File and screen share over Ethernet cable between two Macs that each have their own WiFi?

[ElectronGuru]

I’ve got two Mac mini’s, with independent WiFi’s, load balancing to get around data caps. They are close together and connected via an Ethernet cable.

I want to LAN them together in such a way that I can make one headless and connect to it for screen and file sharing over Ethernet. Headless one is older so firewire 800 and GB Ethernet are it’s best ports. Screen and file services should NOT broadcast on its WiFi.

I’m ok buying hw or sw to make this reliable but would prefer using built in tools. Both Macs are Mojave. I’ve googled for several hours and most pages are about sharing internet. The rest cover making a connection but seem to expect one network, being the same network for internet.

How do I enable file and screen sharing and have it recognize self assigned or manual IPs on Ethernet while ignoring active WiFi networks?

 

[Mikael H]

Without knowing the details for those applications, one way to achieve what you describe would be to assign addresses in a subnet separate from the ones used by the respective WiFi connections to the wired network interfaces, and not define a gateway address for these interfaces. In this way only traffic intended for the other machine would ever go across the wired connection.

The second question is how to prevent these protocols to be presented across WiFi. One way may be to utilize the pf firewall included in macOS: Block traffic to the relevant ports on the WiFi interface IP while still allowing such traffic to the wired interface.