RFC7624]</dt>
        <dd>
<span class="refAuthor">Barnes, R.</span>, <span class="refAuthor">Schneier, B.</span>, <span class="refAuthor">Jennings, C.</span>, <span class="refAuthor">Hardie, T.</span>, <span class="refAuthor">Trammell, B.</span>, <span class="refAuthor">Huitema, C.</span>, and <span class="refAuthor">D. Borkmann</span>, <span class="refTitle">"Confidentiality in the Face of Pervasive Surveillance: A Threat Model and Problem Statement"</span>, <span class="seriesInfo">RFC 7624</span>, <span class="seriesInfo">DOI 10.17487/RFC7624</span>, <time datetime="2015-08" class="refDate">August 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7624">https://www.rfc-editor.org/info/rfc7624</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7626">[RFC7626]</dt>
        <dd>
<span class="refAuthor">Bortzmeyer, S.</span>, <span class="refTitle">"DNS Privacy Considerations"</span>, <span class="seriesInfo">RFC 7626</span>, <span class="seriesInfo">DOI 10.17487/RFC7626</span>, <time datetime="2015-08" class="refDate">August 2015</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7626">https://www.rfc-editor.org/info/rfc7626</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7721">[RFC7721]</dt>
        <dd>
<span class="refAuthor">Cooper, A.</span>, <span class="refAuthor">Gont, F.</span>, and <span class="refAuthor">D. Thaler</span>, <span class="refTitle">"Security and Privacy Considerations for IPv6 Address Generation Mechanisms"</span>, <span class="seriesInfo">RFC 7721</span>, <span class="seriesInfo">DOI 10.17487/RFC7721</span>, <time datetime="2016-03" class="refDate">March 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7721">https://www.rfc-editor.org/info/rfc7721</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7754">[RFC7754]</dt>
        <dd>
<span class="refAuthor">Barnes, R.</span>, <span class="refAuthor">Cooper, A.</span>, <span class="refAuthor">Kolkman, O.</span>, <span class="refAuthor">Thaler, D.</span>, and <span class="refAuthor">E. Nordmark</span>, <span class="refTitle">"Technical Considerations for Internet Service Blocking and Filtering"</span>, <span class="seriesInfo">RFC 7754</span>, <span class="seriesInfo">DOI 10.17487/RFC7754</span>, <time datetime="2016-03" class="refDate">March 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7754">https://www.rfc-editor.org/info/rfc7754</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="RFC7858">[RFC7858]</dt>
        <dd>
<span class="refAuthor">Hu, Z.</span>, <span class="refAuthor">Zhu, L.</span>, <span class="refAuthor">Heidemann, J.</span>, <span class="refAuthor">Mankin, A.</span>, <span class="refAuthor">Wessels, D.</span>, and <span class="refAuthor">P. Hoffman</span>, <span class="refTitle">"Specification for DNS over Transport Layer Security (TLS)"</span>, <span class="seriesInfo">RFC 7858</span>, <span class="seriesInfo">DOI 10.17487/RFC7858</span>, <time datetime="2016-05" class="refDate">May 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7858">https://www.rfc-editor.org/info/rfc7858</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7871">[RFC7871]</dt>
        <dd>
<span class="refAuthor">Contavalli, C.</span>, <span class="refAuthor">van der Gaast, W.</span>, <span class="refAuthor">Lawrence, D.</span>, and <span class="refAuthor">W. Kumari</span>, <span class="refTitle">"Client Subnet in DNS Queries"</span>, <span class="seriesInfo">RFC 7871</span>, <span class="seriesInfo">DOI 10.17487/RFC7871</span>, <time datetime="2016-05" class="refDate">May 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7871">https://www.rfc-editor.org/info/rfc7871</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7873">[RFC7873]</dt>
        <dd>
<span class="refAuthor">Eastlake 3rd, D.</span> and <span class="refAuthor">M. Andrews</span>, <span class="refTitle">"Domain Name System (DNS) Cookies"</span>, <span class="seriesInfo">RFC 7873</span>, <span class="seriesInfo">DOI 10.17487/RFC7873</span>, <time datetime="2016-05" class="refDate">May 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7873">https://www.rfc-editor.org/info/rfc7873</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC7929">[RFC7929]</dt>
        <dd>
<span class="refAuthor">Wouters, P.</span>, <span class="refTitle">"DNS-Based Authentication of Named Entities (DANE) Bindings for OpenPGP"</span>, <span class="seriesInfo">RFC 7929</span>, <span class="seriesInfo">DOI 10.17487/RFC7929</span>, <time datetime="2016-08" class="refDate">August 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7929">https://www.rfc-editor.org/info/rfc7929</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8446">[RFC8446]</dt>
        <dd>
