I started writing my own rtnetlink client (sigh) before I came up for air and found our answer: https://github.com/dotcloud/docker/blob/master/pkg/netlink/netlink_linux.go#L644

```
package main

import "github.com/dotcloud/docker/pkg/netlink"

func main() {
	routes, _ := netlink.NetworkGetRoutes()
	for _, route := range routes {
		if route.Default {
			println(route.Iface.Name)
			return
		}
	}
}
$ go build route.go
$ ./route
enp0s3
```

Yeah?