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308 lines
9.4 KiB
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<title>Host Access Control</title>
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alink="#0000FF">
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<table summary="Header navigation table" width="100%" border="0" cellpadding="0"
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cellspacing="0">
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<tr>
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<th colspan="3" align="center">Slackware Linux Essentials</th>
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</tr>
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<tr>
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<td width="10%" align="left" valign="bottom"><a href="security.html"
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accesskey="P">Prev</a></td>
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<td width="80%" align="center" valign="bottom">Chapter 14 Security</td>
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<td width="10%" align="right" valign="bottom"><a href="security-current.html"
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accesskey="N">Next</a></td>
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</tr>
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</table>
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<hr align="LEFT" width="100%" />
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</div>
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<div class="SECT1">
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<h1 class="SECT1"><a id="SECURITY-HOST" name="SECURITY-HOST">14.2 Host Access
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Control</a></h1>
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<div class="SECT2">
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<h2 class="SECT2"><a id="SECURITY-HOST-IPTABLES" name="SECURITY-HOST-IPTABLES">14.2.1 <tt
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class="COMMAND">iptables</tt></a></h2>
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<p><tt class="COMMAND">iptables</tt> is the packet filtering configuration program for
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Linux 2.4 and above. The 2.4 kernel (2.4.5, to be exact) was first introduced into
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Slackware (as an option) in version 8.0 and was made the default in Slackware 8.1. This
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section only covers the basics of its usage and you should check <a
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href="http://www.netfilter.org/" target="_top">http://www.netfilter.org/</a> for more
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details. These commands can be entered into <tt
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class="FILENAME">/etc/rc.d/rc.firewall</tt>, which has to be set as executable for these
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rules to take effect at startup. Note that incorrect <tt class="COMMAND">iptables</tt>
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commands can essentially lock you out of your own machine. Unless you are 100% confident
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in your skills, always ensure you have local access to the machine.</p>
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<p>The first thing most people should do is set the default policy for each inbound chain
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to DROP:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd class="USERINPUT">iptables -P INPUT DROP</kbd>
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<samp class="PROMPT">#</samp> <kbd class="USERINPUT">iptables -P FORWARD DROP</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>When everything is denied, you can start allowing things. The first thing to allow is
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any traffic for sessions which are already established:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>So as not to break any applications that communicate using the loopback address, it is
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usually wise to add a rule like this:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -A INPUT -s 127.0.0.0/8 -d 127.0.0.0/8 -i lo -j ACCEPT</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>This rules allows any traffic to and from 127.0.0.0/8 (127.0.0.0 - 127.255.255.255) on
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the loopback (<tt class="FILENAME">lo</tt>) interface. When creating rules, it is a good
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idea to be as specific as possible, to make sure that your rules do not inadvertently
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allow anything evil. That said, rules that allow too little mean more rules and more
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typing.</p>
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<p>The next thing to do would be to allow access to specific services running on your
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machine. If, for example, you wanted to run a web server on your machine, you would use a
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rule similar to this:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -A INPUT -p tcp --dport 80 -i ppp0 -j ACCEPT</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>This will allow access from any machine to port 80 on your machine via the <tt
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class="FILENAME">ppp0</tt> interface. You may want to restrict access to this service so
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that only certain machines can access it. This rule allows access to your web service
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from <tt class="HOSTID">64.57.102.34</tt>:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -A INPUT -p tcp -s 64.57.102.34 --dport 80 -i ppp0 -j ACCEPT</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>Allowing ICMP traffic can be useful for diagnostic purposes. To do this, you would use
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a rule like this:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -A INPUT -p icmp -j ACCEPT</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>Most people will also want to set up Network Address Translation (NAT) on their
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gateway machine, so that other machines on their network can access the Internet through
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it. You would use the following rule to do this:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>You will also need to enable IP forwarding. You can do this temporarily, using the
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following command:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="SCREEN">
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<samp class="PROMPT">#</samp> <kbd
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class="USERINPUT">echo 1 > /proc/sys/net/ipv4/ip_forward</kbd>
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</pre>
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</td>
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</tr>
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</table>
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<p>To enable IP forwarding on a more permanent basis (i.e. so that the change is kept
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after a reboot), you will need to open the file <tt
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class="FILENAME">/etc/rc.d/rc.inet2</tt> in your favorite editor and change the following
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line:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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IPV4_FORWARD=0
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</pre>
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</td>
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</tr>
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</table>
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<p>...to this:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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IPV4_FORWARD=1
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</pre>
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</td>
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</tr>
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</table>
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<p>For more information on NAT, see the <a
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href="http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO.txt" target="_top">NAT
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HOWTO</a>.</p>
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</div>
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<div class="SECT2">
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<h2 class="SECT2"><a id="SECURITY-HOST-TCPWRAPPERS"
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name="SECURITY-HOST-TCPWRAPPERS">14.2.2 <tt class="COMMAND">tcpwrappers</tt></a></h2>
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<p><tt class="COMMAND">tcpwrappers</tt> controls access to daemons at the application
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level, rather than at the IP level. This can provide an extra layer of security at times
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when IP-level access controls (e.g. Netfilter) are not functioning correctly. For
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example, if you recompile the kernel but forget to include iptables support, your IP
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level protection will fail but tcpwrappers will still help protect your system.</p>
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<p>Access to services protected by tcpwrappers can be controlled using <tt
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class="FILENAME">/etc/hosts.allow</tt> and <tt class="FILENAME">/etc/hosts.deny</tt>.</p>
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<p>The majority of people would have a single line in their <tt
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class="FILENAME">/etc/hosts.deny</tt> file to deny access to all daemons by default. This
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line would be:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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ALL : ALL
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</pre>
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</td>
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</tr>
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</table>
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<p>When this is done, you can concentrate on allowing access to services for specified
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hosts, domains, or IP ranges. This can be done in the <tt
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class="FILENAME">/etc/hosts.allow</tt> file, which follows the same format.</p>
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<p>A lot of people would start by accepting all connections from <tt
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class="HOSTID">localhost</tt>. This can be achieved using:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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ALL : 127.0.0.1
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</pre>
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</td>
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</tr>
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</table>
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<p>To allow access to SSHd from <tt class="HOSTID">192.168.0.0/24</tt>, you could use
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either of the following rules:</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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sshd : 192.168.0.0/24
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sshd : 192.168.0.
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</pre>
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</td>
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</tr>
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</table>
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<p>It is also possible to restrict access to hosts in certain domains. This can be done
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using the following rule (note that this relies on the reverse DNS entry for the
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connecting host being trustworthy, so I would recommand against its use on
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Internet-connected hosts):</p>
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<table border="0" bgcolor="#E0E0E0" width="100%">
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<tr>
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<td>
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<pre class="PROGRAMLISTING">
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sshd : .slackware.com
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</pre>
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</td>
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</tr>
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</table>
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</div>
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</div>
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<div class="NAVFOOTER">
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<hr align="LEFT" width="100%" />
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<table summary="Footer navigation table" width="100%" border="0" cellpadding="0"
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cellspacing="0">
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<tr>
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<td width="33%" align="left" valign="top"><a href="security.html"
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accesskey="P">Prev</a></td>
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<td width="34%" align="center" valign="top"><a href="index.html"
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accesskey="H">Home</a></td>
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<td width="33%" align="right" valign="top"><a href="security-current.html"
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accesskey="N">Next</a></td>
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</tr>
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<tr>
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<td width="33%" align="left" valign="top">Security</td>
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<td width="34%" align="center" valign="top"><a href="security.html"
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accesskey="U">Up</a></td>
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<td width="33%" align="right" valign="top">Keeping Current</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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