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Footprinting (also known as reconnaissance) is the technique used for gathering information about computer systems and the entities they belong to. To get this information, a hacker might use various tools and technologies. This information is very useful to a hacker who is trying to crack a whole system.
When used in the computer security lexicon, "Footprinting" generally refers to one of the pre-attack phases; tasks performed prior to doing the actual attack. Some of the tools used for Footprinting are Sam Spade, nslookup, traceroute, Nmap and neotrace.
Techniques used for Footprinting
- DNS queries
- Network enumeration
- Network queries
- Operating system identification
- Organizational queries
- Ping sweeps
- Point of contact queries
- Port Scanning
- Registrar queries (WHOIS queries)
- SNMP queries
- World Wide Web spidering
Uses of Footprinting
It allows a hacker to gain information about the target system. This information can be used to carry out further attacks on the system. That is is the reason by which it may be named a Pre-Attack, since all the information is reviewed in order to get a complete and successful resolution of the attack.
Crawling is the process of surfing the internet to get the required information about the target. The sites surfed can include the target’s website, blogs and social networks. The information obtained by this method will be helpful in other methods too.
WHOIS is a web application used to get information about the target website, such as the administrator's e-mail address and details about the registration. WHOIS is a very large database and contains information of approximately all the websites. It can be searched by domain name. 
Search engines such as Google can also be used to gather information about the target system. It depends on how well one knows how to use search engines to collect information. If used properly, the attacker can gather lots of information about a company, its career and its policies etc.
Information can also be gathered using the command Tracert ("traceroute"), which is used to trace a path between a user and the target system on the networks. That way it becomes clear where a request is being forwarded and through which devices. In Linux systems, the tracepath and traceroute commands are also available for doing traceroute operations.
SNS stands for Social Networking Sites. SNS can be used for footprinting. One method is to compile a list of employees working under the target organization and to look for their details which can again give information about the type and number of staff, systems and security measures which are installed. Websites such as Linkedin, Twitter etc. can be used to gather more information about the target. On the basis of the information gathered social networking can also be performed.
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