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<div class="chapter" title="Chapter7.BIND 9 Security Considerations">
<div class="titlepage"><div><div><h2 class="title">
<a name="Bv9ARM.ch07"></a>Chapter7.<acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl>
<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
<dt><span class="sect1"><a href="Bv9ARM.ch07.html#idp2139776"><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span></a></span></dt>
<dd><dl>
<dt><span class="sect2"><a href="Bv9ARM.ch07.html#idp2145288">The <span class="command"><strong>chroot</strong></span> Environment</a></span></dt>
<dt><span class="sect2"><a href="Bv9ARM.ch07.html#idp2149928">Using the <span class="command"><strong>setuid</strong></span> Function</a></span></dt>
</dl></dd>
<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
</dl>
</div>
<div class="sect1" title="Access Control Lists">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
<p>
Access Control Lists (ACLs) are address match lists that
you can set up and nickname for future use in <span class="command"><strong>allow-notify</strong></span>,
<span class="command"><strong>allow-query</strong></span>, <span class="command"><strong>allow-query-on</strong></span>,
<span class="command"><strong>allow-recursion</strong></span>, <span class="command"><strong>allow-recursion-on</strong></span>,
<span class="command"><strong>blackhole</strong></span>, <span class="command"><strong>allow-transfer</strong></span>,
etc.
</p>
<p>
Using ACLs allows you to have finer control over who can access
your name server, without cluttering up your config files with huge
lists of IP addresses.
</p>
<p>
It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
control access to your server. Limiting access to your server by
outside parties can help prevent spoofing and denial of service (DoS) attacks against
your server.
</p>
<p>
Here is an example of how to properly apply ACLs:
</p>
<pre class="programlisting">
// Set up an ACL named "bogusnets" that will block
// RFC1918 space and some reserved space, which is
// commonly used in spoofing attacks.
acl bogusnets {
0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
};
// Set up an ACL called our-nets. Replace this with the
// real IP numbers.
acl our-nets { x.x.x.x/24; x.x.x.x/21; };
options {
...
...
allow-query { our-nets; };
allow-recursion { our-nets; };
...
blackhole { bogusnets; };
...
};
zone "example.com" {
type master;
file "m/example.com";
allow-query { any; };
};
</pre>
<p>
This allows recursive queries of the server from the outside
unless recursion has been previously disabled.
</p>
<p>
For more information on how to use ACLs to protect your server,
see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
</p>
<p>
<a class="ulink" 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>
</p>
</div>
<div class="sect1" title="Chroot and Setuid">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="idp2139776"></a><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span>
</h2></div></div></div>
<p>
On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
in a <span class="emphasis"><em>chrooted</em></span> environment (using
the <span class="command"><strong>chroot()</strong></span> function) by specifying
the "<code class="option">-t</code>" option for <span class="command"><strong>named</strong></span>.
This can help improve system security by placing
<acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
the damage done if a server is compromised.
</p>
<p>
Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
We suggest running as an unprivileged user when using the <span class="command"><strong>chroot</strong></span> feature.
</p>
<p>
Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span class="command"><strong>chroot</strong></span> sandbox,
<span class="command"><strong>/var/named</strong></span>, and to run <span class="command"><strong>named</strong></span> <span class="command"><strong>setuid</strong></span> to
user 202:
</p>
<p>
<strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
</p>
<div class="sect2" title="The chroot Environment">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp2145288"></a>The <span class="command"><strong>chroot</strong></span> Environment</h3></div></div></div>
<p>
In order for a <span class="command"><strong>chroot</strong></span> environment
to
work properly in a particular directory
(for example, <code class="filename">/var/named</code>),
you will need to set up an environment that includes everything
<acronym class="acronym">BIND</acronym> needs to run.
From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
the root of the filesystem. You will need to adjust the values of
options like
like <span class="command"><strong>directory</strong></span> and <span class="command"><strong>pid-file</strong></span> to account
for this.
</p>
<p>
Unlike with earlier versions of BIND, you typically will
<span class="emphasis"><em>not</em></span> need to compile <span class="command"><strong>named</strong></span>
statically nor install shared libraries under the new root.
However, depending on your operating system, you may need
to set up things like
<code class="filename">/dev/zero</code>,
<code class="filename">/dev/random</code>,
<code class="filename">/dev/log</code>, and
<code class="filename">/etc/localtime</code>.
</p>
</div>
<div class="sect2" title="Using the setuid Function">
<div class="titlepage"><div><div><h3 class="title">
<a name="idp2149928"></a>Using the <span class="command"><strong>setuid</strong></span> Function</h3></div></div></div>
<p>
Prior to running the <span class="command"><strong>named</strong></span> daemon,
use
the <span class="command"><strong>touch</strong></span> utility (to change file
access and
modification times) or the <span class="command"><strong>chown</strong></span>
utility (to
set the user id and/or group id) on files
to which you want <acronym class="acronym">BIND</acronym>
to write.
</p>
<div class="note" title="Note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
Note that if the <span class="command"><strong>named</strong></span> daemon is running as an
unprivileged user, it will not be able to bind to new restricted
ports if the server is reloaded.
</div>
</div>
</div>
<div class="sect1" title="Dynamic Update Security">
<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 class="command"><strong>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 class="command"><strong>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 class="command"><strong>allow-update</strong></span>
option should
list only TSIG key names, not IP addresses or network
prefixes. Alternatively, the new <span class="command"><strong>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>
</div>
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