[lwip-devel] [bug #31367] Cannot send a UDP datagram to a different valid subnet

[Bill Auerbach]

Follow-up Comment #2, bug #31367 (project lwip):

I use both types broadcasts - this test was global.  Subnet broadcasts do
work because I use that with a utility that scans devices per PC NIC.

I don't have a problem adding the received broadcast entry to the ARP table
before replying.  This isn't needed in Windows. I found this problem from the
beginning and thought it was how it was supposed to be.  I have another
embedded device as well as Windows both which do send unicast UDP to a subnet
other than the subnet of the device or PC.  As for the embedded device, I have
no information about its stack except to know that it replies to my broadcasts
with a unicast address.  When I saw it work this way and it works in Windows
is what prompted me to post this bug report.  At least it documents
information that lwIP isn't completely compatible with IP addressing of other
stacks.

As to the ARP table, I can see that this is not easy. Perhaps one broadcast
ARP entry be kept - most likely the broadcast will be followed by a reply. 
One is better than none and would solve this UDP broadcast-reply-unicast
scenario.  Or a udp_sendto_hwa where we pass in the IP address and MAC address
of the device.

Until I tried this I've been replying with a global broadcast which is really
not very efficient to all the devices that will have to process these
messages.

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