Stateful vs Stateless Packet Filtering
The core difference between traditional ACLs and stateful firewalls.
Stateless filtering evaluates every packet in isolation against a static rule set, with no awareness of connection state.
Stateful firewalls track connection state in a table, automatically permitting return traffic for established sessions without an explicit inbound rule.
Most modern firewalls, including pfSense and enterprise NGFWs, default to stateful inspection for performance and security reasons.