Stand-a-lone NAT PGM

[FBSD_User]

I have been looking for NAT program which is not part of some other
program like NATD is part of IPFW or IPNAT is part of IPFILTER or
NAT of user ppp. I can not find any NAT program in the FBSD ports
collection unless I mis-understood some description.  If anybody
knows of one please let me know. Yes I know that I can set firewall
rule to pass all packets just to use NAT function, but I don't want
overhead of firewall logic, just simple NAT like PPP NAT function
with out the PPP stuff.

I was wondering if the NAT logic code from user ppp could be copied
and made into stand-a-lone NAT program. My programming ability is
not great so I am asking for opinions  on weather this is technical
possible?  Thanks