Network interface fallback? I have 2 internet interfaces: Ethernet and Wifi.
Ethernet is the office network (read: completely closed) and Wifi is way more permissive. 

The ethernet is used for back-office systems (internal access only. addresses are not public) and the wifi for everything else. Now, what i do is:
1.Set Wifi as default network.
2.Server not found? Set Ethernet as default network
3.Content blocked? go back to 1.

Is there a way to make this auto?

Edit1:
Clarification: the back-office systems are not accessible via regular web, only via intranet
Sure, since all networks are active, you simply have to set up network ranges so that each destination is routed to the correct network.

If you can manage the DNS locally, you can have your system point to your local DNS server that can look up all the local host names and then get you an external DNS as well.

Or you could just use IP addresses locally and then your computer knows how to get to all the machines over all of the adapters.

You could even have the wired network have two IP addresses on one connection, so it’s really flexible on the macOS end to match whatever network setup you choose to set up.



In your case, set the WiFi to be the default network setting - drag it higher in the list in the network setup, and have the ethernet lower than the WiFi and then the default route is to WiFi



you don’t need network locations to set this up unless you want to have a simpler setup when you leave the office. Here is the guide on setting service order and location setup if you wish to automate switching from a split network to a unified one:


https://support.apple.com/en-us/HT202480#serviceorder