Netfilter is a host-based firewall for Linux operating systems. It is included as part of the Linux distribution and is activated by default. This firewall is controlled by the program called iptables. Netfilter filtering takes place at the kernel.
www.cyberciti.biz/faq/rhel-fedorta-linux-iptables-firewall-configuration-tutorial/
Covers using IPTABLES with the 2.4 Linux kernel. If you created the proxy server, all you have to do is add a few more IPTABLES commands to enhance the firewalling functionality of the system.
www.aboutdebian.com/firewall.htm
Answers to some common questions about firewalls.
windows.microsoft.com/en-US/windows-vista/Firewall-frequently-asked-questions