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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Chapter 7. BIND 9 Security Considerations</title>
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<meta name="generator" content="DocBook XSL Stylesheets V1.71.1">
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<link rel="start" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
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<link rel="up" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
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<link rel="prev" href="Bv9ARM.ch06.html" title="Chapter 6. BIND 9 Configuration Reference">
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<link rel="next" href="Bv9ARM.ch08.html" title="Chapter 8. Troubleshooting">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<div class="navheader">
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<table width="100%" summary="Navigation header">
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<tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
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<tr>
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<td width="20%" align="left">
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<a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td>
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<th width="60%" align="center"> </th>
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<td width="20%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a>
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</td>
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</tr>
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</table>
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<hr>
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</div>
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<div class="chapter" lang="en">
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<div class="titlepage"><div><div><h2 class="title">
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<a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl>
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<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
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<dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2603082"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
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<dd><dl>
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<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603232">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
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<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603291">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
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</dl></dd>
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<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
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</dl>
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</div>
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<div class="sect1" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
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<p>
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Access Control Lists (ACLs) are address match lists that
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you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
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<span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
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<span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
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<span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
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etc.
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</p>
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<p>
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Using ACLs allows you to have finer control over who can access
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your name server, without cluttering up your config files with huge
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lists of IP addresses.
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</p>
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<p>
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It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
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control access to your server. Limiting access to your server by
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outside parties can help prevent spoofing and denial of service (DoS) attacks against
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your server.
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</p>
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<p>
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Here is an example of how to properly apply ACLs:
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</p>
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<pre class="programlisting">
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// Set up an ACL named "bogusnets" that will block
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// RFC1918 space and some reserved space, which is
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// commonly used in spoofing attacks.
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acl bogusnets {
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0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
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10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
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};
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// Set up an ACL called our-nets. Replace this with the
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// real IP numbers.
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acl our-nets { x.x.x.x/24; x.x.x.x/21; };
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options {
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...
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...
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allow-query { our-nets; };
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allow-recursion { our-nets; };
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...
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blackhole { bogusnets; };
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...
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};
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zone "example.com" {
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type master;
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file "m/example.com";
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allow-query { any; };
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};
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</pre>
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<p>
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This allows recursive queries of the server from the outside
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unless recursion has been previously disabled.
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</p>
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<p>
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For more information on how to use ACLs to protect your server,
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see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
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</p>
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<p>
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<a href="ftp://ftp.auscert.org.au/pub/auscert/advisory/AL-1999.004.dns_dos" target="_top">ftp://ftp.auscert.org.au/pub/auscert/advisory/AL-1999.004.dns_dos</a>
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</p>
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</div>
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<div class="sect1" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="id2603082"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
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</h2></div></div></div>
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<p>
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On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
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in a <span class="emphasis"><em>chrooted</em></span> environment (using
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the <span><strong class="command">chroot()</strong></span> function) by specifying
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the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
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This can help improve system security by placing
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<acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
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the damage done if a server is compromised.
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</p>
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<p>
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Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
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ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
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We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
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</p>
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<p>
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Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
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<span><strong class="command">/var/named</strong></span>, and to run <span><strong class="command">named</strong></span> <span><strong class="command">setuid</strong></span> to
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user 202:
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</p>
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<p>
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<strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
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</p>
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<div class="sect2" lang="en">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id2603232"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
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<p>
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In order for a <span><strong class="command">chroot</strong></span> environment
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to
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work properly in a particular directory
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(for example, <code class="filename">/var/named</code>),
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you will need to set up an environment that includes everything
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<acronym class="acronym">BIND</acronym> needs to run.
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From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
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the root of the filesystem. You will need to adjust the values of
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options like
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like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
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for this.
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</p>
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<p>
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Unlike with earlier versions of BIND, you typically will
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<span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
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statically nor install shared libraries under the new root.
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However, depending on your operating system, you may need
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to set up things like
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<code class="filename">/dev/zero</code>,
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<code class="filename">/dev/random</code>,
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<code class="filename">/dev/log</code>, and
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<code class="filename">/etc/localtime</code>.
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</p>
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</div>
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<div class="sect2" lang="en">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="id2603291"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
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<p>
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Prior to running the <span><strong class="command">named</strong></span> daemon,
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use
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the <span><strong class="command">touch</strong></span> utility (to change file
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access and
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modification times) or the <span><strong class="command">chown</strong></span>
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utility (to
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set the user id and/or group id) on files
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to which you want <acronym class="acronym">BIND</acronym>
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to write.
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</p>
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<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
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<h3 class="title">Note</h3>
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Note that if the <span><strong class="command">named</strong></span> daemon is running as an
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unprivileged user, it will not be able to bind to new restricted
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ports if the server is reloaded.
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</div>
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</div>
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</div>
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<div class="sect1" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
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<p>
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Access to the dynamic
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update facility should be strictly limited. In earlier versions of
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<acronym class="acronym">BIND</acronym>, the only way to do this was
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based on the IP
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address of the host requesting the update, by listing an IP address
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or
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network prefix in the <span><strong class="command">allow-update</strong></span>
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zone option.
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This method is insecure since the source address of the update UDP
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packet
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is easily forged. Also note that if the IP addresses allowed by the
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<span><strong class="command">allow-update</strong></span> option include the
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address of a slave
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server which performs forwarding of dynamic updates, the master can
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be
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trivially attacked by sending the update to the slave, which will
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forward it to the master with its own source IP address causing the
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master to approve it without question.
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</p>
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<p>
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For these reasons, we strongly recommend that updates be
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cryptographically authenticated by means of transaction signatures
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(TSIG). That is, the <span><strong class="command">allow-update</strong></span>
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option should
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list only TSIG key names, not IP addresses or network
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prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
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option can be used.
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</p>
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<p>
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Some sites choose to keep all dynamically-updated DNS data
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in a subdomain and delegate that subdomain to a separate zone. This
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way, the top-level zone containing critical data such as the IP
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addresses
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of public web and mail servers need not allow dynamic update at
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all.
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</p>
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</div>
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</div>
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<div class="navfooter">
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<hr>
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left">
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<a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td>
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<td width="20%" align="center"> </td>
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<td width="40%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a>
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</td>
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||
</tr>
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<tr>
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<td width="40%" align="left" valign="top">Chapter 6. <acronym class="acronym">BIND</acronym> 9 Configuration Reference </td>
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<td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td>
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<td width="40%" align="right" valign="top"> Chapter 8. Troubleshooting</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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