It comes in hand when analyzing traffic to forward a copy of the traffic to a specific IP where a machine is listening and running Wireshark & stuff. It’s very useful for routers that don’t have the capabilities to run network analysis tools (like DD-WRT).
Just run the following commands replacing the xxx.xxx.xxx.xxx field with the IP of your listening machine:
# iptables -t mangle -A POSTROUTING -d 0.0.0.0/0 -j ROUTE --tee --gw xxx.xxx.xxx.xxx
# iptables -t mangle -A PREROUTING -s 0.0.0.0/0 -j ROUTE --tee --gw xxx.xxx.xxx.xxx