<?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><?dbtimestamp format="Y/m/d"?></pubdate> <copyright> <year>2002-2009</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> <caution> <para><emphasis role="bold">This article applies to Shorewall 4.4 and later. If you are running a version of Shorewall earlier than Shorewall 4.4.0 then please see the documentation for that release.</emphasis></para> </caution> <section id="Introduction"> <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 <acronym>IP</acronym> address</para> </listitem> <listitem> <para>Connection through Cable Modem, <acronym>DSL</acronym>, <acronym>ISDN</acronym>, Frame Relay, dial-up... or connected to a <acronym>LAN</acronym> and you simply wish to protect your Linux system from other systems on that <acronym>LAN</acronym>.</para> </listitem> </itemizedlist> <section id="System"> <title>System Requirements</title> <para>Shorewall requires that you have the <command>iproute</command>/<command>iproute2</command> package installed (on<trademark> RedHat</trademark>, the package is called <command>iproute</command>). You can tell if this package is installed by the presence of an <command>ip</command> program on your firewall system. As root, you can use the <command>which</command> command to check for this program:</para> <programlisting>[root@gateway root]# <command>which ip</command> /sbin/ip [root@gateway root]#</programlisting> </section> <section id="Before"> <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 <trademark>Windows</trademark> system, you must save them as <trademark>Unix</trademark> files if your editor supports that option or you must run them through <command>dos2unix</command> before trying to use them. Similarly, if you copy a configuration file from your <trademark>Windows</trademark> hard drive to a floppy disk, you must run <command>dos2unix</command> against the copy before using it with Shorewall. <itemizedlist> <listitem> <para><ulink url="http://www.simtel.net/pub/pd/51438.html"><trademark>Windows</trademark> Version of <command>dos2unix</command></ulink></para> </listitem> <listitem> <para><ulink url="http://www.megaloman.com/%7Ehany/software/hd2u/">Linux Version of <command>dos2unix</command></ulink></para> </listitem> </itemizedlist></para> </caution> </section> <section id="Conventions"> <title>Conventions</title> <para>Points at which configuration changes are recommended are flagged with <inlinegraphic fileref="images/BD21298_.gif" format="GIF"/>.</para> <para>Configuration notes that are unique to Debian and it's derivatives are marked with <inlinegraphic fileref="images/openlogo-nd-25.png" format="GIF"/>.</para> </section> </section> <section id="PPTP"> <title>PPTP/ADSL</title> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF"/></para> <para>If you have an <acronym>ADSL</acronym> Modem and you use <acronym>PPTP</acronym> to communicate with a server in that modem, you must make the changes recommended <ulink url="PPTP.htm#PPTP_ADSL">here</ulink> in addition to those detailed below. <acronym>ADSL</acronym> with <acronym>PPTP</acronym> is most commonly found in Europe, notably in Austria.</para> </section> <section id="Concepts"> <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>, you can find the Samples as follows:</para> <orderedlist> <listitem> <para>If you installed using an <acronym>RPM</acronym>, the samples will be in the <filename class="directory">Samples/one-interface</filename> subdirectory of the Shorewall documentation directory. If you don't know where the Shorewall documentation directory is, you can find the samples using this command:</para> <programlisting>~# rpm -ql shorewall | fgrep one-interface /usr/share/doc/packages/shorewall/Samples/one-interface /usr/share/doc/packages/shorewall/Samples/one-interface/interfaces /usr/share/doc/packages/shorewall/Samples/one-interface/policy /usr/share/doc/packages/shorewall/Samples/one-interface/rules /usr/share/doc/packages/shorewall/Samples/one-interface/zones ~#</programlisting> </listitem> <listitem> <para>If you installed using the tarball, the samples are in the <filename class="directory">Samples/one-interface</filename> directory in the tarball.</para> </listitem> <listitem> <para><graphic align="left" fileref="images/openlogo-nd-25.png"/>If you installed using a Shorewall 4.x .deb, the samples are in <emphasis role="bold"><filename class="directory">/usr/share/doc/shorewall/examples/one-interface</filename>..</emphasis> You do not need the shorewall-doc package to have access to the samples.</para> </listitem> </orderedlist> <warning> <para><emphasis role="bold">Note to Debian Users</emphasis></para> <para>You will find that your <filename class="directory">/etc/shorewall</filename> directory is empty. This is intentional. If you need configuration files other than those found in <emphasis role="bold"><filename class="directory">/usr/share/doc/shorewall/examples/one-interface</filename>, </emphasis> they may be found on your system in the directory <filename class="directory">/usr/share/doc/shorewall/default-config</filename>. Simply copy the files you need from that directory to <filename class="directory">/etc/shorewall</filename> and modify the copies.