Networking with two NICs on Ubuntu

WiringHarness asked:

My embedded hardware running Ubuntu 16.04LTS has two NICs, eth0 and eth1. In production, eth0 is connected and should use as the gateway for addresses outside the /24 subnet. For diagnostics or field upgrades, I want to be able to connect eth1 to my Windows PC and use Windows 10 Internet Connection Sharing, which by default provides a gateway of
Because there is no wifi and I’m trying to keep it simple, I’ve uninstalled the Ubuntu NetworkManager and I’m just using /etc/network/interfaces.

The modest goal is for whichever interface is active to set the gateway, and a bonus would be able to set one as a higher priority with a metric command, but I could live without that. I was hoping to be able to avoid setting the routing table with specific commands.

The /etc/network/interfaces file of my dreams looks like this:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
  metric 100

auto eth1
iface eth1 inet static
  metric 50

plus whatever is needed to make it actually work, since it doesn’t currently meet expectations. At the moment it works fine when eth0 is plugged in but when only eth1 is connected, it can see the local subnet but it does not route anything through the gateway.

I feel like this is not very complicated, and yet I can’t seem to find a simple solution in similar postings

My answer:

You’ve got your metric lines reversed. Lower numbers have higher priority.

Also you could just use DHCP for eth1, as Windows ICS has a built in DHCP server.

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