What type of firewall maintains the state of active connections?

A stateful firewall is designed to keep track of the state of active connections. It monitors the traffic that flows through it, understanding the context of the communication by maintaining a state table that records details about ongoing connections. This capability allows the stateful firewall to make more informed decisions about which packets to allow through or block, based on whether they are part of an established connection or not.

When a packet arrives, the stateful firewall checks its state table to see if the packet is part of an existing connection. If it is, the firewall allows it through, whereas if the packet does not match any entry in the table, it is typically blocked unless it is a new connection that meets certain criteria set by security policies. This provides a level of security that is more refined than that of stateless or packet filtering firewalls, which do not maintain any information about the state of active connections.

By utilizing this stateful connection tracking, stateful firewalls can prevent certain types of attacks, such as session hijacking, while also allowing legitimate users to establish and maintain their connections efficiently.