A firewall is a combination of hardware and software, used to provide security to a network. It is used to protect an internal network or intranet against unauthorized access from the Internet or other outside networks. It restricts inbound and outbound access and can analyze all traffic between an internal network and the Internet. Users can configure a firewall to pass or block packets from specific IP addresses and ports.

Which of the following tools works as a firewall for the Linux 2.4 kernel?