On Sun, 28 Jun 2020 16:51:47 +0200
"R.Wieser"  wrote:

> Ahem,
>
> > That depends - if both networks have DHCP servers then just
> > configure the new interface to use DHCP (probably default),
> > plug it in and watch it connect.
>
> That sounds easy enough.

 It *should* be that easy - worst case you have to add a config file
for the USB ethernet.

> Part of my problem is that I have no idea what the RPi OS does when it
> sees two network interfaces.   Under Windows it would blabber all kinds
> of (discovery) broadcasts and data over both - which is not what I want
> to have happen.

 If DHCP is enabled on the interface (almost certainly) then it will
broadcast a DHCP discover and listen for an offer, once all that has
settled down and it has an IP address it will settle back and listen on it
with any active services (probably only ssh) that haven't been configured
not to (every service I can think of can be configured to listen on a
specific address and/or interface so you can arrange for it to provide
selected services to one network and not the other if you want).

> > You would have to do extra work to get packets passed between them.
>
> Which would be falling right into the target-zone of my programming
> hobby. :-)

 It's not much work - set a flag on the Pi and tell the two subnets
that they can find each other via the Pi. Until you do that the Pi is the
only thing that can see and be seen on both subnets.

> > You don't need to disable anything, you would need to enable
> > routing for there to be connections between the interfaces.
>
> That sounds good.
>
> Perhaps even good enough to continue in this direction ...

 I think you'll find it the path of least resistance, and the path
that offers maximum future possibilities.

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