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<?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>
<!--$Id$-->
<articleinfo>
<title>Shorewall and Bridged Firewalls</title>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Eastep</surname>
</author>
</authorgroup>
<pubdate>2005-10-21</pubdate>
<copyright>
<year>2004</year>
<year>2005</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 3.0 and
later. If you are running a version of Shorewall earlier than Shorewall
3.0.0 then please see the documentation for that
release.</emphasis></para>
</caution>
<section>
<title>Background</title>
<para>Systems where Shorewall runs normally function as
<firstterm>routers</firstterm>. In the context of the Open System
Interconnect (OSI) reference model, a router operates at layer 3,
Shorewall may also be deployed on a GNU Linux System that acts as a
<firstterm>bridge</firstterm>. Bridges are layer-2 devices in the OSI
model (think of a bridge as an ethernet switch).</para>
<para>Some differences between routers and bridges are:</para>
<orderedlist>
<listitem>
<para>Routers determine packet destination based on the destination IP
address while bridges route traffic based on the destination MAC
address in the ethernet frame.</para>
</listitem>
<listitem>
<para>As a consequence of the first difference, routers can be
connected to more than one IP network while a bridge may be part of
only a single network.</para>
</listitem>
<listitem>
<para>In most configurations, routers don't forward broadcast packets
while a bridges do.</para>
<note>
<para>Section 4 of RFC 1812 describes the conditions under which a
router may or must forward broadcasts.</para>
</note>
</listitem>
</orderedlist>
</section>
<section>
<title>Requirements</title>
<para>Note that if you need a bridge but do not need to restrict the
traffic through the bridge then any version of Shorewall will work. See
the <ulink url="SimpleBridge.html">Simple Bridge documentation</ulink> for
details.</para>
<para>In order to use Shorewall as a bridging firewall:</para>
<itemizedlist>
<listitem>
<para>Your kernel must contain bridge support (CONFIG_BRIDGE=m or
CONFIG_BRIDGE=y).</para>
</listitem>
<listitem>
<para>Your kernel must contain Netfilter physdev match support
(CONFIG_IP_NF_MATCH_PHYSDEV=m or CONFIG_IP_NF_MATCH_PHYSDEV=y).
Physdev match is standard in the 2.6 kernel series but must be patched
into the 2.4 kernels (see <ulink
url="http://bridge.sf.net">http://bridge.sf.net</ulink>). Bering and
Bering uCLibc users must find and install ipt_physdev.o for their
distribution and add <quote>ipt_physdev</quote> to
/etc/modules.</para>
</listitem>
<listitem>
<para>Your iptables must contain physdev match support. iptables 1.2.9
and later contain this support.</para>
</listitem>
<listitem>
<para>You must have the bridge utilities (bridge-utils) package
installed.</para>
</listitem>
</itemizedlist>
</section>
<section>
<title>Application</title>
<para>The following diagram shows a typical application of a
bridge/firewall. There is already an existing router in place whose
internal interface supports a network and you want to insert a firewall
between the router and the systems in the local network. In the example
shown, the network uses RFC 1918 addresses but that is not a requirement;
the bridge would work exactly the same if public IP addresses were used
(remember that the bridge doesn't deal with IP addresses).</para>
<graphic fileref="images/bridge.png" />
<para>There are a several key differences in this setup and a normal
Shorewall configuration:</para>
<itemizedlist>
<listitem>
<para>The Shorewall system (the Bridge/Firewall) has only a single IP
address even though it has two ethernet interfaces! The IP address is
configured on the bridge itself rather than on either of the network
cards.</para>
</listitem>
<listitem>
<para>The systems connected to the LAN are configured with the
router's IP address (192.168.1.254 in the above diagram) as their
default gateway.</para>
</listitem>
<listitem>
<para><command>traceroute</command> doesn't detect the Bridge/Firewall
as an intermediate router.</para>
</listitem>
<listitem>
<para>If the router runs a DHCP server, the hosts connected to the LAN
can use that server without having <command>dhcrelay</command> running
on the Bridge/Firewall.</para>
</listitem>
</itemizedlist>
