Losing a UDP packet to Windows 10


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May 26, 2022
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I am using a python script to enable a remote device to enable its ethernet connection. The serial interface to it is also using UDP and internet addressing. When it wakes up the ethernet and responds it does so over ethernet. I see the UDP response on wireshark, but not in python. The remote device has a choice on which interface to respond on, so it does so on the faster interface. The PC is a remote device on both networks so it goes to a routed solution and picks the faster one. So the response packet comes to the windows machine with an ethernet address different than the one sent to, the local machine has not resolved. The slow and fast paths are on separate class B networks. (different second octet) as is my PC. Changing is not an option. Also there is a router in between. Wireshark running on my PC sees the UDP message arrive, but the OS does not deliver it. MS Firewall is off, (at least it says it is)

How do I prevent that packet from being sent to the bit bucket and instead passed to the application looking for it.
I have tried adding a static rout to the ARP table, which is a probable solution for earlier versions of windows. I no difference in behavior.

Is there a process to allow specific IP address senders through no matter what? (I know, shift to Linux) Adding a static ARP entry should bypass the OS wanting to check to see if it knows the sender. Maybe adding a separate IP address to the network card will allow that network past the OS? What I personally know about ethernet practices is significantly out of date.
This is windows 10, 20H1, and a bunch of proprietary code on other processors. I am only asking about possible changes to the Windows 10 machine.

- - Scott

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