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.

via SHODAN – Computer Search Engine.

What ports does SHODAN index?

The majority of data is collected on web servers at the moment (port 80), but there is also some data from FTP (23), SSH (22) and Telnet (21) services. There are plans underway to expand the index for other services. Let me know if there are specific ports you would like to see included.