Network-Manager on node0 is not working after a fresh installation

This script is the one that configures the DNS in MAAS automatically when network manager connects to a new network.
This does not always seem to work, and we have to manually run:
orange-box-setup-nat
to set the upstream DNS server in MAAS correctly on connecting to a new network.

Deleting the link will mean that it never runs automatically.
It would be interesting to discover what is wrong with the script or of it is a problem with network manager.

link is /etc/network/if-up.d/orange-box-setup-nat

debug information of /var/log/upstart/networking.log:
+ exit 1
run-parts: /etc/network/if-up.d/orange-box-setup-nat exited with return code 1
Failed to bring up eth0.
++ id -u
+ '[' 0 '!=' 0 ']'
+ '[' br0 = lo ']'
++ route -n
++ grep '^10\.14\.'
++ sed -e 's/.* //'
+ internal=br0
++ ip route ls
++ grep '^default'
++ awk '{print $5}'
+ external=
+ '[' -z br0 ']'
+ '[' -z '' ']'
+ echo 'usage: /etc/network/if-up.d/orange-box-setup-nat [INTERNAL_INTERFACE] [EXTERNAL_INTERFACE]'
usage: /etc/network/if-up.d/orange-box-setup-nat [INTERNAL_INTERFACE] [EXTERNAL_INTERFACE]