shorewall_code/docs/FoolsFirewall.xml

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  <articleinfo>
    <title>The Fool's Firewall</title>

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

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

    <pubdate><?dbtimestamp format="Y/m/d"?></pubdate>

    <copyright>
      <year>2009</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, no Front-Cover Texts, and 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>Definition</title>

    <para>Occasionally, we hear from someone who has cabled his firewall's
    external and internal firewall interfaces to the same unmanaged switch (or
    mis-configured managed switch). I call this configuration <firstterm>The
    Fool's Firewall</firstterm>.</para>

    <para>When the external interface supports broadcast, this configuration
    has two very bad drawbacks:</para>

    <orderedlist>
      <listitem>
        <para>It is very insecure</para>
      </listitem>

      <listitem>
        <para>Both the up-stream router and the local systems can send
        incoming packets to the wrong interface.</para>
      </listitem>
    </orderedlist>
  </section>

  <section>
    <title>Security Issue</title>

    <para>Because Fool's firewall is not physically located between the net
    and the local systems, the local systems are exposed to all of the systems
    in the same broadcast domain. Because the local systems (especially those
    running Windows) send broadcasts, those systems can be easily detected by
    using a packet sniffer. Once the systems have been spotted, it is child's
    play to add an IP address in Fool's internal IP network and bypass his
    "Firewall".</para>

    <graphic align="center" fileref="images/Fools.png" />
  </section>

  <section>
    <title>ARP Roulette</title>

    <para>The Linux IP stack implements the <ulink
    url="http://en.wikipedia.org/wiki/Host_model">weak host model.</ulink> As
    a result, it exhibits some unexpected behavior with respect to ARP. It
    will respond to ARP 'who-has' requests received on
    <emphasis>any</emphasis> interface and not just on the interface owning
    the address. So when the upstream router sends a 'who-has' request for
    Fool's external IP address, the response may come from his
    <emphasis>internal</emphasis> interface (and reflect the MAC address of
    that interface). When that happens, packets from the net start entering
    the firewall's internal interface.</para>

    <graphic fileref="images/Foolsa.png" />

    <para>A similar problem can occur when a local system sends to the
    "Firewall" or to the Net. The packets may arrive on the firewall through
    the <emphasis>external</emphasis> interface.</para>
  </section>
</article>