In computing, a stateful firewall (any firewall that performs stateful packet inspection (SPI) or stateful inspection) is a firewall that keeps track of the state of network connections (such as TCP streams, UDP communication) traveling across it. The firewall is programmed to distinguish legitimate packets for different types of connections. Only packets matching a known active connection will be allowed by the firewall; others will be rejected.
Stateful inspection, also referred to as Dynamic Packet Filtering, is considered a standard for enterprise-class network security solutions. Nir Zuk invented and patented the technology during his time at Checkpoint, and also coined the term stateful inspection in the use of its FireWall-1 in 1994.