Determining Which Ports Are Active

To restrict the use of open ports, you need to know which ports are active on your system and which programs are using them. Although no single tool provides all the information you need, you can piece together the answers with a few tools and investigative skills. The instructions in this section show you how to combine the information provided by several tools to trace the program that is listening on a particular port.

When port sleuthing, it's often useful to have a list of programs that commonly use a particular port. Table 19-1 presented a list of the most common ports; a complete listing of the official well-known port assignments is at http://www.iana.org/assignments/port-numbers. When tracking down

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