Right now, I'm using a 3-interface box to bridge different segments of 
my home network and connect them to my cable modem.  (The cable modem 
has its own NAT function.)  That's fine for v4; however, I'd like to 
pick up v6 packets there and tunnel them somewhere via 6to4.  I suspect 
that it isn't possible to do that; I'd be happy to find out I'm wrong.  
(If utterly necessary, I could bridge two interfaces, and have NetBSD 
NAT out the third interface, to the cable modem.  For various reasons, 
I'd rather not do that.)

		--Steven M. Bellovin, http://www.cs.columbia.edu/~smb