Widespread suspicion that Chinese authorities are using packet sniffers to block search engines, Web-based e-mail and Internet sites has raised questions over just what is being targeted.
Checks over the past two days turned up three major problem areas.
One is Internet-based e-mail sent via big-name servers such as Yahoo! or Hotmail. The second is overseas search engines such as Google or MSN. The third is everyday Internet sites, such as scmp.com or CNN.com, that carry China content.
The sniffer software, which blocks only when it finds certain strings of words, causes mixed results for users who try to call up Web sites featuring topics that Beijing would prefer were not discussed - Tibet and Taiwan, for example.
The same is true for search engine requests for 'Falun Gong' or even national leaders - the engine may stop when it turns up a search result that is too controversial. E-mail about Tibet, Taiwan, human rights or other topics that have caused rifts between China and its critics may also be blocked.
In many cases, sites stop processing mid-stream, prompting a 'page cannot be displayed' message on the browser. But some computers fail only some of the time, and others never do.
While encouraging Internet use as a business tool, the authorities have long blocked sites they think will incite dissent. Traditionally, they have blocked the sites of overseas media as well as sites belonging to separatist groups, human rights organisations, Taiwanese authorities and overseas China commentators.
But some of the news site blocks have come down over the past year, easing frustration among young and professional people who use the English-language Web.
The packet sniffers have yet to train their noses on other communication channels. Instant messenger conversations, for example via ICQ or Hotmail, proceed freely despite the use of politically volatile or pornographic language. It is the same for mobile phone short messages. Notes sent via small, private overseas-based e-mail exchanges also meet no resistance.