Demo/sockets/mcast.py

   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()