</para> </warning> <para>As each file is introduced, I suggest that you look at the actual file on your system and that you look at the <ulink url="configuration_file_basics.htm#Manpages">man page</ulink> for that file. For example, to look at the man page for the <filename>/etc/shorewall/zones</filename> file, type <command>man shorewall-zones</command> at a shell prompt.</para> <para>Note: Beginning with Shorewall 4.4.20.1, there are versions of the sample files that are annotated with the corresponding manpage contents. These files have names ending in '.annotated'. You might choose to look at those files instead.</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 two zones are defined:</para> <programlisting>#ZONE TYPE OPTIONS IN OUT # OPTIONS OPTIONS fw firewall net ipv4</programlisting> <para>Shorewall zones are defined in <ulink url="manpages/shorewall-zones.html"><filename>/etc/shorewall/zones</filename></ulink>.</para> <para>Note that Shorewall recognizes the firewall system as its own zone. When the <filename>/etc/shorewall/zones</filename> file is processed, the name of the firewall zone (<quote>fw</quote> in the above example) is stored in the shell variable <firstterm>$FW</firstterm> which may be used to refer to the firewall zone throughout the Shorewall configuration.</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="manpages/shorewall-policy.html"><filename>/etc/shorewall/policy</filename></ulink> file.</para> </listitem> <listitem> <para>You define exceptions to those default policies in the <ulink url="manpages/shorewall-rules.html"><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 there is a <ulink url="shorewall_extension_scripts.htm">common action</ulink> defined for the policy in <filename>/etc/shorewall/actions</filename> or <filename>/usr/share/shorewall/actions.std</filename> then that action is performed before the policy is applied. The purpose of the common action is two-fold:</para> <itemizedlist> <listitem> <para>It silently drops or rejects harmless common traffic that would otherwise clutter up your log — Broadcasts for example.</para> </listitem> <listitem> <para>If ensures that traffic critical to correct operation is allowed through the firewall — ICMP <emphasis>fragmentation-needed</emphasis> for example.</para> </listitem> </itemizedlist> <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>The word <firstterm>info</firstterm> in the LOG LEVEL column for the last two policies indicates that packets dropped or rejected under those policies should be <ulink url="shorewall_logging.html">logged at that level</ulink>.</para> <para>At this point, edit your <filename>/etc/shorewall/policy</filename> and make any changes that you wish.</para> </section> <section id="External"> <title>External Interface</title> <para>The firewall has a single network interface. Where Internet connectivity is through a cable or <acronym>DSL</acronym> <quote>Modem</quote>, the <emphasis>External Interface</emphasis> will be the Ethernet adapter (<filename class="devicefile">eth0</filename>) that is connected to that <quote>Modem</quote> <emphasis role="underline">unless</emphasis> you connect via <emphasis>Point-to-Point Protocol over Ethernet</emphasis> (<acronym>PPPoE</acronym>) or <emphasis>Point-to-Point Tunneling Protocol</emphasis> (<acronym>PPTP</acronym>) in which case the External Interface will be a <acronym>PPP</acronym> interface (e.g., <filename class="devicefile">ppp0</filename>). If you connect via a regular modem, your External Interface will also be <filename class="devicefile">ppp0</filename>. If you connect using <acronym>ISDN</acronym>, your external interface will be <filename class="devicefile">ippp0</filename>.</para> <caution> <para>Be sure you know which interface is your external interface. Many hours have been spent floundering by users who have configured the wrong interface. If you are unsure, then as root type <command>ip route ls</command> at the command line. The device listed in the last (default) route should be your external interface.</para> <para>Example:</para> <programlisting>root@lists:~# ip route ls 192.168.2.2 dev tun0 proto kernel scope link src 192.168.2.1 10.13.10.0/24 dev tun1 scope link 192.168.2.0/24 via 192.168.2.2 dev tun0 206.124.146.0/24 dev eth0 proto kernel scope link src 206.124.146.176 10.10.10.0/24 dev tun1 scope link default via 206.124.146.254 dev <emphasis role="bold">eth0</emphasis> root@lists:~# </programlisting> <para>In that example, <filename class="devicefile">eth0</filename> is the external interface.</para> </caution> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF"/></para> <para>The Shorewall one-interface sample configuration assumes that the external interface is <filename class="devicefile">eth0</filename>. If your configuration is different, you will have to modify the sample <filename>/etc/shorewall/interfaces</filename> 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 <filename class="devicefile">ppp0</filename> or <filename class="devicefile">ippp0</filename>, you can replace the <quote>detect</quote> in the second column with <quote>-</quote> (minus the quotes).