A marketing executive at Shanghai Online, who also asked not to be named, said use of its Chinese-language search engine has surged this month.
The government's early blocking of entire sites was technologically fairly primitive and involved placing filters at what were then a handful of international gateways -- where China's piece of the Internet merged with the broader global network.
But gateways are proliferating, and the site-specific blocks are too easily sidestepped by proxy servers, computers abroad that help mask a user's true destination.
The new, selective filtering technologies make it much more difficult to access forbidden information, Clark and others said. Yet they are technologically more difficult to administer -- and more costly. Data requests are free to leave China, but many incoming Web pages are blocked based on certain keywords.
"The Chinese have been working on these new technologies for years," Clark said.
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