Apache reverse proxy detectable?

Question

I setup a simple reverse proxy with mod_rewrite [P]. Is this detectable in any way? I did not see any proxy related headers passed back to the client. Curious if there's any other way someone could detect that the request has been proxied.

Answer 1

If the upstream is unreachable or just loses a single connection, the user will get a proxy-specific 502 response code.

Assuming that your RewriteRule is only proxying for a subset of the URL space in the domain, you may have subtle header differences, differences in the handling of file caching and encoding (especially if the content on the upstream server is a different web server or a different kind of content, like PHP for instance), or even clock differences in the returned responses.

In most cases, a user will not know or care that a request has been proxied, and will not notice with a cursory glance at the headers. However, a user the knows what to look for and is determined to find evidence that a request is being proxied will almost certainly be able to do so, either through careful comparison of response behaviors and handling of edge case and error-inducing requests, or by stressing the server into simply throwing a 502.