1 # Send/receive UDP multicast packets.

     2 # Requires that your OS kernel supports IP multicast.

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

     4 #

     5 # Usage:

     6 #   mcast -s (sender)

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

     8 #   mcast    (receivers)

     9 

    10 MYPORT = 8123

    11 MYGROUP = '225.0.0.250'

    12 

    13 import sys

    14 import time

    15 import struct

    16 from socket import *

    17 

    18 

    19 # Main program

    20 def main():

    21     flags = sys.argv[1:]

    22     #

    23     if flags:

    24         sender(flags[0])

    25     else:

    26         receiver()

    27 

    28 

    29 # Sender subroutine (only one per local area network)

    30 def sender(flag):

    31     s = socket(AF_INET, SOCK_DGRAM)

    32     if flag == '-b':

    33         s.setsockopt(SOL_SOCKET, SO_BROADCAST, 1)

    34         mygroup = '<broadcast>'

    35     else:

    36         mygroup = MYGROUP

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

    38         s.setsockopt(IPPROTO_IP, IP_MULTICAST_TTL, ttl)

    39     while 1:

    40         data = repr(time.time())

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

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

    43         time.sleep(1)

    44 

    45 

    46 # Receiver subroutine (as many as you like)

    47 def receiver():

    48     # Open and initialize the socket

    49     s = openmcastsock(MYGROUP, MYPORT)

    50     #

    51     # Loop, printing any data we receive

    52     while 1:

    53         data, sender = s.recvfrom(1500)

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

    55         print sender, ':', repr(data)

    56 

    57 

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

    59 def openmcastsock(group, port):

    60     # Import modules used only here

    61     import string

    62     import struct

    63     #

    64     # Create a socket

    65     s = socket(AF_INET, SOCK_DGRAM)

    66     #

    67     # Allow multiple copies of this program on one machine

    68     # (not strictly needed)

    69     s.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)

    70     #

    71     # Bind it to the port

    72     s.bind(('', port))

    73     #

    74     # Look up multicast group address in name server

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

    76     group = gethostbyname(group)

    77     #

    78     # Construct binary group address

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

    80     grpaddr = 0

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

    82     #

    83     # Construct struct mreq from grpaddr and ifaddr

    84     ifaddr = INADDR_ANY

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

    86     #

    87     # Add group membership

    88     s.setsockopt(IPPROTO_IP, IP_ADD_MEMBERSHIP, mreq)

    89     #

    90     return s

    91 

    92 

    93 main()