forked from extern/shorewall_code
c31f899749
git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@9561 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
725 lines
24 KiB
XML
725 lines
24 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
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"http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd">
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<article>
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<!--$Id$-->
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<articleinfo>
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<title>Shorewall-perl and Bridged Firewalls</title>
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<authorgroup>
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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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</authorgroup>
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<pubdate><?dbtimestamp format="Y/m/d"?></pubdate>
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<copyright>
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<year>2007</year>
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<year>2009</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 url="GnuCopyright.htm">GNU Free Documentation
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License</ulink></quote>.</para>
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</legalnotice>
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</articleinfo>
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<caution>
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<para><emphasis role="bold">This article applies to Shorewall-perl 4.3 and
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later.</emphasis></para>
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</caution>
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<section id="Background">
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<title>Background</title>
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<para>Systems where Shorewall runs normally function as
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<firstterm>routers</firstterm>. In the context of the Open System
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Interconnect (OSI) reference model, a router operates at layer 3,
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Shorewall may also be deployed on a GNU Linux System that acts as a
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<firstterm>bridge</firstterm>. Bridges are layer 2 devices in the OSI
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model (think of a bridge as an Ethernet switch).</para>
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<para>Some differences between routers and bridges are:</para>
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<orderedlist>
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<listitem>
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<para>Routers determine packet destination based on the destination IP
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address, while bridges route traffic based on the destination MAC
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address in the Ethernet frame.</para>
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</listitem>
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<listitem>
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<para>As a consequence of the first difference, routers can be
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connected to more than one IP network while a bridge may be part of
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only a single network.</para>
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</listitem>
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<listitem>
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<para>In most configurations, routers don't forward broadcast packets
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while bridges do.</para>
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<note>
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<para>Section 4 of RFC 1812 describes the conditions under which a
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router may or must forward broadcasts.</para>
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</note>
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</listitem>
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</orderedlist>
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</section>
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<section id="Requirements">
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<title>Requirements</title>
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<para>Note that if you need a bridge but do not need to restrict the
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traffic through the bridge then any version of Shorewall will work. See
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the <ulink url="SimpleBridge.html">Simple Bridge documentation</ulink> for
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details.</para>
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<para>In order to use Shorewall as a bridging firewall:</para>
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<itemizedlist>
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<listitem>
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<para>Your kernel must contain bridge support (CONFIG_BRIDGE=m or
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CONFIG_BRIDGE=y).</para>
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</listitem>
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<listitem>
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<para>Your kernel must contain bridge/netfilter integration
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(CONFIG_BRIDGE_NETFILTER=y).</para>
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</listitem>
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<listitem>
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<para>Your kernel must contain Netfilter physdev match support
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(CONFIG_IP_NF_MATCH_PHYSDEV=m or CONFIG_IP_NF_MATCH_PHYSDEV=y).
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Physdev match is standard in the 2.6 kernel series but must be patched
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into the 2.4 kernels (see <ulink
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url="http://bridge.sf.net">http://bridge.sf.net</ulink>). Bering and
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Bering uCLibc users must find and install ipt_physdev.o for their
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distribution and add <quote>ipt_physdev</quote> to
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/etc/modules.</para>
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</listitem>
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<listitem>
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<para>Your iptables must contain physdev match support and must
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support multiple instances of '-m physdev' in a single rule. iptables
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1.3.6 and later contain this support.</para>
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</listitem>
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<listitem>
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<para>You must have the bridge utilities (bridge-utils) package
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installed.</para>
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</listitem>
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</itemizedlist>
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</section>
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<section id="Application">
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<title>Application</title>
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<para>The following diagram shows a typical application of a
