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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
<article id="usefull_links">
<!--$Id$-->
<articleinfo>
<title>Introduction</title>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
<copyright>
<year>2003-2007</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 type="" url="Copyright.htm">GNU Free Documentation
License</ulink></quote>.</para>
</legalnotice>
</articleinfo>
<section id="Intro">
<title>Introduction</title>
<para>The information in this document applies only to 4.x releases of
Shorewall.</para>
<section id="Glossary">
<title>Glossary</title>
<itemizedlist>
<listitem>
<para><ulink url="http://www.netfilter.org">Netfilter</ulink> - the
packet filter facility builtinto the 2.4 and later Linux
kernels.</para>
</listitem>
<listitem>
<para>ipchains - the packet filter facility builtinto the 2.2 Linux
kernels. Also the name of the utility program used to configure and
control that facility. Netfilter can be used in ipchains
compatibility mode.</para>
</listitem>
<listitem>
<para>iptables - the utility program used to configure and control
Netfilter. The term <quote>iptables</quote> is often used to refer
to the combination of iptables+Netfilter (with Netfilter not in
ipchains compatibility mode).</para>
</listitem>
<listitem>
<para>iptables-restore - a program included with iptables that
allows for atomic installation of a set of Netfilter rules. This is
a much more efficient way to install a rule set than running the
iptables utility once for each rule in the rule set.</para>
</listitem>
<listitem>
<para>ifconfig - An obsolete program included in the net-utils
package. ifconfig was used to configure network interfaces.</para>
</listitem>
<listitem>
<para>route - An obsolete program included in the net-utils package.
route was used to configure routing.</para>
</listitem>
<listitem>
<para>ip - A program included in the iproute2 package. ip replaces
ifconfig and route in modern Linux systems.</para>
</listitem>
<listitem>
<para>tc - A program included in the iproute2 package. tc is used to
configure QOS/Traffic Shaping on Linux systems.</para>
</listitem>
</itemizedlist>
</section>
<section id="Shorewall">
<title>What is Shorewall?</title>
<para>The Shoreline Firewall, more commonly known as
<quote>Shorewall</quote>, is high-level tool for configuring Netfilter.
You describe your firewall/gateway requirements using entries in a set
of configuration files. Shorewall reads those configuration files and
with the help of the iptables, iptables-restore, ip and tc utilities,
Shorewall configures Netfilter and the Linux networking subsystem to
match your requirements. Shorewall can be used on a dedicated firewall
system, a multi-function gateway/router/server or on a standalone
GNU/Linux system. Shorewall does not use Netfilter's ipchains
compatibility mode and can thus take advantage of Netfilter's connection
state tracking capabilities.</para>
<para>Shorewall is not a daemon. Once Shorewall has configured the Linux
networking subsystem, its job is complete and there is no
<quote>Shorewall process</quote> left running in your system. The <ulink
url="starting_and_stopping_shorewall.htm">/sbin/shorewall program can be
used at any time to monitor the Netfilter firewall</ulink>.</para>
<para>Shorewall is not the easiest to use of the available iptables
configuration tools but I believe that it is the most flexible and
powerful. So if you are looking for a simple point-and-click
set-and-forget Linux firewall solution that requires a minimum of
networking knowledge, I would encourage you to check out the following
alternatives:</para>
<itemizedlist>
<listitem>
<para><ulink
url="http://www.kmyfirewall.org/">kmyfirewall</ulink></para>
</listitem>
<listitem>
<para><ulink
url="http://www.fs-security.com/">firestarter</ulink></para>
</listitem>
</itemizedlist>
<para>If you are looking for a Linux firewall solution that can handle
complex and fast changing network environments then Shorewall is a
logical choice.</para>
</section>
</section>
<section id="Concepts">
<title>Shorewall Concepts</title>
<para>The configuration files for Shorewall are contained in the directory
<filename class="directory">/etc/shorewall</filename> -- for simple
setups, you will only need to deal with a few of them.</para>
<para>Shorewall views the network where it is running as being composed of
a set of <firstterm>zones</firstterm>. In the <ulink
url="three-interface.htm">three-interface sample configuration</ulink> for
example, the following zone names are used:</para>
<programlisting>#NAME DESCRIPTION
fw The firewall itself
net The Internet
loc Your Local Network
dmz Demilitarized Zone</programlisting>
<para>Zones are declared and given a type in the <ulink
url="manpages/shorewall-zones.html"><filename
class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
file.Here is the <ulink url="manpages/shorewall-zones.html"><filename
class="directory">/etc/shorewall/</filename><filename>zones</filename></ulink>
file from the three-interface sample:</para>
<programlisting>#ZONE TYPE OPTIONS IN OUT
# OPTIONS OPTIONS
fw firewall
net ipv4
loc ipv4
dmz ipv4
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
<para>Note that Shorewall recognizes the firewall system as its own zone.
