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 31<tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
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 42<div class="chapter" lang="en">
 43<div class="titlepage"><div><div><h2 class="title">
 44<a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
 45<div class="toc">
 46<p><b>Table of Contents</b></p>
 47<dl>
 48<dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
 49<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>
 50<dd><dl>
 51<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603232">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
 52<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603291">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
 53</dl></dd>
 54<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
 55</dl>
 56</div>
 57<div class="sect1" lang="en">
 58<div class="titlepage"><div><div><h2 class="title" style="clear: both">
 59<a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
 60<p>
 61          Access Control Lists (ACLs) are address match lists that
 62          you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
 63          <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
 64          <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
 65          <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
 66          etc.
 67        </p>
 68<p>
 69          Using ACLs allows you to have finer control over who can access
 70          your name server, without cluttering up your config files with huge
 71          lists of IP addresses.
 72        </p>
 73<p>
 74          It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
 75          control access to your server. Limiting access to your server by
 76          outside parties can help prevent spoofing and denial of service (DoS) attacks against
 77          your server.
 78        </p>
 79<p>
 80          Here is an example of how to properly apply ACLs:
 81        </p>
 82<pre class="programlisting">
 83// Set up an ACL named "bogusnets" that will block
 84// RFC1918 space and some reserved space, which is
 85// commonly used in spoofing attacks.
 86acl bogusnets {
 87        0.0.0.0/8;  192.0.2.0/24; 224.0.0.0/3;
 88        10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
 89};
 90
 91// Set up an ACL called our-nets. Replace this with the
 92// real IP numbers.
 93acl our-nets { x.x.x.x/24; x.x.x.x/21; };
 94options {
 95  ...
 96  ...
 97  allow-query { our-nets; };
 98  allow-recursion { our-nets; };
 99  ...
100  blackhole { bogusnets; };
101  ...
102};
103
104zone "example.com" {
105  type master;
106  file "m/example.com";
107  allow-query { any; };
108};
109</pre>
110<p>
111          This allows recursive queries of the server from the outside
112          unless recursion has been previously disabled.
113        </p>
114<p>
115          For more information on how to use ACLs to protect your server,
116          see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
117        </p>
118<p>
119          <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>
120        </p>
121</div>
122<div class="sect1" lang="en">
123<div class="titlepage"><div><div><h2 class="title" style="clear: both">
124<a name="id2603082"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
125</h2></div></div></div>
126<p>
127          On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
128          in a <span class="emphasis"><em>chrooted</em></span> environment (using
129          the <span><strong class="command">chroot()</strong></span> function) by specifying
130          the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
131          This can help improve system security by placing
132          <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
133          the damage done if a server is compromised.
134        </p>
135<p>
136          Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
137          ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
138          We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
139        </p>
140<p>
141          Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
142          <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
143          user 202:
144        </p>
145<p>
146          <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
147        </p>
148<div class="sect2" lang="en">
149<div class="titlepage"><div><div><h3 class="title">
150<a name="id2603232"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
151<p>
152            In order for a <span><strong class="command">chroot</strong></span> environment
153            to
154            work properly in a particular directory
155            (for example, <code class="filename">/var/named</code>),
156            you will need to set up an environment that includes everything
157            <acronym class="acronym">BIND</acronym> needs to run.
158            From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
159            the root of the filesystem.  You will need to adjust the values of
160            options like
161            like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
162            for this.
163          </p>
164<p>
165            Unlike with earlier versions of BIND, you typically will
166            <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
167            statically nor install shared libraries under the new root.
168            However, depending on your operating system, you may need
169            to set up things like
170            <code class="filename">/dev/zero</code>,
171            <code class="filename">/dev/random</code>,
172            <code class="filename">/dev/log</code>, and
173            <code class="filename">/etc/localtime</code>.
174          </p>
175</div>
176<div class="sect2" lang="en">
177<div class="titlepage"><div><div><h3 class="title">
178<a name="id2603291"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
179<p>
180            Prior to running the <span><strong class="command">named</strong></span> daemon,
181            use
182            the <span><strong class="command">touch</strong></span> utility (to change file
183            access and
184            modification times) or the <span><strong class="command">chown</strong></span>
185            utility (to
186            set the user id and/or group id) on files
187            to which you want <acronym class="acronym">BIND</acronym>
188            to write.
189          </p>
190<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
191<h3 class="title">Note</h3>
192            Note that if the <span><strong class="command">named</strong></span> daemon is running as an
193            unprivileged user, it will not be able to bind to new restricted
194            ports if the server is reloaded.
195          </div>
196</div>
197</div>
198<div class="sect1" lang="en">
199<div class="titlepage"><div><div><h2 class="title" style="clear: both">
200<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
201<p>
202          Access to the dynamic
203          update facility should be strictly limited.  In earlier versions of
204          <acronym class="acronym">BIND</acronym>, the only way to do this was
205          based on the IP
206          address of the host requesting the update, by listing an IP address
207          or
208          network prefix in the <span><strong class="command">allow-update</strong></span>
209          zone option.
210          This method is insecure since the source address of the update UDP
211          packet
212          is easily forged.  Also note that if the IP addresses allowed by the
213          <span><strong class="command">allow-update</strong></span> option include the
214          address of a slave
215          server which performs forwarding of dynamic updates, the master can
216          be
217          trivially attacked by sending the update to the slave, which will
218          forward it to the master with its own source IP address causing the
219          master to approve it without question.
220        </p>
221<p>
222          For these reasons, we strongly recommend that updates be
223          cryptographically authenticated by means of transaction signatures
224          (TSIG).  That is, the <span><strong class="command">allow-update</strong></span>
225          option should
226          list only TSIG key names, not IP addresses or network
227          prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
228          option can be used.
229        </p>
230<p>
231          Some sites choose to keep all dynamically-updated DNS data
232          in a subdomain and delegate that subdomain to a separate zone. This
233          way, the top-level zone containing critical data such as the IP
234          addresses
235          of public web and mail servers need not allow dynamic update at
236          all.
237        </p>
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