Iptables forwarding interfaces through VPN interface back and forth

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http://serverfault.com – I am trying to recreate this behaviour using iptables under ubuntu : Let's say we have 3 interfaces : eth0, eth1 and eth2. I must forward the traffic through a VPN interface vpn0. Then, I must assure that the incoming traffic through the VPN is redirected correctly to the right interface ( eth0, eth1 or eth2 ). I have searched long for an answer, but didn't find anything truly usefull. I have little experience with iptables and a starting point would be very appreciated. (HowTos)