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Add some advice about Martians

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git-svn-id: https://shorewall.svn.sourceforge.net/svnroot/shorewall/trunk@5399 fbd18981-670d-0410-9b5c-8dc0c1a9a2bb
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teastep 2007-02-11 18:35:38 +00:00
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docs/MultiISP.xml
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@ -597,6 +597,59 @@ done</programlisting>
of failure of one of your Internet connections</emphasis>.</para>
</section>
<section id="Martians">
<title>Martians</title>
<para>One problem that often arises with Multi-ISP configuration is
'Martians'. If your internet interfaces are configured with the
<emphasis role="bold">routefilter</emphasis> option in
<filename>/etc/shorewall/interfaces</filename> (remember that if you set
that option, you should also select <emphasis
role="bold">logmartians</emphasis>), then things may not work correctly
and you will see messages like this:</para>
<programlisting>Feb 9 17:23:45 gw.ilinx kernel: martian source 206.124.146.176 from 64.86.88.116, on dev eth1
Feb 9 17:23:45 gw.ilinx kernel: ll header: 00:a0:24:2a:1f:72:00:13:5f:07:97:05:08:00</programlisting>
<para>The above message is somewhat awkwardly phrased. The source IP in
this incoming packet was 64.86.88.116 and the destination IP address is
206.124.146.176. Another gotcha is that the incoming packet has already
had the destination IP address changed for DNAT or because the original
outgoing connection was altered by an entry in
<filename>/etc/shorewall/masq</filename> (SNAT or Masquerade). So the
destination IP address (206.124.146.176) may not have been the
destination IP address in the packet as it was initially
received.</para>
<para>There a couple of common causes for these problems:</para>
<orderedlist>
<listitem>
<para>You have connected both of your external interfaces to the
same hub/switch. Connecting multiple firewall interfaces to a common
hub or switch is always a bad idea that will result in
hard-to-diagnose problems.</para>
</listitem>
<listitem>
<para>You are specifying both the <emphasis
role="bold">loose</emphasis> and <emphasis
role="bold">balance</emphasis> options on your provider(s). This
causes individual connections to ping-pong back and forth between
the interfaces which is guaranteed to cause problems.</para>
</listitem>
<listitem>
<para>You are redirecting traffic from the local system out of one
interface or the other using packet marking in your
<filename>/etc/shorewall/tcrules</filename> file. A better approach
is to configure the application to use the appropriate local IP
address (the IP address of the interface that you want the
application to use). See <link linkend="Local">below</link>.</para>
</listitem>
</orderedlist>
</section>
<section>
<title>Example</title>
@ -621,17 +674,19 @@ net eth1 detect …</programlisting>
<programlisting>#SOURCE DESTINATION POLICY LIMIT:BURST
net net DROP</programlisting>
<para>Regardless of whether you have masqueraded hosts or not, <emphasis
role="bold">YOU MUST ADD THESE TWO ENTRIES TO
<filename>/etc/shorewall/masq</filename></emphasis>:</para>
<para>Regardless of whether you have masqueraded hosts or not, the
following entries are required in
<filename>/etc/shorewall/masq</filename> if you plan to redirect
connections from the firewall using entries in
<filename>/etc/shorewall/tcrules</filename>.</para>
<programlisting>#INTERFACE SUBNET ADDRESS
eth0 130.252.99.27 206.124.146.176
eth1 206.124.146.176 130.252.99.27</programlisting>
<para>Those entries ensure that traffic originating on the firewall
always has the source IP address corresponding to the interface that it
is routed out of.</para>
<para>Those entries ensure that traffic originating on the firewall and
redirected via packet marks always has the source IP address
corresponding to the interface that it is routed out of.</para>
<note>
<para>If you have a Dynamic IP address on either of the interfaces,
@ -679,13 +734,16 @@ eth1 eth2 130.252.99.27</programlisting>
2:P &lt;local network&gt; 0.0.0.0/0 tcp 25</programlisting>
</section>
<section>
<section id="Local">
<title>Applications running on the Firewall</title>
<para>Experience has shown that in some cases, problems occur with
applications running on the firewall itself. When this happens, it is
suggested that you have the application use specific local IP addresses
rather than 0.</para>
applications running on the firewall itself. This is especially true
when you have specified <emphasis role="bold">routefilter</emphasis> on
your external interfaces in /etc/shorewall/interfaces (see <link
linkend="Martians">above</link>). When this happens, it is suggested
that you have the application use specific local IP addresses rather
than 0.</para>
<para>Examples:</para>