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bridge/firewall. There is already an existing router in place whose
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internal interface supports a network, and you want to insert a firewall
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between the router, and the systems in the local network. In the example
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shown, the network uses RFC 1918 addresses but that is not a requirement;
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the bridge would work exactly the same if public IP addresses were used
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(remember that the bridge doesn't deal with IP addresses).</para>
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<graphic fileref="images/bridge.png" />
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<para>There are a several key differences in this setup and a normal
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Shorewall configuration:</para>
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<itemizedlist>
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<listitem>
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<para>The Shorewall system (the Bridge/Firewall) has only a single IP
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address even though it has two Ethernet interfaces! The IP address is
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configured on the bridge itself, rather than on either of the network
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cards.</para>
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</listitem>
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<listitem>
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<para>The systems connected to the LAN are configured with the
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router's IP address (192.168.1.254 in the above diagram) as their
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default gateway.</para>
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</listitem>
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<listitem>
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<para><command>traceroute</command> doesn't detect the Bridge/Firewall
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as an intermediate router.</para>
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</listitem>
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<listitem>
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<para>If the router runs a DHCP server, the hosts connected to the LAN
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can use that server without having <command>dhcrelay</command> running
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on the Bridge/Firewall.</para>
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</listitem>
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</itemizedlist>
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<warning>
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<para>Inserting a bridge/firewall between a router and a set of local
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hosts only works if those local hosts form a single IP network. In the
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above diagram, all of the hosts in the loc zone are in the
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192.168.1.0/24 network. If the router is routing between several local
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networks through the same physical interface (there are multiple IP
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networks sharing the same LAN), then inserting a bridge/firewall between
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the router and the local LAN won't work.</para>
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</warning>
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<para>There are other possibilities here -- there could be a hub or switch
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between the router and the Bridge/Firewall and there could be other
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systems connected to that switch. All of the systems on the local side of
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the <emphasis role="bold">router</emphasis> would still be configured with
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IP addresses in 192.168.1.0/24 as shown below.<graphic
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fileref="images/bridge3.png" /></para>
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</section>
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<section id="Bridge">
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<title>Configuring the Bridge</title>
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<para>Configuring the bridge itself is quite simple and uses the
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<command>brctl</command> utility from the bridge-utils package. Bridge
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configuration information may be found at <ulink
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url="http://bridge.sf.net">http://bridge.sf.net</ulink>.</para>
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<para>Unfortunately, many Linux distributions don't have good bridge
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configuration tools, and the network configuration GUIs don't detect the
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presence of bridge devices. Here is an excerpt from a Debian
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<filename>/etc/network/interfaces</filename> file for a two-port bridge
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with a static IP address:</para>
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<blockquote>
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<programlisting>auto br0
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iface br0 inet static
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address 192.168.1.253
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netmask 255.255.255.0
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network 192.168.1.0
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broadcast 192.168.1.255
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pre-up /sbin/ip link set eth0 up
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pre-up /sbin/ip link set eth1 up
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pre-up /usr/sbin/brctl addbr br0
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pre-up /usr/sbin/brctl addif br0 eth0
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pre-up /usr/sbin/brctl addif br0 eth1
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pre-down /usr/sbin/brctl delif br0 eth0
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pre-down /sbin/ip link set eth0 down
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pre-down /usr/sbin/brctl delif br0 eth1
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pre-down /sbin/ip link set eth1 down
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post-down /usr/sbin/brctl delbr br0</programlisting>
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</blockquote>
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<para>While it is not a requirement to give the bridge an IP address,
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doing so allows the bridge/firewall to access other systems and allows the
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bridge/firewall to be managed remotely. The bridge must also have an IP
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address for REJECT rules and policies to work correctly — otherwise REJECT
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behaves the same as DROP. It is also a requirement for bridges to have an
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IP address if they are part of a <link
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linkend="bridge-router">bridge/router</link>.</para>
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<important>
