This is one of the earlier papers that looked specifically for regional variation in China’s internet censorship; as I mentioned when reviewing
Large-scale Spatiotemporal Characterization of Inconsistencies in the World’s Largest Firewall, assuming that censorship is monolithic is unwise in general and especially so for a country as large, diverse, and technically sophisticated as China. This paper concentrates on variation in DNS-based blockade: they probed 187 DNS servers in 29 Chinese cities (concentrated, like the population, toward the east of the country) for a relatively small number of sites, both highly likely and highly unlikely to be censored within China.
The results reported are maybe better described as
inconsistencies among DNS servers than
regional variation. For instance, there are no sites called out as accessible from one province but not another. Rather, roughly the same set of sites is blocked in all locales, but all of the blocking is somewhat leaky, and some DNS servers are more likely to leak—regardless of the site—than others. The type of DNS response when a site is blocked also varies from server to server and site to site; observed behaviors include no response at all, an error response, or (most frequently) a success response with an incorrect IP address. Newer papers (e.g.  ) have attempted to explain some of this in terms of the large-scale network topology within China, plus periodic
outages when nothing is filtered at all, but I’m not aware of any wholly compelling analysis (and short of a major leak of internal policy documents, I doubt we can ever have one).
There’s also an excellent discussion of the practical and ethical problems with this class of research. I suspect this was largely included to justify the author’s choice to only look at DNS filtering, despite its being well-known that China also uses several other techniques for online censorship. It nonetheless provides valuable background for anyone wondering about methodological choices in this kind of paper. To summarize:
Many DNS servers accept queries from the whole world, so they can be probed directly from a researcher’s computer; however, they might vary their response depending on the apparent location of the querent, their use of UDP means it’s hard to tell censorship by the server itself from censorship by an intermediate DPI router, and there’s no way to know the geographic distribution of their intended clientele.
Studying most other forms of filtering requires measurement clients within the country of interest. These can be dedicated proxy servers of various types, or computers volunteered for the purpose. Regardless, the researcher risks inflicting legal penalties (or worse) on the operators of the measurement clients; even if the censorship authority normally takes no direct action against people who merely try to access blocked material, they might respond to a sufficiently high volume of such attempts.
Dedicated proxy servers are often blacklisted by sites seeking to reduce their exposure to spammers, scrapers, trolls, and DDoS attacks; a study relying exclusively on such servers will therefore tend to overestimate censorship.
Even in countries with a strong political commitment to free expression, there are some things that are illegal to download or store; researchers must take care not to do so, and the simplest way to do that is to avoid retrieving anything other than text.