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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#id2593181"><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#id2593326">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
52 <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2593386">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-recursion</strong></span>,
64 <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
68 Using ACLs allows you to have finer control over who can access
69 your name server, without cluttering up your config files with huge
70 lists of IP addresses.
73 It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
74 control access to your server. Limiting access to your server by
75 outside parties can help prevent spoofing and denial of service (DoS) attacks against
79 Here is an example of how to properly apply ACLs:
81 <pre class="programlisting">
82 // Set up an ACL named "bogusnets" that will block RFC1918 space
83 // and some reserved space, which is commonly used in spoofing attacks.
85 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;
86 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
89 // Set up an ACL called our-nets. Replace this with the real IP numbers.
90 acl our-nets { x.x.x.x/24; x.x.x.x/21; };
94 allow-query { our-nets; };
95 allow-recursion { our-nets; };
97 blackhole { bogusnets; };
103 file "m/example.com";
104 allow-query { any; };
108 This allows recursive queries of the server from the outside
109 unless recursion has been previously disabled.
112 For more information on how to use ACLs to protect your server,
113 see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
116 <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>
119 <div class="sect1" lang="en">
120 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
121 <a name="id2593181"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
122 </h2></div></div></div>
124 On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym> in a <span class="emphasis"><em>chrooted</em></span> environment
125 (using the <span><strong class="command">chroot()</strong></span> function) by specifying the "<code class="option">-t</code>"
126 option. This can help improve system security by placing <acronym class="acronym">BIND</acronym> in
127 a "sandbox", which will limit the damage done if a server is
131 Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
132 ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
133 We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
136 Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox,
137 <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
141 <strong class="userinput"><code>/usr/local/bin/named -u 202 -t /var/named</code></strong>
143 <div class="sect2" lang="en">
144 <div class="titlepage"><div><div><h3 class="title">
145 <a name="id2593326"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
147 In order for a <span><strong class="command">chroot</strong></span> environment
149 work properly in a particular directory
150 (for example, <code class="filename">/var/named</code>),
151 you will need to set up an environment that includes everything
152 <acronym class="acronym">BIND</acronym> needs to run.
153 From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
154 the root of the filesystem. You will need to adjust the values of
156 like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
160 Unlike with earlier versions of BIND, you typically will
161 <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
162 statically nor install shared libraries under the new root.
163 However, depending on your operating system, you may need
164 to set up things like
165 <code class="filename">/dev/zero</code>,
166 <code class="filename">/dev/random</code>,
167 <code class="filename">/dev/log</code>, and
168 <code class="filename">/etc/localtime</code>.
171 <div class="sect2" lang="en">
172 <div class="titlepage"><div><div><h3 class="title">
173 <a name="id2593386"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
175 Prior to running the <span><strong class="command">named</strong></span> daemon,
177 the <span><strong class="command">touch</strong></span> utility (to change file
179 modification times) or the <span><strong class="command">chown</strong></span>
181 set the user id and/or group id) on files
182 to which you want <acronym class="acronym">BIND</acronym>
185 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
186 <h3 class="title">Note</h3>
187 Note that if the <span><strong class="command">named</strong></span> daemon is running as an
188 unprivileged user, it will not be able to bind to new restricted
189 ports if the server is reloaded.
193 <div class="sect1" lang="en">
194 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
195 <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
197 Access to the dynamic
198 update facility should be strictly limited. In earlier versions of
199 <acronym class="acronym">BIND</acronym>, the only way to do this was
201 address of the host requesting the update, by listing an IP address
203 network prefix in the <span><strong class="command">allow-update</strong></span>
205 This method is insecure since the source address of the update UDP
207 is easily forged. Also note that if the IP addresses allowed by the
208 <span><strong class="command">allow-update</strong></span> option include the
210 server which performs forwarding of dynamic updates, the master can
212 trivially attacked by sending the update to the slave, which will
213 forward it to the master with its own source IP address causing the
214 master to approve it without question.
217 For these reasons, we strongly recommend that updates be
218 cryptographically authenticated by means of transaction signatures
219 (TSIG). That is, the <span><strong class="command">allow-update</strong></span>
221 list only TSIG key names, not IP addresses or network
222 prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
226 Some sites choose to keep all dynamically-updated DNS data
227 in a subdomain and delegate that subdomain to a separate zone. This
228 way, the top-level zone containing critical data such as the IP
230 of public web and mail servers need not allow dynamic update at
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