<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> <article id="standalone"> <!--$Id$--> <articleinfo> <title>Standalone Firewall</title> <authorgroup> <author> <firstname>Tom</firstname> <surname>Eastep</surname> </author> </authorgroup> <pubdate>2004-01-06</pubdate> <copyright> <year>2002-2004</year> <holder>Thomas M. Eastep</holder> </copyright> <legalnotice> <para>Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled <quote><ulink url="GnuCopyright.htm">GNU Free Documentation License</ulink></quote>.</para> </legalnotice> </articleinfo> <section> <title>Introduction</title> <para>Setting up Shorewall on a standalone Linux system is very easy if you understand the basics and follow the documentation.</para> <para>This guide doesn't attempt to acquaint you with all of the features of Shorewall. It rather focuses on what is required to configure Shorewall in one of its most common configurations:</para> <itemizedlist> <listitem> <para>Linux system</para> </listitem> <listitem> <para>Single external IP address</para> </listitem> <listitem> <para>Connection through Cable Modem, DSL, ISDN, Frame Relay, dial-up...</para> </listitem> </itemizedlist> <section> <title>Requirements</title> <para>Shorewall requires that you have the iproute/iproute2 package installed (on RedHat, the package is called <emphasis>iproute</emphasis>). You can tell if this package is installed by the presence of an <emphasis role="bold">ip</emphasis> program on your firewall system. As root, you can use the <quote>which</quote> command to check for this program:</para> <programlisting>[root@gateway root]# which ip /sbin/ip [root@gateway root]#</programlisting> </section> <section> <title>Before you start</title> <para>I recommend that you read through the guide first to familiarize yourself with what's involved then go back through it again making your configuration changes.</para> <caution> <para>If you edit your configuration files on a Windows system, you must save them as Unix files if your editor supports that option or you must run them through dos2unix before trying to use them. Similarly, if you copy a configuration file from your Windows hard drive to a floppy disk, you must run dos2unix against the copy before using it with Shorewall.</para> <simplelist> <member><ulink url="http://www.simtel.net/pub/pd/51438.html">Windows Version of dos2unix</ulink></member> <member><ulink url="http://www.megaloman.com/~hany/software/hd2u/">Linux Version of dos2unix</ulink></member> </simplelist> </caution> </section> <section> <title>Conventions</title> <para>Points at which configuration changes are recommended are flagged with <inlinegraphic fileref="images/BD21298_.gif" format="GIF" />.</para> </section> </section> <section> <title>PPTP/ADSL</title> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>If you have an ADSL Modem and you use PPTP to communicate with a server in that modem, you must make the <ulink url="PPTP.htm#PPTP_ADSL">changes recommended here</ulink> <emphasis role="underline">in addition to those described in the steps below</emphasis>. ADSL with PPTP is most commonly found in Europe, notably in Austria.</para> </section> <section> <title>Shorewall Concepts</title> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>The configuration files for Shorewall are contained in the directory <filename class="directory">/etc/shorewall</filename> -- for simple setups, you only need to deal with a few of these as described in this guide. After you have <ulink url="Install.htm">installed Shorewall</ulink>, <emphasis role="bold">download the <ulink url="http://www1.shorewall.net/pub/shorewall/Samples/">one-interface sample</ulink>, un-tar it (tar -zxvf one-interface.tgz) and and copy the files to /etc/shorewall (they will replace files with the same names that were placed in /etc/shorewall during Shorewall installation)</emphasis>.</para> <para>As each file is introduced, I suggest that you look through the actual file on your system -- each file contains detailed configuration instructions and default entries.</para> <para>Shorewall views the network where it is running as being composed of a set of <emphasis>zones</emphasis>. In the one-interface sample configuration, only one zone is defined:</para> <informaltable> <tgroup cols="2"> <thead> <row> <entry align="center">Name</entry> <entry align="center">Description</entry> </row> </thead> <tbody> <row> <entry>net</entry> <entry>The Internet</entry> </row> </tbody> </tgroup> </informaltable> <para>Shorewall zones are defined in <ulink url="Documentation.htm#Zones"><filename>/etc/shorewall/zones</filename></ulink>.