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Date: Mon, 1 Nov 2004 08:27:50 -0800
From: Aaron Nichols <adnichols@gmail.com>
To: Bill Eccles <bill.lists@eccles.net>
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References: <200410312349.08193.4711@chello.at>
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cc: freebsd-questions@freebsd.org
Subject: Re: ipfw configuration to intercept SMTP traffic
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> I believe you'll have one additional problem to resolve. Even if you
> successfully modify the destination IP address and get it pointed to
> the upstream server, the source IP will be unmodified and will still
> be the originator. Since the source IP is unmodified - the upstream
> mail server will send an ACK back to the originators IP (not yours)
> which will most likely get discarded and the connection will fail.
> Most sane TCP/IP stacks will reject an ACK from an IP address to which
> it did not send a request. Since the ACK is not going to run back
> through your host (thus allowing natd another go at reversing the
> translation) this likely wont work.

Sorry all - I had missed the post regarding use of the -proxy_rule
option, which may address this issue.

Didn't mean to futher confuse the issue. 

Aaron