From owner-freebsd-net@FreeBSD.ORG  Wed Aug 20 21:34:42 2008
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Date: Wed, 20 Aug 2008 23:34:20 +0200 (CEST)
From: Adrian Penisoara <ady@freebsd.ady.ro>
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To: Javier Ubillos <jav@sics.se>
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Sender: Adrian Penisoara <ady@ady.ro>
Cc: freebsd-net@freebsd.org
Subject: Re: Override default ICMP (and other protocols) default replies.
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Hi,

On Wed, 20 Aug 2008, Javier Ubillos wrote:

> Hi freebsd-net.
> (Sorry for cross posting. This time I think I found the right forum for
> my question)
>
> I'm implementing a NAT (1 ip - 1 ip) like router. (it's not actually
> NAT, but it's a good analogy for this case).
>
> I have chosen to use pcaplib to pick up the packets. I have an
> implementation which picks up the packets, inspects them, rewrites the
> destination/source ip-addresses and sends them out on the repective
> interface.

Umm, this is going parallel to the real network stack.
Why not try to "hijack" the packets fro the kernel to the userland process 
with a feature like divert in ipfw(8) ?

>
> The problem I'm facing however is that my interfaces are answering to
> e.g. icmp-echo (ping) automatically, and I don't know how to turn this
> behaviour off.

This is a normal TCP/IP network stack feature in the kernel.
You may also find that connecting to one of the open ports on the machine 
will trigger a similar effect.

You need to cut off that packet before entering the upper network 
application layer in the kernel -- see suggestion above.

Regards,
Adrian.