Grant,

>> :-) Which is what my initial question started with.   Should I just plug
>> a ethernet dongle into an USB port, or are other (and perhaps better)
>> options available ?
>
> There are a number of options.  Some are more practical than others.

:-)   Now I still know nothing.

The thing is I know very few options, and I named both ("hat" and dongle).
However, I do know that using an USB dongle will throttle the speed to that
of the USB port.  I have no idea if any of the other possibilities will do
better.

> That makes me think that you want hosts A and C to communicate with each
> other across some routed network.

No, no intermediate network.    The hosts are always in the same room.

> I thought you /explicitly/ wanted hosts A and C to NOT be able to talk to
> each other at all.

I did, and stil do.  But I also said /directly/.

Instead of an UUCP program dropping a file somewhere where the other side
can read it I was thinking of a program which reads a data paccket from one
interface and that writes to the other - in the process dropping all headers
(containing the MAC, IP, port) in either direction (replaced by ones from
the RPi).  It would be real-time communication, but definitily not directly.

Hmmmm... Does that UUCP program allow one interface to write a file while
the other interface is reading from it at the same time ?   That would be
comparable.

> Typically, in scenarios like the one you're asking about, it's okay if the
> services listen on multiple interfaces on B.

True.  As long as that process does not inadvertedly creates some kind of an
U-turn allowing data to pass from one interface to the other (allowing a
direct A-to-C communication).     Yeah I know, that sounds a bit paranoid,
but its better to make sure that that can't happen than (much) later pay the
price for it.

> Presuming you are using the same protocol on A and C, in some ways having
> Samba (et al.) on B listen on multiple interfaces is required.

Agreed.  But I would not have any qualms to install SAMBA twice, once for
each interface, only coming together on the file-storage level.

> Learning opportunities are good.  But you may want to avoid painting
> yourself into a corner and making things harder on yourself than is
> strictly necessary.

The former doesn't automatically follow from the latter.

> Why re-create programs if you can use existing programs.

Why else ?    Because I want to see if I can do it myself. :-)    Its also a
good(?) way to get to know the OS a bit under its surface. Besides, if that
fails I can always fall back on stuff that others wrote.

> Especially if said existing programs will happily work with the isolation
> that you desire.

They won't. But as I have quite a bit of experience with programming and
TCP/IP on a Windows machine I don't think that writing a program for it*
will give me much trouble.

*simple file-transfer first, maybe automatic clipboard exchange later.

Regards,
Rudy Wieser

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