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4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<div class="chapter" lang="en">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<div class="titlepage"><div><div><h2 class="title">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<a name="Bv9ARM.ch07"></a>Chapter�7.�<acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<div class="toc">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<p><b>Table of Contents</b></p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dl>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2604198"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dd><dl>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2604347">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2604407">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync</dl></dd>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
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4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync</div>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<div class="sect1" lang="en">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync Access Control Lists (ACLs) are address match lists that
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync etc.
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync </p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync Using ACLs allows you to have finer control over who can access
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync your name server, without cluttering up your config files with huge
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync lists of IP addresses.
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync </p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync control access to your server. Limiting access to your server by
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync outside parties can help prevent spoofing and denial of service (DoS) attacks against
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync your server.
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync </p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync Here is an example of how to properly apply ACLs:
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync </p>
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync<pre class="programlisting">
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync// Set up an ACL named "bogusnets" that will block
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync// RFC1918 space and some reserved space, which is
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync// commonly used in spoofing attacks.
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsyncacl bogusnets {
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync 0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync};
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync// Set up an ACL called our-nets. Replace this with the
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync// real IP numbers.
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsyncacl our-nets { x.x.x.x/24; x.x.x.x/21; };
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsyncoptions {
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync ...
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync ...
4fd606d1f5abe38e1f42c38de1d2e895166bd0f4vboxsync 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 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" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id2604198"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">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><strong class="command">chroot()</strong></span> function) by specifying
the "<code class="option">-t</code>" option for <span><strong class="command">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><strong class="command">chroot</strong></span> feature.
</p>
<p>
Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
<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
user 202:
</p>
<p>
<strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2604347"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
<p>
In order for a <span><strong class="command">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><strong class="command">directory</strong></span> and <span><strong class="command">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><strong class="command">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" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2604407"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
<p>
Prior to running the <span><strong class="command">named</strong></span> daemon,
use
the <span><strong class="command">touch</strong></span> utility (to change file
access and
modification times) or the <span><strong class="command">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" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
Note that if the <span><strong class="command">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" 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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