netmon - a simple network connectivity monitor
Copyright © 2008-2016 Mikko Rasa, Mikkosoft Productions

Netmon keeps track of the machine's network connection status.  It can
passively monitor network traffic with libpcap or actively ping a target
machine.  It can also run an external command when the network goes down,
for example to reset an ADSL modem in an effort to bring the connection
back up.

Command-line options:
  -f
    Run in foreground (by default netmon daemonizes itself)

  -v
    Be more verbose

  -p HOST
    Send a ping to HOST every second

  -s NUM
    Reports packet loss statistics every NUM seconds (default 1800, only
    works if pinging is enabled)

  -i NAME
    Monitor packets on interface NAME

  -c CMD
    Run CMD if the connection goes down

  -t NUM[,NUM]
    Sets the delay and interval (in seconds) for running the trigger
    command (default 60 and 600)