Comment 6 for bug 1172566

so the bug here is actually in 'get_default_route_ip' of the postinst for maas-region-controller-min and maas-region-controller.

They run with 'set -e' (which i'm never a fan of), and then have:

get_default_route_ip() {
   while read Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT; do [ "$Mask" = "00000000" ] && break
   done < /proc/net/route
   interface="$Iface"
   ipaddr=$(LC_ALL=C /sbin/ip -4 addr list dev "$interface" scope global)
   ipaddr=${ipaddr#* inet }
   ipaddr=${ipaddr%%/*}
        echo $ipaddr
}

That method has many issues.
a.) will stomp over global variables 'iface' 'Destination' Gateway, Flags .... and 'interface'. luckily it is only run in a subshell, so thats not really a problem here.
b.) if interface is empty it runs : ip -4 addr list dev "" scope global

ip fails, thus the method fails, and thus the dpkg postinst fails (thanks to set -e).

the simpllist fix is just to add:
 [ -n "$interface" ] || return 0

right after the loop that reads /proc/net/route.