A port scan is a series of messages sent by someone attempting to break into a computer to learn which computer network services, each associated with a "well-known" port number, the computer provides. Port scanning, a favorite approach of computer cracker, gives the assailant an idea where to probe for weaknesses. Essentially, a port scan consists of sending a message to each port, one at a time. The kind of response received indicates whether the port is used and can therefore be probed for weakness.

Types of port scans include:

* vanilla - An attempt to connect to all ports (there are 65,536)

* Strobe - An attempt to connect to only selected ports (typically, under 20)

* Stealth scan - Several techniques for scanning that attempt to prevent the request for connection being logged

* FTP Bounce Scan - Attempts that are directed through an File Transfer Protocol server to disguise the cracker's location

* Fragmented Packets - Scans by sending packet fragments that can get through simple packet filters in a firewall

* UDP - Scans for open User Datagram Protocol ports

* Sweep - Scans the same port on a number of computers

[Quelle: searchsecurity]