</para> <para>Shorewall also recognizes the firewall system as its own zone - by default, the firewall itself is known as <emphasis role="bold">fw</emphasis>.</para> <para>Rules about what traffic to allow and what traffic to deny are expressed in terms of zones.</para> <itemizedlist> <listitem> <para>You express your default policy for connections from one zone to another zone in the <ulink url="Documentation.htm#Policy"><filename>/etc/shorewall/policy</filename></ulink> file.</para> </listitem> <listitem> <para>You define exceptions to those default policies in the <ulink url="Documentation.htm#Rules"><filename>/etc/shorewall/rules</filename></ulink> file.</para> </listitem> </itemizedlist> <para>For each connection request entering the firewall, the request is first checked against the <filename><filename>/etc/shorewall/rules</filename></filename> file. If no rule in that file matches the connection request then the first policy in <filename>/etc/shorewall/policy</filename> that matches the request is applied. If that policy is REJECT or DROP the request is first checked against the rules in /etc/shorewall/common if that file exists; otherwise the rules in <filename>/etc/shorewall/common.def</filename> are checked.</para> <para>The <filename>/etc/shorewall/policy</filename> file included with the one-interface sample has the following policies:</para> <programlisting>#SOURCE ZONE DESTINATION ZONE POLICY LOG LEVEL LIMIT:BURST fw net ACCEPT net all DROP info all all REJECT info</programlisting> <para>The above policy will:</para> <orderedlist> <listitem> <para>allow all connection requests from the firewall to the internet</para> </listitem> <listitem> <para>drop (ignore) all connection requests from the internet to your firewall</para> </listitem> <listitem> <para>reject all other connection requests (Shorewall requires this catchall policy).</para> </listitem> </orderedlist> <para>At this point, edit your <filename>/etc/shorewall/policy</filename> and make any changes that you wish.</para> </section> <section> <title>External Interface</title> <para>The firewall has a single network interface. Where Internet connectivity is through a cable or DSL <quote>Modem</quote>, the <emphasis>External Interface</emphasis> will be the ethernet adapter (<emphasis role="bold">eth0</emphasis>) that is connected to that <quote>Modem</quote> <emphasis role="underline">unless</emphasis> you connect via <emphasis>Point-to-Point Protocol over Ethernet</emphasis> (PPPoE) or <emphasis>Point-to-Point Tunneling Protocol</emphasis> (PPTP) in which case the External Interface will be a <emphasis role="bold">ppp0</emphasis>. If you connect via a regular modem, your External Interface will also be <emphasis role="bold">ppp0</emphasis>. If you connect using ISDN, your external interface will be <emphasis role="bold">ippp0</emphasis>.</para> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>The Shorewall one-interface sample configuration assumes that the external interface is <emphasis role="bold">eth0</emphasis>. If your configuration is different, you will have to modify the sample /etc/shorewall/interfaces file accordingly. While you are there, you may wish to review the list of options that are specified for the interface. Some hints:</para> <tip> <para>If your external interface is <emphasis role="bold">ppp0</emphasis> or <emphasis role="bold">ippp0</emphasis>, you can replace the <quote>detect</quote> in the second column with <quote>-</quote>.</para> </tip> <tip> <para>If your external interface is <emphasis role="bold">ppp0</emphasis> or <emphasis role="bold">ippp0</emphasis> or if you have a static IP address, you can remove <quote>dhcp</quote> from the option list.</para> </tip> <tip> <para>If you specify <emphasis>norfc1918</emphasis> for your external interface, you will want to check the <ulink url="errata.htm">Shorewall Errata</ulink> periodically for updates to the <filename>/etc/shorewall/rfc1918 file</filename>. Alternatively, you can <ulink url="myfiles.htm#RFC1918">strip down your <filename>/etc/shorewall/rfc1918</filename> file as I do</ulink>.</para> </tip> </section> <section> <title>IP Addresses</title> <para>RFC 1918 reserves several <emphasis>Private</emphasis> IP address ranges for use in private networks:</para> <programlisting>10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255</programlisting> <para>These addresses are sometimes referred to as <emphasis>non-routable</emphasis> because the Internet backbone routers will not forward a packet whose destination address is reserved by RFC 1918. In some cases though, ISPs are assigning these addresses then using <emphasis>Network Address Translation</emphasis> to rewrite packet headers when forwarding to/from the internet.