
Howto: Copy/Tee/Clone network traffic using iptables - bluesmoon
http://www.bjou.de/blog/2008/05/howto-copyteeclone-network-traffic-using-iptables/
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there
this can be done easily with pf using the dup-to flag of a match statement.

using a span port or other physical network tap would probably be a better
solution than these things in production, though.

