In computing, a stateful firewall (any firewall that performs stateful packet inspection or stateful inspection) is a firewall that keeps track of the state of network connections (such as TCP streams) traveling across it. The firewall is programmed to know what legitimate packets are for different types of connections. Only packets which match a known connection state will be allowed by the firewall; others will be rejected.
Early attempts at producing firewalls operated at the application level of the seven-layer OSI model but this required too much CPU speed. Packet filters operate at the network layer (layer-3) and function more efficiently because they only look at the header part of a packet. However, pure packet filters have no concept of state as defined by computer science using the term finite state machine and are subject to spoofing attacks and other exploits.
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