Problem receiving UDP broadcast packets.

[Thomas Heller]

Am 20.04.2011 00:21, schrieb Grant Edwards:
> I'm have problems figuring out how to receive UDP broadcast packets on
> Linux.
[...]
>
> 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
[...]
> 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.
>

You must set the network interface to promiscous mode on the receiving side:

os.system("ifconfig eth0 promisc")

Thomas