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To: "John H. Aughey" <jha@freefall.freebsd.org>
cc: freebsd-hackers@freefall.freebsd.org
Subject: Re: Pre/Post processing IP packets 
In-reply-to: Your message of "Fri, 14 Mar 1997 09:45:03 PST."
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Date: Fri, 14 Mar 1997 20:27:02 +0000
From: Brian Somers <brian@awfulhak.demon.co.uk>
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> I want to be able to pre-process and post-process IP packets which
> are received by a network interface or are going out a network
> interface.  Basically I want to be able to look at an IP packet
> immediately after it's received by an interface, fiddle around with
> the packet, and then pass the modified packet back to the kernel
> for further processing.  In the same way I want to look at a packet
> that is going out an interface, fiddle around with the packet, and
> then pass the modified packet back to the kernel for actual
> transmission.

Have a look on http://www.srv.com/~cmott.  The natd program is a good
example of using divert(4) sockets.

The idea is that you tell the firewall software to "divert" certain
packets to a given raw socket.  You then write a program that sits
on that raw socket, plays with packets and re-injects them into the
ip handling routines in the kernel.

I'm going to make a "port" of natd soon, but I've been quite busy
lately :|
-- 
Brian <brian@awfulhak.demon.co.uk>, <brian@freebsd.org>
      <http://www.awfulhak.demon.co.uk/>
Don't _EVER_ lose your sense of humour....