a good tutorial on IPtables Tutorial (http://iptables-tutorial.frozentux.net/iptables-tutorial.html) with sample scripts for various implementations, eg
firewall, DMZ, DHCP.

Scripts are native BASH, so systems with Borne Shell or equiv will easily implement these rules

the attachment is a trivial set of rules to allow LAN access
and control WAN access.

File would be placed locate and named /etc/sysconfig/iptables