Configuring Iptables For NAT

So you want to setup iptables for Natting:

echo 1 > /proc/sys/net/ipv4/ip_forward

Then setup your iptables to do the mangling (internal = eth1, external = eth0):

/sbin/iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
/sbin/iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
/sbin/iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT

For Me, tun3 with eth0 as the internal interface

/sbin/iptables -t nat -A POSTROUTING -o tun3 -j MASQUERADE
/sbin/iptables -A FORWARD -i tun3 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
/sbin/iptables -A FORWARD -i eth0 -o tun3 -j ACCEPT