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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#id2598990"><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#id2599072">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
52 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2599268">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 RFC1918 space
84 // and some reserved space, which is commonly used in spoofing attacks.
86 0.0.0.0/8; 1.0.0.0/8; 2.0.0.0/8; 192.0.2.0/24; 224.0.0.0/3;
87 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
90 // Set up an ACL called our-nets. Replace this with the real IP numbers.
91 acl our-nets { x.x.x.x/24; x.x.x.x/21; };
95 allow-query { our-nets; };
96 allow-recursion { our-nets; };
98 blackhole { bogusnets; };
104 file "m/example.com";
105 allow-query { any; };
109 This allows recursive queries of the server from the outside
110 unless recursion has been previously disabled.
113 For more information on how to use ACLs to protect your server,
114 see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
117 <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 <div class="sect1" lang="en">
121 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
122 <a name="id2598990"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
123 </h2></div></div></div>
125 On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
126 in a <span class="emphasis"><em>chrooted</em></span> environment (using
127 the <span><strong class="command">chroot()</strong></span> function) by specifying
128 the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
129 This can help improve system security by placing
130 <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
131 the damage done if a server is compromised.
134 Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
135 ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
136 We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
139 Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
140 <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
144 <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
146 <div class="sect2" lang="en">
147 <div class="titlepage"><div><div><h3 class="title">
148 <a name="id2599072"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
150 In order for a <span><strong class="command">chroot</strong></span> environment
152 work properly in a particular directory
153 (for example, <code class="filename">/var/named</code>),
154 you will need to set up an environment that includes everything
155 <acronym class="acronym">BIND</acronym> needs to run.
156 From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
157 the root of the filesystem. You will need to adjust the values of
159 like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
163 Unlike with earlier versions of BIND, you typically will
164 <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
165 statically nor install shared libraries under the new root.
166 However, depending on your operating system, you may need
167 to set up things like
168 <code class="filename">/dev/zero</code>,
169 <code class="filename">/dev/random</code>,
170 <code class="filename">/dev/log</code>, and
171 <code class="filename">/etc/localtime</code>.
174 <div class="sect2" lang="en">
175 <div class="titlepage"><div><div><h3 class="title">
176 <a name="id2599268"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
178 Prior to running the <span><strong class="command">named</strong></span> daemon,
180 the <span><strong class="command">touch</strong></span> utility (to change file
182 modification times) or the <span><strong class="command">chown</strong></span>
184 set the user id and/or group id) on files
185 to which you want <acronym class="acronym">BIND</acronym>
188 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
189 <h3 class="title">Note</h3>
190 Note that if the <span><strong class="command">named</strong></span> daemon is running as an
191 unprivileged user, it will not be able to bind to new restricted
192 ports if the server is reloaded.
196 <div class="sect1" lang="en">
197 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
198 <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
200 Access to the dynamic
201 update facility should be strictly limited. In earlier versions of
202 <acronym class="acronym">BIND</acronym>, the only way to do this was
204 address of the host requesting the update, by listing an IP address
206 network prefix in the <span><strong class="command">allow-update</strong></span>
208 This method is insecure since the source address of the update UDP
210 is easily forged. Also note that if the IP addresses allowed by the
211 <span><strong class="command">allow-update</strong></span> option include the
213 server which performs forwarding of dynamic updates, the master can
215 trivially attacked by sending the update to the slave, which will
216 forward it to the master with its own source IP address causing the
217 master to approve it without question.
220 For these reasons, we strongly recommend that updates be
221 cryptographically authenticated by means of transaction signatures
222 (TSIG). That is, the <span><strong class="command">allow-update</strong></span>
224 list only TSIG key names, not IP addresses or network
225 prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
229 Some sites choose to keep all dynamically-updated DNS data
230 in a subdomain and delegate that subdomain to a separate zone. This
231 way, the top-level zone containing critical data such as the IP
233 of public web and mail servers need not allow dynamic update at
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