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Date: Sat, 25 Sep 1999 02:50:06 -0700
To: freebsd-questions@FreeBSD.ORG
From: Joe Bo <ibjoe@home.com>
Subject: ipfw, natd and RFC1918
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Hi,

I'm running v3.2 with ipfw and natd on a 2 nic machine
as a gateway for a RFC1918 network of windows PCs.

I changed the firewall type to "simple", and my internal
network could no longer get internet access.

of course in rc.firewall I have:
$fwcmd add divert natd all from any to any via ${natd_interface}
as the first line.

The problem was the 
$fwcmd add deny all from any to 172.16.0.0:255.240.0.0 via ${oif}
line from the "simple" firewall prototype, as per the distribution:

# Stop RFC1918 nets on the outside interface
$fwcmd add deny all from 192.168.0.0:255.255.0.0 to any via ${oif}
$fwcmd add deny all from any to 192.168.0.0:255.255.0.0 via ${oif}
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any via ${oif}
$fwcmd add deny all from any to 172.16.0.0:255.240.0.0 via ${oif} <-------
$fwcmd add deny all from 10.0.0.0:255.0.0.0 to any via ${oif}
$fwcmd add deny all from any to 10.0.0.0:255.0.0.0 via ${oif}

but is all this necessary? at
http://www.bellnetworks.net/cs/showrec.php3?story_id=3
it is stated that

# Stop RFC1918 nets on the outside interface 
$fwcmd add deny all from 192.168.0.0:255.255.0.0 to any in recv ${oif}
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any in recv ${oif}
$fwcmd add deny all from 10.0.0.0:255.0.0.0 to any in recv ${oif}

is appropriate.

My questions:

Is it true that
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any in recv ${oif}
is sufficient to stop (172.16) RFC1918 nets on the outside interface?

If so, why is it done by
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any via ${oif}
$fwcmd add deny all from any to 172.16.0.0:255.240.0.0 via ${oif}
in the FreeBSD distribution example (is there some advantage)?

and, as a side question, what is the difference between
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any in recv ${oif}
and
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any in via ${oif}
and
$fwcmd add deny all from 172.16.0.0:255.240.0.0 to any recv ${oif}
if any (or do they all say the same thing in different ways)?

thanks to all who can comment on this...

Joe



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