forward NAT rules

http://www.linuxtopia.org/online_books/centos_linux_guides/centos_linux_security_guide/s1-firewall-ipt-fwd.html

By default, the IPv4 policy in Red Hat Enterprise Linux kernels disables support for IP forwarding, which prevents boxes running Red Hat Enterprise Linux from functioning as dedicated edge routers. To enable IP forwarding, run the following command:

sysctl -w net.ipv4.ip_forward=1