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1N/A<title>Chapter�7.�BIND 9 Security Considerations</title>
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1N/A</div>
1N/A<div class="chapter" lang="en">
1N/A<div class="titlepage"><div><div><h2 class="title">
1N/A<a name="Bv9ARM.ch07"></a>Chapter�7.�<acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
1N/A<div class="toc">
1N/A<p><b>Table of Contents</b></p>
1N/A<dl>
1N/A<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
1N/A<dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2603690"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
1N/A<dd><dl>
1N/A<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603771">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
1N/A<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603831">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
1N/A</dl></dd>
1N/A<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
1N/A</dl>
1N/A</div>
1N/A<div class="sect1" lang="en">
1N/A<div class="titlepage"><div><div><h2 class="title" style="clear: both">
1N/A<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
1N/A<p>
1N/A Access Control Lists (ACLs) are address match lists that
1N/A you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
1N/A <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
1N/A <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
1N/A <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
1N/A etc.
1N/A </p>
1N/A<p>
1N/A Using ACLs allows you to have finer control over who can access
1N/A your name server, without cluttering up your config files with huge
1N/A lists of IP addresses.
1N/A </p>
1N/A<p>
1N/A It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
1N/A control access to your server. Limiting access to your server by
1N/A outside parties can help prevent spoofing and denial of service (DoS) attacks against
1N/A your server.
1N/A </p>
1N/A<p>
1N/A Here is an example of how to properly apply ACLs:
1N/A </p>
1N/A<pre class="programlisting">
1N/A// Set up an ACL named "bogusnets" that will block
1N/A// RFC1918 space and some reserved space, which is
1N/A// commonly used in spoofing attacks.
1N/Aacl bogusnets {
1N/A 0.0.0.0/8; 1.0.0.0/8; 2.0.0.0/8; 192.0.2.0/24;
1N/A 224.0.0.0/3; 10.0.0.0/8; 172.16.0.0/12;
1N/A 192.168.0.0/16;
1N/A};
1N/A
1N/A// Set up an ACL called our-nets. Replace this with the
1N/A// real IP numbers.
1N/Aacl our-nets { x.x.x.x/24; x.x.x.x/21; };
1N/Aoptions {
1N/A ...
1N/A ...
1N/A allow-query { our-nets; };
1N/A allow-recursion { our-nets; };
1N/A ...
1N/A blackhole { bogusnets; };
1N/A ...
1N/A};
1N/A
1N/Azone "example.com" {
1N/A type master;
1N/A file "m/example.com";
1N/A allow-query { any; };
1N/A};
1N/A</pre>
1N/A<p>
1N/A This allows recursive queries of the server from the outside
1N/A unless recursion has been previously disabled.
1N/A </p>
1N/A<p>
1N/A For more information on how to use ACLs to protect your server,
1N/A see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
1N/A </p>
1N/A<p>
1N/A <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>
1N/A </p>
1N/A</div>
1N/A<div class="sect1" lang="en">
1N/A<div class="titlepage"><div><div><h2 class="title" style="clear: both">
1N/A<a name="id2603690"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
1N/A</h2></div></div></div>
1N/A<p>
1N/A On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
1N/A in a <span class="emphasis"><em>chrooted</em></span> environment (using
1N/A the <span><strong class="command">chroot()</strong></span> function) by specifying
1N/A the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
1N/A This can help improve system security by placing
1N/A <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
1N/A the damage done if a server is compromised.
1N/A </p>
1N/A<p>
1N/A Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
1N/A ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
1N/A We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
1N/A </p>
1N/A<p>
1N/A Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
1N/A <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
1N/A user 202:
1N/A </p>
1N/A<p>
1N/A <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
1N/A </p>
1N/A<div class="sect2" lang="en">
1N/A<div class="titlepage"><div><div><h3 class="title">
1N/A<a name="id2603771"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
1N/A<p>
1N/A In order for a <span><strong class="command">chroot</strong></span> environment
1N/A to
1N/A work properly in a particular directory
1N/A (for example, <code class="filename">/var/named</code>),
1N/A you will need to set up an environment that includes everything
1N/A <acronym class="acronym">BIND</acronym> needs to run.
1N/A From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
1N/A the root of the filesystem. You will need to adjust the values of
1N/A options like
1N/A like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
1N/A for this.
1N/A </p>
1N/A<p>
1N/A Unlike with earlier versions of BIND, you typically will
1N/A <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
1N/A statically nor install shared libraries under the new root.
1N/A However, depending on your operating system, you may need
1N/A to set up things like
1N/A <code class="filename">/dev/zero</code>,
1N/A <code class="filename">/dev/random</code>,
1N/A <code class="filename">/dev/log</code>, and
1N/A <code class="filename">/etc/localtime</code>.
1N/A </p>
1N/A</div>
1N/A<div class="sect2" lang="en">
1N/A<div class="titlepage"><div><div><h3 class="title">
1N/A<a name="id2603831"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
1N/A<p>
1N/A Prior to running the <span><strong class="command">named</strong></span> daemon,
1N/A use
1N/A the <span><strong class="command">touch</strong></span> utility (to change file
1N/A access and
1N/A modification times) or the <span><strong class="command">chown</strong></span>
1N/A utility (to
1N/A set the user id and/or group id) on files
1N/A to which you want <acronym class="acronym">BIND</acronym>
1N/A to write.
1N/A </p>
1N/A<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
1N/A<h3 class="title">Note</h3>
1N/A Note that if the <span><strong class="command">named</strong></span> daemon is running as an
1N/A unprivileged user, it will not be able to bind to new restricted
1N/A ports if the server is reloaded.
1N/A </div>
1N/A</div>
1N/A</div>
1N/A<div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
<p>
Access to the dynamic
update facility should be strictly limited. In earlier versions of
<acronym class="acronym">BIND</acronym>, the only way to do this was
based on the IP
address of the host requesting the update, by listing an IP address
or
network prefix in the <span><strong class="command">allow-update</strong></span>
zone option.
This method is insecure since the source address of the update UDP
packet
is easily forged. Also note that if the IP addresses allowed by the
<span><strong class="command">allow-update</strong></span> option include the
address of a slave
server which performs forwarding of dynamic updates, the master can
be
trivially attacked by sending the update to the slave, which will
forward it to the master with its own source IP address causing the
master to approve it without question.
</p>
<p>
For these reasons, we strongly recommend that updates be
cryptographically authenticated by means of transaction signatures
(TSIG). That is, the <span><strong class="command">allow-update</strong></span>
option should
list only TSIG key names, not IP addresses or network
prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
option can be used.
</p>
<p>
Some sites choose to keep all dynamically-updated DNS data
in a subdomain and delegate that subdomain to a separate zone. This
way, the top-level zone containing critical data such as the IP
addresses
of public web and mail servers need not allow dynamic update at
all.
</p>
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