Proxy Port detection

[John Madden <chiwawa999 () yahoo com>]

Hi,

In our enterprise we have URL filtering capabilities
and we restrict the usual sites (Porn, Sports,
Gambling etc..)

We do not use a proxy, so everyone goes directly to
the internet. 

I believe that some users put in their proxy settings
an anonymous proxy using port 80 (which is obviously
allowed) and in that manner avoid the restriction of
the URL filtering.

First thoughts:

- Blocking all the anonymous proxy is imposible and  
would be a full time job
- The use of a proxy is not an option right now

Is there any way to detect this type of traffic
(HTTP-Proxy) ?

I'm sure someone had this problem before...

Any help would be appreciated.

Thanks


                
__________________________________ 
Do you Yahoo!? 
Yahoo! Mail - Helps protect you from nasty viruses. 
http://promotions.yahoo.com/new_mail