Web search engines, such as Google and Bing, are great for finding websites. But what if you’re interested in finding computers running a certain piece of software (such as Apache)? Or if you want to know which version of Microsoft IIS is the most popular? Or you want to see how many anonymous FTP servers there are? Maybe a new vulnerability came out and you want to see how many hosts it could infect? Traditional web search engines don’t let you answer those questions.
So what does SHODAN index then? Good question. The bulk of the data is taken from ‘banners’, which are meta-data the server sends back to the client. This can be information about the server software, what options the service supports, a welcome message or anything else that the client would like to know before interacting with the server.