On Sun, 28 Jun 2020 22:22:31 +0200
"R.Wieser"  wrote:

> Dennis,
>
> > You still aren't clear on WHAT this communication IS...
>
> Ah.  The answer to that is simple: Anything I want.    Datapackets
> normally do not care what they contain. :-)

 OK so take it step by step. First achieve the necessary physical
connection then decide on what mode of data transfer you want to permit.
Examples:

 Machines on A or B can place files somewhere where they can be seen
by both A and B. Solution Samba and/or NFS. Obvious variations where A and
B have separate writable areas.

 Machines on A and B can have a variety of secure file drops that
they can make available to machines on A or B selectively - sftp or
similar, but you can torture Samba into it if you are good at Windows
networking.

 Machines on A can reach selected services on B (and vice-versa) via
a proxy on the Pi - the proxy can be as simple as a firewall based packet
forwarding service or as complex as squid.

 Any combination of the above.

 The Pi provides the illusion of a transparent connection between A
and B but in reality intercepts, filters and potentially modifies any and
all traffic between them. This is done with the firewall capabilities of
Linux and is as complex as you want to make it plus about 10% usually.

> And that is all I intended the connection with the RPi as a middle-man to
> be: To be able to send data either way, but without either of the two
> sides being able to see each other.   No more, no less.

 The devil is in the details - the ones above are just a handful of
examples that sprang to mind. The main thing is you need the physical
connections working so that you can ping the pi from either network. Nothing
else happens until you make it but that's the starting point upon which all
the rest is built - so get that going first while you work out exactly what
you want to try and pull off first - it's all possible, it's all open
source and some of it is a tad fiddly.

--
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