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3.2.10 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,
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