How to check if network connection or remote ROS Master is down?

edit

I would also like to add my input on this matter! The above solution is working fine and its native to ROS, however it stopped working for me when I started using Gstreamer to get a UDP webcam stream off of a RPI.

A bit more about the setup:

• ROS Noetic on Linux 20.04
• Static Network 10.0.0.x/24 ( 255.255.255.0 )

• Laptop ( master ) address 10.0.0.1

                           Laptop (ROS master) <---- ethernet -----> RPI <--- USB camera 
     UDP stream receivever <-/                                        \-> UDP stream sender
  

using rosgraph.is_master_online() would detect the connection, however when the ethernet cable was disconnected, everything ROS related on the RPI side would halt.

What solved that issue, was an approach that slightly deviated from the pure ROS methods. I created a function following this tutorial that would check if there is a connection with the master:

 def ping_master(self):
    # try to ping the master IP
    try:
        socket.setdefaulttimeout(1)
        # if we do not receive data for 1 second except will kick in

        soc = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        # AF_INET: address family
        # SOCK_STREAM: type for TCP

        host = "10.0.0.1"
        port = 11311

        server_address = (host, port)
        soc.connect(server_address)

    except OSError as error:
        return False
        # function returns false value
        # disconnection is detected

    else:
        soc.close()
        # closing the connection after the
        # communication with the master is complete
        return True

Do not forget to change the host and port based on your network setup!

And call it in the main loop like so:

while not rospy.is_shutdown():
    try:
        mainRate.sleep()
    except rospy.ROSInterruptException:  # shutdown
        break


    if self.ping_master():

        print("{}: {} - Ros Master Is Online".format(time.strftime("%d.%m.%Y-%H:%M:%S"),self.node_name))
        # do stuff here when master is online 

    else:

        print("{}: {} - Ros Master Went Offine".format(time.strftime("%d.%m.%Y-%H:%M:%S"),self.node_name))
        # do stuff here when master is offline 


    rospy.on_shutdown(self.on_shutdown_cb)

Hope this helps someone in the future!