Does anybody have any knowledge on how to get a user behind a proxy found in vbulletin while he is browsing the site and then have a custom msg in the header that says: "you are behind a proxy, our site doesn't allow this, whatever".

not being a php wiz, but i know the basics of how to get this done.

1: check for proxy in the domain host of the user, trivial.

2: check with a tcp/ip socket on the normal proxy sockets (1080 afaik), this might not always work, and well, it will produce alot of extra traffic. users running firewalls that pop up connection attempt messages will panic and think you are trying to hack them or some shit.

id advise against checking for proxies, but thats just me. alot of isp's use proxies to cut down on their traffic, so most of their (non-techy) users will have their web browser set up to use said isp's proxy server.
id also check with the hosting company/isp about this sort of stuff, they might not aprove of it either.

I think I've released a few php resources on vBulletin.org regarding the use of ALT_IP from vBulletin and storing it with each post. And by including it in emails from new members.

You can see if they match, if they mismatch chances are the user is behind a non-transp. proxy.

Perhaps you could use a conditional like:

if IP != ALTIP exit;

A user behind a proxy isn't necessarily a bad user. As elmo said alot of ISPs do use proxies and the user, in some cases, has no say in the matter. You could be loosing good users by not allowing proxies.

A user behind a proxy isn't necessarily a bad user. As elmo said alot of ISPs do use proxies and the user, in some cases, has no say in the matter. You could be loosing good users by not allowing proxies.

Exactly, a proxy doesnt mean you automatically are trying to do something evil.
isp's have proxies, alot of companies use proxies also.

Which is why i allow it on vBulletin.nl, but .. if a user is misbehaving I do check against his IP and try to find duplicate accounts and the alt_ip function does help him prevent sign up through multiple proxies.