The four techniques used by firewalls to control access and enforce a security policy are Service control, Direction control, User control and Behavior control. Service control regulates the types of Internet services that can be accessed inbound or outbound which can be done by funneling traffic on the basis of an IP address, protocol or port numbers. The can be done by making use of proxy software or host server software. Direction control regulates the direction in which particular service requests may be initialized and are allowed to flow via through the firewall. A User control manages or authorizes admission to a service according to which entity is trying to access that specified service. A User control typically applied to access inside a perimeter but also can be applied to incoming external traffic. Finally there is Behavior control which manages how specific services are used such as filtering of emails.
