# Send/receive UDP multicast packets.

# Requires that your OS kernel supports IP multicast.

# This is built-in on SGI, still optional for most other vendors.

#

# Usage:

#   mcast -s (sender)

#   mcast -b (sender, using broadcast instead multicast)

#   mcast    (receivers)

MYPORT = 8123

MYGROUP = '225.0.0.250'

import sys

import time

import struct

from socket import *

# Main program

def main():

    flags = sys.argv[1:]

    #

    if flags:

        sender(flags[0])

    else:

        receiver()

# Sender subroutine (only one per local area network)

def sender(flag):

    s = socket(AF_INET, SOCK_DGRAM)

    if flag == '-b':

        s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1)

        mygroup = '<broadcast>'

    else:

        mygroup = MYGROUP

        ttl = struct.pack('b', 1)               # Time-to-live

        s.setsockopt(IPPROTO_IP, IP_MULTICAST_TTL, ttl)

    while 1:

        data = repr(time.time())

##              data = data + (1400 - len(data)) * '\0'

        s.sendto(data, (mygroup, MYPORT))

        time.sleep(1)

# Receiver subroutine (as many as you like)

def receiver():

    # Open and initialize the socket

    s = openmcastsock(MYGROUP, MYPORT)

    #

    # Loop, printing any data we receive

    while 1:

        data, sender = s.recvfrom(1500)

        while data[-1:] == '\0': data = data[:-1] # Strip trailing \0's

        print sender, ':', repr(data)

# Open a UDP socket, bind it to a port and select a multicast group

def openmcastsock(group, port):

    # Import modules used only here

    import string

    import struct

    #

    # Create a socket

    s = socket(AF_INET, SOCK_DGRAM)

    #

    # Allow multiple copies of this program on one machine

    # (not strictly needed)

    s.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)

    #

    # Bind it to the port

    s.bind(('', port))

    #

    # Look up multicast group address in name server

    # (doesn't hurt if it is already in ddd.ddd.ddd.ddd format)

    group = gethostbyname(group)

    #

    # Construct binary group address

    bytes = map(int, string.split(group, "."))

    grpaddr = 0

    for byte in bytes: grpaddr = (grpaddr << 8) | byte

    #

    # Construct struct mreq from grpaddr and ifaddr

    ifaddr = INADDR_ANY

    mreq = struct.pack('ll', htonl(grpaddr), htonl(ifaddr))

    #

    # Add group membership

    s.setsockopt(IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq)

    #

    return s

main()