proxy_rule is missing in kernel nat?

[lsrz2006 at sina.com]

                        I had using natd for a proxy for a while time, using the command:
natd -p 3333 -proxy_only -proxy_rule "server 192.168.6.6 proto tcp"

but when I use this in kernel nat config, I get a error infomation:

# ipfw nat 20 config proxy_only proxy_rule server 192.168.6.6ipfw: unrecognised option ``proxy_rule''
I'd readed the src code in /usr/src/sbin/ipfw and can't find the LibAliasProxyRule() function which is in the natd.c.
Does I have a mistake?