Stateless Firewall

A stateless firewall inspects each packet in isolation, applying rules based on header fields such as source/destination IP, port, and protocol, but it does not maintain any record of past flows or connection state. This makes it extremely fast and memory-efficient — well suited to high-throughput environments like ISP edges, ACLs on routers and switches, or AWS Network ACLs — but it forces administrators to write explicit rules for both directions of every flow. Stateless filters are vulnerable to spoofing, fragmentation tricks, and traffic that exploits the lack of session context. Modern designs typically combine stateless ACLs at the network edge with stateful firewalls deeper in the architecture.