We run a small network with a Squid proxy. Over the last few weeks, the network has been slowing down and completely stopping for no apparent reason. We discovered that it was due to some users running ultrasurf. We were able to block ultrasurf from connecting to its severs using something we found online (which we added to our squid.conf). Now although squid doesnt let ultrasurf connect, just leaving it on practically stops the network completely.