Comment by jeroenhd
Do raw TCP proxies still get used often? I'd imagine most proxies you'd want to detect are full HTTP proxies and this formula won't detect those.
I suppose it's possible botnets ("residential proxies") may get detected this way if they're using SOCKS to forward requests?
Still, this looks like an interesting signal to add to a system like Anubis to increase the difficulty for suspicious traffic sources.
This does very reliably detect TOR traffic, though you can just download a list of exit nodes if that's what you want.
I think for stealth TCP proxies are more common since you can use your own TLS fingerprints and all of that, with something like an HTTP proxy you'd need to set up your requests to match with the TLS fingerprint that the proxy is using, although I guess the proxy could make the TLS look the same? There are other ways of detecting HTTP proxies like for example comparing with the RTT of websockets or something like that, the idea is that there will always be at least one thing with RTT from the proxy and at least the RTT for one thing from the client that must go trough the proxy, you measure the difference between the two and there you have it.