Which of the following is a firewall that keeps track of the state of network connections traveling across it?
A . Stateful firewall
B . Stateless packet filter firewall
C . Circuit-level proxy firewall
D . Application gateway firewall
A stateful firewall 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 connection state will be allowed by the firewall; others will be rejected. Answer option B is incorrect. A stateless packet filter firewall allows direct connections from the external network to hosts on the internal network and is included with router configuration software or with Open Source operating systems.
Answer option C is incorrect. It applies security mechanisms when a TCP or UDP connection is established.
Answer option D is incorrect. An application gateway firewall applies security mechanisms to specific applications, such as FTP and Telnet servers.