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<TITLE>The Answer Guy 33:
IP and Sendmail Masquerading over a Cablemodem</TITLE>
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ip masquerading
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<H4>"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<H4 align="center">By James T. Dennis,
<a href="mailto:answerguy@ssc.com">answerguy@ssc.com</a>
<BR>Starshine Technical Services, <A HREF="http://www.starshine.org/">http://www.starshine.org/</A>
</H4>
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<H3><img src="../../gx/dennis/qbub.gif" alt="(?)"
width="50" height="28" align="left" border="0"
>IP and Sendmail Masquerading over a Cablemodem</H3>
<p><strong>From Marty Leisner on 22 Sep 1998 </strong></p>
<!-- begin body -->
<p><strong>
I read your column in the May LG. (I'm behind on my reading
<IMG SRC="../../gx/dennis/smily.gif" ALT=":-)" height="24" width="20"
align="middle">)
</strong></p>
<p><strong>
I recently (last month) got a cable modem and hooked up a
masquerading firewall...
</strong></p>
<p><strong>
On the firewall machine, I have the rule:
</strong></p>
<pre><strong>ipfwadm -F -p deny
ipfwadm -F -a m -S 192.168.0.0/24 -D 0.0.0.0/0
</strong></pre>
<p><strong>
I got this of the IP-masquerade howto...
</strong></p>
<p><strong>
I'm not sure if its the same as the rule:
<br><code>ipfwadm -F -a accept -m -S 192.168.1.0/24 -D any</code>
</strong></p>
<blockquote><img src="../../gx/dennis/bbub.gif" alt="(!)"
width="50" height="28" align="left" border="0"
>Mine is similar, all 253 of the 192.168.1.* through the
192.168.254.* class C address blocks are reserved for
"private net" addressing (use behind proxying firewalls,
masquerading/NAT (network address translation) routers,
and on disconnected LAN's).
</blockquote>
<blockquote>
I've heard conflicting reports about using 192.168.0.* and
192.168.255.* (the first and the last of this range).
So I don't recommend it. If you needed a very large network
of "private net" (RFC 1918 --- aka RFC 1597) addresses you
could also use 172.16.*.* through 172.31.*.* --- that's
sixteen adjacent class B networks, or your could use
10.*.*.* --- a full class A.
</blockquote>
<p><strong><img src="../../gx/dennis/qbub.gif" alt="(?)"
width="50" height="28" align="left" border="0"
>Also, you sendmail .mc:
</strong></p>
<pre><strong>
-- FEATURE(always_add_domain)dnl
FEATURE(allmasquerade)dnl
FEATURE(always_add_domain)dnl
FEATURE(masquerade_envelope)dnl
MASQUERADE_AS($YOURHOST)dnl
</strong></pre>
<p><strong>
adds always_add_domain twice...
</strong></p>
<blockquote><img src="../../gx/dennis/bbub.gif" alt="(!)"
width="50" height="28" align="left" border="0"
>That's just a typo.
</blockquote>
<p><strong><img src="../../gx/dennis/qbub.gif" alt="(?)"
width="50" height="28" align="left" border="0"
>Is $YOURHOST defined someplace (I just went through the work
of configuring sendmail a few weeks ago).
</strong></p>
<blockquote><img src="../../gx/dennis/bbub.gif" alt="(!)"
width="50" height="28" align="left" border="0"
>I used $YOURHOST as a marker for my readers to fill in
with <EM>their</EM> sendmail name. Mine is "starshine.org"
--- yours is a subdomain off of "rr.com" I expected
people to clue into that; though I probably should
explicitly pointed it out.
</blockquote>
<p><strong><img src="../../gx/dennis/qbub.gif" alt="(?)"
width="50" height="28" align="left" border="0"
>The Feynman problem solving Algorithm
</strong></p>
<strong><ol>
<li>Write down the problem
<li>Think real hard
<li>Write down the answer
</ol></strong>
<p><strong>
--- Murray Gell-mann in the NY Times
</strong></p>
<blockquote><img src="../../gx/dennis/bbub.gif" alt="(!)"
width="50" height="28" align="left" border="0"
>He forgot to show his work in step two!
</blockquote>
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<H5 align="center"><a href="http://www.linuxgazette.com/ssc.copying.html"
>Copyright ©</a> 1998, James T. Dennis <BR>
Published in <I>Linux Gazette</I> Issue 33 October 1998</H5>
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