when clicking through the portal, you are usually invisibiy redirected to another site, usually hidden somewhere within a frameset, as not to reveal the URL which is really used. this hidden site checks whether you entered it through the portal by checking the HTTP Referer header.
You can easily assist your browser in simulation entry via the right portal, by using this patched version of mod_headers from apache 1.3.19. (or just the patch)
<IfModule mode_proxy.c> ProxyRequests On AllowConnect 1 <Directory proxy:*> Order deny,allow Deny from all Allow from 192.168 Allow from 127.0.0.1 Header setif Referer http://*.hiddensite.net/|http://portalsite.com/start/ <Directory> <IfModule>The line
Deny from all is necesary to disallow people from
the evil outside world to violate your private ip addresses space via this proxy.
Allow from 127.0.0.1 is necesary to allow you to access
the proxy server if it is running on the same machine you are using for
Header setif Referer ... tells the apache proxy to
check if the target URL of the current request matches
and if it matches, set the
Referer header to the value after the vertical bar '