This should help....
What are ports?
Ports are used by a computer to control which service is accessed when establishing a connection. If you are communicating with Secure Design for example and you are sending e-mail, your computer establishes a connection to port 25 (SMTP) however if you are accessing a web page, you must connect to port 80 (http). Ports on a computer range from 1 to 65535. Ports under 1024 are reserved for system processes such as mail and web servers. Ports above 1024 are often used for outbound connections.
What is port scanning?
When establishing a connection to a server, your computer specifies the server address and the target port number. When the request is made, the server responds by allowing the connection or responding with a "port closed" message.
Port scanning is a method of probing a computer to see what ports are open. This is usually a brute force operation where one simply tries to establish a connection to each and every port on the target computer. When a connection is established, the caller makes note of the port number and continues on. The caller can then examine these ports later to see if any known security holes exist.
Should I care?
In short, YES. Even if you are on a basic dialup internet account, you need to take precautions to ensure your computer is not broken into. Don't make the assumption that nobody will find your "one little computer in the vast expanse of the internet." Many programs exist that will allow miscreants to automatically scan large blocks of internet addresses. Some only look for Windows file sharing ports, while others look for any open port. Once your computer is found, you are at the mercy of their whim.
If your computer is insecure, files can be stolen from your disk drive. If you have used an online banking program, or personal finance software, your bank account numbers and credit card information can be stolen. Passwords are also stored on your computer's disk drive and can be stolen just as easily by copying the password file.
Even if you have no "important" information worth stealing from your computer, you are still at risk of another type of break-in. A cracker may also install a Denial of Service zombie, which will hijack your computer and instruct it to send garbage packets to a target site. Several large scale e-commerce businesses such as Ebay, Yahoo, Etrade, and others were attacked around Feb 7-8, 2000 by using hundreds or even thousands of hijacked computers. In this case, the theft was not one of information, but a theft of service by hijacking the computer's services.
In the course of the US DoJ investigation into recent Denial of Service attacks, the FBI confiscated a server from home businesses that were used in the attack. This server were hijacked and used in the attack without the knowledge of the owners. Even though this is an extreme case, the owner was left without their server in this example of why security is important. If the owner had taken steps to ensure their server's security, they would have avoided this situation.
When checking your computer for open ports, you may find that you have several open ports or services running that you never use. While this is mainly a Unix/Linux issue, it may also apply to Windows NT or Windows 9x users as well. The fewer ports you leave open, the lower your risk to breakins is. In short, less is more. After running a port scan, you should review the results and close any services that you don't need. More information on how to do this in Linux (or UNIX in general) is available. This port scan will also search for known problems such as Windows file sharing, and Denial of Service zombie programs such as Trinoo, Stacheldraht, and others. If these are found, you should take steps to eliminate these programs.
