Stateful packet inspection

In computing, a stateful firewall is a network firewall that tracks the operating state and characteristics of network connections traversing it. The firewall is configured to distinguish legitimate packets for different types of connections. Only packets matching a known active connection are allowed to pass the firewall.

Stateful packet inspection (SPI), also referred to as dynamic packet filtering, is a security feature often included in business networks.

Stateful firewall technology was introduced by Check Point Software with the FireWall-1 product in 1994.

Before the development of stateful firewalls, firewalls were stateless. A stateless firewall treats each network frame or packet individually. Such packet filters operate at the OSI Network Layer (layer 3) and function more efficiently because they only look at the header part of a packet. They do not keep track of the packet context such as the nature of the traffic. Such a firewall has no way of knowing if any given packet is part of an existing connection, is trying to establish a new connection, or is just a rogue packet. Modern firewalls are connection-aware (or state-aware), offering network administrators finer-grained control of network traffic.

The classic example of a network operation that may fail with a stateless firewall is the (FTP). By design, such protocols need to be able to open connections to arbitrary high ports to function properly. Since a stateless firewall has no way of knowing that the packet destined to the protected network (to some host's destination port 4970, for example) is part of a legitimate FTP session, it will drop the packet. Stateful firewalls with application inspection solve this problem by maintaining a table of open connections, inspecting the payload of some packets and intelligently associating new connection requests with existing legitimate connections.

Early attempts at producing firewalls operated at the application layer, which is the very top of the seven-layer OSI model. This method required exorbitant amounts of computing power and is not commonly used in modern implementations.

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