Strange things which should never happen (was Re: RFC 1918)

[Sean Donelan <sean () donelan com>]

In article <Pine.BSF.4.21.0007141956280.95155-100000 () overlord e-gerbil net>, "Richard A. Steenbergen" <ras () 
e-gerbil net> wrote:
> I still have not seen a single compelling arguement which says you gain
> one bit more security by filtering RFC1918-source'd packets. It is useless
> at best, and disruptive at worst.

I don't know my TCP/IP stack well enough, but what happens when a host
with multiple interfaces, one of which is assigned an RFC1918 address,
receives an packet through another interface with a source address the
same RFC1918 address.  Are the stacks smart enough to realize the packet
is really an external packet, or will they assume the packet came from
inside.