User Manual Part 2

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: Place a check in this box to enable SPI. SPI ("stateful packet inspection" also known as "dynamic packet filtering") helps

to prevent cyber attacks 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.