File: rfc9276.html

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<!DOCTYPE html>
<html lang="en" class="BCP RFC">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 9276: Guidance for NSEC3 Parameter Settings</title>
<meta content="Wes Hardaker" name="author">
<meta content="Viktor Dukhovni" name="author">
<meta content="
       NSEC3 is a DNSSEC mechanism providing proof of nonexistence by
asserting that there are no names that exist between two domain names
within a zone.  Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domain name pairs.  This document provides
guidance on setting NSEC3 parameters based on recent operational
deployment experience.  This document updates RFC 5155 with
guidance about selecting NSEC3 iteration and salt parameters. 
    " name="description">
<meta content="xml2rfc 3.12.10" name="generator">
<meta content="DNSSEC" name="keyword">
<meta content="DNS" name="keyword">
<meta content="NSEC3" name="keyword">
<meta content="NSEC" name="keyword">
<meta content="Denial of Existence" name="keyword">
<meta content="9276" name="rfc.number">
<!-- Generator version information:
  xml2rfc 3.12.10
    Python 3.6.15
    appdirs 1.4.4
    ConfigArgParse 1.4.1
    google-i18n-address 2.4.0
    html5lib 1.0.1
    intervaltree 3.0.2
    Jinja2 2.11.3
    kitchen 1.2.6
    lxml 4.7.1
    MarkupSafe 2.0.1
    pycairo 1.15.1
    pycountry 19.8.18
    pyflakes 2.1.1
    PyYAML 5.4.1
    requests 2.24.0
    setuptools 40.5.0
    six 1.14.0
    WeasyPrint 52.5
-->
<link href="rfc9276.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;
  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;
}
/* Fix PDF info block run off issue */
@media print {
  #identifiers dd {
    float: none;
  }
}
#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;
  }
  pre.breakable {
    break-inside: auto;
  }
  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;
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<table class="ears">
<thead><tr>
<td class="left">RFC 9276</td>
<td class="center">NSEC3 Parameter Settings</td>
<td class="right">August 2022</td>
</tr></thead>
<tfoot><tr>
<td class="left">Hardaker &amp; Dukhovni</td>
<td class="center">Best Current Practice</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/rfc9276" class="eref">9276</a></dd>
<dt class="label-bcp">BCP:</dt>
<dd class="bcp">236</dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc5155" class="eref">5155</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Best Current Practice</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2022-08" class="published">August 2022</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">W. Hardaker</div>
<div class="org">USC/ISI</div>
</div>
<div class="author">
      <div class="author-name">V. Dukhovni</div>
<div class="org">Bloomberg, L.P.</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 9276</h1>
<h1 id="title">Guidance for NSEC3 Parameter Settings</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">NSEC3 is a DNSSEC mechanism providing proof of nonexistence by
asserting that there are no names that exist between two domain names
within a zone.  Unlike its counterpart NSEC, NSEC3 avoids directly
disclosing the bounding domain name pairs.  This document provides
guidance on setting NSEC3 parameters based on recent operational
deployment experience.  This document updates RFC 5155 with
guidance about selecting NSEC3 iteration and salt parameters.<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 memo documents an Internet Best Current Practice.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further information
            on BCPs is available in Section 2 of RFC 7841.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <span><a href="https://www.rfc-editor.org/info/rfc9276">https://www.rfc-editor.org/info/rfc9276</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) 2022 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Revised BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Revised BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.1.2.1">
                <p id="section-toc.1-1.1.2.1.1" class="keepWithNext"><a href="#section-1.1" class="xref">1.1</a>.  <a href="#name-requirements-notation" class="xref">Requirements Notation</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1"><a href="#section-2" class="xref">2</a>.  <a href="#name-nsec3-parameter-value-discu" class="xref">NSEC3 Parameter Value Discussions</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.1">
                <p id="section-toc.1-1.2.2.1.1" class="keepWithNext"><a href="#section-2.1" class="xref">2.1</a>.  <a href="#name-algorithms" class="xref">Algorithms</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.2">
