Is SPI built-in?

[kingsfan]

The marketing page for the router speaks to it's Stateful Packet Inspection (SPI) capability in the firewall, but I do not see a setting to turn this on. Is this on by default?

 

[RMerlin]

The marketing page for the router speaks to it's Stateful Packet Inspection (SPI) capability in the firewall, but I do not see a setting to turn this on. Is this on by default?

While SPI sound very technically impressive, it's nothing really special. It's not an option that can be enabled or disabled, it's part of the firewall rules of pretty much any Linux-based firewall, through two simple rules:

Chain FORWARD (policy DROP)
target     prot opt source               destination         
...snip...
ACCEPT     all  --  anywhere             anywhere            state RELATED,ESTABLISHED 
DROP       all  --  anywhere             anywhere            state INVALID

Packets with a related or established state are allowed in. Packets with an invalid state are dropped. The rest has to go through the firewall rules to determine what happens to them (forwarded or dropped).