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Concept
Port scanner
Summary
A port scanner is an application designed to probe a server or host for open ports. Such an application may be used by administrators to verify security policies of their networks and by attackers to identify network services running on a host and exploit vulnerabilities.
A port scan or portscan is a process that sends client requests to a range of server port addresses on a host, with the goal of finding an active port; this is not a nefarious process in and of itself. The majority of uses of a port scan are not attacks, but rather simple probes to determine services available on a remote machine.
To portsweep is to scan multiple hosts for a specific listening port. The latter is typically used to search for a specific service, for example, an SQL-based computer worm may portsweep looking for hosts listening on TCP port 1433.
The design and operation of the Internet is based on the Internet Protocol Suite, commonly also called TCP/IP. In this system, network services are referenced using two components: a host address and a port number. There are 65535 distinct and usable port numbers, numbered 1..65535. (Port zero is not a usable port number.) Most services use one, or at most a limited range of, port numbers.
Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host.
The result of a scan on a port is usually generalized into one of three categories:
Open or Accepted: The host sent a reply indicating that a service is listening on the port.
Closed or Denied or Not Listening: The host sent a reply indicating that connections will be denied to the port.
Filtered, Dropped or Blocked: There was no reply from the host.
Open ports present two vulnerabilities of which administrators must be wary:
Security and stability concerns associated with the program responsible for delivering the service - Open ports.
Security and stability concerns associated with the operating system that is running on the host - Open or Closed ports.
Official source
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