                <p id="section-toc.1-1.2.2.2.1"><a href="#section-2.2" class="xref">2.2</a>.  <a href="#name-flags" class="xref">Flags</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.3">
                <p id="section-toc.1-1.2.2.3.1"><a href="#section-2.3" class="xref">2.3</a>.  <a href="#name-iterations" class="xref">Iterations</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.2.2.4">
                <p id="section-toc.1-1.2.2.4.1"><a href="#section-2.4" class="xref">2.4</a>.  <a href="#name-salt" class="xref">Salt</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1"><a href="#section-3" class="xref">3</a>.  <a href="#name-recommendations-for-deployi" class="xref">Recommendations for Deploying and Validating NSEC3 Records</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3.2.1">
                <p id="section-toc.1-1.3.2.1.1"><a href="#section-3.1" class="xref">3.1</a>.  <a href="#name-best-practice-for-zone-publ" class="xref">Best Practice for Zone Publishers</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3.2.2">
                <p id="section-toc.1-1.3.2.2.1"><a href="#section-3.2" class="xref">3.2</a>.  <a href="#name-recommendation-for-validati" class="xref">Recommendation for Validating Resolvers</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.3.2.3">
                <p id="section-toc.1-1.3.2.3.1"><a href="#section-3.3" class="xref">3.3</a>.  <a href="#name-recommendation-for-primary-" class="xref">Recommendation for Primary and Secondary Relationships</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1"><a href="#section-4" class="xref">4</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1"><a href="#section-5" class="xref">5</a>.  <a href="#name-operational-considerations" class="xref">Operational Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1"><a href="#section-6" class="xref">6</a>.  <a href="#name-iana-considerations" class="xref">IANA Considerations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1"><a href="#section-7" class="xref">7</a>.  <a href="#name-references" class="xref">References</a></p>
<ul class="compact toc ulBare ulEmpty">
<li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.1">
                <p id="section-toc.1-1.7.2.1.1"><a href="#section-7.1" class="xref">7.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a></p>
</li>
              <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.7.2.2">
                <p id="section-toc.1-1.7.2.2.1"><a href="#section-7.2" class="xref">7.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a></p>
</li>
            </ul>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1"><a href="#appendix-A" class="xref">Appendix A</a>.  <a href="#name-deployment-measurements-at-" class="xref">Deployment Measurements at Time of Publication</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1"><a href="#appendix-B" class="xref">Appendix B</a>.  <a href="#name-computational-burdens-of-pr" class="xref">Computational Burdens of Processing NSEC3 Iterations</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1"><a href="#appendix-C" class="xref"></a><a href="#name-acknowledgments" class="xref">Acknowledgments</a></p>
</li>
          <li class="compact toc ulBare ulEmpty" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1"><a href="#appendix-D" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</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">As with NSEC <span>[<a href="#RFC4035" class="xref">RFC4035</a>]</span>, NSEC3
<span>[<a href="#RFC5155" class="xref">RFC5155</a>]</span> provides proof of
nonexistence that consists of signed DNS records establishing the
nonexistence of a given name or associated Resource Record Type
(RRTYPE) in a DNSSEC-signed zone <span>[<a href="#RFC4035" class="xref">RFC4035</a>]</span>.  However, in the case of NSEC3, the names of valid nodes in the zone are obfuscated through
(possibly multiple iterations of) hashing (currently only
SHA-1 is in use on the Internet).<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">
  NSEC3 also provides "opt-out support", allowing for blocks of
  unsigned delegations to be covered by a single NSEC3 record.  Use of
  the opt-out feature allows large registries to only sign as many
  NSEC3 records as there are signed DS or other Resource Record sets
  (RRsets) in the zone; with opt-out, unsigned delegations don't
  require additional NSEC3 records.  This sacrifices the tamper-
  resistance of the proof of nonexistence offered by NSEC3 in order to
  reduce memory and CPU overheads.<a href="#section-1-2" class="pilcrow">¶</a></p>
<p id="section-1-3">NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex.  These parameters are the
hash algorithm, the processing flags, the number of hash iterations, and