</para> </tip> <tip> <para>If your external interface is <filename class="devicefile">ppp0</filename> or <filename class="devicefile">ippp0</filename> or if you have a static IP address, you can remove <quote>dhcp</quote> from the option list.</para> </tip> </section> <section id="Addresses"> <title>IP Addresses</title> <para>Before going further, we should say a few words about <emphasis>Internet Protocol</emphasis> (<acronym>IP</acronym>) addresses. Normally, your <emphasis>Internet Service Provider</emphasis> (<acronym>ISP</acronym>) will assign you a single <acronym>IP</acronym> address. That address can be assigned statically, by the <emphasis>Dynamic Host Configuration Protocol</emphasis> (<acronym>DHCP</acronym>), through the establishment of your dial-up connection, or during establishment of your other type of <acronym>PPP</acronym> (<acronym>PPPoA</acronym>, <acronym>PPPoE</acronym>, etc.) connection.</para> <para><emphasis role="bold">RFC-1918</emphasis> reserves several <emphasis>Private</emphasis> <acronym>IP</acronym> 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 <emphasis role="bold">RFC-1918</emphasis>. In some cases though, <acronym>ISP</acronym>s are assigning these addresses then using <emphasis>Network Address Translation</emphasis> <emphasis>- </emphasis><acronym>NAT</acronym>) to rewrite packet headers when forwarding to/from the Internet.</para> </section> <section id="Logging"> <title>Logging</title> <para>Shorewall does not maintain a log itself but rather relies on your <ulink url="shorewall_logging.html">system's logging configuration</ulink>. The following <ulink url="manpages/shorewall.html">commands</ulink> rely on knowing where Netfilter messages are logged:</para> <itemizedlist> <listitem> <para><command>shorewall show log</command> (Displays the last 20 Netfilter log messages)</para> </listitem> <listitem> <para><command>shorewall logwatch</command> (Polls the log at a settable interval</para> </listitem> <listitem> <para><command>shorewall dump</command> (Produces an extensive report for inclusion in Shorewall problem reports)</para> </listitem> </itemizedlist> <para>It is important that these commands work properly because when you encounter connection problems when Shorewall is running, the first thing that you should do is to look at the Netfilter log; with the help of <ulink url="FAQ.htm#faq17">Shorewall FAQ 17</ulink>, you can usually resolve the problem quickly.</para> <para>The Netfilter log location is distribution-dependent:</para> <itemizedlist> <listitem> <para>Debian and its derivatives log Netfilter messages to <filename>/var/log/kern.log</filename>.</para> </listitem> <listitem> <para>Recent <trademark>SuSE/OpenSuSE</trademark> releases come preconfigured with syslog-ng and log netfilter messages to <filename>/var/log/firewall</filename>.</para> </listitem> <listitem> <para>For other distributions, Netfilter messages are most commonly logged to <filename>/var/log/messages</filename>.</para> </listitem> </itemizedlist> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF"/></para> <para>If you are running a distribution that logs Netfilter messages to a log other than <filename>/var/log/messages</filename>, then modify the LOGFILE setting in <filename>/etc/shorewall/shorewall.conf</filename> to specify the name of your log.</para> <important> <para>The LOGFILE setting does not control where the Netfilter log is maintained -- it simply tells the /sbin/<filename>shorewall</filename> utility where to find the log.</para> </important> </section> <section id="Modules"> <title>Kernel Module Loading</title> <para>Beginning in Shorewall 4.4.7, <filename>/etc/shorewall/shorewall.conf</filename> contains a LOAD_HELPERS_ONLY option which is set to <option>Yes</option> in the samples. This causes Shorewall to attempt to load the modules listed in <filename>/usr/share/shorewall/helpers</filename>. In addition, it sets <emphasis role="bold">sip_direct_media=0</emphasis> when loading the nf_conntrack_sip module. That setting is somewhat less secure than <emphasis role="bold">sip_direct_media=1</emphasis>, but it generally makes VOIP through the firewall work much better.</para> <para>The modules in <filename>/usr/share/shorewall/helpers</filename> are those that are not autoloaded. If your kernel does not support module autoloading and you want Shorewall to attempt to load all netfilter modules that it might require, then set LOAD_HELPERS_ONLY=No. That will cause Shorewall to try to load the modules listed in <filename>/usr/share/shorewall/modules</filename>. That file does not set <emphasis role="bold">sip_direct_media=0</emphasis>.</para> <para>If you need to modify either <filename>/usr/share/shorewall/helpers</filename> or <filename>/usr/share/shorewall/modules</filename> then copy the file to <filename>/etc/shorewall</filename> and modify the copy.</para> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF"/></para> <para>Modify the setting of LOAD_HELPER_ONLY as necessary.</para> </section> <section id="Open"> <title>Enabling other Connections</title> <para>Shorewall includes a collection of macros that can be used to quickly allow or deny services. You can find a list of the macros included in your version of Shorewall using the command <command>ls <filename>/usr/share/shorewall/macro.