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Join Date: Feb 2007
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Are your VPN users merely using your services/servers as a gateway? In this case, you need to block all HTTP traffic and use an HTTP Reverse Proxy server. With this reverse proxy server you can do tricks so that all requests go through a page which displays an ad and then redirects the user to the page they requested.
Or are you looking to serve your site's hosted web pages in a special way for those users connected through VPNs? In this case, you need to identify the IP address range for those users and you can use a conditional server-side include on all pages.