<para>There are other possibilities here -- there could be a hub or switch
between the router and the Bridge/Firewall and there could be other
systems connected to that switch. All of the systems on the local side of
the <emphasis role="bold">router</emphasis> would still be configured with
IP addresses in 192.168.1.0/24 as shown below.<graphic
fileref="images/bridge3.png" /></para>
</section>
<section>
<title>Configuring the Bridge</title>
<para>Configuring the bridge itself is quite simple and uses the
<command>brctl</command> utility from the bridge-utils package. Bridge
configuration information may be found at <ulink
url="http://bridge.sf.net">http://bridge.sf.net</ulink>.</para>
<para>Unfortunately, many Linux distributions don't have good bridge
configuration tools and the network configuration GUIs don't detect the
presence of bridge devices. Here is an excerpt from a Debian
<filename>/etc/network/interfaces</filename> file for a two-port bridge
with a static IP address:</para>
<blockquote>
<programlisting>auto br0
iface br0 inet static
address 192.168.1.253
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
pre-up /sbin/ip link set eth0 up
pre-up /sbin/ip link set eth1 up
pre-up /usr/sbin/brctl addbr br0
pre-up /usr/sbin/brctl addif br0 eth0
pre-up /usr/sbin/brctl addif br0 eth1</programlisting>
</blockquote>
<para>While it is not a requirement to give the bridge an IP address,
doing so allows the bridge/firewall to access other systems and allows the
bridge/firewall to be managed remotely. The bridge must also have an IP
address for REJECT rules and policies to work correctly — otherwise REJECT
behaves the same as DROP. It is also a requirement for bridges to have an
IP address if they are part of a <link
linkend="bridge-router">bridge/router</link>.</para>
<important>
<para>Get your bridge configuration working first, including bridge
startup at boot, before you configure and start Shorewall.</para>
</important>
<para>The bridge may have its IP address assigned via DHCP. Here's an
example of an /etc/sysconfig/network/ifcfg-br0 file from a
<trademark>SUSE</trademark> system:</para>
<blockquote>
<programlisting>BOOTPROTO='dhcp'
REMOTE_IPADDR=''
STARTMODE='onboot'
UNIQUE='3hqH.MjuOqWfSZ+C'
WIRELESS='no'
MTU=''</programlisting>
</blockquote>
<para>Here's an /etc/sysconfig/network-scripts/ifcfg-br0 file for a
<trademark>Mandriva</trademark> system:</para>
<blockquote>
<programlisting>DEVICE=br0
BOOTPROTO=dhcp
ONBOOT=yes</programlisting>
</blockquote>
<para>On both the SUSE and Mandriva systems, a separate script is required
to configure the bridge itself.</para>
<para>Here are scripts that I used on a <trademark>Suse</trademark> 9.1
system.</para>
<blockquote>
<para><filename>/etc/sysconfig/network/ifcfg-br0</filename></para>
<programlisting>BOOTPROTO='dhcp'
REMOTE_IPADDR=''
STARTMODE='onboot'
UNIQUE='3hqH.MjuOqWfSZ+C'
WIRELESS='no'
MTU=''</programlisting>
<para><filename>/etc/init.d/bridge</filename><programlisting>#!/bin/sh
################################################################################
# Script to create a bridge
#
# (c) 2004 - Tom Eastep (teastep@shorewall.net)
#
# Modify the following variables to match your configuration
#
#### BEGIN INIT INFO
# Provides: bridge
# Required-Start: coldplug
# Required-Stop:
# Default-Start: 2 3 5
# Default-Stop: 0 1 6
# Description: starts and stops a bridge
### END INIT INFO
#
# chkconfig: 2345 05 89
# description: GRE/IP Tunnel
#
################################################################################
PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
INTERFACES="eth1 eth0"
BRIDGE="br0"
MODULES="tulip"
do_stop() {
echo "Stopping Bridge $BRIDGE"
brctl delbr $BRIDGE
for interface in $INTERFACES; do
ip link set $interface down
done
}
do_start() {
echo "Starting Bridge $BRIDGE"
for module in $MODULES; do
modprobe $module
done
sleep 5
for interface in $INTERFACES; do
ip link set $interface up
done
brctl addbr $BRIDGE
for interface in $INTERFACES; do
brctl addif $BRIDGE $interface
done
}
case "$1" in
start)
do_start
;;
stop)
do_stop
;;
restart)
do_stop
sleep 1
do_start
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
esac
exit 0</programlisting></para>
</blockquote>
<para>Axel Westerhold has contributed this example of configuring a bridge
with a static IP address on a Fedora System (Core 1 and Core 2 Test 1).
