Tips 23: Who owns this port ?

Several utilities exist to check which ports are open, who is connected to your system and even what process owns a port number.

First a few ground rules. Ports below 1024 are reserved for common services, and only root can use them.
Standard port numbers can be found in /etc/services. The maximum number of ports is 65k, so you have more than enough Internet ports for all your services.
Here are some useful utilities. Netstat is a command that will list both the open ports and who is connected to your system. You should run it like this:

netstat -an | more

This way you can find out who is connected to which service.

Another interesting command is the fuser program. This program can tell you which user and process owns a
port. For example, the following command will tell you who owns port 6000:

fuser -v -n tcp 6000