forked from extern/shorewall_code
86bbae3111
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@1463 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
230 lines
12 KiB
XML
230 lines
12 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<article id="usefull_links">
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<!--$Id$-->
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<articleinfo>
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<title>Introduction</title>
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<author>
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<firstname>Tom</firstname>
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<surname>Eastep</surname>
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</author>
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<pubdate>2004-07-10</pubdate>
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<copyright>
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<year>2003-2004</year>
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<holder>Thomas M. Eastep</holder>
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</copyright>
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<legalnotice>
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<para>Permission is granted to copy, distribute and/or modify this
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document under the terms of the GNU Free Documentation License, Version
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1.2 or any later version published by the Free Software Foundation; with
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no Invariant Sections, with no Front-Cover, and with no Back-Cover
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Texts. A copy of the license is included in the section entitled
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<quote><ulink type="" url="Copyright.htm">GNU Free Documentation License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<section>
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<title>Introduction</title>
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<para>The information in this document applies only to 2.0.x releases of
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Shorewall.</para>
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<section>
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<title>Glossary</title>
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<itemizedlist>
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<listitem>
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<para><ulink url="http://www.netfilter.org">Netfilter</ulink> - the
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packet filter facility built into the 2.4 and later Linux kernels.</para>
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</listitem>
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<listitem>
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<para>ipchains - the packet filter facility built into the 2.2 Linux
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kernels. Also the name of the utility program used to configure and
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control that facility. Netfilter can be used in ipchains
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compatibility mode.</para>
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</listitem>
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<listitem>
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<para>iptables - the utility program used to configure and control
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Netfilter. The term <quote>iptables</quote> is often used to refer
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to the combination of iptables+Netfilter (with Netfilter not in
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ipchains compatibility mode).</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>What is Shorewall?</title>
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<para>The Shoreline Firewall, more commonly known as <quote>Shorewall</quote>,
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is high-level tool for configuring Netfilter. You describe your
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firewall/gateway requirements using entries in a set of configuration
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files. Shorewall reads those configuration files and with the help of
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the iptables utility, Shorewall configures Netfilter to match your
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requirements. Shorewall can be used on a dedicated firewall system, a
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multi-function gateway/router/server or on a standalone GNU/Linux
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system. Shorewall does not use Netfilter's ipchains compatibility
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mode and can thus take advantage of Netfilter's connection state
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tracking capabilities.</para>
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<para>Shorewall is not a daemon. Once Shorewall has configured
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Netfilter, it's job is complete and there is no <quote>Shorewall
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process</quote> left running in your system. The <ulink
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url="starting_and_stopping_shorewall.htm">/sbin/shorewall program can be
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used at any time to monitor the Netfilter firewall</ulink>.</para>
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</section>
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</section>
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<section>
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<title>Shorewall Concepts</title>
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<para>The configuration files for Shorewall are contained in the directory
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<filename class="directory">/etc/shorewall</filename> -- for simple
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setups, you will only need to deal with a few of them.</para>
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<para>Shorewall views the network where it is running as being composed of
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a set of zones. In the <ulink url="three-interface.htm">three-interface
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sample configuration</ulink> for example, the following zone names are
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used: <informaltable frame="all" pgwide="0"><tgroup align="left" cols="2"><thead
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valign="middle"><row valign="middle"><entry align="left">Name</entry><entry
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align="left">Description</entry></row></thead><tbody valign="middle"><row
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valign="middle"><entry align="left"><varname>net</varname></entry><entry
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align="left">The Internet</entry></row><row valign="middle"><entry
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align="left"><varname>loc</varname></entry><entry align="left">Your Local
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Network</entry></row><row valign="middle"><entry align="left"><varname>dmz</varname></entry><entry
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align="left">Demilitarized Zone</entry></row></tbody></tgroup></informaltable>Zones
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are defined in the <ulink url="Documentation.htm#Zones"><filename
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class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
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file.</para>
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<para>Shorewall also recognizes the firewall system as its own zone - by
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default, the firewall itself is known as <emphasis role="bold"><varname>fw</varname></emphasis>.</para>
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<para>Rules about what traffic to allow and what traffic to deny are
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expressed in terms of zones. <itemizedlist spacing="compact"><listitem><para>You
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express your default policy for connections from one zone to another zone
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in the <ulink url="Documentation.htm#Policy"><filename class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
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file. The choices for policy are:</para><itemizedlist><listitem><para>ACCEPT
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- Accept the connection.</para></listitem><listitem><para>DROP - Ignore
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the connection request.</para></listitem><listitem><para>REJECT - Return
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an appropriate error to the connection request.</para></listitem></itemizedlist><para>Connection
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request logging may be specified as part of a policy and it is
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conventional to log DROP and REJECT policies.</para></listitem><listitem><para>You
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define exceptions to those default policies in the <ulink