Note that these files also configure the bridge itself so there is no need
for a separate bridge config script.</para>
<blockquote>
<para><filename>/etc/sysconfig/network-scripts/ifcfg-br0:</filename></para>
<programlisting>DEVICE=br0
TYPE=Bridge
IPADDR=192.168.50.14
NETMASK=255.255.255.0
ONBOOT=yes</programlisting>
<para><filename>/etc/sysconfig/network-scripts/ifcfg-eth0:</filename><programlisting>DEVICE=eth0
TYPE=ETHER
BRIDGE=br0
ONBOOT=yes</programlisting><filename>/etc/sysconfig/network-scripts/ifcfg-eth1:</filename><programlisting>DEVICE=eth1
TYPE=ETHER
BRIDGE=br0
ONBOOT=yes</programlisting></para>
</blockquote>
<para>Florin Grad at <trademark>Mandriva</trademark> provides this script
for configuring a bridge:</para>
<blockquote>
<programlisting>#!/bin/sh
# chkconfig: 2345 05 89
# description: Layer 2 Bridge
#
[ -f /etc/sysconfig/bridge ] &amp;&amp; . /etc/sysconfig/bridge
PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
do_stop() {
echo "Stopping Bridge"
for i in $INTERFACES $BRIDGE_INTERFACE ; do
ip link set $i down
done
brctl delbr $BRIDGE_INTERFACE
}
do_start() {
echo "Starting Bridge"
for i in $INTERFACES ; do
ip link set $i up
done
brctl addbr br0
for i in $INTERFACES ; do
ip link set $i up
brctl addif br0 $i
done
ifup $BRIDGE_INTERFACE
}
case "$1" in
start)
do_start
;;
stop)
do_stop
;;
restart)
do_stop
sleep 1
do_start
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
esac
exit 0</programlisting>
<para>The <filename>/etc/sysconfig/bridge file</filename>:</para>
<programlisting>BRIDGE_INTERFACE=br0 #The name of your Bridge
INTERFACES="eth0 eth1" #The physical interfaces to be bridged</programlisting>
</blockquote>
<para>Andrzej Szelachowski contributed the following.</para>
<blockquote>
<programlisting>Here is how I configured bridge in Slackware:
1) I had to compile bridge-utils (It's not in the standard distribution)
2) I've created rc.bridge in /etc/rc.d:
#########################
#! /bin/sh
ifconfig eth0 0.0.0.0
ifconfig eth1 0.0.0.0
#ifconfig lo 127.0.0.1 #this line should be uncommented if you don't use rc.inet1
brctl addbr most
brctl addif most eth0
brctl addif most eth1
ifconfig most 192.168.1.31 netmask 255.255.255.0 up
#route add default gw 192.168.1.1 metric 1 #this line should be uncommented if
#you don't use rc.inet1
#########################
3) I made rc.brige executable and added the following line to /etc/rc.d/rc.local
/etc/rc.d/rc.bridge </programlisting>
</blockquote>
<para>Joshua Schmidlkofer writes:</para>
<blockquote>
<programlisting>Bridge Setup for Gentoo
#install bridge-utils
emerge bridge-utils
## create a link for net.br0
cd /etc/init.d
ln -s net.eth0 net.br0
# Remove net.eth*, add net.br0 and bridge.
rc-update del net.eth0
rc-update del net.eth1
rc-update add net.br0 default
rc-update add bridge boot
/etc/conf.d/bridge:
#bridge contains the name of each bridge you want created.
