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This map shows the locations of more than 16,000,000 open DNS resolvers discovered by John Kristoff during a survey of the IPv4 space between April 29 and June 14 of 2007. A DNS server is "open" if it accepts queries from external locations and forwards those queries on to authoritative nameservers. Open DNS servers are sometimes used, in conjunction with spoofed source addresses, to execute massive DNS-based DDoS attacks.

Each pixel in the original 4096 x 4096 image (3.5 MB) represents a single /24 network containing up to 256 hosts. The color of each pixel indicates the number of open DNS servers found there, ranging from 1 (blue) to 256 (red). Reserved and unallocated areas are shown in grey.

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