If you're using Network Manager (opposed to the command-line /etc/network/interfaces file), you should use the Network Manager dispatcher scripts instead.

Simply place your script in the /etc/NetworkManager/dispatcher.d/ directory, similar to the if-down.d approach. Scripting with Network Manager dispatcher scripts is rather easy and I suggest to read this (basic) example based as posted on the Arch Wiki:

#!/bin/sh



INTERFACE=$1  # The interface which is brought up or down

STATUS=$2     # The new state of the interface

USERNAME=gert # ENTER YOUR USERNAME HERE



case "$STATUS" in

    'up') # $INTERFACE is up

    # you could do something here...

    ;;

    'down') # $INTERFACE is down

    # Check for other active interfaces and only act on all down

    if [ ! `nm-tool|grep State|cut -f2 -d' '` = "connected" ]; then

        /bin/su -l ${USERNAME} -c 'DISPLAY=:0 /usr/bin/zenity --info --text="all network interfaces down"'

    fi

    ;;

esac

For acting on a specific network, see this answer.

And make sure to restart Network Manager to pick up with this new script.

sudo service network-manager restart