bridge="br0"
# bridge_&lt;bridge&gt;_devices contains the devices to use at bridge startup.
bridge_br0_devices="eth0 eth1"
/etc/conf.d/net
iface_br0="10.0.0.1 broadcast 10.0.0.255 netmask 255.255.255.0"
#for dhcp:
#iface_br0="dhcp"
#comment this out if you use dhcp.
gateway="eth0/10.0.0.1" </programlisting>
</blockquote>
<para>Users who successfully configure bridges on other distributions,
with static or dynamic IP addresses, are encouraged to send <ulink
url="mailto:webmaster@shorewall.net">me</ulink> their configuration so I
can post it here.</para>
</section>
<section>
<title>Configuring Shorewall</title>
<para>Bridging in Shorewall is enabled using the BRIDGING option in
<filename>/etc/shorewall/shorewall.conf</filename>:</para>
<programlisting>BRIDGING=Yes</programlisting>
<para>In the scenario pictured above, there would probably be two zones
defined -- one for the internet and one for the local LAN so in
<filename>/etc/shorewall/zones</filename>:</para>
<programlisting>#ZONE TYPE OPTIONS
fw firewall
net ipv4
loc ipv4
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
<para>A conventional two-zone policy file is appropriate here —
<filename>/etc/shorewall/policy</filename>:</para>
<programlisting>#SOURCE DEST POLICY LOG LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
<para>Only the bridge device itself is configured with an IP address so
only that device is defined to Shorewall in
<filename>/etc/shorewall/interfaces</filename>:</para>
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
- br0 192.168.1.255
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
<para>The zones are defined using the
<filename>/etc/shorewall/hosts</filename> file. Assuming that the router
is connected to <filename class="devicefile">eth0</filename> and the
switch to <filename class="devicefile">eth1</filename>:</para>
<programlisting>#ZONE HOST(S) OPTIONS
net br0:eth0
loc br0:eth1
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE</programlisting>
<para>When Shorewall is stopped, you want to allow only local traffic
through the bridge —
<filename><filename>/etc/shorewall/routestopped</filename></filename>:</para>
<programlisting>#INTERFACE HOST(S) OPTIONS
br0 192.168.1.0/24 routeback
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
<para>The <filename>/etc/shorewall/rules</filename> file from the
two-interface sample is a good place to start for defining a set of
firewall rules.</para>
</section>
<section id="bridge-router">
<title>Combination Router/Bridge</title>
<para>A system running Shorewall doesn't have to be exclusively a bridge
or a router -- it can act as both. Here's an example:<graphic
fileref="images/bridge2.png" /></para>
<para>This is basically the same setup as shown in the <ulink
url="shorewall_setup_guide.htm">Shorewall Setup Guide</ulink> with the
exception that the DMZ is bridged rather than using Proxy ARP. Changes in
the configuration shown in the Setup Guide are as follows:</para>
<orderedlist>
<listitem>
<para>The <filename>/etc/shorewall/proxyarp</filename> file is empty
in this configuration.</para>
</listitem>
<listitem>
<para>The <filename>/etc/shorewall/interfaces</filename> file is as
follows:<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
- br0 detect routefilter
loc eth1 detect</programlisting></para>
</listitem>
<listitem>
<para>The <filename>/etc/shorewall/hosts</filename> file would
have:</para>
<programlisting>#ZONE HOSTS OPTIONS
net br0:eth0
dmz br0:eth2</programlisting>
</listitem>
<listitem>
<para>The DMZ systems need a route to the 192.168.201.0/24 network via
192.0.2.176 to enable them to communicate with the local
network.</para>
</listitem>
</orderedlist>
</section>
<section>
<title>Limitations</title>
<para>Bridging doesn't work with some wireless cards — see <ulink
url="http://bridge.sf.net">http://bridge.sf.net</ulink>.</para>
</section>
<section>
<title>Other Links</title>
<itemizedlist>
<listitem>
<para><ulink
url="http://wiki.buenosaireslibre.org/HowTos_2fBridgedFirewall">Here
is an article in Spanish </ulink>detailing bridging a public and local
network using Shorewall. This is another router/bridge
configuration.</para>
</listitem>
</itemizedlist>
</section>
</article>
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