What's unclear, though, is just how many of the proxy servers were set up for the explicit purpose of helping Iranians circumvent Internet censorship and how many were up and running before the recent communications crackdown there, according to James Cowie, CTO of Renesys Corp., a Manchester, N.H.-based Internet monitoring firm.

A can allow someone to access a Web site or a Web service anonymously. For instance, a user in Iran trying to access an external news site that might have been blocked within the country can configure a browser to access the site via a proxy service. The user's browser doesn't contact the Web site directly, nor does it directly receive any content from the Web site, because the requests and responses are passed through the proxy server. All the user needs is the IP address of the proxy server and a port number to access it.

Software is available that allows pretty much any user to turn his or her computer -- or someone else's computer -- into a proxy system.

Since began in Iran about a week ago, supporters of the Iranian opposition movement have been propagating lists of available proxy servers to Iranians via Twitter and numerous Web sites. An inspection of about 2,000 such servers shows that while many are hosted in the U.S and Western Europe, proxy services have also been available from numerous other countries, including China, India, Russia, Romania, Bulgaria and Vietnam.

While that geographic spread might suggest widespread support for the opposition movement in Iran, the reality might be different, Cowie said. In many cases, computers that host proxy services are previously compromised machines with malware installed on them that make them proxy servers. "I suspect that many of these hosts may not be aware that they are running open proxies," he said.