</para> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>Before starting Shorewall, you should look at the IP address of your external interface and if it is one of the above ranges, you should remove the <quote>norfc1918</quote> option from the entry in <filename>/etc/shorewall/interfaces</filename>.</para> </section> <section> <title>Enabling other Connections</title> <para>If you wish to enable connections from the internet to your firewall, the general format of a rule in <filename>/etc/shorewall/rules</filename> is:</para> <programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) ACCEPT net fw <emphasis><protocol></emphasis> <emphasis><port></emphasis></programlisting> <example> <title>You want to run a Web Server and a POP3 Server on your firewall system:</title> <para><programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) ACCEPT net fw tcp 80 ACCEPT net fw tcp 110</programlisting></para> </example> <para>If you don't know what port and protocol a particular application uses, see <ulink url="ports.htm">here</ulink>.</para> <important> <para>I don't recommend enabling telnet to/from the internet because it uses clear text (even for login!). If you want shell access to your firewall from the internet, use SSH:</para> <programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) ACCEPT net fw tcp 22</programlisting> </important> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>At this point, edit /etc/shorewall/rules to add other connections as desired.</para> </section> <section> <title>Starting and Stopping Your Firewall</title> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF" /></para> <para>The <ulink url="Install.htm">installation procedure</ulink> configures your system to start Shorewall at system boot but beginning with Shorewall version 1.3.9 startup is disabled so that your system won't try to start Shorewall before configuration is complete. Once you have completed configuration of your firewall, you can enable Shorewall startup by removing the file <filename>/etc/shorewall/startup_disabled</filename>.</para> <important> <para><emphasis role="bold">Users of the .deb package must edit <filename>/etc/default/shorewall</filename> and set <quote>startup=1</quote>.</emphasis></para> </important> <para>The firewall is started using the <quote><command>shorewall start</command></quote> command and stopped using <quote><command>shorewall stop</command></quote>. When the firewall is stopped, routing is enabled on those hosts that have an entry in <filename><ulink url="Documentation.htm#Routestopped">/etc/shorewall/routestopped</ulink></filename>. A running firewall may be restarted using the <quote><command>shorewall restart</command></quote> command. If you want to totally remove any trace of Shorewall from your Netfilter configuration, use <quote><command>shorewall clear</command></quote>.</para> <warning> <para>If you are connected to your firewall from the internet, do not issue a <quote><command>shorewall stop</command></quote> command unless you have added an entry for the IP address that you are connected from to <ulink url="Documentation.htm#Routestopped"><filename>/etc/shorewall/routestopped</filename></ulink>. Also, I don't recommend using <quote><command>shorewall restart</command></quote>; it is better to create an <emphasis><ulink url="configuration_file_basics.htm#Configs">alternate configuration</ulink></emphasis> and test it using the <ulink url="starting_and_stopping_shorewall.htm"><quote><command>shorewall try</command></quote> command</ulink>.</para> </warning> </section> <section> <title>Additional Recommended Reading</title> <para>I highly recommend that you review the <ulink url="configuration_file_basics.htm">Common Configuration File Features page</ulink> -- it contains helpful tips about Shorewall features than make administering your firewall easier.</para> </section> <appendix> <title>Revision History</title> <para><revhistory><revision><revnumber>1.5</revnumber><date>2003-01-05</date><authorinitials>TE</authorinitials><revremark>Standards Changes</revremark></revision><revision><revnumber>1.4</revnumber><date>2003-12-30</date><authorinitials>TE</authorinitials><revremark>Add tip about /etc/shorewall/rfc1918 updates.</revremark></revision><revision><revnumber>1.3</revnumber><date>2003-11-15</date><authorinitials>TE</authorinitials><revremark>Initial Docbook Conversion</revremark></revision></revhistory></para> </appendix> </article>