debian | promisc interface auto up on boot

auto enp3s0
iface enp3s0 inet manual
up ifconfig enp3s0 up
up ifconfig enp3s0 promisc

ATTENTION: Check it with

netstat -i

The promisc interface should show up with BMPRU (P stands for promisc).
P appears only if the interface is wired and connected to a switch.