the salt.  Each of these has security and operational considerations
that impact both zone owners and validating resolvers.  This document
provides some best-practice recommendations for setting the NSEC3
parameters.<a href="#section-1-3" class="pilcrow">¶</a></p>
<div id="requirements-notation">
<section id="section-1.1">
        <h3 id="name-requirements-notation">
<a href="#section-1.1" class="section-number selfRef">1.1. </a><a href="#name-requirements-notation" class="section-name selfRef">Requirements Notation</a>
        </h3>
<p id="section-1.1-1">
   The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>", "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>", "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>", "<span class="bcp14">SHOULD NOT</span>", "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>",
   "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document are to be interpreted as
   described in BCP 14 <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span> 
   when, and only when, they appear in all capitals, as shown here.<a href="#section-1.1-1" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="nsec3-parameter-value-discussions">
<section id="section-2">
      <h2 id="name-nsec3-parameter-value-discu">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-nsec3-parameter-value-discu" class="section-name selfRef">NSEC3 Parameter Value Discussions</a>
      </h2>
<p id="section-2-1">The following sections describe the background of the parameters for
the NSEC3 and NSEC3PARAM RRTYPEs.<a href="#section-2-1" class="pilcrow">¶</a></p>
<div id="algorithms">
<section id="section-2.1">
        <h3 id="name-algorithms">
<a href="#section-2.1" class="section-number selfRef">2.1. </a><a href="#name-algorithms" class="section-name selfRef">Algorithms</a>
        </h3>
<p id="section-2.1-1">The algorithm field is not discussed by this document.  Readers are
encouraged to read <span>[<a href="#RFC8624" class="xref">RFC8624</a>]</span> for guidance about DNSSEC algorithm
usage.<a href="#section-2.1-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="flags">
<section id="section-2.2">
        <h3 id="name-flags">
<a href="#section-2.2" class="section-number selfRef">2.2. </a><a href="#name-flags" class="section-name selfRef">Flags</a>
        </h3>
<p id="section-2.2-1">The NSEC3PARAM flags field currently contains only reserved and
unassigned flags.  However, individual NSEC3 records contain the
"Opt-Out" flag <span>[<a href="#RFC5155" class="xref">RFC5155</a>]</span> that specifies whether that NSEC3 record
provides proof of nonexistence.  In general, NSEC3 with the Opt-Out
flag enabled should only be used in large, highly dynamic zones with a
small percentage of signed delegations.  Operationally, this allows
for fewer signature creations when new delegations are inserted into a
zone.  This is typically only necessary for extremely large
registration points providing zone updates faster than real-time
signing allows or when using memory-constrained hardware.

  Operators considering the use of NSEC3 are advised to carefully
  weigh the costs and benefits of choosing NSEC3 over NSEC.  Smaller
  zones, or large but relatively static zones, are encouraged to not
  use the opt-opt flag and to take advantage of DNSSEC's
  authenticated denial of existence.<a href="#section-2.2-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="iterations">
<section id="section-2.3">
        <h3 id="name-iterations">
<a href="#section-2.3" class="section-number selfRef">2.3. </a><a href="#name-iterations" class="section-name selfRef">Iterations</a>
        </h3>
<p id="section-2.3-1">NSEC3 records are created by first hashing the input domain and then
repeating that hashing using the same algorithm a number of times based on the
iteration parameter in the NSEC3PARAM and NSEC3 records.


The first hash with NSEC3 is typically sufficient to discourage zone
enumeration performed by "zone walking" an unhashed NSEC chain.<a href="#section-2.3-1" class="pilcrow">¶</a></p>
<p id="section-2.3-2">Note that <span>[<a href="#RFC5155" class="xref">RFC5155</a>]</span> describes the Iterations field as follows<a href="#section-2.3-2" class="pilcrow">¶</a></p>
<blockquote id="section-2.3-3">The
Iterations field defines the number of additional times the hash
function has been performed.<a href="#section-2.3-3" class="pilcrow">¶</a>
</blockquote>
<p id="section-2.3-4">This means that an NSEC3 record with an
Iterations field of 0 actually requires one hash iteration.<a href="#section-2.3-4" class="pilcrow">¶</a></p>
<p id="section-2.3-5">Only determined parties
with significant resources are likely to try and uncover hashed
values, regardless of the number of additional iterations performed.
