How do sites know that a user is behind a proxy?

Question

SSH

I configured the browser through work with Socks5. The connection was raised on the local computer:
ssh -D 8080 [email protected]_ip
But the sites still see that I'm behind a proxy. How is that? :) How do they see through this tunnel? How can you not be seen? :)

Answer 1

As far as I know, some proxies do not encrypt their use in any way and give out your real IP and your IP in the header.

Answer 2

The IP from which the connection comes belongs to the data center, this can be tracked.

Answer 3

Methods of anonymity in the network. Part 2. Data leaks. - there are answers to all your questions.
I think that in your case there is a leak through DNS queries.