The quickest and probably the easiest way to set an interface to promiscuous mode in Linux is to run the following command and add the same to the /etc/rc.local file to survive a reboot.
ifconfig eth0 up promisc
Another option to make it permanent is to edit and update the network configuration file namely /etc/network/interfaces
Remove the lines related to eth0 and update the file to look something like
iface eth0 inet manual
up ifconfig eth0 up promisc
down ifconfig eth0 down -promisc