Stretch WIFI client

Wireless interfaces are also managed with /etc/network/interfaces.d/. For example, to connect as a client to a WPA network with DHCP.

Install wpa_supplicant:

apt-get update && apt-get install wpasupplicant -y

Run:

wpa_passphrase youressid yourpassword

This will output something like:

 network={
 	ssid="youressid"
 	#psk="yourpassword"
 	psk=151790fab3bf3a1751a269618491b54984e192aa19319fc667397d45ec8dee5b
 }

Use the PSK in your network interfaces file.

echo "auto wlan0
iface wlan0 inet dhcp
    wpa-ssid youressid
    wpa-psk 151790fab3bf3a1751a269618491b54984e192aa19319fc667397d45ec8dee5b" > /etc/network/interfaces.d/wlan0

To make this take effect immediately:

service networking restart

For more information on configuring wifi, see Debian's guide here.