I believe you are looking for NAT part of netbsd. Am not sure about 1.2 
but it's in -current

On Thu, 26 Jun 1997, Alicia da Conceicao wrote:

> Greetings NetBSD users:
> 
> I've have a TCP/IP lan which I've set up with with non-routable Intranet
> IP numbers (192.168.0.X).  One of the computers on the lan is an i386 box
> running NetBSD 1.2, with PPP and no IP forwarding.  That computer has a
> dedicated PPP connection to the Internet with a static IP number, which 
> through the use of various proxies (Apache, Sendmail, Named, etc.) is the
> Internet gateway and firewall for the Intranet lan.
> 
>      Anyways, although this solution works, it is not very scalable, and
> is quite restrictive in that proxies have to be installed for every
> addition Internet service I wish to use on the lan.  :-(  To solve this
> problem, I wish to replace all of those proxies on the gateway with a
> single transparent proxy that works with address translation, so that
> the lan IP numbers (192.168.0.X) of Internet packets going through the
> gateway get translated to the "real" static IP number of the gateway.
> The Cisco 700 series routers are capable of this, but I was hoping to
> do the same with an i386 NetBSD box.  Better yet, I would also like
> some packet filtering abilites with the transparent proxy for added
> security.
> 
>      Does anyone have any suggestions on what software / configuration
> can be used to set up a transparent proxy with address translation that
> can be installed on an i386 NetBSD lan gateway?  Any help would be
> appreciated.
> 
> Sincerely, Alicia.
> _____                                                              
>  Name: Alicia da Conceicao         Bus: Media-City                  _
> Email: alicia@media-city.com       WWW: http://www.media-city.com  | |-+ /
> Phone: 416-410-6855                                                | | | \__/
> Smail: 52 Clinton Street, Toronto, Ontario, M6J-2Y3, Canada        ~~~~~
> 
> 


..............          ....................................
Thomas T. Thai          Infomedia Interactive Communications
tom@iic.net             TEL 612.376.9090 * FAX 612.376.9087