Handle interfaces configured by DHCP

Binary package hint: ucarp

The ucarp if-up script requires $IF_ADDRESS which is only provided by ifup if listed in /etc/network/interfaces (nothing set dynamically is populated into the variables). In order to allow for DHCP configured real interfaces, the if-up script can be altered to grab the IP address itself, by adding:

if [ -z "$IF_ADDRESS" ]; then
        IF_ADDRESS="`ifconfig $IFACE | sed -rn 's/^.*inet addr:([0-9.]+) .*/\1/p'`"
fi

to /etc/network/if-up.d/ucarp