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Troubleshooting"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <div class="navheader"> <table width="100%" summary="Navigation header"> <tr><th colspan="3" align="center">Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</th></tr> <tr> <td width="20%" align="left"> <a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td> <th width="60%" align="center"> </th> <td width="20%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a> </td> </tr> </table> <hr> </div> <div class="chapter" lang="en"> <div class="titlepage"><div><div><h2 class="title"> <a name="Bv9ARM.ch07"></a>Chapter 7. <acronym class="acronym">BIND</acronym> 9 Security Considerations</h2></div></div></div> <div class="toc"> <p><b>Table of Contents</b></p> <dl> <dt><span class="sect1"><a href="Bv9ARM.ch07.html#Access_Control_Lists">Access Control Lists</a></span></dt> <dt><span class="sect1"><a href="Bv9ARM.ch07.html#id2597645"><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span></a></span></dt> <dd><dl> <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2597722">The <span><strong class="command">chroot</strong></span> Environment</a></span></dt> <dt><span class="sect2"><a href="Bv9ARM.ch07.html#id2597782">Using the <span><strong class="command">setuid</strong></span> Function</a></span></dt> </dl></dd> <dt><span class="sect1"><a href="Bv9ARM.ch07.html#dynamic_update_security">Dynamic Update Security</a></span></dt> </dl> </div> <div class="sect1" lang="en"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="Access_Control_Lists"></a>Access Control Lists</h2></div></div></div> <p> Access Control Lists (ACLs), are address match lists that you can set up and nickname for future use in <span><strong class="command">allow-notify</strong></span>, <span><strong class="command">allow-query</strong></span>, <span><strong class="command">allow-query-on</strong></span>, <span><strong class="command">allow-recursion</strong></span>, <span><strong class="command">allow-recursion-on</strong></span>, <span><strong class="command">blackhole</strong></span>, <span><strong class="command">allow-transfer</strong></span>, etc. </p> <p> Using ACLs allows you to have finer control over who can access your name server, without cluttering up your config files with huge lists of IP addresses. </p> <p> It is a <span class="emphasis"><em>good idea</em></span> to use ACLs, and to control access to your server. Limiting access to your server by outside parties can help prevent spoofing and denial of service (DoS) attacks against your server. </p> <p> Here is an example of how to properly apply ACLs: </p> <pre class="programlisting"> // Set up an ACL named "bogusnets" that will block RFC1918 space // and some reserved space, which is commonly used in spoofing attacks. acl bogusnets { 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; 10.0.0.0/8; 172.16.0.0/12; 192.168.0.0/16; }; // Set up an ACL called our-nets. Replace this with the real IP numbers. acl our-nets { x.x.x.x/24; x.x.x.x/21; }; options { ... ... allow-query { our-nets; }; allow-recursion { our-nets; }; ... blackhole { bogusnets; }; ... }; zone "example.com" { type master; file "m/example.com"; allow-query { any; }; }; </pre> <p> This allows recursive queries of the server from the outside unless recursion has been previously disabled. </p> <p> For more information on how to use ACLs to protect your server, see the <span class="emphasis"><em>AUSCERT</em></span> advisory at: </p> <p> <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> </p> </div> <div class="sect1" lang="en"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="id2597645"></a><span><strong class="command">Chroot</strong></span> and <span><strong class="command">Setuid</strong></span> </h2></div></div></div> <p> On UNIX servers, it is possible to run <acronym class="acronym">BIND</acronym> in a <span class="emphasis"><em>chrooted</em></span> environment (using the <span><strong class="command">chroot()</strong></span> function) by specifying the "<code class="option">-t</code>" option. This can help improve system security by placing <acronym class="acronym">BIND</acronym> in a "sandbox", which will limit the damage done if a server is compromised. </p> <p> Another useful feature in the UNIX version of <acronym class="acronym">BIND</acronym> is the ability to run the daemon as an unprivileged user ( <code class="option">-u</code> <em class="replaceable"><code>user</code></em> ). We suggest running as an unprivileged user when using the <span><strong class="command">chroot</strong></span> feature. </p> <p> Here is an example command line to load <acronym class="acronym">BIND</acronym> in a <span><strong