If an adversary really wants to expend significant CPU resources to
mount an offline dictionary attack on a zone's NSEC3 chain, they'll
likely be able to find most of the "guessable" names despite any
level of additional hashing iterations.<a href="#section-2.3-5" class="pilcrow">¶</a></p>
<p id="section-2.3-6">Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans.  They
are often recorded in many other network logs such as email logs,
certificate transparency logs, web page links, intrusion-detection
systems, malware scanners, email archives, etc.  Many times a simple
dictionary of commonly used domain names prefixes (www, mail,
imap, login, database, etc.) can be used to quickly reveal a large
number of labels within a zone.  Because of this, there are increasing
performance costs yet diminishing returns associated with applying
additional hash iterations beyond the first.<a href="#section-2.3-6" class="pilcrow">¶</a></p>
<p id="section-2.3-7">Although <span><a href="https://www.rfc-editor.org/rfc/rfc5155#section-10.3" class="relref">Section 10.3</a> of [<a href="#RFC5155" class="xref">RFC5155</a>]</span> specifies the upper bounds for the
number of hash iterations to use, there is no published guidance for
zone owners about good values to select.  Recent academic studies
have shown that NSEC3 hashing provides only moderate
protection <span>[<a href="#GPUNSEC3" class="xref">GPUNSEC3</a>]</span> <span>[<a href="#ZONEENUM" class="xref">ZONEENUM</a>]</span>.<a href="#section-2.3-7" class="pilcrow">¶</a></p>
</section>
</div>
<div id="salt">
<section id="section-2.4">
        <h3 id="name-salt">
<a href="#section-2.4" class="section-number selfRef">2.4. </a><a href="#name-salt" class="section-name selfRef">Salt</a>
        </h3>
<p id="section-2.4-1">NSEC3 records provide an additional salt value, which can be
combined with a Fully Qualified Domain Name (FQDN) to influence the resulting hash, but properties
of this extra salt are complicated.<a href="#section-2.4-1" class="pilcrow">¶</a></p>
<p id="section-2.4-2">In cryptography, salts generally add a layer of protection against
offline, stored dictionary attacks by combining the value to be hashed
with a unique "salt" value. This prevents adversaries from building up
and remembering a single dictionary of values that can translate a
hash output back to the value that it was derived from.<a href="#section-2.4-2" class="pilcrow">¶</a></p>
<p id="section-2.4-3">In the case of DNS, the situation is different because the hashed
names placed in NSEC3 records are always implicitly "salted" by
hashing the FQDN from each zone. Thus, no
single pre-computed table works to speed up dictionary attacks
against multiple target zones. An attacker is always required to
compute a complete dictionary per zone, which is expensive in both
storage and CPU time.<a href="#section-2.4-3" class="pilcrow">¶</a></p>
<p id="section-2.4-4">To understand the role of the additional NSEC3 salt field, we have to
consider how a typical zone walking attack works. Typically, the attack
has two phases: online and offline. In the online phase, an attacker
"walks the zone" by enumerating (almost) all hashes listed in NSEC3
records and storing them for the offline phase. Then, in the offline
cracking phase, the attacker attempts to crack the underlying hash. In
this phase, the additional salt value raises the cost of the attack
only if the salt value changes during the online phase of the
attack. In other words, an additional, constant salt value does not
change the cost of the attack.<a href="#section-2.4-4" class="pilcrow">¶</a></p>
<p id="section-2.4-5">Changing a zone's salt value requires the construction of a complete
new NSEC3 chain.  This is true both when re-signing the entire zone at
once and when incrementally signing it in the background where the new
salt is only activated once every name in the chain has been
completed. As a result, re-salting is a very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical and renders the additional salt
field functionally useless.<a href="#section-2.4-5" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="recommendations-for-deploying-and-validating-nsec3-records">
<section id="section-3">
      <h2 id="name-recommendations-for-deployi">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-recommendations-for-deployi" class="section-name selfRef">Recommendations for Deploying and Validating NSEC3 Records</a>
      </h2>
<p id="section-3-1">The following subsections describe recommendations for the different
operating realms within the DNS.<a href="#section-3-1" class="pilcrow">¶</a></p>
<div id="best-practice-for-zone-publishers">
<section id="section-3.1">
        <h3 id="name-best-practice-for-zone-publ">
<a href="#section-3.1" class="section-number selfRef">3.1. </a><a href="#name-best-practice-for-zone-publ" class="section-name selfRef">Best Practice for Zone Publishers</a>
        </h3>
<p id="section-3.1-1">First, if the operational or security features of NSEC3 are not
needed, then NSEC <span class="bcp14">SHOULD</span> be used in preference to NSEC3. NSEC3
requires greater computational power (see <a href="#computationalburdens" class="xref">Appendix B</a>)
