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Subject: Re: The problem with MTU <1500
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You may use this patch to ipfw
http://www.freebsd.org/cgi/query-pr.cgi?pr=103454
Or you may use ng_patch netgraph node (man ng_patch(4) should give you some
examples)
The simpliest way to do what you want is pf rule
 scrub in on em0 no-df

performance of all those methods you should check by youre self.