Transproxy and ipfw

[Rohit Neupane]

Hi,
`ipfw add 50 fwd 127.0.0.1,3128 tcp from any to any 80`  returns ipfw: 
getsockopt(IP_FW_ADD): Invalid argument

I'm running FreeBSD 4.6 with the default kernel. I guess 
IPFIREWALL_FORWARD option is enabled in kernel.
Do i need to enable it in /etc/rc.conf? if so then how?

Thanks a lot.
Regards,
Rohit Neupane