<span class="refAuthor">Rescorla, E.</span>, <span class="refTitle">"The Transport Layer Security (TLS) Protocol Version 1.3"</span>, <span class="seriesInfo">RFC 8446</span>, <span class="seriesInfo">DOI 10.17487/RFC8446</span>, <time datetime="2018-08" class="refDate">August 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8446">https://www.rfc-editor.org/info/rfc8446</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8484">[RFC8484]</dt>
        <dd>
<span class="refAuthor">Hoffman, P.</span> and <span class="refAuthor">P. McManus</span>, <span class="refTitle">"DNS Queries over HTTPS (DoH)"</span>, <span class="seriesInfo">RFC 8484</span>, <span class="seriesInfo">DOI 10.17487/RFC8484</span>, <time datetime="2018-10" class="refDate">October 2018</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8484">https://www.rfc-editor.org/info/rfc8484</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>
<dt id="RFC8744">[RFC8744]</dt>
        <dd>
<span class="refAuthor">Huitema, C.</span>, <span class="refTitle">"Issues and Requirements for Server Name Identification (SNI) Encryption in TLS"</span>, <span class="seriesInfo">RFC 8744</span>, <span class="seriesInfo">DOI 10.17487/RFC8744</span>, <time datetime="2020-07" class="refDate">July 2020</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8744">https://www.rfc-editor.org/info/rfc8744</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8890">[RFC8890]</dt>
        <dd>
<span class="refAuthor">Nottingham, M.</span>, <span class="refTitle">"The Internet is for End Users"</span>, <span class="seriesInfo">RFC 8890</span>, <span class="seriesInfo">DOI 10.17487/RFC8890</span>, <time datetime="2020-08" class="refDate">August 2020</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8890">https://www.rfc-editor.org/info/rfc8890</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8932">[RFC8932]</dt>
        <dd>
<span class="refAuthor">Dickinson, S.</span>, <span class="refAuthor">Overeinder, B.</span>, <span class="refAuthor">van Rijswijk-Deij, R.</span>, and <span class="refAuthor">A. Mankin</span>, <span class="refTitle">"Recommendations for DNS Privacy Service Operators"</span>, <span class="seriesInfo">BCP 232</span>, <span class="seriesInfo">RFC 8932</span>, <span class="seriesInfo">DOI 10.17487/RFC8932</span>, <time datetime="2020-10" class="refDate">October 2020</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8932">https://www.rfc-editor.org/info/rfc8932</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC9000">[RFC9000]</dt>
        <dd>
<span class="refAuthor">Iyengar, J., Ed.</span> and <span class="refAuthor">M. Thomson, Ed.</span>, <span class="refTitle">"QUIC: A UDP-Based Multiplexed and Secure Transport"</span>, <span class="seriesInfo">RFC 9000</span>, <span class="seriesInfo">DOI 10.17487/RFC9000</span>, <time datetime="2021-05" class="refDate">May 2021</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc9000">https://www.rfc-editor.org/info/rfc9000</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="ripe-qname-measurements">[ripe-qname-measurements]</dt>
        <dd>
<span class="refAuthor">de Vries, W.</span>, <span class="refTitle">"Making the DNS More Private with QNAME Minimisation"</span>, <time datetime="2019-04" class="refDate">April 2019</time>, <span>&lt;<a href="https://labs.ripe.net/Members/wouter_de_vries/make-dns-a-bit-more-private-with-qname-minimisation">https://labs.ripe.net/Members/wouter_de_vries/make-dns-a-bit-more-private-with-qname-minimisation</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="sidn-entrada">[sidn-entrada]</dt>
        <dd>
<span class="refAuthor">Hesselman, C.</span>, <span class="refAuthor">Jansen, J.</span>, <span class="refAuthor">Wullink, M.</span>, <span class="refAuthor">Vink, K.</span>, and <span class="refAuthor">M. Simon</span>, <span class="refTitle">"A privacy framework for 'DNS big data' applications"</span>, <time datetime="2014-11" class="refDate">November 2014</time>, <span>&lt;<a href="https://www.sidnlabs.nl/downloads/yBW6hBoaSZe4m6GJc_0b7w/2211058ab6330c7f3788141ea19d3db7/SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf">https://www.sidnlabs.nl/downloads/yBW6hBoaSZe4m6GJc_0b7w/2211058ab6330c7f3788141ea19d3db7/SIDN_Labs_Privacyraamwerk_Position_Paper_V1.4_ENG.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="thomas-ditl-tcp">[thomas-ditl-tcp]</dt>
        <dd>
<span class="refAuthor">Thomas, M.</span> and <span class="refAuthor">D. Wessels</span>, <span class="refTitle">"An Analysis of TCP Traffic in Root Server DITL Data"</span>, <span class="refContent">DNS-OARC 2014 Fall Workshop</span>, <time datetime="2014-10" class="refDate">October 2014</time>, <span>&lt;<a href="https://indico.dns-oarc.net/event/20/session/2/contribution/15/material/slides/1.pdf">https://indico.dns-oarc.net/event/20/session/2/contribution/15/material/slides/1.pdf</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="tor-leak">[tor-leak]</dt>
