Daniel,
The subnet broadcast address does not have to be highest IP address in the
subnet. This just happens to norm and what the ifconfig command sets by
default. However, you can set the broadcast address to something
else. Also, by using the iproute2 tools, you can actually create multiple
broadcast addresses on a given physical interface. I suspect this is what
is happening on your FC4 and FC5 systems. Run the command "ip addr show"
and it should show all addresses bound to an interface.
With regard to your need to capture any broadcast on the local subnet, you
can use the packet interface (man 7 packet) to read raw packets from the
interface. You application will receive a copy of the packet before it
hits the IP layer and possibly dropped.
Hope this helps,
Mark
Hi,
>> 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.
I agree with you but... I've been doing these tests on my C-class network
having the same problem for other classes.
For an A-class network, 255.255.255.255, 10.20.1.255, 10.20.255.255 and
10.255.255.255 are all broadcast addresses within the same network. However,
my controller (and Ubuntu) only accepts packets sent to the local broadcast
address or 255.255.255.255.
On the other hand, FC4/5 has not this limitation.
>> 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.
My router correctly forwards any packet. I've verified this from Ethereal.
>> 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.
Thank you very much for your reply,
Daniel.
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