Problem receiving UDP broadcast packets.

Dan Stromberg drsalists at gmail.com
Wed Apr 20 01:52:01 CEST 2011


On Tue, Apr 19, 2011 at 4:40 PM, Irmen de Jong <irmen.NOSPAM at xs4all.nl> wrote:
> 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.
>
> Cheers
> Irmen.

I'm guessing there are two different subnets on the same physical
cable - which is a little unusual, but not impossible.



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