for both authoritative servers and validating clients.  Specifically,
there is a nontrivial complexity in finding matching NSEC3 records to
randomly generated prefixes within a DNS zone.  NSEC mitigates this
concern.  If NSEC3 must be used, then an iterations count of 0 <span class="bcp14">MUST</span> be
used to alleviate computational burdens.  Note that extra iteration
counts other than 0 increase the impact of CPU-exhausting DoS attacks,
and also increase the risk of interoperability problems.<a href="#section-3.1-1" class="pilcrow">¶</a></p>
<p id="section-3.1-2">Note that deploying NSEC with minimally covering NSEC records
<span>[<a href="#RFC4470" class="xref">RFC4470</a>]</span> also incurs a cost, and zone owners should measure the
computational difference in deploying either <span>[<a href="#RFC4470" class="xref">RFC4470</a>]</span> or NSEC3.<a href="#section-3.1-2" class="pilcrow">¶</a></p>
<p id="section-3.1-3">In short, for all zones, the recommended NSEC3 parameters are as shown
below:<a href="#section-3.1-3" class="pilcrow">¶</a></p>
<div class="alignLeft art-text artwork" id="section-3.1-4">
<pre>
; SHA-1, no extra iterations, empty salt:
;
bcp.example. IN NSEC3PARAM 1 0 0 -
</pre><a href="#section-3.1-4" class="pilcrow">¶</a>
</div>
<p id="section-3.1-5">For small zones, the use of opt-out-based NSEC3 records is <span class="bcp14">NOT RECOMMENDED</span>.<a href="#section-3.1-5" class="pilcrow">¶</a></p>
<p id="section-3.1-6">For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out <span class="bcp14">MAY</span> be used.<a href="#section-3.1-6" class="pilcrow">¶</a></p>
<p id="section-3.1-7">Operators <span class="bcp14">SHOULD NOT</span> use a salt by indicating a zero-length salt value
instead (represented as a "-" in the presentation format).<a href="#section-3.1-7" class="pilcrow">¶</a></p>
<p id="section-3.1-8">If salts are used, note that since the NSEC3PARAM RR is not used by
validating resolvers (see <span><a href="https://www.rfc-editor.org/rfc/rfc5155#section-4" class="relref">Section 4</a> of [<a href="#RFC5155" class="xref">RFC5155</a>]</span>), the iterations and
salt parameters can be changed without the need to wait for RRsets to
expire from caches.  A complete new NSEC3 chain needs to be
constructed and the full zone needs to be re-signed.<a href="#section-3.1-8" class="pilcrow">¶</a></p>
</section>
</div>
<div id="recommendation-for-validating-resolvers">
<section id="section-3.2">
        <h3 id="name-recommendation-for-validati">
<a href="#section-3.2" class="section-number selfRef">3.2. </a><a href="#name-recommendation-for-validati" class="section-name selfRef">Recommendation for Validating Resolvers</a>
        </h3>
<p id="section-3.2-1">Because there has been a large growth of open (public) DNSSEC
validating resolvers that are subject to compute resource constraints
when handling requests from anonymous clients, this document
recommends that validating resolvers reduce their iteration count
limits over time.  

Specifically, validating
resolver operators and validating resolver software implementers are
encouraged to continue evaluating NSEC3 iteration count deployment
trends and lower their acceptable iteration limits over time.
Because
treating a high iterations count as insecure leaves zones subject to
attack, validating resolver operators and validating resolver software
implementers are further encouraged to lower their default 
limit for returning SERVFAIL when processing NSEC3 parameters
containing large iteration count values.  
See
<a href="#deploymentmeasurements" class="xref">Appendix A</a> for measurements taken near the time of
publication of this document and potential starting points.<a href="#section-3.2-1" class="pilcrow">¶</a></p>
<p id="section-3.2-2">Validating resolvers <span class="bcp14">MAY</span> return an insecure response to their clients
when processing NSEC3 records with iterations larger
than 0.
Note also that a validating resolver returning an insecure response
<span class="bcp14">MUST</span> still validate the signature over the NSEC3 record to ensure
the iteration count was not altered since record publication (see
<span><a href="https://www.rfc-editor.org/rfc/rfc5155#section-10.3" class="relref">Section 10.3</a> of [<a href="#RFC5155" class="xref">RFC5155</a>]</span>).<a href="#section-3.2-2" class="pilcrow">¶</a></p>
<p id="section-3.2-3">Validating resolvers <span class="bcp14">MAY</span> also return a SERVFAIL response when
processing NSEC3 records with iterations larger than 0.  Validating
resolvers <span class="bcp14">MAY</span> choose to ignore authoritative server responses with
iteration counts greater than 0, which will likely result in
returning a SERVFAIL to the client when no acceptable responses are
received from authoritative servers.<a href="#section-3.2-3" class="pilcrow">¶</a></p>
<p id="section-3.2-4">Validating resolvers returning an insecure or SERVFAIL answer to their
client after receiving and validating an unsupported NSEC3 parameter
from the authoritative server(s) <span class="bcp14">SHOULD</span> return an Extended DNS
Error (EDE) <span>[<a href="#RFC8914" class="xref">RFC8914</a>]</span> EDNS0 option of value 27.