The name of the zone designating the firewall itself (usually 'fw' as
shown in the above file) is stored in the shell variable
$<firstterm>FW</firstterm> which may be used throughout the Shorewall
configuration to refer to the firewall zone.</para>
<para>The simplest way to define the hosts in a zone is to associate the
zone with a network interface using the <ulink
url="manpages/shorewall-interfaces.html"><filename>/etc/shorewall/interfaces</filename></ulink>
file. In the three-interface sample, the three zones are defined using
that file as follows:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
net eth0 detect dhcp,routefilter,norfc1918
loc eth1 detect
dmz eth2 detect</programlisting>
<para>The above file defines the <emphasis>net</emphasis> zone as all IPv4
hosts interfacing to the firewall through eth0, the
<emphasis>loc</emphasis> zone as all IPv4 hosts interfacing through eth1
and the <emphasis>dmz</emphasis> as all IPv4 hosts interfacing through
eth2. It is important to note that the composition of a zone is defined in
terms of a combination of addresses <emphasis role="bold">and</emphasis>
interfaces. When using the <ulink
url="manpages/shorewall-interfaces.html"><filename>/etc/shorewall/interfaces</filename></ulink>
file to define a zone, all addresses are included; when you want to define
a zone that contains a limited subset of the IPv4 address space, you use
the <ulink
url="manpages/shorewall-hosts.html"><filename>/etc/shorewall/hosts</filename></ulink>
file.</para>
<para>Rules about what traffic to allow and what traffic to deny are
expressed in terms of zones. <itemizedlist spacing="compact">
<listitem>
<para>You express your default policy for connections from one zone
to another zone in the <ulink
url="manpages/shorewall-policy.html"><filename
class="directory">/etc/shorewall/</filename><filename>policy</filename></ulink>
file. The basic choices for policy are:</para>
<itemizedlist>
<listitem>
<para>ACCEPT - Accept the connection.</para>
</listitem>
<listitem>
<para>DROP - Ignore the connection request.</para>
</listitem>
<listitem>
<para>REJECT - Return an appropriate error to the connection
request.</para>
</listitem>
</itemizedlist>
<para>Connection request logging may be specified as part of a
policy and it is conventional (and highly recommended) to log DROP
and REJECT policies.</para>
</listitem>
<listitem>
<para>You define exceptions to these default policies in the <ulink
url="manpages/shorewall-rules.html"><filename
class="directory">/etc/shorewall/</filename><filename>rules</filename></ulink>
file.</para>
</listitem>
<listitem>
<para>You only need concern yourself with connection requests. You
don't need to define rules for handling traffic that is part of an
established connection is and in most cases you don't have to worry
about how related connections are handled (ICMP error packets and
<ulink url="FTP.html">related TCP connection requests such as used
by FTP</ulink>).</para>
</listitem>
</itemizedlist>For each connection request entering the firewall, the
request is first checked against the <filename
class="directory">/etc/shorewall/</filename><filename>rules</filename>
file. If no rule in that file matches the connection request then the
first policy in <filename
class="directory">/etc/shorewall/</filename><filename>policy</filename>
that matches the request is applied. If there is a default action defined
for the policy in /etc/shorewall/actions (or
<filename>/usr/share/shorewall/actions.std</filename>) then that action is
invoked before the policy is enforced. In the standard Shorewall
distribution, the DROP policy has a default action called <emphasis
role="bold">Drop</emphasis> and the REJECT policy has a default action
called <emphasis role="bold">Reject</emphasis>. Default actions are used
primarily to discard packets silently so that they don't clutter up your
log.</para>
<para>The <filename
class="directory">/etc/shorewall/</filename><filename>policy</filename>
file included with the three-interface sample has the following policies:
<programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info</programlisting>In the three-interface
sample, the line below is included but commented out. If you want your
firewall system to have full access to servers on the Internet, uncomment
that line. <programlisting>#SOURCE DEST POLICY LOG LEVEL LIMIT:BURST
$FW net ACCEPT</programlisting> The above policy will:
<itemizedlist>
<listitem>
<para>Allow all connection requests from your local network to the
Internet</para>
</listitem>
<listitem>
<para>Drop (ignore) all connection requests from the Internet to
your firewall or local networks; these ignored connection requests
will be logged using the <emphasis>info</emphasis> syslog priority
