What does you imac do with traffic on port 65299?

If I had incoming traffic on port 65299 I would forward it as port 80 to one of the devices on my network that has a web server.

I'm not sure if you've tried this, or if it'll work, but have you tried using a software port scanner rather than a website?

I used to use the one on the portforward website years ago, though I notice they charge for it these days. But I'm sure there is a free version that will work on the imac.