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<para>Get your bridge configuration working first, including bridge
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startup at boot, before you configure and start Shorewall.</para>
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</important>
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<para>The bridge may have its IP address assigned via DHCP. Here's an
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example of an /etc/sysconfig/network/ifcfg-br0 file from a
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<trademark>SUSE</trademark> system:</para>
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<blockquote>
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<programlisting>BOOTPROTO='dhcp'
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REMOTE_IPADDR=''
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STARTMODE='onboot'
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UNIQUE='3hqH.MjuOqWfSZ+C'
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WIRELESS='no'
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MTU=''</programlisting>
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</blockquote>
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<para>Here's an /etc/sysconfig/network-scripts/ifcfg-br0 file for a
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<trademark>Mandriva</trademark> system:</para>
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<blockquote>
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<programlisting>DEVICE=br0
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BOOTPROTO=dhcp
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ONBOOT=yes</programlisting>
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</blockquote>
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<para>On both the <trademark>SUSE</trademark> and Mandriva systems, a
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separate script is required to configure the bridge itself.</para>
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<para>Here are scripts that I used on a <trademark>SUSE</trademark> 9.1
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system.</para>
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<blockquote>
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<para><filename>/etc/sysconfig/network/ifcfg-br0</filename></para>
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<programlisting>BOOTPROTO='dhcp'
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REMOTE_IPADDR=''
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STARTMODE='onboot'
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UNIQUE='3hqH.MjuOqWfSZ+C'
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WIRELESS='no'
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MTU=''</programlisting>
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<para><filename>/etc/init.d/bridge</filename><programlisting>#!/bin/sh
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################################################################################
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# Script to create a bridge
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#
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# (c) 2004 - Tom Eastep (teastep@shorewall.net)
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#
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# Modify the following variables to match your configuration
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#
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#### BEGIN INIT INFO
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# Provides: bridge
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# Required-Start: coldplug
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# Required-Stop:
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# Default-Start: 2 3 5
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# Default-Stop: 0 1 6
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# Description: starts and stops a bridge
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### END INIT INFO
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#
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# chkconfig: 2345 05 89
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# description: GRE/IP Tunnel
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#
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################################################################################
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PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
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INTERFACES="eth1 eth0"
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BRIDGE="br0"
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MODULES="tulip"
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do_stop() {
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echo "Stopping Bridge $BRIDGE"
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brctl delbr $BRIDGE
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for interface in $INTERFACES; do
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ip link set $interface down
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done
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}
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do_start() {
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echo "Starting Bridge $BRIDGE"
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for module in $MODULES; do
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modprobe $module
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done
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sleep 5
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for interface in $INTERFACES; do
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ip link set $interface up
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done
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brctl addbr $BRIDGE
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for interface in $INTERFACES; do
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brctl addif $BRIDGE $interface
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done
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}
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case "$1" in
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start)
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do_start
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;;
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stop)
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do_stop
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;;
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restart)
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do_stop
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sleep 1
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do_start
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;;
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*)
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echo "Usage: $0 {start|stop|restart}"
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exit 1
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esac
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exit 0</programlisting></para>
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</blockquote>
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<para>Axel Westerhold has contributed this example of configuring a bridge
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with a static IP address on a Fedora System (Core 1 and Core 2 Test 1).
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Note that these files also configure the bridge itself, so there is no
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need for a separate bridge config script.</para>
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<blockquote>
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<para><filename>/etc/sysconfig/network-scripts/ifcfg-br0:</filename></para>
