The word firewall is one of the most used ones to define OPNsense; even with a lot of other features, it is very common to hear from someone curious about your network topology asking, which firewall are you using in the network? The firewall feature is so important that it defines a whole network security platform. Let's find out why, beginning with the basics.

A stateful firewall

Every connection that a stateful firewall permits to pass will create a connection state, which means that the firewall will monitor all the connection information, such as the source, the destination, the protocol, the port number, and the protocol state. The protocols that a stateful firewall can handle are the ones that run on layers 3 and 4, using the OSI model as a reference. OPNsense running only with core features is considered a stateful firewall.

For example, monitoring the connection states will prevent common attacks that use the packet spoofing...