Problem receiving UDP broadcast packets.

Irmen de Jong irmen.NOSPAM at
Tue Apr 19 19:40:50 EDT 2011

On 20-4-2011 1:21, Grant Edwards wrote:
> If I don't call bind(), then the broadcast packets go out the wrong
> interface on the sending machine.

Fair enough.

Next issue then: as far as I know, broadcast packets are by default not routed across
subnets by gateways. Which is a good thing.

That would explain why your receiver doesn't see the packets unless its interface IP
address is in the same subnet as the sender's.

However it doesn't explain (for me) why the tcpdump program running on that same
receiver machine still happily spits out received packets. Unless the routing between
the subnets is somehow done on the receiving machine itself? My knowledge of networks
and TCP/IP ends here I'm afraid.


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