GFW (Great FireWall) is a technology, which China used it to shield content transmission between overseas and China ,targeting on content transmission not meeting the requirements of Chinese government, especially political contents.
Early on, in about September 2002, the main methods GFW shield sites were IP blocking and DNS poisoning. When you access a website which is on the blacklist of GFW, it will return a 404 error; later, your IP will then be unable to access oversea sites for 5-10 minutes as punishment.
But, soon, users discovered that the famous search engine Google has a feature, which is called “cached pages”. You can view the cache of web pages you want to visit through “cached page” link on Google. Although the original site can not be accessed, you can still view it through Google and bypass the GFW. Therefore, GFW then block the keywords used in the url of Google’s cached page, so that all of Google’s cached page can not be accessed.
Although you can not access the site’s content, visitors are still aware of the existence of this article. Soon, GFW start to block search result pages for some special keywords on Google. For example, in a certain period, all surnames of Chinese top leaders are shielded. At that time, the General Secretary of Chinese Communist Party is Hu Jingtao, therefore, if you searched any words associated with “Hu” in Google, such as carrots(spell as Hu Luo Bo in Chinese), the search page would be blocked. (As in English, if GFW take “hu” as a blocked search keywords, then you can not search “human”, “shut” etc. also)
Because China’s internet development has lagged behind, it did not have many IP addresses. Some colleges and universities only had one or a few IPs, all students accessed to foreign websites via the same IP. If anybody searched a keyword on GFW’s black list, then all students would lose connection to foreign websites and it would take a short period to recover.
To improve service, Google began to move servers to China and established Google.cn. Google once occupied about 30% of China’s search market share. But after Kai-fu Lee’s leave, Google’s relationship with the Chinese government deteriorated rapidly.
Since the Chinese government’s long time censoring of Google.cn’s contents. There are also some occasional events happened. In 2009, CCTV, the national TV station, attack Google for distribution of sexual contents. CCTV interviewed an accuser, who claim to be a student, but the student was then confirmed as a CCTV intern. In 2010, the government-backed Chinese hackers have attacked Gmail, try to steal some of the opposition’s mailbox. After that, Google decided to leave China.
Before Google’s exiting, GFW has also shielded Google.com, through deterioration (such as accessing one minute break 15 seconds), to reduce the amount of Google users. Shortly after the exit, China directly blocked all Google servers. In 2014, China also blocked gmail.
Other Google’s peripheral products, such as youtube, google plus, maps, drive, hangout, etc., have all been banned by China soon after being launched.