This tells the kernel to load the device upon boot
iface eth1 inet manual
This tells the kernel that this interface will be configured manually. There is not ip and no dhcp for it – we want it just to be there.
up ifconfig $IFACE 0.0.0.0 up
The up means that if the interface comes up, this command should be run. the $IFACE in there will be replaced by eth1. But to be generic, i just left it like that, as this will work with any network device. So the command that runs really is ifconfig eth1 0.0.0.0 up which again brings the interface up and configured it to have NO IP.
up ip link set $IFACE promisc on
This works the same was the command before, so it will actually run ip link set eth1 promisc on which will enable the promisc mode on the interface. So now we are done. We have an interface that is up, unconfigured and in promisc mode.
down ip link set $IFACE promisc off
down ifconfig $IFACE down
This does the same as the two up commands, they are just ran when the interface is going down. So this unconfigured the network card.