Stateful firewalls monitor and filter traffic dynamically, providing more thorough packet inspection than stateless firewalls. They can track connection states and are more secure, but slower. Stateless firewalls are simpler and faster but less secure. The appropriate firewall depends on the size of the network.
A stateful firewall is a computer or router that dynamically monitors and filters traffic passing through it, an architecture known as stateful packet inspection (SPI) or dynamic packet filtering. It allows you to inspect data packets more thoroughly than stateless firewalls, which can only monitor traffic based on static values, such as the source address of the packet. Stateful firewalls are used when security is preferred over speed.
The Internet protocol, and networks in general, used to communicate between computers is built in layers. Most traffic that passes through a firewall will have a header, or initial packet, that identifies what it’s for, where it’s going, and what kind of traffic it is. A stateless firewall can only look at the header of a packet, which is at the most superficial level. The stateful firewall can delve into other layers of the protocol and provide more information about the packet, thus making it more dynamic.
A stateless firewall typically examines the traffic that passes through it and filters it using information such as the address it’s going to, the address it’s coming from, and other predefined statistics. It is the simplest type of firewall and the easiest to use; most software-based firewalls use this technology. It’s not as secure as a stateful firewall, but it’s usually faster because it doesn’t have to process as much information.
Not only can a stateful firewall examine a packet more thoroughly, eliminating the possibility of a packet pretending to be what it isn’t and causing damage, but it can also track the connection states of incoming and outgoing traffic. It will keep information in a table, known as the state table, which allows traffic to be filtered and routed based on more detailed information, such as packet size and what part of the connection process it is in. This makes stateful firewalls more efficient as they don’t have to review packets for each part of the connection, they can simply check the state table; a much faster process, at least for security reasons. Overall, they are more secure than stateless firewalls, but are generally slower.
Each type of firewall has its appropriate uses. For a home user who has only one computer, a stateless firewall, which is built into most operating systems anyway, will do; a stateful firewall can slow down a system. For larger networks, such as large companies or institutions, the stateful firewall would be the best choice. Any loss in speed is usually compensated for by the fact that the firewall is hardware and has its own processor and memory.
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