mirror of
https://gitlab.com/shorewall/code.git
synced 2024-11-14 03:34:31 +01:00
95 lines
3.3 KiB
XML
95 lines
3.3 KiB
XML
<?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>
|
|
<!--$Id$-->
|
|
|
|
<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, 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>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>
|