I think this package does exactly what you need: https://pkg.go.dev/github.com/google/gopacket/routing. Works on my machine (error handling left to the reader)

    router, _ := routing.New()
    iface, _, _, _ := router.Route(net.ParseIP("8.8.8.8"))
    fmt.Println(iface.Name)

this prints my Ethernet interface as expected. It doesn't make any requests, it just figures out where to route a packet. I guess it interfaces with the OS routing table.

▲ rvermeulen98 6 hours ago | parent [-]

Thanks for sharing! This is definitely something I will look into, I am all in favor to simplify the current implementation of finding the "default" OS network interface.

	▲	contingencies 3 hours ago \| parent [-]
		You'd better use the default route and not some random IP, particularly DNS IPs which people often meddle with. `# IPv4 default route only uname Darwin$ route -n get 0.0.0.0 \| grep interface \| cut -d ':' -f2 Linux$ route -nv \|grep ^0.0.0.0 \| awk '{print $NF}'`