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Limiting access to your server by 76 outside parties can help prevent spoofing and denial of service (DoS) attacks against 77 your server. 78 </p> 79<p> 80 Here is an example of how to properly apply ACLs: 81 </p> 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. 86acl bogusnets { 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; 89}; 90 91// Set up an ACL called our-nets. Replace this with the 92// real IP numbers. 93acl our-nets { x.x.x.x/24; x.x.x.x/21; }; 94options { 95 ... 96 ... 97 allow-query { our-nets; }; 98 allow-recursion { our-nets; }; 99 ... 100 blackhole { bogusnets; }; 101 ... 102}; 103 104zone "example.com" { 105 type master; 106 file "m/example.com"; 107 allow-query { any; }; 108}; 109</pre> 110<p> 111 This allows recursive queries of the server from the outside 112 unless recursion has been previously disabled. 113 </p> 114</div> 115<div class="sect1" lang="en"> 116<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 117<a name="id2606376"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span> 118</h2></div></div></div> 119<p> 120 On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym> 121 in a <span class="emphasis"><em>chrooted</em></span> environment (using 122 the <span><strong class="command">chroot()</strong></span> function) by specifying 123 the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>. 124 This can help improve system security by placing 125 <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit 126 the damage done if a server is compromised. 127 </p> 128<p> 129 Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the 130 ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ). 131 We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature. 132 </p> 133<p> 134 Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox, 135 <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 136 user 202: 137 </p> 138<p> 139 <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong> 140 </p> 141<div class="sect2" lang="en"> 142<div class="titlepage"><div><div><h3 class="title"> 143<a name="id2606457"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div> 144<p> 145 In order for a <span><strong class="command">chroot</strong></span> environment 146 to 147 work properly in a particular directory 148 (for example, <code class="filename">/var/named</code>), 149 you will need to set up an environment that includes everything 150 <acronym class="acronym">BIND</acronym> needs to run. 151 From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is 152 the root of the filesystem. You will need to adjust the values of 153 options like 154 like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account 155 for this. 156 </p> 157<p> 158 Unlike with earlier versions of BIND, you typically will 159 <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span> 160 statically nor install shared libraries under the new root. 161 However, depending on your operating system, you may need 162 to set up things like 163 <code class="filename">/dev/zero</code>, 164 <code class="filename">/dev/random</code>, 165 <code class="filename">/dev/log</code>, and 166 <code class="filename">/etc/localtime</code>. 167 </p> 168</div> 169<div class="sect2" lang="en"> 170<div class="titlepage"><div><div><h3 class="title"> 171<a name="id2606517"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div> 172<p> 173 Prior to running the <span><strong class="command">named</strong></span> daemon, 174 use 175 the <span><strong class="command">touch</strong></span> utility (to change file 176 access and 177 modification times) or the <span><strong class="command">chown</strong></span> 178 utility (to 179 set the user id and/or group id) on files 180 to which you want <acronym class="acronym">BIND</acronym> 181 to write. 182 </p> 183<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 184<h3 class="title">Note</h3> 185 Note that if the <span><strong class="command">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 </div> 189</div> 190</div> 191<div class="sect1" lang="en"> 192<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 193<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div> 194<p> 195 Access to the dynamic 196 update facility should be strictly limited. In earlier versions of 197 <acronym class="acronym">BIND</acronym>, the only way to do this was 198 based on the IP 199 address of the host requesting the update, by listing an IP address 200 or 201 network prefix in the <span><strong class="command">allow-update</strong></span> 202 zone option. 203 This method is insecure since the source address of the update UDP 204 packet 205 is easily forged. Also note that if the IP addresses allowed by the 206 <span><strong class="command">allow-update</strong></span> option include the 207 address of a slave 208 server which performs forwarding of dynamic updates, the master can 209 be 210 trivially attacked by sending the update to the slave, which will 211 forward it to the master with its own source IP address causing the 212 master to approve it without question. 213 </p> 214<p> 215 For these reasons, we strongly recommend that updates be 216 cryptographically authenticated by means of transaction signatures 217 (TSIG). That is, the <span><strong class="command">allow-update</strong></span> 218 option should 219 list only TSIG key names, not IP addresses or network 220 prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span> 221 option can be used. 222 </p> 223<p> 224 Some sites choose to keep all dynamically-updated DNS data 225 in a subdomain and delegate that subdomain to a separate zone. This 226 way, the top-level zone containing critical data such as the IP 227 addresses 228 of public web and mail servers need not allow dynamic update at 229 all. 230 </p> 231</div> 232</div> 233<div class="navfooter"> 234<hr> 235<table width="100%" summary="Navigation footer"> 236<tr> 237<td width="40%" align="left"> 238<a accesskey="p" href="Bv9ARM.ch06.html">Prev</a>�</td> 239<td width="20%" align="center">�</td> 240<td width="40%" align="right">�<a accesskey="n" href="Bv9ARM.ch08.html">Next</a> 241</td> 242</tr> 243<tr> 244<td width="40%" align="left" valign="top">Chapter�6.�<acronym class="acronym">BIND</acronym> 9 Configuration Reference�</td> 245<td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td> 246<td width="40%" align="right" valign="top">�Chapter�8.�Troubleshooting</td> 247</tr> 248</table> 249</div> 250<p style="text-align: center;">BIND 9.10.2-P4</p> 251</body> 252</html> 253