        <dd>
<span class="refAuthor">Tor</span>, <span class="refTitle">"Tor FAQs: I keep seeing these warnings about SOCKS and DNS information leaks. Should I worry?"</span>, <span>&lt;<a href="https://www.torproject.org/docs/faq.html.en#WarningsAboutSOCKSandDNSInformationLeaks">https://www.torproject.org/docs/faq.html.en#WarningsAboutSOCKSandDNSInformationLeaks</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="yanbin-tsudik">[yanbin-tsudik]</dt>
      <dd>
<span class="refAuthor">Yanbin, L.</span> and <span class="refAuthor">G. Tsudik</span>, <span class="refTitle">"Towards Plugging Privacy Leaks in Domain Name System"</span>, <time datetime="2010-06" class="refDate">June 2010</time>, <span>&lt;<a href="https://arxiv.org/abs/0910.2472">https://arxiv.org/abs/0910.2472</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<div id="updates-since-rfc7626">
<section id="appendix-A">
      <h2 id="name-updates-since-rfc-7626">
<a href="#appendix-A" class="section-number selfRef">Appendix A. </a><a href="#name-updates-since-rfc-7626" class="section-name selfRef">Updates since RFC 7626</a>
      </h2>
<p id="appendix-A-1">Many references were updated. Discussions of encrypted transports, including
DoT and DoH, and sections on DNS payload, authentication of servers, and blocking of services were added.

With the publishing of
<span>[<a href="#RFC7816" class="xref">RFC7816</a>]</span> on QNAME minimization, text, references, and initial attempts to
measure deployment were added to reflect this.  The text and references on the
Snowden revelations were updated.<a href="#appendix-A-1" class="pilcrow">¶</a></p>
<p id="appendix-A-2">The "Risks Overview" section was changed to "Scope" to help clarify the risks
being considered.  Text on cellular network DNS, blocking, and
security was added.  Considerations for recursive resolvers were collected and placed
together.  A discussion on resolver selection was added.<a href="#appendix-A-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="acknowledgments">
<section id="appendix-B">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="appendix-B-1">Thanks to <span class="contact-name">Nathalie Boulvard</span> and to the CENTR members for the original work
   that led to this document. Thanks to <span class="contact-name">Ondrej Sury</span> for the interesting
   discussions. Thanks to <span class="contact-name">Mohsen Souissi</span> and <span class="contact-name">John Heidemann</span> for proofreading and
   to <span class="contact-name">Paul Hoffman</span>, <span class="contact-name">Matthijs Mekking</span>, <span class="contact-name">Marcos Sanz</span>, <span class="contact-name">Francis Dupont</span>,
   <span class="contact-name">Allison Mankin</span>, and <span class="contact-name">Warren Kumari</span> for proofreading, providing technical
   remarks, and making many readability improvements. Thanks to <span class="contact-name">Dan York</span>,
   <span class="contact-name">Suzanne Woolf</span>, <span class="contact-name">Tony Finch</span>, <span class="contact-name">Stephen Farrell</span>, <span class="contact-name">Peter Koch</span>, <span class="contact-name">Simon Josefsson</span>, and
   <span class="contact-name">Frank Denis</span> for good written contributions. Thanks to <span class="contact-name">Vittorio Bertola</span> and
   <span class="contact-name">Mohamed Boucadair</span> for a detailed review of the -bis. And thanks to the IESG
   members for the last remarks.<a href="#appendix-B-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="contributions">
<section id="appendix-C">
      <h2 id="name-contributions">
<a href="#name-contributions" class="section-name selfRef">Contributions</a>
      </h2>
<p id="appendix-C-1"><span class="contact-name">Sara Dickinson</span> and <span class="contact-name">Stephane Bortzmeyer</span> were the original authors of the
   document, and their contribution to the initial draft of this document is greatly appreciated.<a href="#appendix-C-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="appendix-D">
      <h2 id="name-authors-address">
<a href="#name-authors-address" class="section-name selfRef">Author's Address</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Tim Wicinski (<span class="role">editor</span>)</span></div>
<div dir="auto" class="left">
<span class="locality">Elkins</span>, <span class="region">WV</span> <span class="postal-code">26241</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:tjw.ietf@gmail.com" class="email">tjw.ietf@gmail.com</a>
</div>
</address>
</section>
</div>
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