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Wednesday, March 14, 2012

Google and Facebook blocked by the Danish CP filter

In the morning of 1 March 2012, about 8000 websites, including Google and Facebook, were blocked by the Danish CP filter. When the customers of the affected ISPs, Siminn Denmark and Tele Greenland, made Google searches or accessed their Facebook pages, they were met by the STOP page for the Danish CP filter. The STOP page warns people that they are trying to access websites with CP content, and that even viewing such content is illegal under the Danish law. The Danish CP filter is implemented using DNS hijacking (DNS redirection). The participating ISPs (which are all mainstream ISPs with private customers in Denmark) receive a list of domains to be blocked from the Danish police, and the ISPs implement this list in their DNS resolvers. This is done in a completely automatic process.

The police made a serious error when they added the 8000 legitimate domains to the blocking list, including and The error only affected two smaller ISPs in Denmark, because they were the first to do the daily CP list update on their DNS resolvers, but this was sheer luck. It could just as well have affected TDC, the largest Danish ISP. The vigilant technical support staff at Siminn Denmark immediately alerted the Danish police about the error, so that the blocking list was not pushed to the rest of the Danish ISPs.