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url="Documentation.htm#Rules"><filename class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
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file.</para></listitem><listitem><para>You only need concern yourself with
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connection requests. You don't need to define rules for how traffic
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that is part of an established connection is handled and in most cases you
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don't have to worry about how related connections are handled (ICMP
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error packets and <ulink url="FTP.html">related TCP connection requests
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such as used by FTP</ulink>).</para></listitem></itemizedlist>For each
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connection request entering the firewall, the request is first checked
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against the <filename class="directory">/etc/shorewall/</filename><filename>rules</filename>
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file. If no rule in that file matches the connection request then the
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first policy in <filename class="directory">/etc/shorewall/</filename><filename>policy</filename>
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that matches the request is applied. If there is a common action defined
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for the policy in /etc/shorewall/actions (or <filename>/usr/share/shorewall/actions.std</filename>)
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then that action is invoked before the policy is enforces. In the standard
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Shorewall distribution, the DROP policy has a common action called
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<emphasis role="bold">Drop</emphasis> and the REJECT policy has a common
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action called <emphasis role="bold">Reject</emphasis>. Common actions are
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used primarily to discard</para>
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<para>The <filename class="directory">/etc/shorewall/</filename><filename>policy</filename>
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file included with the three-interface sample has the following policies:
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<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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loc net ACCEPT
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net all DROP info
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all all REJECT info</programlisting>In the three-interface
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sample, the line below is included but commented out. If you want your
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firewall system to have full access to servers on the internet, uncomment
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that line. <programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
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fw net ACCEPT</programlisting> The above policy will:
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<itemizedlist><listitem><para>Allow all connection requests from your
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local network to the internet</para></listitem><listitem><para>Drop
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(ignore) all connection requests from the internet to your firewall or
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local network; these ignored connection requests will be logged using the
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<emphasis>info</emphasis> syslog priority (log level).</para></listitem><listitem><para>Optionally
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accept all connection requests from the firewall to the internet (if you
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uncomment the additional policy)</para></listitem><listitem><para>reject
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all other connection requests; these rejected connection requests will be
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logged using the <emphasis>info</emphasis> syslog priority (log level).</para></listitem></itemizedlist></para>
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<para>The simplest way to define a zone is to associate the zone with a
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network interface using the <ulink url="Documentation.htm#Interfaces"><filename>/etc/shorewall/interfaces</filename></ulink>
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file. In the three-interface sample, the three zones are defined using
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that file as follows:</para>
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<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
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net eth0 detect dhcp,routefilter,norfc1918
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loc eth1 detect
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dmz eth2 detect</programlisting>
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<para>The above file defines the net zone as all hosts interfacing to the
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firewall through eth0, the loc zone as all hosts interfacing through eth1
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and the dmz as all hosts interfacing through eth2.</para>
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<para>To illustrate how rules provide exceptions to policies, suppose that
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you have the polcies listed above but you want to be able to connect to
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your firewall from the internet using Secure Shell (SSH). Recall that SSH
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connects uses TCP port 22.</para>
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<programlisting>#ACTION SOURCE DEST PROTO DEST
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# PORT(S)
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ACCEPT net fw tcp 22</programlisting>
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<para>So although you have a policy of ignoring all connection attempts
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from the net zone (from the internet), the above exception to that policy
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allows you to connect to the SSH server running on your firewall.</para>
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<para>Because Shorewall makes no assumptions about what traffic you want
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accepted, there are certain rules (exceptions) that need to be added to
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almost any configuration.</para>
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<itemizedlist>
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<listitem>
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<para>The <ulink url="shorewall_quickstart_guide.htm">QuickStart
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guildes</ulink> provide links to download pre-populated files for use
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in common setups and the <ulink url="shorewall_setup_guide.htm">Shorewall
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Setup Guide</ulink> shows you examples for use with other more complex
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setups.</para>
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</listitem>
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<listitem>
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<para>To keep your <ulink url="shorewall_logging.html">firewall log</ulink>
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from filling up with useless noise, Shorewall provides <ulink
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url="User_defined_Actions.html">common actions</ulink> that silently
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discard or reject such noise before it can be logged. As with
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everything in Shorewall, you can alter the behavior of these common
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actions (or do away with them entirely) as you see fit.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section>
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<title>License</title>
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<para>This program is free software; you can redistribute it and/or modify
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it under the terms of <ulink url="http://www.gnu.org/licenses/gpl.html">Version
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2 of the GNU General Public License</ulink> as published by the Free
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Software Foundation.</para>
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<para>This program is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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for more detail.</para>
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<para>You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software Foundation,
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Inc., 675 Mass Ave, Cambridge, MA 02139, USA</para>
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</section>
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</article> |