Validating resolvers that choose to ignore a response with an
unsupported iteration count (and that do not validate the signature) <span class="bcp14">MUST NOT</span> return this EDE option.<a href="#section-3.2-4" class="pilcrow">¶</a></p>
<p id="section-3.2-5">Note that this specification updates <span>[<a href="#RFC5155" class="xref">RFC5155</a>]</span> by significantly
decreasing the requirements originally specified in <span><a href="https://www.rfc-editor.org/rfc/rfc5155#section-10.3" class="relref">Section 10.3</a> of [<a href="#RFC5155" class="xref">RFC5155</a>]</span>. See the Security
Considerations (<a href="#security-considerations" class="xref">Section 4</a>) for arguments on how to
handle responses with non-zero iteration count.<a href="#section-3.2-5" class="pilcrow">¶</a></p>
</section>
</div>
<div id="recommendation-for-primary-secondary-relationships">
<section id="section-3.3">
        <h3 id="name-recommendation-for-primary-">
<a href="#section-3.3" class="section-number selfRef">3.3. </a><a href="#name-recommendation-for-primary-" class="section-name selfRef">Recommendation for Primary and Secondary Relationships</a>
        </h3>
<p id="section-3.3-1">Primary and secondary authoritative servers for a zone that are not
being run by the same operational staff and/or using the same software
and configuration must take into account the potential differences in
NSEC3 iteration support.<a href="#section-3.3-1" class="pilcrow">¶</a></p>
<p id="section-3.3-2">Operators of secondary services should advertise the parameter limits
that their servers support. Correspondingly, operators of primary
servers need to ensure that their secondaries support the NSEC3
parameters they expect to use in their zones.  To ensure reliability,
after primaries change their iteration counts, they should query their
secondaries with known nonexistent labels to verify the secondary
servers are responding as expected.<a href="#section-3.3-2" class="pilcrow">¶</a></p>
</section>
</div>
</section>
</div>
<div id="security-considerations">
<section id="section-4">
      <h2 id="name-security-considerations">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-4-1">This entire document discusses security considerations with various
parameter selections of NSEC3 and NSEC3PARAM fields.<a href="#section-4-1" class="pilcrow">¶</a></p>
<p id="section-4-2">The point where a validating resolver returns insecure versus the point
where it returns SERVFAIL must be considered carefully.  Specifically,
when a validating resolver treats a zone as insecure above a
particular value (say 100) and returns SERVFAIL above a higher point
(say 500), it leaves the zone subject to attacker-in-the-middle
attacks as if it were unsigned between these values.



  Thus, validating resolver operators and software implementers
  <span class="bcp14">SHOULD</span> set the point above which a zone is treated as
  insecure for certain values of NSEC3 iterations to the same as the
  point where a validating resolver begins returning SERVFAIL.<a href="#section-4-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="operational-considerations">
<section id="section-5">
      <h2 id="name-operational-considerations">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-operational-considerations" class="section-name selfRef">Operational Considerations</a>
      </h2>
<p id="section-5-1">This entire document discusses operational considerations with various
parameter selections of NSEC3 and NSEC3PARAM fields.<a href="#section-5-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="iana-considerations">
<section id="section-6">
      <h2 id="name-iana-considerations">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-6-1">IANA has allocated the following code in the First Come First Served
     range <span>[<a href="#RFC8126" class="xref">RFC8126</a>]</span> of the "Extended DNS Error Codes" registry within the "Domain Name System
(DNS) Parameters" registry:<a href="#section-6-1" class="pilcrow">¶</a></p>
<span class="break"></span><dl class="dlCompact dlParallel" id="section-6-2">
        <dt id="section-6-2.1">INFO-CODE:</dt>
        <dd style="margin-left: 1.5em" id="section-6-2.2">27<a href="#section-6-2.2" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-2.3">Purpose:</dt>
        <dd style="margin-left: 1.5em" id="section-6-2.4">Unsupported NSEC3 iterations value<a href="#section-6-2.4" class="pilcrow">¶</a>
</dd>
        <dd class="break"></dd>
<dt id="section-6-2.5">Reference:</dt>
        <dd style="margin-left: 1.5em" id="section-6-2.6">RFC 9276<a href="#section-6-2.6" class="pilcrow">¶</a>
</dd>
      <dd class="break"></dd>
</dl>
</section>
</div>
<section id="section-7">
      <h2 id="name-references">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-7.1">
