It looks like some folks are hosting proxies on Google's AppEngine.
I noticed http://proxysun5.appspot.com/ when I caught it doing naughty things to one of my web sites. Unfortunately, I cannot easily IP block it because it's IP is within Google's IP address space. It attacked from 126.96.36.199, which is part of Google's 188.8.131.52/18 range.
I found a good writeup on Labnol about how to do this, Setup your own Proxy Server in 5 Minutes for Free.
What do you think?
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It sounds good, but it seems that you are limited in many ways and you have to mess around a little to get things working, appengine only supports things like python / java. There is no support for PHP at Google appengine yet, - Issues - googleappengine - but there are workarounds such as this - Run PHP on the Google App Engine - which still requires more time / messing around to get things working.
Could be it's too technical for people to bother setting up compared to setting up a php proxy. Especially when anyone can just drop a php proxy script on a server and have a proxy running without any real knowledge of what they're doing.What do you think?