This is an odd one.
First, THIS IS NOT A NETWORKING ISSUE. This is an "rdp host behavior" that can be seen, from one persective, via the behavior of an RDP app. (see What happens to a Windows 7 workstation that acts as an RDP host? for the bigger story)
Why is it when I rdp from one machine to another (both of them Win7 Pro), on the same subnet, apps on the "host" machine lose udp to the local subnet?
Our office computers, which run as VPI virtual machines (win7) run a chat app that is udp based. It is for chat within the office.
When we access these systems via gotomypc, the chat app works fine.
But we don't do that normally. We use rdp (native remote desktop on Win stations).
When we use remote desktop to connect to these instances, the chat app loses visibility to the local subnet. (I can repro the problem just using rdp from a machine on the local subnet, but first noticed it when vpning in from off site.)
The chat app runs on udp 8168
Another way of asking the question is: What happens to UDP networking/routing on a host machine when a remote system does remote desktop to the host?
Is it possible that there is a virtual network interface that only exists when the machine is an rdp host (and that the chat app loses connection to the main network interface), or ?
How to chase?
Note: Fiddler does not see udp traffic, and is not helpful. nslookup is not relevant, as the chat app uses all ip addresses and not domain names (it is for local chat on local subnet only)