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Stateful inspection is a technique used in computer network firewalls for protecting a network from unauthorized access. Also sometimes known as dynamic filtering, the method is capable of inspecting an entire data packet before it enters the network. In this way, every packet entering any interface on the firewall is checked completely for validity against the types of connections that are allowed to pass through to the other side. The process gets its name because it not only inspects the data packets, but also monitors the state of a connection that has been established and allowed through the firewall.
The idea for stateful inspection was first devised by Check Point® software, back in the mid 1990s. Prior to Check Point's® Firewall-1 INSPECT™ engine software, firewalls monitored the application layer, at the top of the open systems interconnection (OSI) model. This tended to be very taxing on a computer's processor, so packet inspection moved down the OSI model's layers to the third layer, the network layer. Early packet inspection only checked the header information, the addressing and protocol information, of packets and had no way of distinguishing the state of the packet, such as whether it was a new connection request.
In a stateful inspection firewall, the resource-friendly and speedy packet filtering method is merged somewhat with the more detailed application information. This gives some context to the packet, thereby providing more information from which to base security decisions. To store all of this information, the firewall needs to establish a table, which then defines the state of the connection. The details of every connection, including the address information, ports and protocols, as well as the sequencing information for the packets, are then stored in the table. The only time resources are strained at all is during the initial entry into the state table; after that, every other packet matched against that state uses hardly any computing resources.
The stateful inspection process begins when the first packet requesting a connection is captured and inspected. The packet is matched against the firewall's rules, where it is checked against an array of possible authorization parameters which are endlessly customizable in order to support previously unknown, or as yet to be developed, software, services and protocols. The captured packet initializes the handshake, and the firewall sends a response back to the requesting user acknowledging a connection. Now that the table has been populated with state information for the connection, the next packet from the client is matched against the connection state. This continues until the connection either times out or is terminated, and the table is cleared of the state information for that connection.
This brings about one of the issues faced by the stateful inspection firewall the denial of service attack. With this type of attack, the security isn't compromised in as much as the firewall is bombarded with numerous initial packets requesting a connection, forcing the state table to fill up with requests. Once full, the state table can no longer accept any requests, and so all other connection requests are blocked. Another attack method against a stateful firewall takes advantage of the firewall's rules to block incoming traffic, but allow any outgoing traffic. An attacker can trick a host on the secure side of the firewall into asking for connections from the outside, effectively opening up any services on the host for the attacker to use.