Setup a Gateway

cat /proc/sys/net/ipv4/ip_forward

1

And make it permanent:

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

vi /etc/sysctl.conf

grep forwa /etc/sysctl.conf

sysctl -p /etc/sysctl.conf

cat /proc/sys/net/ipv4/ip_forward

/etc/init.d/network restart

cat /proc/sys/net/ipv4/ip_forward

Iptables:

-A FORWARD -i eth0 -j ACCEPT

where eth0 is internal nic

*nat

:PREROUTING ACCEPT [209:15828]

:POSTROUTING ACCEPT [47:3576]

:OUTPUT ACCEPT [53:4036]

-A POSTROUTING -o eth1 -j MASQUERADE

COMMIT

Or from cmd line

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

iptables -A FORWARD -i eth1 -j ACCEPT