Hi Matus,
yes. However, you would need filter that would detect the used protocol. I'm afraid it's currently impossible to push such filter to squid w/o patching and recompiling it. Also, I'm not 100% sure that it's easy to detect ssl negotiation and refuse connection if it's not used (note that TLS negotiation is in some cases requested after initisl handshake) Last, when SSL is used, you even can't tell what protocol is inside of it.
thanks, then I will look to lock at least some dynamic IP addresses in order to prevent abuse by my users. ;-)
Florian
