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    31. 

  31. <tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr>
    32. 

  32. <tr>
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    42. 

  42. <div class="chapter" lang="en">
    43. 

  43. <div class="titlepage"><div><div><h2 class="title">
    44. 

  44. <a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div>
    45. 

  45. <div class="toc">
    46. 

  46. <p><b>Table of Contents</b></p>
    47. 

  47. <dl>
    48. 

  48. <dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt>
    49. 

  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. 

  50. <dd><dl>
    51. 

  51. <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603232">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt>
    52. 

  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. 

  53. </dl></dd>
    54. 

  54. <dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt>
    55. 

  55. </dl>
    56. 

  56. </div>
    57. 

  57. <div class="sect1" lang="en">
    58. 

  58. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    59. 

  59. <a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div>
    60. 

  60. <p>
    61. 

  61.           Access Control Lists (ACLs) are address match lists that
    62. 

  62.           you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>,
    63. 

  63.           <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>,
    64. 

  64.           <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>,
    65. 

  65.           <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>,
    66. 

  66.           etc.
    67. 

  67.         </p>
    68. 

  68. <p>
    69. 

  69.           Using ACLs allows you to have finer control over who can access
    70. 

  70.           your name server, without cluttering up your config files with huge
    71. 

  71.           lists of IP addresses.
    72. 

  72.         </p>
    73. 

  73. <p>
    74. 

  74.           It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to
    75. 

  75.           control access to your server. Limiting access to your server by
    76. 

  76.           outside parties can help prevent spoofing and denial of service (DoS) attacks against
    77. 

  77.           your server.
    78. 

  78.         </p>
    79. 

  79. <p>
    80. 

  80.           Here is an example of how to properly apply ACLs:
    81. 

  81.         </p>
    82. 

  82. <pre class="programlisting">
    83. 

  83. // Set up an ACL named "bogusnets" that will block
    84. 

  84. // RFC1918 space and some reserved space, which is
    85. 

  85. // commonly used in spoofing attacks.
    86. 

  86. acl bogusnets {
    87. 

  87.         0.0.0.0/8;  192.0.2.0/24; 224.0.0.0/3;
    88. 

  88.         10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16;
    89. 

  89. };
    90. 

  90. 

  91. // Set up an ACL called our-nets. Replace this with the
    92. 

  92. // real IP numbers.
    93. 

  93. acl our-nets { x.x.x.x/24; x.x.x.x/21; };
    94. 

  94. options {
    95. 

  95.   ...
    96. 

  96.   ...
    97. 

  97.   allow-query { our-nets; };
    98. 

  98.   allow-recursion { our-nets; };
    99. 

  99.   ...
    100. 

  100.   blackhole { bogusnets; };
    101. 

  101.   ...
    102. 

  102. };
    103. 

  103. 

  104. zone "example.com" {
    105. 

  105.   type master;
    106. 

  106.   file "m/example.com";
    107. 

  107.   allow-query { any; };
    108. 

  108. };
    109. 

  109. </pre>
    110. 

  110. <p>
    111. 

  111.           This allows recursive queries of the server from the outside
    112. 

  112.           unless recursion has been previously disabled.
    113. 

  113.         </p>
    114. 

  114. <p>
    115. 

  115.           For more information on how to use ACLs to protect your server,
    116. 

  116.           see the <span class="emphasis"><em>AUSCERT</em></span> advisory at:
    117. 

  117.         </p>
    118. 

  118. <p>
    119. 

  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. 

  120.         </p>
    121. 

  121. </div>
    122. 

  122. <div class="sect1" lang="en">
    123. 

  123. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    124. 

  124. <a name="id2603082"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span>
    125. 

  125. </h2></div></div></div>
    126. 

  126. <p>
    127. 

  127.           On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym>
    128. 

  128.           in a <span class="emphasis"><em>chrooted</em></span> environment (using
    129. 

  129.           the <span><strong class="command">chroot()</strong></span> function) by specifying
    130. 

  130.           the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>.
    131. 

  131.           This can help improve system security by placing
    132. 

  132.           <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit
    133. 

  133.           the damage done if a server is compromised.
    134. 

  134.         </p>
    135. 

  135. <p>
    136. 

  136.           Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the
    137. 

  137.           ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ).
    138. 

  138.           We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature.
    139. 

  139.         </p>
    140. 

  140. <p>
    141. 

  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. 

  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. 

  143.           user 202:
    144. 

  144.         </p>
    145. 

  145. <p>
    146. 

  146.           <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong>
    147. 

  147.         </p>
    148. 

  148. <div class="sect2" lang="en">
    149. 

  149. <div class="titlepage"><div><div><h3 class="title">
    150. 

