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<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Good morning,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I have a Linux machine running with two NICs on it (+ the loopback
interface), which I would like to use as a firewall for my home network.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I would like to understand a bit more about iptables and
wonder if any of you could help me.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>In IPTABLES, there are three main queues of packets, namely
INPUT, FORWARD and OUTPUT.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I believe:<o:p></o:p></span></font></p>
<ul style='margin-top:0in' type=disc>
<li class=MsoNormal style='mso-list:l1 level1 lfo1'><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>INPUT and OUTPUT exist for both
eth0 and eth1, but FORWARD is shared between the two.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l1 level1 lfo1'><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>When testing packets in the
INPUT queue, only the input interface can be tested for, the output
interface won’t be set.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l1 level1 lfo1'><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>When testing packets in the OUTPUT
queue, only the output interface can be tested for, the input interface won’t
be set.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l1 level1 lfo1'><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>Unless “echo 1 >
/proc/sys/net/ipv4/ip_forward” is executed, packets will not be forwarded
from one interface to the other.<o:p></o:p></span></font></li>
</ul>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>One more related question:<o:p></o:p></span></font></p>
<ul style='margin-top:0in' type=disc>
<li class=MsoNormal style='mso-list:l0 level1 lfo2'><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>Does TCPDUMP show only arriving
packets or does it also show packets being emitted from the interface
being watched? Maybe I am using the wrong tool to watched the traffic go
by.<o:p></o:p></span></font></li>
</ul>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Thank you so much,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<div>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Osiris Pedroso</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>--<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Ph: (707) 658-3500</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>AOL AIM: osirisPedroso</span></font><o:p></o:p></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><a href="http://www.swoptimizer.com/">http://www.SWoptimizer.com</a></span></font><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'> <o:p></o:p></span></font></p>
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12.0pt'><o:p> </o:p></span></font></p>
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