From owner-freebsd-hackers@FreeBSD.ORG  Thu Aug 16 02:42:49 2007
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Date: Wed, 15 Aug 2007 21:42:44 -0500
From: Eric Anderson <anderson@freebsd.org>
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Subject: Modifying bridged traffic
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What is the easiest way to play with modifying data in-transit within an 
ethernet bridge?

For instance, say I have something like this:

[BOX 1] <----> [ BOX 2 ] <----> [ BOX 3 ]

And BOX 2 is a FreeBSD box with bridging enabled between two ethernet 
interfaces, how can I parse/modify the ethernet frames as they pass through?

Eric