I am a ed-technologist in a 4 school supervisory union in Vermont. The district has been using a Sonicwall firewall with content filtering for a couple years. They recently noticed a lot of traffic tunneling through the firewall by students using https proxies. Sonicwall has not been much help with the problem.

I have used Squid and Dansguardian in the past. Will these programs easily block https? What distribution would you suggest for ease of use and administration? It would be nice if Dansguardian and Squid came preinstalled, so all we had to do was tweak it. Thanks! -Will

hmm.....

May be I am not getting you clearly but..
If you are provinding acces through direct proxy ie. student provided with IP and Port for proxy then disable every other port. Next, squid can block unwanted ports on its own .
There are software like packages like smoothwall etc.. they may be of your help!

totally blocking https would be counter productive as it would perclude the use of 90% of webmail based email accounts (hotmail, gmail etc)

Yes that is true. I was facing the similar situation wherein people are using https proxies to browse the net. The only solution that I could figure out in that moment is to write an acl in squid that filters the word 'proxy'. Though this is not a solution but it somewhat solved my purpose for the time being. A better solution might exists.

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Prasanta