*</filename></command>.</para> <para>If you wish to enable connections from the Internet to your firewall and you find an appropriate macro in <filename>/etc/shorewall/macro.*</filename>, the general format of a rule in <filename>/etc/shorewall/rules</filename> is:</para> <programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) <<emphasis>macro</emphasis>>(ACCEPT) net $FW</programlisting> <important> <para>Be sure to add your rules after the line that reads <emphasis role="bold">SECTION NEW.</emphasis></para> </important> <example id="Example1"> <title>You want to run a Web Server and a IMAP Server on your firewall system:</title> <programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) Web(ACCEPT) net $FW IMAP(ACCEPT)net $FW</programlisting> </example> <para>You may also choose to code your rules directly without using the pre-defined macros. This will be necessary in the event that there is not a pre-defined macro that meets your requirements. In that case 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 id="Example2"> <title>You want to run a Web Server and a IMAP Server on your firewall system:</title> <para><programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) ACCEPT net $FW tcp 80 ACCEPT net $FW tcp 143</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 <acronym>SSH</acronym>:</para> <programlisting>#ACTION SOURCE DESTINATION PROTO DEST PORT(S) SSH(ACCEPT) net $FW </programlisting> </important> <para><inlinegraphic fileref="images/BD21298_.gif" format="GIF"/></para> <para>At this point, edit <filename>/etc/shorewall/rules</filename> to add other connections as desired.</para> </section> <section id="Starting"> <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 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 must edit /etc/shorewall/shorewall.conf and set STARTUP_ENABLED=Yes.<graphic align="left" fileref="images/openlogo-nd-25.png"/></para> <important> <para>Users of the .deb package must edit <filename>/etc/default/shorewall</filename> and set <varname>startup=1.</varname></para> </important> <important> <para>You must enable startup by editing <filename>/etc/shorewall/shorewall.conf</filename> and setting <varname>STARTUP_ENABLED=Yes.</varname></para> </important> <para>While you are editing <filename>shorewall.conf</filename>, it is a good idea to check the value of the SUBSYSLOCK option. You can find a description of this option by typing 'man shorewall.conf' at a shell prompt and searching for SUBSYSLOCK.</para> <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="manpages/shorewall-routestopped.html">/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="manpages/shorewall-routestopped.html"><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></ulink> command.</para> </warning> <para>The firewall will start after your network interface has been brought up. This leaves a small window between the time that the network interface is working and when the firewall is controlling connections through that interface. If this is a concern, you can close that window by installing the <ulink url="Shorewall-init.html">Shorewall Init Package</ulink>.</para> </section> <section id="Problems"> <title>If it Doesn't Work</title> <itemizedlist> <listitem> <para>Re-check each of the items flagged with a red arrow above.</para> </listitem> <listitem> <para>Check your <ulink url="shorewall_logging.html">log</ulink>.</para> </listitem> <listitem> <para>Check the <ulink url="troubleshoot.htm">Troubleshooting Guide</ulink>.</para> </listitem> <listitem> <para>Check the <ulink url="FAQ.htm">FAQ</ulink>.</para> </listitem> </itemizedlist> </section> <section> <title>Disabling your existing Firewall</title> <para>Before starting Shorewall for the first time, it's a good idea to stop your existing firewall. On older Redhat/CentOS/Fedora:</para> <programlisting><command>service iptables stop</command></programlisting> <para>On recent Fedora systems that run systemd, the command is:</para> <programlisting><command>systemctl stop iptables.service</command></programlisting> <para>If you are running SuSE, use Yast or Yast2 to stop SuSEFirewall.</para> <para>On other systems that use a classic SysV init system:</para> <programlisting><command>/etc/init.d/iptables stop</command></programlisting> <para>Once you have Shorewall running to your satisfaction, you should totally disable your existing firewall. On older Redhat/CentOS/Fedora:</para> <programlisting><command>chkconfig --del iptables</command></programlisting> <para>On Debian systems:</para> <programlisting><command>update-rc.d iptables disable</command></programlisting> <para>On recent Fedora system running systemd:</para> <programlisting><command>systemctl disable iptables.service</command></programlisting> <para><inlinegraphic fileref="images/BD21298_.gif"/></para> <para>At this point, disable your existing firewall service.</para> </section> <section id="Other"> <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. Also, <ulink url="starting_and_stopping_shorewall.htm">Operating Shorewall and Shorewall Lite</ulink> contains a lot of useful operational hints.</para> </section> </article>