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fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<a name="Bv9ARM.ch07"></a>Chapter�7.�<acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<div class="toc">
af40ebed6257e4ac1996144530b3de317cf4da11Tinderbox User<p><b>Table of Contents</b></p>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<dl>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2606320"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<dd><dl>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2606401">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2606460">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein</dl></dd>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein</dl>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein</div>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<div class="sect1" lang="en">
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<div class="titlepage"><div><div><h2 class="title" style="clear: both">
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<p>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User Access Control Lists (ACLs) are address match lists that
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews etc.
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews </p>
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews<p>
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews Using ACLs allows you to have finer control over who can access
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews your name server, without cluttering up your config files with huge
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews lists of IP addresses.
48abcd3eb789fdd24a2e0a6155b25e6979a39ae0Mark Andrews </p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<p>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein control access to your server. Limiting access to your server by
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein outside parties can help prevent spoofing and denial of service (DoS) attacks against
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein your server.
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein </p>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein Here is an example of how to properly apply ACLs:
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein </p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<pre class="programlisting">
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein// Set up an ACL named "bogusnets" that will block
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein// RFC1918 space and some reserved space, which is
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User// commonly used in spoofing attacks.
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox Useracl bogusnets {
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein 0.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
af40ebed6257e4ac1996144530b3de317cf4da11Tinderbox User 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User};
5a4557e8de2951a2796676b5ec4b6a90caa5be14Mark Andrews
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews// Set up an ACL called our-nets. Replace this with the
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews// real IP numbers.
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austeinacl our-nets { x.x.x.x/24; x.x.x.x/21; };
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrewsoptions {
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein ...
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein ...
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews allow-query { our-nets; };
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein allow-recursion { our-nets; };
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein ...
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews blackhole { bogusnets; };
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein ...
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User};
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox Userzone "example.com" {
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews type master;
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User file "m/example.com";
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein allow-query { any; };
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein};
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User</pre>
fd2597f75693a2279fdf588bd40dfe2407c42028Tinderbox User<p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein This allows recursive queries of the server from the outside
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein unless recursion has been previously disabled.
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein </p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein</div>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<div class="sect1" lang="en">
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<div class="titlepage"><div><div><h2 class="title" style="clear: both">
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<a name="id2606320"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein</h2></div></div></div>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein<p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein in a <span class="emphasis"><em>chrooted</em></span> environment (using
71c66a876ecca77923638d3f94cc0783152b2f03Mark Andrews the <span><strong class="command">chroot()</strong></span> function) by specifying
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
cd32f419a8a5432fbb139f56ee73cbf68b9350ccTinderbox User This can help improve system security by placing
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein the damage done if a server is compromised.
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein </p>
1700442a7751c2bbdafe2d039cebbd8316496957Tinderbox User<p>
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
60e5e10f8d2e2b0c41e8abad38cacd867caa6ab2Rob Austein 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="id2606401"></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="id2606460"></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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