Shodan

Shodan is a search engine that lets you find specific computers (routers, servers, etc.) using a variety of filters. 

Shodan will searching all the 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. 

For example, following is a FTP banner:

220 kcg.cz FTP server (Version 6.00LS) ready.

This tells us a potential name of the server (kcg.cz), the type of FTP server (Solaris ftpd) and its version (6.00LS). For HTTP a banner looks like:

HTTP/1.0 200 OK
Date: Tue, 16 Feb 2010 10:03:04 GMT
Server: Apache/1.3.26 (Unix) AuthMySQL/2.20 PHP/4.1.2 mod_gzip/1.3.19.1a mod_ssl/2.8.9 OpenSSL/0.9.6g
Last-Modified: Wed, 01 Jul 1998 08:51:04 GMT
ETag: "135074-61-3599f878"
Accept-Ranges: bytes
Content-Length: 97
Content-Type: text/html 

1. Search input box: where all the magic happens
2. Country map: click on a country to only see computers from there in the results
3. Service filter: decide which services should be searched
4. Options bar: click on it to see the above filters

The interface should be familiar to anybody who's used search engines before. Onwards to actually using SHODAN!

To get things started, search for computers that contain 'Apache' in their banner: Apache

1. Search input: the query we used for searching, in this case 'apache'.
2. Save: store useful search queries.
3. Export: export up to 1,000 results in XML format.
4. Result info: shows how many total hosts were found for the query.
5. Country breakdown: highlights the countries that had the most matches for your query.
6. Left side of results: contains the IP, operating system, date when it was added and the country the computer is in. If there's domain information available then that is shown as well.
7. Right side of results: shows the banner with the search term(s) highlighted.