nginx – Is it possible to detect that site is using reverse proxy?

Not in the general case.

It is sometimes possible to deduce that a proxy server is being used by looking at the headers returned with your request, but those will typically be legitimate reverse proxies.

As mentioned by others, proxies are very commonly used to speed up access, provide redundancy and even to merge different back-ends into a single site – all of these are entirely legitimate purposes.

It is entirely practical (common even) for many web servers to act as reverse proxies, or indeed to reverse proxy some content while directly serving other content. Both Apache and NGINX offer modules to do this.

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