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What is a Stateful Firewall?

By Solomon Branch
Updated May 16, 2024
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A stateful firewall is a computer or router that can monitor and filter the traffic coming across it dynamically, an architecture known as stateful packet inspection (SPI) or dynamic packet filtering. It allows for packets of data to be inspected more thoroughly than stateless firewalls, which can only monitor traffic based on static values, such as the address where the packet originated. Stateful firewalls are used when security is preferred over speed.

The protocol of the Internet, and networks in general, used to communicate between computers is built in layers. Most traffic that comes through a firewall will have a header, or initial packet, that identifies what it is for, where it’s going and what type of traffic it is. A stateless firewall can only look at the header of a packet, which is located in the most superficial layer. The stateful firewall can go deeper into other layers of the protocol and tell more about the packet, thus making it more dynamic.

A stateless firewall will typically look at traffic that comes across it and filter it using such information as the address where it is headed, the address where it came from and other pre-defined statistics. It is the simplest type of firewall and the easiest to use; most software-based firewalls use this technology. It not as secure as a stateful firewall, but it is typically faster because it doesn’t have to process as much information.

A stateful firewall can not only examine a packet more deeply, eliminating the chance a packet is pretending to be what it’s not and possibly cause damage, it can also keep track of incoming and outgoing traffic’s connection states. It will keep the information in a table, known as a state table, which lets it filter out and route traffic based on more detailed information, such as size of the packet and what part of the connection process it is in. This makes stateful firewalls more efficient in that they don’t have to re-inspect the packets for every part of the connection, they can simply check the state table; a much faster process, at least for security purposes. Overall, they are more secure than stateless firewalls but are generally slower.

Each type of firewall has its appropriate uses. For a home-based user who only has one computer, a stateless firewall, which is built into most operating systems anyway, will do fine; a stateful firewall can slow down a system. For larger networks, such as a large businesses or institutions, the stateful firewall would be the better choice. Any loss in speed is usually made up for by the fact the firewall is hardware and has its own processor and memory.

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