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Troubleshooting"> 27</head> 28<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 29<div class="navheader"> 30<table width="100%" summary="Navigation header"> 31<tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr> 32<tr> 33<td width="20%" align="left"> 34<a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td> 35<th width="60%" align="center"> </th> 36<td width="20%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a> 37</td> 38</tr> 39</table> 40<hr> 41</div> 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> 45<div class="toc"> 46<p><b>Table of Contents</b></p> 47<dl> 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#id2603082"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt> 50<dd><dl> 51<dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2603232">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt> 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</dl></dd> 54<dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt> 55</dl> 56</div> 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> 60<p> 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>, 66 etc. 67 </p> 68<p> 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. 72 </p> 73<p> 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 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<p> 115 For more information on how to use ACLs to protect your server, 116 see the <span class="emphasis"><em>AUSCERT</em></span> advisory at: 117 </p> 118<p> 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 </p> 121</div> 122<div class="sect1" lang="en"> 123<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 124<a name="id2603082"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span> 125</h2></div></div></div> 126<p> 127 On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym> 128 in a <span class="emphasis"><em>chrooted</em></span> environment (using 129 the <span><strong class="command">chroot()</strong></span> function) by specifying 130 the "<code class="option">-t</code>" option for <span><strong class="command">named</strong></span>. 131 This can help improve system security by placing 132 <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit 133 the damage done if a server is compromised. 134 </p> 135<p> 136 Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the 137 ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ). 138 We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature. 139 </p> 140<p> 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 <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 user 202: 144 </p> 145<p> 146 <strong class="userinput"><code>/usr/local/sbin/named -u 202 -t /var/named</code></strong> 147 </p> 148<div class="sect2" lang="en"> 149<div class="titlepage"><div><div><h3 class="title"> 150<a name="id2603232"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div> 151<p> 152 In order for a <span><strong class="command">chroot</strong></span> environment 153 to 154 work properly in a particular directory 155 (for example, <code class="filename">/var/named</code>), 156 you will need to set up an environment that includes everything 157 <acronym class="acronym">BIND</acronym> needs to run. 158 From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is 159 the root of the filesystem. You will need to adjust the values of 160 options like 161 like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account 162 for this. 163 </p> 164<p> 165 Unlike with earlier versions of BIND, you typically will 166 <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span> 167 statically nor install shared libraries under the new root. 168 However, depending on your operating system, you may need 169 to set up things like 170 <code class="filename">/dev/zero</code>, 171 <code class="filename">/dev/random</code>, 172 <code class="filename">/dev/log</code>, and 173 <code class="filename">/etc/localtime</code>. 174 </p> 175</div> 176<div class="sect2" lang="en"> 177<div class="titlepage"><div><div><h3 class="title"> 178<a name="id2603291"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div> 179<p> 180 Prior to running the <span><strong class="command">named</strong></span> daemon, 181 use 182 the <span><strong class="command">touch</strong></span> utility (to change file 183 access and 184 modification times) or the <span><strong class="command">chown</strong></span> 185 utility (to 186 set the user id and/or group id) on files 187 to which you want <acronym class="acronym">BIND</acronym> 188 to write. 189 </p> 190<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> 191<h3 class="title">Note</h3> 192 Note that if the <span><strong class="command">named</strong></span> daemon is running as an 193 unprivileged user, it will not be able to bind to new restricted 194 ports if the server is reloaded. 195 </div> 196</div> 197</div> 198<div class="sect1" lang="en"> 199<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 200<a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div> 201<p> 202 Access to the dynamic 203 update facility should be strictly limited. In earlier versions of 204 <acronym class="acronym">BIND</acronym>, the only way to do this was 205 based on the IP 206 address of the host requesting the update, by listing an IP address 207 or 208 network prefix in the <span><strong class="command">allow-update</strong></span> 209 zone option. 210 This method is insecure since the source address of the update UDP 211 packet 212 is easily forged. Also note that if the IP addresses allowed by the 213 <span><strong class="command">allow-update</strong></span> option include the 214 address of a slave 215 server which performs forwarding of dynamic updates, the master can 216 be 217 trivially attacked by sending the update to the slave, which will 218 forward it to the master with its own source IP address causing the 219 master to approve it without question. 220 </p> 221<p> 222 For these reasons, we strongly recommend that updates be 223 cryptographically authenticated by means of transaction signatures 224 (TSIG). That is, the <span><strong class="command">allow-update</strong></span> 225 option should 226 list only TSIG key names, not IP addresses or network 227 prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span> 228 option can be used. 229 </p> 230<p> 231 Some sites choose to keep all dynamically-updated DNS data 232 in a subdomain and delegate that subdomain to a separate zone. This 233 way, the top-level zone containing critical data such as the IP 234 addresses 235 of public web and mail servers need not allow dynamic update at 236 all. 237 </p> 238</div> 239</div> 240<div class="navfooter"> 241<hr> 242<table width="100%" summary="Navigation footer"> 243<tr> 244<td width="40%" align="left"> 245<a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td> 246<td width="20%" align="center"> </td> 247<td width="40%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a> 248</td> 249</tr> 250<tr> 251<td width="40%" align="left" valign="top">Chapter 6. <acronym class="acronym">BIND</acronym> 9 Configuration Reference </td> 252<td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td> 253<td width="40%" align="right" valign="top"> Chapter 8. Troubleshooting</td> 254</tr> 255</table> 256</div> 257</body> 258</html>