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6.3.10. Firewall
The device provides a tight firewall by virtue of the way NAT works. Unless you configure the router to the contrary, the NAT does not
respond to unsolicited incoming requests on any port, thereby making your LAN invisible to Internet cyber attacks. However, some network
applications cannot run with a tight firewall. Those applications need to selectively open ports in the firewall to function correctly. The
options on this page control several ways of opening the firewall to address the needs of specific types of applications.
Enable SPI: Check this box to enable SPI. SPI ("stateful packet inspection" also known as "dynamic packet filtering") helps to prevent
cyberattacks by tracking more state per session. It validates that the traffic passing through that session conforms to the protocol.
When the protocol is TCP, SPI checks that packet sequence numbers are within the valid range for the session, discarding those
packets that do not have valid sequence numbers. Whether SPI is enabled or not, the router always tracks TCP connection states and
ensures that each TCP packet's flags are valid for the current state.