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<programlisting>DEVICE=br0
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TYPE=Bridge
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IPADDR=192.168.50.14
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NETMASK=255.255.255.0
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ONBOOT=yes</programlisting>
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<para><filename>/etc/sysconfig/network-scripts/ifcfg-eth0:</filename><programlisting>DEVICE=eth0
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TYPE=ETHER
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BRIDGE=br0
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ONBOOT=yes</programlisting><filename>/etc/sysconfig/network-scripts/ifcfg-eth1:</filename><programlisting>DEVICE=eth1
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TYPE=ETHER
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BRIDGE=br0
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ONBOOT=yes</programlisting></para>
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</blockquote>
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<para>Florin Grad at <trademark>Mandriva</trademark> provides this script
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for configuring a bridge:</para>
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<blockquote>
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<programlisting>#!/bin/sh
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# chkconfig: 2345 05 89
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# description: Layer 2 Bridge
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#
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[ -f /etc/sysconfig/bridge ] && . /etc/sysconfig/bridge
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PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
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do_stop() {
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echo "Stopping Bridge"
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for i in $INTERFACES $BRIDGE_INTERFACE ; do
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ip link set $i down
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done
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brctl delbr $BRIDGE_INTERFACE
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}
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do_start() {
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echo "Starting Bridge"
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for i in $INTERFACES ; do
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ip link set $i up
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done
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brctl addbr br0
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for i in $INTERFACES ; do
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ip link set $i up
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brctl addif br0 $i
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done
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ifup $BRIDGE_INTERFACE
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}
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case "$1" in
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start)
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do_start
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;;
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stop)
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do_stop
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;;
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restart)
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do_stop
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sleep 1
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do_start
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;;
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*)
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echo "Usage: $0 {start|stop|restart}"
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exit 1
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esac
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exit 0</programlisting>
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<para>The <filename>/etc/sysconfig/bridge file</filename>:</para>
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<programlisting>BRIDGE_INTERFACE=br0 #The name of your Bridge
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INTERFACES="eth0 eth1" #The physical interfaces to be bridged</programlisting>
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</blockquote>
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<para>Andrzej Szelachowski contributed the following.</para>
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<blockquote>
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<programlisting>Here is how I configured bridge in Slackware:
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1) I had to compile bridge-utils (It's not in the standard distribution)
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2) I've created rc.bridge in /etc/rc.d:
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#########################
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#! /bin/sh
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ifconfig eth0 0.0.0.0
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ifconfig eth1 0.0.0.0
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#ifconfig lo 127.0.0.1 #this line should be uncommented if you don't use rc.inet1
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brctl addbr most
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brctl addif most eth0
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brctl addif most eth1
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ifconfig most 192.168.1.31 netmask 255.255.255.0 up
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#route add default gw 192.168.1.1 metric 1 #this line should be uncommented if
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#you don't use rc.inet1
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#########################
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3) I made rc.bridge executable and added the following line to /etc/rc.d/rc.local
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/etc/rc.d/rc.bridge </programlisting>
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</blockquote>
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<para>Joshua Schmidlkofer writes:</para>
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<blockquote>
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<programlisting>Bridge Setup for Gentoo
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#install bridge-utils
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emerge bridge-utils
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## create a link for net.br0
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cd /etc/init.d
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ln -s net.eth0 net.br0
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# Remove net.eth*, add net.br0 and bridge.
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rc-update del net.eth0
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rc-update del net.eth1
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rc-update add net.br0 default
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rc-update add bridge boot
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/etc/conf.d/bridge:
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#bridge contains the name of each bridge you want created.
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bridge="br0"
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# bridge_<bridge>_devices contains the devices to use at bridge startup.
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bridge_br0_devices="eth0 eth1"
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/etc/conf.d/net
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iface_br0="10.0.0.1 broadcast 10.0.0.255 netmask 255.255.255.0"
