Port scanning is the process of determining which of the many TCP (or UDP or other protocol) ports are being used by a particular computer. Some of the previous questions I've answered have defined what TCP ports are. Port scanning is often one of the first things an attacker will do when attempting to penetrate a particular computer. Tools such as Nmap (available from http://www.insecure.org) provide an automated mechanism for an attacker to not only scan the machine to find out what ports are "open" (meaning being used), but also help to identify what operating system is being used by the machine. Other tools also provide port scanning. Nessus (available from http://www.nessus.org) integrates Nmap with it to do a comprehensive port scan and also vulnerability checking. Used by the security professional, the results of Nessus can help in determining what security patches need to be applied or what services need to be disabled. Used by the attacker, the results can help in determining what exploit attempts will work and which ones will not.
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