I'm using multicast UDP over localhost to implement a loose collection of cooperative programs running on a single machine. The following code works well on Mac OSX, Windows and linux. The flaw is that the code will receive UDP packets outside of the localhost network as well. For example, sendSock.sendto(pkt, ('192.168.0.25', 1600)) is received by my test machine when sent from another box on my network.

import platform, time, socket, select

addr = ("239.255.2.9", 1600)

sendSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sendSock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 24)
sendSock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_IF, 
    socket.inet_aton("127.0.0.1"))

recvSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
recvSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, True)
if hasattr(socket, 'SO_REUSEPORT'):
    recvSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, True)

recvSock.bind(("0.0.0.0", addr[1]))
status = recvSock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, 
    socket.inet_aton(addr[0]) + socket.inet_aton("127.0.0.1"));

while 1:
    pkt = "Hello host: {1} time: {0}".format(time.ctime(), platform.node())
    print "SEND to: {0} data: {1}".format(addr, pkt)
    r = sendSock.sendto(pkt, addr)

    while select.select([recvSock], [], [], 0)[0]:
        data, fromAddr = recvSock.recvfrom(1024)
        print "RECV from: {0} data: {1}".format(fromAddr, data)

    time.sleep(2)

I've attempted to recvSock.bind(("127.0.0.1", addr[1])), but that prevents the socket from receiving any multicast traffic. Is there a proper way to configure recvSock to only accept multicast packets from the 127/24 network, or do I need to test the address of each received packet?