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Shodan is a search engine for everything on the internet — web cams, water treatment facilities, yachts, medical devices, traffic lights, wind turbines, license plate readers, smart TVs, refrigerators, anything and everything you could possibly imagine that’s plugged into the internet (and often shouldn’t be). Google and other search engines, by comparison, index only the web.
The best way to understand what Shodan does is to read founder John Matherly’s book on the subject. The basic algorithm is short and sweet:
1. Generate a random IPv4 address
2. Generate a random port to test from the list of ports that Shodan understands
3. Check the random IPv4 address on the random port and grab a banner
4. Goto 1