I just did a fresh install of Ubuntu 16.04 (Xenial). I've configured
/etc/network/interfaces to bring up the wlan interface automatically using DHCP:
auto lo iface lo inet loopback auto wlp2s0 iface wlp2s0 inet dhcp wpa-ssid SSID wpa-psk "password"
This sort of works: the IP is configured normally, but the default gateway is not set. After boot I always have to run
route add default gw 192.168.1.1 manually. Any idea of what can possibly be wrong? I could swear this setup used to work fine on older versions...