RPi
More than 1 year has passed since last update.
/etc/network/interfaces.d
auto lo
iface lo inet loopback

auto eth0
allow-hotplug eth0
iface eth0 inet dhcp
 metric 0

auto wlan0
allow-hotplug wlan0
iface wlan0 inet dhcp
 wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf
 metric 1

http://takuya-1st.hatenablog.jp/entry/2017/10/02/194834
https://qiita.com/yosi-q/items/c677e4650b22ffffa806
http://www005.upp.so-net.ne.jp/develop-tom/deb/network-deb.html
http://blog.0108note.com/entry/2016/03/11/093000