Detecting if user is using proxies or vpn [duplicate]

This ques开发者_高级运维tion already has answers here: How do you detect a VPN or Proxy connection? [closed] (7 answers) Closed 2 years ago.

It's possible to grab x-forward info from user. Is there a way to detect a user is using VPN versus straight connection? (i.e ipsec-nat-t, ESP or max-forward?)

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Not really, you might get somewhere if the vpn modified the actual tcp packets and left a signature which is possible, but there isn't much in the way of prebuilt tools to do this, and you'd have to essentially code your own webserver to see it. Such a signature would really be a guess that the user was on a VPN not a certainty, and its entirely possible all popular VPNs are not detectable at all.