<HTML ><HEAD ><TITLE >BIND 9 Security Considerations</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.73 "><LINK REL="HOME" TITLE="BIND 9 Administrator Reference Manual" HREF="Bv9ARM.html"><LINK REL="PREVIOUS" TITLE="BIND 9 Configuration Reference" HREF="Bv9ARM.ch06.html"><LINK REL="NEXT" TITLE="Troubleshooting" HREF="Bv9ARM.ch08.html"></HEAD ><BODY CLASS="chapter" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >BIND 9 Administrator Reference Manual</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="Bv9ARM.ch06.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="Bv9ARM.ch08.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="chapter" ><H1 ><A NAME="ch07" >Chapter 7. <SPAN CLASS="acronym" >BIND</SPAN > 9 Security Considerations</A ></H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT >7.1. <A HREF="Bv9ARM.ch07.html#Access_Control_Lists" >Access Control Lists</A ></DT ><DT >7.2. <A HREF="Bv9ARM.ch07.html#AEN4218" ><B CLASS="command" >chroot</B > and <B CLASS="command" >setuid</B > (for UNIX servers)</A ></DT ><DT >7.3. <A HREF="Bv9ARM.ch07.html#dynamic_update_security" >Dynamic Update Security</A ></DT ></DL ></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="Access_Control_Lists" >7.1. Access Control Lists</A ></H1 ><P >Access Control Lists (ACLs), are address match lists that you can set up and nickname for future use in <B CLASS="command" >allow-notify</B >, <B CLASS="command" >allow-query</B >, <B CLASS="command" >allow-recursion</B >, <B CLASS="command" >blackhole</B >, <B CLASS="command" >allow-transfer</B >, etc.</P ><P >Using ACLs allows you to have finer control over who can access your nameserver, without cluttering up your config files with huge lists of IP addresses.</P ><P >It is a <SPAN CLASS="emphasis" ><I CLASS="emphasis" >good idea</I ></SPAN > to use ACLs, and to control access to your server. Limiting access to your server by outside parties can help prevent spoofing and 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, // 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" ><I CLASS="emphasis" >AUSCERT</I ></SPAN > advisory at <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" ><H1 CLASS="sect1" ><A NAME="AEN4218" >7.2. <B CLASS="command" >chroot</B > and <B CLASS="command" >setuid</B > (for UNIX servers)</A ></H1 ><P >On UNIX servers, it is possible to run <SPAN CLASS="acronym" >BIND</SPAN > in a <SPAN CLASS="emphasis" ><I CLASS="emphasis" >chrooted</I ></SPAN > environment (<B CLASS="command" >chroot()</B >) by specifying the "<TT CLASS="option" >-t</TT >" option. This can help improve system security by placing <SPAN CLASS="acronym" >BIND</SPAN > in a "sandbox," which will limit the damage done if a server is compromised.</P ><P >Another useful feature in the UNIX version of <SPAN CLASS="acronym" >BIND</SPAN > is the ability to run the daemon as a nonprivileged user ( <TT CLASS="option" >-u</TT > <TT CLASS="replaceable" ><I >user</I ></TT > ). We suggest running as a nonprivileged user when using the <B CLASS="command" >chroot</B > feature.</P ><P >Here is an example command line to load <SPAN CLASS="acronym" >BIND</SPAN > in a <B CLASS="command" >chroot()</B > sandbox, <B CLASS="command" >/var/named</B >, and to run <B CLASS="command" >named</B > <B CLASS="command" >setuid</B > to user 202:</P ><P ><TT CLASS="userinput" ><B >/usr/local/bin/named -u 202 -t /var/named</B ></TT ></P ><DIV CLASS="sect2" ><H2 CLASS="sect2" ><A NAME="AEN4241" >7.2.1. The <B CLASS="command" >chroot</B > Environment</A ></H2 ><P >In order for a <B CLASS="command" >chroot()</B > environment to work properly in a particular directory (for example, <TT CLASS="filename" >/var/named</TT >), you will need to set up an environment that includes everything <SPAN CLASS="acronym" >BIND</SPAN > needs to run. From <SPAN CLASS="acronym" >BIND</SPAN >'s point of view, <TT CLASS="filename" >/var/named</TT > is the root of the filesystem. You will need to adjust the values of options like like <B CLASS="command" >directory</B > and <B CLASS="command" >pid-file</B > to account for this. </P ><P > Unlike with earlier versions of BIND, you will typically <SPAN CLASS="emphasis" ><I CLASS="emphasis" >not</I ></SPAN > need to compile <B CLASS="command" >named</B > statically nor install shared libraries under the new root. However, depending on your operating system, you may need to set up things like <TT CLASS="filename" >/dev/zero</TT >, <TT CLASS="filename" >/dev/random</TT >, <TT CLASS="filename" >/dev/log</TT >, and/or <TT CLASS="filename" >/etc/localtime</TT >. </P ></DIV ><DIV CLASS="sect2" ><H2 CLASS="sect2" ><A NAME="AEN4259" >7.2.2. Using the <B CLASS="command" >setuid</B > Function</A ></H2 ><P >Prior to running the <B CLASS="command" >named</B > daemon, use the <B CLASS="command" >touch</B > utility (to change file access and modification times) or the <B CLASS="command" >chown</B > utility (to set the user id and/or group id) on files to which you want <SPAN CLASS="acronym" >BIND</SPAN > to write. Note that if the <B CLASS="command" >named</B > daemon is running as a nonprivileged user, it will not be able to bind to new restricted ports if the server is reloaded.</P ></DIV ></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="dynamic_update_security" >7.3. Dynamic Update Security</A ></H1 ><P >Access to the dynamic update facility should be strictly limited. In earlier versions of <SPAN CLASS="acronym" >BIND</SPAN > 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 <B CLASS="command" >allow-update</B > 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 <B CLASS="command" >allow-update</B > 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 <B CLASS="command" >allow-update</B > option should list only TSIG key names, not IP addresses or network prefixes. Alternatively, the new <B CLASS="command" >update-policy</B > 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 ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="Bv9ARM.ch06.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="Bv9ARM.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="Bv9ARM.ch08.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><SPAN CLASS="acronym" >BIND</SPAN > 9 Configuration Reference</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Troubleshooting</TD ></TR ></TABLE ></DIV ></BODY ></HTML >