One method used to gain entrance to a network is to scan a server's ports to discover which ports are open. It's good practice therefore to run a port scanner yourself to see which of your ports are vulnerable and secure them. Two of the better-known utilities for port scanning are nmap
The nmap scanner is useful for Linux and other forms of UNIX, and has a broad range of options. The command has a large number of options, which varies depending upon your particular version. To scan a particular host you need to specify the host by either its name or IP address. The nmap command scans the /etc/services file returns a table showing open ports, the protocol that they support and their current service assignment. You can also use nmap to check for any half open ports (they show up in the list), and to check for FTP bounce attacks.
The second valuable scanner is the strobe command. You'll find strobe in UNIX, Linux, Macintosh, and some Windows versions. The strobe command lists all open ports for a specified hostname between 1 and 1024. Although strobe searches for other things, you should see the same open ports in strobe's output as you do in nmap's.
A more complete explanation of port scanning may be found at: www.auditmypc.com/freescan/readingroom/port_scanning.asp. For a listing of these and other port scanners, and their download locations go to: www.mycert.mimos.my/resource/scanner.htm.
Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.
This was first published in December 2003