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  20 <title>Chapter 7. BIND 9 Security Considerations</title>
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  30 <tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
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  41 <div class="chapter">
  42 <div class="titlepage"><div><div><h1 class="title">
  43 <a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h1></div></div></div>
  44 <div class="toc">
  45 <p><b>Table of Contents</b></p>
  46 <dl class="toc">
  47 <dt><span class="section"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
  48 <dt><span class="section"><a href="Bv9ARM.ch07.html#chroot_and_setuid"><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span></a></span></dt>
  49 <dd><dl>
  50 <dt><span class="section"><a href="Bv9ARM.ch07.html#chroot">The <span class="command"><strong>chroot</strong></span> Environment</a></span></dt>
  51 <dt><span class="section"><a href="Bv9ARM.ch07.html#setuid">Using the <span class="command"><strong>setuid</strong></span> Function</a></span></dt>
  52 </dl></dd>
  53 <dt><span class="section"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
  54 </dl>
  55 </div>
  56 <div class="section">
  57 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  58 <a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
  59 <p>
  60           Access Control Lists (ACLs) are address match lists that
  61           you can set up and nickname for future use in <span class="command"><strong>allow-notify</strong></span>,
  62           <span class="command"><strong>allow-query</strong></span>, <span class="command"><strong>allow-query-on</strong></span>,
  63           <span class="command"><strong>allow-recursion</strong></span>, <span class="command"><strong>allow-recursion-on</strong></span>,
  64           <span class="command"><strong>blackhole</strong></span>, <span class="command"><strong>allow-transfer</strong></span>,
  65           etc.
  66         </p>
  67 <p>
  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.
  71         </p>
  72 <p>
  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
  76           your server.
  77         </p>
  78 <p>
  79           Here is an example of how to properly apply ACLs:
  80         </p>
  81 <pre class="programlisting">
  82 // Set up an ACL named "bogusnets" that will block
  83 // RFC1918 space and some reserved space, which is
  84 // commonly used in spoofing attacks.
  85 acl bogusnets {
  86         0.0.0.0/8;  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;
  88 };
  89
  90 // Set up an ACL called our-nets. Replace this with the
  91 // real IP numbers.
  92 acl our-nets { x.x.x.x/24; x.x.x.x/21; };
  93 options {
  94   ...
  95   ...
  96   allow-query { our-nets; };
  97   allow-recursion { our-nets; };
  98   ...
  99   blackhole { bogusnets; };
 100   ...
 101 };
 102
 103 zone "example.com" {
 104   type master;
 105   file "m/example.com";
 106   allow-query { any; };
 107 };
 108 </pre>
 109 <p>
 110           This allows recursive queries of the server from the outside
 111           unless recursion has been previously disabled.
 112         </p>
 113 </div>
 114 <div class="section">
 115 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 116 <a name="chroot_and_setuid"></a><span class="command"><strong>Chroot</strong></span> and <span class="command"><strong>Setuid</strong></span>
 117 </h2></div></div></div>
 118 <p>
 119           On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
 120           in a <span class="emphasis"><em>chrooted</em></span> environment (using
 121           the <span class="command"><strong>chroot()</strong></span> function) by specifying
 122           the "<code class="option">-t</code>" option for <span class="command"><strong>named</strong></span>.
 123           This can help improve system security by placing
 124           <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
 125           the damage done if a server is compromised.
 126         </p>
 127 <p>
 128           Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
 129           ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
 130           We suggest running as an unprivileged user when using the <span class="command"><strong>chroot</strong></span> feature.
 131         </p>
 132 <p>
 133           Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span class="command"><strong>chroot</strong></span> sandbox,
 134           <span class="command"><strong>/var/named</strong></span>, and to run <span class="command"><strong>named</strong></span> <span class="command"><strong>setuid</strong></span> to
 135           user 202:
 136         </p>
 137 <p>
 138           <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
 139         </p>
 140 <div class="section">
 141 <div class="titlepage"><div><div><h3 class="title">
 142 <a name="chroot"></a>The <span class="command"><strong>chroot</strong></span> Environment</h3></div></div></div>
 143 <p>
 144             In order for a <span class="command"><strong>chroot</strong></span> environment
 145             to
 146             work properly in a particular directory
 147             (for example, <code class="filename">/var/named</code>),
 148             you will need to set up an environment that includes everything
 149             <acronym class="acronym">BIND</acronym> needs to run.
