Let's say I make an https GET request to a webserver with curl without specifying any headers.
On the webserver, I see the incoming request has a X-Forwarded-For (XFF) header set. Given that I did not set that header, who set it?
My guess is that my ISP intercepts my request and adds that header to my request. Then, additional links (switches, routers, etc.) on the way to the webserver add their own IP to the XFF header.
Is this correct? Assuming yes, what happens if I set the XFF header in my request? Will the ISP override that header or will it append its IP to that header?