        <h3 id="name-normative-references">
<a href="#section-7.1" class="section-number selfRef">7.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
        <dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03" class="refDate">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC4035">[RFC4035]</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">"Protocol Modifications for the DNS Security Extensions"</span>, <span class="seriesInfo">RFC 4035</span>, <span class="seriesInfo">DOI 10.17487/RFC4035</span>, <time datetime="2005-03" class="refDate">March 2005</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc4035">https://www.rfc-editor.org/info/rfc4035</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="RFC8174">[RFC8174]</dt>
        <dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05" class="refDate">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8914">[RFC8914]</dt>
      <dd>
<span class="refAuthor">Kumari, W.</span>, <span class="refAuthor">Hunt, E.</span>, <span class="refAuthor">Arends, R.</span>, <span class="refAuthor">Hardaker, W.</span>, and <span class="refAuthor">D. Lawrence</span>, <span class="refTitle">"Extended DNS Errors"</span>, <span class="seriesInfo">RFC 8914</span>, <span class="seriesInfo">DOI 10.17487/RFC8914</span>, <time datetime="2020-10" class="refDate">October 2020</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8914">https://www.rfc-editor.org/info/rfc8914</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
<section id="section-7.2">
        <h3 id="name-informative-references">
<a href="#section-7.2" class="section-number selfRef">7.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="GPUNSEC3">[GPUNSEC3]</dt>
        <dd>
<span class="refAuthor">Wander, M.</span>, <span class="refAuthor">Schwittmann, L.</span>, <span class="refAuthor">Boelmann, C.</span>, and <span class="refAuthor">T. Weis</span>, <span class="refTitle">"GPU-Based NSEC3 Hash Breaking"</span>, <span class="seriesInfo">DOI 10.1109/NCA.2014.27</span>, <time datetime="2014-08" class="refDate">August 2014</time>, <span>&lt;<a href="https://doi.org/10.1109/NCA.2014.27">https://doi.org/10.1109/NCA.2014.27</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8126">[RFC8126]</dt>
        <dd>
<span class="refAuthor">Cotton, M.</span>, <span class="refAuthor">Leiba, B.</span>, and <span class="refAuthor">T. Narten</span>, <span class="refTitle">"Guidelines for Writing an IANA Considerations Section in RFCs"</span>, <span class="seriesInfo">BCP 26</span>, <span class="seriesInfo">RFC 8126</span>, <span class="seriesInfo">DOI 10.17487/RFC8126</span>, <time datetime="2017-06" class="refDate">June 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8126">https://www.rfc-editor.org/info/rfc8126</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="RFC8624">[RFC8624]</dt>
        <dd>
<span class="refAuthor">Wouters, P.</span> and <span class="refAuthor">O. Sury</span>, <span class="refTitle">"Algorithm Implementation Requirements and Usage Guidance for DNSSEC"</span>, <span class="seriesInfo">RFC 8624</span>, <span class="seriesInfo">DOI 10.17487/RFC8624</span>, <time datetime="2019-06" class="refDate">June 2019</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8624">https://www.rfc-editor.org/info/rfc8624</a>&gt;</span>. </dd>
<dd class="break"></dd>
<dt id="ZONEENUM">[ZONEENUM]</dt>
      <dd>
<span class="refAuthor">Wang, Z.</span>, <span class="refAuthor">Xiao, L.</span>, and <span class="refAuthor">R. Wang</span>, <span class="refTitle">"An efficient DNSSEC zone enumeration algorithm"</span>, <span class="seriesInfo">DOI 10.2495/MIIT130591</span>, <time datetime="2014-04" class="refDate">April 2014</time>, <span>&lt;<a href="https://doi.org/10.2495/MIIT130591">https://doi.org/10.2495/MIIT130591</a>&gt;</span>. </dd>
<dd class="break"></dd>
</dl>
</section>
</section>
<div id="deploymentmeasurements">
<section id="appendix-A">
      <h2 id="name-deployment-measurements-at-">
<a href="#appendix-A" class="section-number selfRef">Appendix A. </a><a href="#name-deployment-measurements-at-" class="section-name selfRef">Deployment Measurements at Time of Publication</a>
      </h2>
<p id="appendix-A-1">At the time of publication, setting an upper limit of 100 iterations
for treating a zone as insecure is interoperable without significant
problems, but at the same time still enables CPU-exhausting DoS
attacks.<a href="#appendix-A-1" class="pilcrow">¶</a></p>
<p id="appendix-A-2">At the time of publication, returning SERVFAIL beyond 500 iterations
appears to be interoperable without significant problems.<a href="#appendix-A-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="computationalburdens">
<section id="appendix-B">
