shorewall_code/Shorewall-docs2/bridge.xml

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  <articleinfo>
    <title>Shorewall and Bridged Firewalls</title>

    <authorgroup>
      <author>
        <firstname>Tom</firstname>

        <surname>Eastep</surname>
      </author>
    </authorgroup>

    <pubdate>2004-04-12</pubdate>

    <copyright>
      <year>2004</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>

  <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.
    Beginning with Shorewall version 2.0.1, 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>A router cannot forward broadcast packets while a bridge can.</para>
      </listitem>
    </orderedlist>
  </section>

  <section>
    <title>Requirements</title>

    <para>In order to use Shorewall with 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>).</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>

      <listitem>
        <para>You must be running Shorewall 2.0.1 Beta 1 or later.</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&#39;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&#39;s IP address (192.168.1.254 in the above diagram) as their
        default gateway.</para>
      </listitem>

      <listitem>
        <para><command>traceroute</command> doesn&#39;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, Linux distributions don&#39;t have good bridge
    configuration tools and the network configuration GUIs don&#39;t detect
    the presence of bridge devices. You may refer to <ulink
    url="http://shorewall.net/2.0/myfiles.htm">my configuration files</ulink>
    for an example of configuring a three-port bridge at system boot under
    <trademark>SuSE</trademark>. 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.</para>

    <para>The bridge may have its IP address assigned via DHCP. Here&#39;s an
    example of an /etc/sysconfig/network/ifcfg-br0 file from a
    <trademark>SuSE</trademark> system:</para>

    <blockquote>
      <programlisting>BOOTPROTO=&#39;dhcp&#39;
REMOTE_IPADDR=&#39;&#39;
STARTMODE=&#39;onboot&#39;
UNIQUE=&#39;3hqH.MjuOqWfSZ+C&#39;
WIRELESS=&#39;no&#39;
MTU=&#39;&#39;</programlisting>
    </blockquote>

    <para>Here&#39;s an /etc/sysconfig/network-scripts/ifcfg-br0 file for a
    <trademark>Mandrake</trademark> system:</para>

    <blockquote>
      <programlisting>DEVICE=br0
BOOTPROTO=dhcp
ONBOOT=yes</programlisting>
    </blockquote>

    <para>On both the SuSE and Mandrake systems, a separate script is required
    to configure the bridge itself (again see <ulink url="myfiles.htm">my
    configuration files</ulink> for an example - <filename>/etc/init.d/bridge</filename>).</para>

    <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>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   DISPLAY         COMMENTS
net     Net             Internet
loc     Local           Local networks
#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>
    <title>Combination Router/Bridge</title>

    <para>A system running Shorewall doesn&#39;t have to be exclusively a
    bridge or a router -- it can act as both. Here&#39;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 confiiguration.</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>
    </orderedlist>
  </section>

  <section>
    <title>Limitations</title>

    <para>Bridging doesn&#39; t work with some wireless cards — see <ulink
    url="http://bridge.sf.net">http://bridge.sf.net</ulink>.</para>
  </section>
</article>