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#for dhcp:
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#iface_br0="dhcp"
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#comment this out if you use dhcp.
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gateway="eth0/10.0.0.1" </programlisting>
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</blockquote>
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<para>Users who successfully configure bridges on other distributions,
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with static or dynamic IP addresses, are encouraged to send <ulink
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url="mailto:webmaster@shorewall.net">me</ulink> their configuration so I
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can post it here.</para>
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</section>
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|
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<section id="Shorewall">
|
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<title>Configuring Shorewall</title>
|
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|
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<para>As described above, Shorewall bridge support requires the
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<firstterm>physdev match</firstterm> feature of Netfilter/iptables.
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Physdev match allows rules to be triggered based on the bridge port that a
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packet arrived on and/or the bridge port that a packet will be sent over.
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The latter has proved to be problematic because it requires that the
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evaluation of rules be deferred until the destination bridge port is
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known. This deferral has the unfortunate side effect that it makes IPSEC
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Netfilter filtration incompatible with bridges. To work around this
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problem, in kernel version 2.6.20 the Netfilter developers decided to
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remove the deferred processing in two cases:</para>
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|
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<itemizedlist>
|
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<listitem>
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<para>When a packet being sent through a bridge entered the firewall
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on another interface and was being forwarded to the bridge.</para>
|
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</listitem>
|
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|
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<listitem>
|
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<para>When a packet originating on the firewall itself is being sent
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through a bridge.</para>
|
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</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>Notice that physdev match was only weakened with respect to the
|
|
destination bridge port -- it remains fully functional with respect to the
|
|
source bridge port.</para>
|
|
|
|
<para>To deal with the asymmetric nature of the new physdev match,
|
|
Shorewall-perl supports a new type of zone - a <firstterm>Bridge
|
|
Port</firstterm> (BP) zone. Bridge port zones have a number of
|
|
restrictions:</para>
|
|
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>BP zones may only be associated with bridge ports.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>All ports associated with a given BP zone must be on the same
|
|
bridge.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Policies from a non-BP zone to a BP are disallowed.</para>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>Rules where the SOURCE is a non-BP zone and the DEST is a BP
|
|
zone are disallowed.</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>In /etc/shorewall/zones, BP zones are specified using the <emphasis
|
|
role="bold">bport</emphasis> (or <emphasis role="bold">bport4</emphasis>)
|
|
keyword. Shorewall perl requires that BRIDGING=No in
|
|
<filename>shorewall.conf</filename>.</para>
|
|
|
|
<para>In the scenario pictured above, there would probably be two BP 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
|
|
world ipv4
|
|
net:world bport
|
|
loc:world bport
|
|
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE</programlisting>
|
|
|
|
<para>The <emphasis>world</emphasis> zone can be used when defining rules
|
|
whose source zone is the firewall itself (remember that fw-><BP
|
|
zone> rules are not allowed).</para>
|
|
|
|
<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>Bridges use a special syntax in
|
|
<filename>/etc/shorewall/interfaces</filename>. Assuming that the router
|
|
is connected to <filename class="devicefile">eth0</filename> and the
|
|
switch to <filename class="devicefile">eth1</filename>:</para>
|
|
|
|
<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
|
|
world br0 detect bridge
|
|
net br0:eth0
|
|
loc br0:eth1
|
|
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE</programlisting>
|
|
|
|
<para>The <emphasis>world</emphasis> zone is associated with the bridge
|
|
itself which is defined with the <emphasis role="bold">bridge</emphasis>
|
|
option. Bridge port entries may not have any OPTIONS.</para>
|
|
|
|
<note>
|
|
<para>When a bridge is configured without an IP address, the
|
|
<option>optional</option> option must also be specified.</para>
|
|
</note>
|
|
|
|
<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, which is also know as a brouter. 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/zones</filename> file is
|
|
modified:</para>
|
|
|
|
<programlisting>#ZONE TYPE OPTIONS
|
|
fw firewall
|
|
pub ipv4 #zone containing all public addresses
|
|
net:pub bport4
|
|
dmz:pub bport4
|
|
loc ipv4</programlisting>
|
|
</listitem>
|
|
|
|
<listitem>
|
|
<para>The <filename>/etc/shorewall/interfaces</filename> file is as
|
|
follows:<programlisting>#ZONE INTERFACE BROADCAST OPTIONS
|
|
pub br0 detect routefilter,bridge
|
|
net br0:eth0
|
|
dmz br0:eth2
|
|
loc eth1 detect</programlisting></para>
|
|
</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>
|
|
|
|
<listitem>
|
|
<para>This configuration does not support separate fw->dmz and
|
|
fw->net policies/rules; similarly, it does not support separate
|
|
loc->dmz and loc->net rules. This will make it a bit trickier to
|
|
configure the rules. I suggest something like the following:</para>
|
|
|
|
<para><filename>/etc/shorewall/params</filename>:</para>
|
|
|
|
<programlisting>SERVERS=192.0.2.177,192.0.2.178 #IP Addresses of hosts in the DMZ
|
|
DMZ=pub:$SERVERS #Use in place of 'dmz' in rule DEST
|
|
NET=pub:!$SERVERS #Use in place of 'net' in rule DEST</programlisting>
|
|
|
|
<para><filename>/etc/shorewall/policy</filename>:</para>
|
|
|
|
<programlisting>#SOURCE DEST POLICY LEVEL
|
|
loc <emphasis role="bold">pub</emphasis> ACCEPT
|
|
loc $FW REJECT info
|
|
loc all REJECT info
|
|
|
|
$FW <emphasis role="bold">pub</emphasis> REJECT info
|
|
$FW loc REJECT info
|
|
$FW all REJECT info
|
|
|
|
dmz net REJECT info
|
|
dmz $FW REJECT info
|
|
dmz loc REJECT info
|
|
dmz all REJECT info
|
|
|
|
net dmz DROP info
|
|
net $FW DROP info
|
|
net loc DROP info
|
|
net all DROP info
|
|
|
|
# THE FOLLOWING POLICY MUST BE LAST
|
|
all all REJECT info</programlisting>
|
|
|
|
<para><filename>/etc/shorewall/rules</filename>:</para>
|
|
|
|
<programlisting>#ACTION SOURCE DEST PROTO DEST SOURCE
|
|
#
|
|
PORT(S) PORT(S)
|
|
ACCEPT all all icmp 8
|
|
ACCEPT loc $DMZ tcp 25,53,80,443,...
|
|
ACCEPT loc $DMZ udp 53
|
|
ACCEPT loc $NET
|
|
ACCEPT $FW $DMZ udp 53
|
|
ACCEPT $FW $DMZ tcp 53 </programlisting>
|
|
</listitem>
|
|
</orderedlist>
|
|
</section>
|
|
|
|
<section id="Limitations">
|
|
<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>
|
|
</article>
|