Simple nat setup for iptables.
External interface is eth0. Internal interface is eth1.
If you use pppoe connection you may want to replace eth0 by ppp0 for IF_EXTERNAL constant.
Will show following output:
SETTING UP IPTABLES NAT
Loading required stateful/NAT kernel modules...
Configurating system parameters...
External interface: eth0 (172.30.1.38)
Internal interface: eth1 (172.19.255.254)
Loading iptables rule sets...
DONE.