class="command">chroot</strong></span> sandbox, <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 user 202: </p> <p> <strong class="userinput"><code>/usr/local/bin/named -u 202 -t /var/named</code></strong> </p> <div class="sect2" lang="en"> <div class="titlepage"><div><div><h3 class="title"> <a name="id2597722"></a>The <span><strong class="command">chroot</strong></span> Environment</h3></div></div></div> <p> In order for a <span><strong class="command">chroot</strong></span> environment to work properly in a particular directory (for example, <code class="filename">/var/named</code>), you will need to set up an environment that includes everything <acronym class="acronym">BIND</acronym> needs to run. From <acronym class="acronym">BIND</acronym>'s point of view, <code class="filename">/var/named</code> is the root of the filesystem. You will need to adjust the values of options like like <span><strong class="command">directory</strong></span> and <span><strong class="command">pid-file</strong></span> to account for this. </p> <p> Unlike with earlier versions of BIND, you typically will <span class="emphasis"><em>not</em></span> need to compile <span><strong class="command">named</strong></span> statically nor install shared libraries under the new root. However, depending on your operating system, you may need to set up things like <code class="filename">/dev/zero</code>, <code class="filename">/dev/random</code>, <code class="filename">/dev/log</code>, and <code class="filename">/etc/localtime</code>. </p> </div> <div class="sect2" lang="en"> <div class="titlepage"><div><div><h3 class="title"> <a name="id2597782"></a>Using the <span><strong class="command">setuid</strong></span> Function</h3></div></div></div> <p> Prior to running the <span><strong class="command">named</strong></span> daemon, use the <span><strong class="command">touch</strong></span> utility (to change file access and modification times) or the <span><strong class="command">chown</strong></span> utility (to set the user id and/or group id) on files to which you want <acronym class="acronym">BIND</acronym> to write. </p> <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"> <h3 class="title">Note</h3> Note that if the <span><strong class="command">named</strong></span> daemon is running as an unprivileged user, it will not be able to bind to new restricted ports if the server is reloaded. </div> </div> </div> <div class="sect1" lang="en"> <div class="titlepage"><div><div><h2 class="title" style="clear: both"> <a name="dynamic_update_security"></a>Dynamic Update Security</h2></div></div></div> <p> Access to the dynamic update facility should be strictly limited. In earlier versions of <acronym class="acronym">BIND</acronym>, the only way to do this was based on the IP address of the host requesting the update, by listing an IP address or network prefix in the <span><strong class="command">allow-update</strong></span> zone option. This method is insecure since the source address of the update UDP packet is easily forged. Also note that if the IP addresses allowed by the <span><strong class="command">allow-update</strong></span> option include the address of a slave server which performs forwarding of dynamic updates, the master can be trivially attacked by sending the update to the slave, which will forward it to the master with its own source IP address causing the master to approve it without question. </p> <p> For these reasons, we strongly recommend that updates be cryptographically authenticated by means of transaction signatures (TSIG). That is, the <span><strong class="command">allow-update</strong></span> option should list only TSIG key names, not IP addresses or network prefixes. Alternatively, the new <span><strong class="command">update-policy</strong></span> option can be used. </p> <p> Some sites choose to keep all dynamically-updated DNS data in a subdomain and delegate that subdomain to a separate zone. This way, the top-level zone containing critical data such as the IP addresses of public web and mail servers need not allow dynamic update at all. </p> </div> </div> <div class="navfooter"> <hr> <table width="100%" summary="Navigation footer"> <tr> <td width="40%" align="left"> <a accesskey="p" href="Bv9ARM.ch06.html">Prev</a> </td> <td width="20%" align="center"> </td> <td width="40%" align="right"> <a accesskey="n" href="Bv9ARM.ch08.html">Next</a> </td> </tr> <tr> <td width="40%" align="left" valign="top">Chapter 6. <acronym class="acronym">BIND</acronym> 9 Configuration Reference </td> <td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td> <td width="40%" align="right" valign="top"> Chapter 8. Troubleshooting</td> </tr> </table> </div> </body> </html>