 150             From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
 151             the root of the filesystem.  You will need to adjust the values of
 152             options like
 153             like <span class="command"><strong>directory</strong></span> and <span class="command"><strong>pid-file</strong></span> to account
 154             for this.
 155           </p>
 156 <p>
 157             Unlike with earlier versions of BIND, you typically will
 158             <span class="emphasis"><em>not</em></span> need to compile <span class="command"><strong>named</strong></span>
 159             statically nor install shared libraries under the new root.
 160             However, depending on your operating system, you may need
 161             to set up things like
 162             <code class="filename">/dev/zero</code>,
 163             <code class="filename">/dev/random</code>,
 164             <code class="filename">/dev/log</code>, and
 165             <code class="filename">/etc/localtime</code>.
 166           </p>
 167 </div>
 168 <div class="section">
 169 <div class="titlepage"><div><div><h3 class="title">
 170 <a name="setuid"></a>Using the <span class="command"><strong>setuid</strong></span> Function</h3></div></div></div>
 171 <p>
 172             Prior to running the <span class="command"><strong>named</strong></span> daemon,
 173             use
 174             the <span class="command"><strong>touch</strong></span> utility (to change file
 175             access and
 176             modification times) or the <span class="command"><strong>chown</strong></span>
 177             utility (to
 178             set the user id and/or group id) on files
 179             to which you want <acronym class="acronym">BIND</acronym>
 180             to write.
 181           </p>
 182 <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
 183 <h3 class="title">Note</h3>
 184 <p>
 185             If the <span class="command"><strong>named</strong></span> daemon is running as an
 186             unprivileged user, it will not be able to bind to new restricted
 187             ports if the server is reloaded.
 188           </p>
 189 </div>
 190 </div>
 191 </div>
 192 <div class="section">
 193 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
 194 <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
 195 <p>
 196           Access to the dynamic
 197           update facility should be strictly limited.  In earlier versions of
 198           <acronym class="acronym">BIND</acronym>, the only way to do this was
 199           based on the IP
 200           address of the host requesting the update, by listing an IP address
 201           or
 202           network prefix in the <span class="command"><strong>allow-update</strong></span>
 203           zone option.
 204           This method is insecure since the source address of the update UDP
 205           packet
 206           is easily forged.  Also note that if the IP addresses allowed by the
 207           <span class="command"><strong>allow-update</strong></span> option include the
 208           address of a slave
 209           server which performs forwarding of dynamic updates, the master can
 210           be
 211           trivially attacked by sending the update to the slave, which will
 212           forward it to the master with its own source IP address causing the
 213           master to approve it without question.
 214         </p>
 215 <p>
 216           For these reasons, we strongly recommend that updates be
 217           cryptographically authenticated by means of transaction signatures
 218           (TSIG).  That is, the <span class="command"><strong>allow-update</strong></span>
 219           option should
 220           list only TSIG key names, not IP addresses or network
 221           prefixes. Alternatively, the new <span class="command"><strong>update-policy</strong></span>
 222           option can be used.
 223         </p>
 224 <p>
 225           Some sites choose to keep all dynamically-updated DNS data
 226           in a subdomain and delegate that subdomain to a separate zone. This
 227           way, the top-level zone containing critical data such as the IP
 228           addresses
 229           of public web and mail servers need not allow dynamic update at
 230           all.
 231         </p>
 232 </div>
 233 </div>
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 245 <td width="40%" align="left" valign="top">Chapter 6. <acronym class="acronym">BIND</acronym> 9 Configuration Reference </td>
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