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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>
46 <p><b>Table of Contents</b></p>
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#id2603136"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt>
51 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603285">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
52 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603345">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt>
54 <dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
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>
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>,
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.
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
80 Here is an example of how to properly apply ACLs:
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.
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;
91 // Set up an ACL called our-nets. Replace this with the
93 acl our-nets { x.x.x.x/24; x.x.x.x/21; };
97 allow-query { our-nets; };
98 allow-recursion { our-nets; };
100 blackhole { bogusnets; };
106 file "m/example.com";
107 allow-query { any; };
111 This allows recursive queries of the server from the outside
112 unless recursion has been previously disabled.
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:
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>
122 <div class="sect1" lang="en">
123 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
124 <a name="id2603136"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
125 </h2></div></div></div>
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.
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.
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
146 <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
148 <div class="sect2" lang="en">
149 <div class="titlepage"><div><div><h3 class="title">
150 <a name="id2603285"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
152 In order for a <span><strong class="command">chroot</strong></span> environment
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
161 like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
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>.
176 <div class="sect2" lang="en">
177 <div class="titlepage"><div><div><h3 class="title">
178 <a name="id2603345"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
180 Prior to running the <span><strong class="command">named</strong></span> daemon,
182 the <span><strong class="command">touch</strong></span> utility (to change file
184 modification times) or the <span><strong class="command">chown</strong></span>
186 set the user id and/or group id) on files
187 to which you want <acronym class="acronym">BIND</acronym>
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.
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>
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
206 address of the host requesting the update, by listing an IP address
208 network prefix in the <span><strong class="command">allow-update</strong></span>
210 This method is insecure since the source address of the update UDP
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
215 server which performs forwarding of dynamic updates, the master can
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.
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>
226 list only TSIG key names, not IP addresses or network
227 prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
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
235 of public web and mail servers need not allow dynamic update at
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