  150. <a name="id2603232"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div>
    151. 

  151. <p>
    152. 

  152.             In order for a <span><strong class="command">chroot</strong></span> environment
    153. 

  153.             to
    154. 

  154.             work properly in a particular directory
    155. 

  155.             (for example, <code class="filename">/var/named</code>),
    156. 

  156.             you will need to set up an environment that includes everything
    157. 

  157.             <acronym class="acronym">BIND</acronym> needs to run.
    158. 

  158.             From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is
    159. 

  159.             the root of the filesystem.  You will need to adjust the values of
    160. 

  160.             options like
    161. 

  161.             like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account
    162. 

  162.             for this.
    163. 

  163.           </p>
    164. 

  164. <p>
    165. 

  165.             Unlike with earlier versions of BIND, you typically will
    166. 

  166.             <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span>
    167. 

  167.             statically nor install shared libraries under the new root.
    168. 

  168.             However, depending on your operating system, you may need
    169. 

  169.             to set up things like
    170. 

  170.             <code class="filename">/dev/zero</code>,
    171. 

  171.             <code class="filename">/dev/random</code>,
    172. 

  172.             <code class="filename">/dev/log</code>, and
    173. 

  173.             <code class="filename">/etc/localtime</code>.
    174. 

  174.           </p>
    175. 

  175. </div>
    176. 

  176. <div class="sect2" lang="en">
    177. 

  177. <div class="titlepage"><div><div><h3 class="title">
    178. 

  178. <a name="id2603291"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div>
    179. 

  179. <p>
    180. 

  180.             Prior to running the <span><strong class="command">named</strong></span> daemon,
    181. 

  181.             use
    182. 

  182.             the <span><strong class="command">touch</strong></span> utility (to change file
    183. 

  183.             access and
    184. 

  184.             modification times) or the <span><strong class="command">chown</strong></span>
    185. 

  185.             utility (to
    186. 

  186.             set the user id and/or group id) on files
    187. 

  187.             to which you want <acronym class="acronym">BIND</acronym>
    188. 

  188.             to write.
    189. 

  189.           </p>
    190. 

  190. <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
    191. 

  191. <h3 class="title">Note</h3>
    192. 

  192.             Note that if the <span><strong class="command">named</strong></span> daemon is running as an
    193. 

  193.             unprivileged user, it will not be able to bind to new restricted
    194. 

  194.             ports if the server is reloaded.
    195. 

  195.           </div>
    196. 

  196. </div>
    197. 

  197. </div>
    198. 

  198. <div class="sect1" lang="en">
    199. 

  199. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    200. 

  200. <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div>
    201. 

  201. <p>
    202. 

  202.           Access to the dynamic
    203. 

  203.           update facility should be strictly limited.  In earlier versions of
    204. 

  204.           <acronym class="acronym">BIND</acronym>, the only way to do this was
    205. 

  205.           based on the IP
    206. 

  206.           address of the host requesting the update, by listing an IP address
    207. 

  207.           or
    208. 

  208.           network prefix in the <span><strong class="command">allow-update</strong></span>
    209. 

  209.           zone option.
    210. 

  210.           This method is insecure since the source address of the update UDP
    211. 

  211.           packet
    212. 

  212.           is easily forged.  Also note that if the IP addresses allowed by the
    213. 

  213.           <span><strong class="command">allow-update</strong></span> option include the
    214. 

  214.           address of a slave
    215. 

  215.           server which performs forwarding of dynamic updates, the master can
    216. 

  216.           be
    217. 

  217.           trivially attacked by sending the update to the slave, which will
    218. 

  218.           forward it to the master with its own source IP address causing the
    219. 

  219.           master to approve it without question.
    220. 

  220.         </p>
    221. 

  221. <p>
    222. 

  222.           For these reasons, we strongly recommend that updates be
    223. 

  223.           cryptographically authenticated by means of transaction signatures
    224. 

  224.           (TSIG).  That is, the <span><strong class="command">allow-update</strong></span>
    225. 

  225.           option should
    226. 

  226.           list only TSIG key names, not IP addresses or network
    227. 

  227.           prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span>
    228. 

  228.           option can be used.
    229. 

  229.         </p>
    230. 

  230. <p>
    231. 

  231.           Some sites choose to keep all dynamically-updated DNS data
    232. 

  232.           in a subdomain and delegate that subdomain to a separate zone. This
    233. 

  233.           way, the top-level zone containing critical data such as the IP
    234. 

  234.           addresses
    235. 

  235.           of public web and mail servers need not allow dynamic update at
    236. 

  236.           all.
    237. 

  237.         </p>
    238. 

  238. </div>
    239. 

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