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From: Mark Felder <feld@FreeBSD.org>
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Subject: Re: transparent udp proxy
Date: Fri, 31 Oct 2014 09:00:53 -0500
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I'm not sure if this is what you're looking for, but perhaps the
solution is in net/samplicator ?

>From the project's website:

This simple program listens for UDP datagrams on a network port, and
sends copies of these datagrams on to a set of destinations. Optionally,
it can perform sampling, i.e. rather than forwarding every packet,
forward only 1 in N. Another option is that it can "spoof" the IP source
address, so that the copies appear to come from the original source,
rather than the relay. Currently only supports IPv4.