(log level).</para>
</listitem>
<listitem>
<para>Optionally accept all connection requests from the firewall to
the Internet (if you uncomment the additional policy)</para>
</listitem>
<listitem>
<para>reject all other connection requests; these rejected
connection requests will be logged using the
<emphasis>info</emphasis> syslog priority (log level).</para>
</listitem>
</itemizedlist></para>
<para>To illustrate how rules provide exceptions to policies, suppose that
you have the polices listed above but you want to be able to connect to
your firewall from the Internet using Secure Shell (SSH). Recall that SSH
connects uses TCP port 22.</para>
<programlisting>#ACTION SOURCE DEST PROTO DEST
# PORT(S)
ACCEPT net $FW tcp 22</programlisting>
<para>So although you have a policy of ignoring all connection attempts
from the net zone (from the Internet), the above exception to that policy
allows you to connect to the SSH server running on your firewall.</para>
<para>Because Shorewall makes no assumptions about what traffic you want
accepted, there are certain rules (exceptions) that need to be added to
almost any configuration.</para>
<itemizedlist>
<listitem>
<para>The <ulink url="shorewall_quickstart_guide.htm">QuickStart
guides</ulink> point to pre-populated files for use in common setups
and the <ulink url="shorewall_setup_guide.htm">Shorewall Setup
Guide</ulink> shows you examples for use with other more complex
setups.</para>
</listitem>
<listitem>
<para>To keep your <ulink url="shorewall_logging.html">firewall
log</ulink> from filling up with useless noise, Shorewall provides
<ulink url="Actions.html">common actions</ulink> that silently discard
or reject such noise before it can be logged. As with everything in
Shorewall, you can alter the behavior of these common actions (or do
away with them entirely) as you see fit.</para>
</listitem>
</itemizedlist>
</section>
<section id="Compile">
<title>Compile then Execute</title>
<para>Shorewall versions beginning with 3.2.0 use a "compile" then
"execute" approach. The Shorewall configuration compiler reads the
configuration files and generates a shell script. Errors in the
compilation step cause the script to be discarded and the command to be
aborted. If the compilation step doesn't find any errors then the shell
script is executed.</para>
<para>The 'compiled' scripts are placed in the directory <filename
class="directory">/var/lib/shorewall</filename> and are named to
correspond to the command being executed. For example, the command
"/sbin/shorewall start" will generate a script named
<filename>/var/lib/shorewall/.start</filename> and, if the compilation is
error free, that script will then be executed.</para>
</section>
<section id="Packages">
<title>Shorewall Packages</title>
<para>Shorewall 4.0 consists of four packages.</para>
<orderedlist>
<listitem>
<para><emphasis role="bold">Shorewall-common</emphasis>. This package
must be installed on at least one system in your network. That system
must also have Shorewall-shell and/or Shorewall-perl installed.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Shorewall-shell</emphasis>. This package
includes the legacy Shorewall configuration compiler written in Bourne
Shell. This compiler is very portable but suffers from performance
problems and has become hard to maintain.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Shorewall-perl</emphasis>. An alternative
to Shorewall-shell written in the Perl language. This compiler is
highly portable to those Unix-like platforms that support Perl
(including Cygwin) and is the compiler of choice for new Shorewall
installations. Scripts created using Shorewall-perl use
iptables-restore to install the generated Netfilter rule set.</para>
</listitem>
<listitem>
<para><emphasis role="bold">Shorewall-lite</emphasis>. Shorewall
allows for central administration of multiple firewalls through use of
Shorewall lite. The full Shorewall product (along with Shorewall-shell
and/or Shorewall-perl) are installed on a central administrative
system where compiled Shorewall scripts are generated. These scripts
are copied to the firewall systems where they run under the control of
Shorewall-lite.</para>
</listitem>
</orderedlist>
</section>
<section id="License">
<title>License</title>
<para>This program is free software; you can redistribute it and/or modify
it under the terms of <ulink
url="http://www.gnu.org/licenses/gpl.html">Version 2 of the GNU General
Public License</ulink> as published by the Free Software
Foundation.</para>
<para>This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more detail.</para>
<para>You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.</para>
</section>
</article>
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