Using a 2-enet-port Unix host as a bridge instead of a router?

[Malcolm Carlock]

Anytime you've got a Unix host with N >= 2 ethernet (or other network)
interfaces that can handle IP traffic, you've pretty much got a router
on your hands if you want it.

My question:  is it possible for such a machine to be used as a bridge,
rather than a router, and still act as a "normal" IP host, albeit with two
ports on the same logical IP network (and still run Un*x, or whatever) ?

I'm not familiar with any vendors' equipment or software that supports
such a scheme, and it does seem pretty far-fetched:  having to generate
ethernet frames with "fake" source addresses in them (is this even possible
with the majority of ethernet interfaces?) while still listening for one's
own traffic, etc.  However, if a PC can be used as a bridge, then it does
seem within the realm of possibility to do it with a Un*x host.

Seems like it would definitely be a major load on the CPU;  a router only
needs to look at packets that other machines have explicitly forwarded to
it.  A machine running as a bridge would have to look at EVERY packet.

Anyone ever heard of this being done?  No, I'm not interested in trying to
implement such a scheme, but the "has it ever been done" question is likely
to arise soon in some local networking matters (whew, don't ask.)

Thanks in advance,


Malcolm L. Carlock              Internet:  malc at unrvax.unr.edu
                                UUCP:      uunet!unrvax!malc