      <h2 id="name-computational-burdens-of-pr">
<a href="#appendix-B" class="section-number selfRef">Appendix B. </a><a href="#name-computational-burdens-of-pr" class="section-name selfRef">Computational Burdens of Processing NSEC3 Iterations</a>
      </h2>
<p id="appendix-B-1">The queries per second (QPS) of authoritative servers will decrease due
to computational overhead when processing DNS requests for zones
containing higher NSEC3 iteration counts.  The table below
shows the drop in QPS for various iteration counts.<a href="#appendix-B-1" class="pilcrow">¶</a></p>
<span id="name-drop-in-qps-for-various-ite"></span><div id="iteration-counts-table">
<table class="center" id="table-1">
        <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-drop-in-qps-for-various-ite" class="selfRef">Drop in QPS for Various Iteration Counts</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">Iterations</th>
            <th class="text-left" rowspan="1" colspan="1">QPS [% of 0 Iterations QPS]</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">0</td>
            <td class="text-left" rowspan="1" colspan="1">100%</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">10</td>
            <td class="text-left" rowspan="1" colspan="1">89%</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">20</td>
            <td class="text-left" rowspan="1" colspan="1">82%</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">50</td>
            <td class="text-left" rowspan="1" colspan="1">64%</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">100</td>
            <td class="text-left" rowspan="1" colspan="1">47%</td>
          </tr>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">150</td>
            <td class="text-left" rowspan="1" colspan="1">38%</td>
          </tr>
        </tbody>
      </table>
</div>
</section>
</div>
<div id="qps">
<section id="appendix-C">
      <h2 id="name-acknowledgments">
<a href="#name-acknowledgments" class="section-name selfRef">Acknowledgments</a>
      </h2>
<p id="appendix-C-1">The authors would like to thank the participants in the dns-operations discussion, which took place on mattermost hosted by DNS-OARC.<a href="#appendix-C-1" class="pilcrow">¶</a></p>
<p id="appendix-C-2">Additionally, the following people contributed text or review comments
to this document:<a href="#appendix-C-2" class="pilcrow">¶</a></p>
<ul class="normal">
<li class="normal" id="appendix-C-3.1">
          <p id="appendix-C-3.1.1"><span class="contact-name">Vladimir Cunat</span><a href="#appendix-C-3.1.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.2">
          <p id="appendix-C-3.2.1"><span class="contact-name">Tony Finch</span><a href="#appendix-C-3.2.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.3">
          <p id="appendix-C-3.3.1"><span class="contact-name">Paul Hoffman</span><a href="#appendix-C-3.3.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.4">
          <p id="appendix-C-3.4.1"><span class="contact-name">Warren Kumari</span><a href="#appendix-C-3.4.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.5">
          <p id="appendix-C-3.5.1"><span class="contact-name">Alexander Mayrhofer</span><a href="#appendix-C-3.5.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.6">
          <p id="appendix-C-3.6.1"><span class="contact-name">Matthijs Mekking</span><a href="#appendix-C-3.6.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.7">
          <p id="appendix-C-3.7.1"><span class="contact-name">Florian Obser</span><a href="#appendix-C-3.7.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.8">
          <p id="appendix-C-3.8.1"><span class="contact-name">Petr Spacek</span><a href="#appendix-C-3.8.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.9">
          <p id="appendix-C-3.9.1"><span class="contact-name">Paul Vixie</span><a href="#appendix-C-3.9.1" class="pilcrow">¶</a></p>
</li>
        <li class="normal" id="appendix-C-3.10">
          <p id="appendix-C-3.10.1"><span class="contact-name">Tim Wicinski</span><a href="#appendix-C-3.10.1" class="pilcrow">¶</a></p>
</li>
      </ul>
</section>
</div>
<div id="authors-addresses">
<section id="appendix-D">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Wes Hardaker</span></div>
<div dir="auto" class="left"><span class="org">USC/ISI</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:ietf@hardakers.net" class="email">ietf@hardakers.net</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Viktor Dukhovni</span></div>
<div dir="auto" class="left"><span class="org">Bloomberg, L.P.</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:ietf-dane@dukhovni.org" class="email">ietf-dane@dukhovni.org</a>
</div>
</address>
</section>
</div>
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