filtering router

An internetwork router that selectively prevents the passage of data packets according to a security policy. (See: guard.)

Tutorial: A router usually has two or more physical connections to networks or other systems; and when the router receives a packet on one of those connections, it forwards the packet on a second connection. A filtering router does the same; but it first decides, according to some security policy, whether the packet should be forwarded at all. The policy is implemented by rules (packet filters) loaded into the router. The rules mostly involve values of data packet control fields (especially IP source and destination addresses and TCP port numbers) [R2179]. A filtering router may be used alone as a simple firewall or be used as a component of a more complex firewall.