[uClinux-dev] Some broadcasted UDP packets are dropped

[Gavin Lambert]

Quoth Daniel Berenguer [dberenguer at usapiens.com]:
> I'm testing a simple application that listens for incoming
> broadcasted UDP packets on a given port. This application
> behaves in different ways depending on the system:
> 
> 1. Ubuntu PC and ARM7TDMI : See packets sent to these broadcast
> addresses: 255.255.255.255 and 192.168.1.255. Packets to
> 192.255.255.255 are dropped. 

192.255.255.255 is not a broadcast address, so that makes sense.

> 2. FC4 and FC5 PC's : See packets sent to these broadcast
> addresses: 255.255.255.255, 192.168.1.255 and 192.255.255.255

That's weird.

> All these machines have 255.255.255.0 as netmask and
> 192.168.1.255 as broadcast address.

Which means that the only two broadcast addresses are 255.255.255.255
and 192.168.1.255.  There are no others.  Since routers are required to
drop (or not forward) broadcast packets you cannot do an internetwork
broadcast.

> Anyway, I need my ARM7 to read any broadcasted packet, even
> those not addressed to the local broadcast address. My
> embedded module should communicate in fact with third-party
> applications that broadcast messages to different addresses.

Any other addresses are not broadcasts.  They might be multicasts, but
not broadcasts.  At least not for that network.