how ipfw firewall is implemented in the kernel

[Biks N]

Hi,

Can anyone please help me understand how the IPFW firewall is
implemented in the kernel.

I have created new ACTIONS in ipfw. I have already implemented in the userland.

Now i need to check the IPFW rule list (in ip_input.c and in
ip_output.c) and call a custom routine if there is a match to those
rules.

I would really appreciate if anyone could point me to right direction/reference.

thanks