I haven’t done this, so this is a guess at what happens.

I would expect that when you enable Internet sharing that MacOS turns into a router. The virtual Ethernet port from your Nerves device becomes one of its LAN ports. On the Nerves side, that means that you’d want to configure "usb0" as a wired Ethernet connection so it does DHCP and sets up routes. In your application config:

config :vintage_net,
  config: [
    {"usb0", %{type: VintageNetEthernet, ipv4: %{method: :dhcp}}}
  ]

The reason you lose connectivity is that MacOS wants to change the IP address of the Nerves device to something in the range it gives out on it’s “LAN”. The Nerves device doesn’t know this, though, and still uses its original IP address.

It seems like VintageNetDirect could be made clever enough to figure out if “Internet Sharing” is on and just work. I never thought of doing this before. It would be a little magical and surprising to me, but I totally understand why it would be convenient. Cool idea.