Question

I have a Computer with 5 interfaces: 3 with public IPs and 2 local IPs. I am trying to send “HELLO” to a UDP server, but I would like to change the interface to one of the local IP addresses. I did some research and this is the line I need to add in python:

sock.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton("64.195.10.11"))

with 64.195.10.11 being the IP I want to change to

So my python code looks like this:

import socket

MCAST_GRP = '224.0.0.1'
MCAST_PORT = 10222

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.SOL_IP, socket.IP_MULTICAST_IF, socket.inet_aton("64.195.10.10"))
sock.sendto("HELLO", (MCAST_GRP, MCAST_PORT))

It works perfectly when I change between my 3 public IP addresses. However, it does not work (packet is not delivered) if I put a local IP in the argument. I am not sure if the script does not send, or the server does not receive. What is happening?

SL

Edit: I am listening using two ways:

import socket
import sys

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(("", 10222))
sock.setsockopt(socket.IPPROTO_IP,
                                 socket.IP_ADD_MEMBERSHIP,
                                 socket.inet_aton("224.0.0.1") +
                                 socket.inet_aton("64.195.10.11"))
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_LOOP, 0)

while True:
    print >>sys.stderr, '\nwaiting to receive message'
    data, address = sock.recvfrom(1024)

    print >>sys.stderr, 'received %s bytes from %s' % (len(data), address)
    print >>sys.stderr, data

    print >>sys.stderr, 'sending acknowledgement to', address
    sock.sendto('ack', address)

and using twisted:

from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor
from twisted.application.internet import MulticastServer

class MulticastServerUDP(DatagramProtocol):
    def __init__ (self, group, name):
        self.group = group
        self.name = name
    def startProtocol(self):
        print '%s Started Listening' % self.group
        self.transport.joinGroup(self.group)

    def datagramReceived(self, datagram, address):
        print "%s Received:"%self.name + repr(datagram) + repr(address)

reactor.listenMulticast(10222, MulticastServerUDP('224.0.0.1', 'SERVER1'), listenMultiple = True)             
reactor.run()

Answer 1

The answer: my firewall did not accept udp packet from that RFC1918 IP address.