Problem receiving UDP broadcast packets.

[Grant Edwards]

I'm have problems figuring out how to receive UDP broadcast packets on
Linux.

Here's the receiving code:

------------------------------receive.py-------------------------------
#!/usr/bin/python
import socket

host = ''
port = 5010

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
s.bind((host, port))

while 1:
    try:
        message = s.recv(8192)
        print "Got data: %s" % repr(message)
    except KeyboardInterrupt:
        break
----------------------------------------------------------------------

Here's the sending code:

--------------------------------send.py-------------------------------
#!/usr/bin/python
import sys,socket,time

host = sys.argv[1]
port = 5010

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
s.bind((host,port))

s.sendto(str(time.time()), ('255.255.255.255', port))
----------------------------------------------------------------------


On the receiving machine, I've used tcpdump to verify that broadcast
packets are being seen and have a destination IP of 255.255.255.255 and
destination MAC of ff:ff:ff:ff:ff:ff

03:05:09.187327 IP 10.0.0.1.5010 > 255.255.255.255.5010: UDP, length 13
        0x0000:  ffff ffff ffff 0018 e708 2033 0800 4500
        0x0010:  0029 0000 4000 4011 30c4 0a00 0001 ffff
        0x0020:  ffff 1392 1392 0015 6e6e 3133 3033 3235
        0x0030:  3131 3830 2e34 3500 0000
03:05:09.407508 IP 10.0.0.1.5010 > 255.255.255.255.5010: UDP, length 13
        0x0000:  ffff ffff ffff 0018 e708 2033 0800 4500
        0x0010:  0029 0000 4000 4011 30c4 0a00 0001 ffff
        0x0020:  ffff 1392 1392 0015 6c6c 3133 3033 3235
        0x0030:  3131 3830 2e36 3700 0000
03:05:09.615962 IP 10.0.0.1.5010 > 255.255.255.255.5010: UDP, length 13
        0x0000:  ffff ffff ffff 0018 e708 2033 0800 4500
        0x0010:  0029 0000 4000 4011 30c4 0a00 0001 ffff
        0x0020:  ffff 1392 1392 0015 6b6a 3133 3033 3235
        0x0030:  3131 3830 2e38 3800 0000

But, the receiving Python program never sees any packets unless the
_source_ IP address in the packets is on the same subnet as the
receiving machine.  In this test case, the receiving machine has an IP
address of 172.16.12.34/16.  If I change the receiving machine's IP
address to 10.0.0.123, then the receiving program sees the packets.

Even though the destination address is 255.255.255.255, the receiving
machine appears to discard the packets based on the _source_ IP.  Can
anybody provide example Python code for Linux that receives UDP
broadcast packets regardless of their source IP address?

This probably is more of a Linux networking question than a Python
question, but I'm hoping somebody has solved this problem in Python.

-- 
Grant Edwards               grant.b.edwards        Yow! I want my nose in
                                  at               lights!
                              gmail.com