A
listener? On the same machine as the website? That should fail because of trying to start two server sockets on the same port, but apparently
there is some magic that makes that not be the case nowadays[
^]. So I would guess that you aren't allowed to do that to combine with a socket started by a different process.
I'd also direct you to the note in that topic:
By default, multiple listeners can listen to a specific port. However, only one of the listeners can perform operations on the network traffic sent to the port. If more than one listener attempts to bind to a particular port, then the one with the more specific IP address handles the network traffic sent to that port. You can use the ExclusiveAddressUse property to prevent multiple listeners from listening to a specific port.
That is, you can't listen in to requests and still have them processed by the original destination, anyway. To do that you need to hook into the network stack which I believe requires P/Invoke calls.