Re: Using NM to 'relay' an internet connection

On Wed, 2017-03-22 at 13:41 +0000, colin helliwell ln-systems com
We have an embedded system that we're working through getting
set up on. There's a mode of operation we'd like to explore, and I'm
wondering if NM has such capabilities 'built-in':
Imagine two units - both have Ethernet and are on the same LAN
internet connectivity for them is NAT'd via a ADSL router/modem or
such-like, using the Ethernet. 
But Unit A also has a GSM modem, and if the ADSL goes down then it
switch over to use GSM for Internet. 
What we'd like is, if the ADSL goes down, that Unit B can open 'some
sort of
tunnel' to Unit A and have its traffic 'bridged in some way' to Unit
A and
then to/from the internet via Unit A's GSM connection. 
It would want to be secure i.e. only Unit B could use Unit A, not
anyone on
the LAN.

I use the words 'relay'/'tunnel'/'bridged' in a *generic* sense, as
often imply specific mechanisms. 

Is this something that NM can provide and manage...? 


For one there is "ipv4.method=shared" to use IPv4-NAT and 
"ipv6.method=shared" for IPv6 prefix delegation. But that doesn't sound
like a good fit in your scenario.

Maybe setup a tunnel between Unit A and B. That may be a VPN, some kind
of IP tunnel [1] or maybe MACSec [2]. All this is supported by NM,
MACSec sounds most interesting to me.
(note that IP tunnels and MACSec are a recent feature, you may not have
it in your version).
And then setup IP routing accordingly accros the tunnel.



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