How do I find out what network services are running or listing under Linux?

To view a list of network services that are running or listening on a Linux system, you can use the netstat command. The netstat command can be used to display a list of active network connections, as well as a list of network services that are listening for incoming connections.

Here's the basic syntax for the netstat command:

netstat [-options]

To view a list of active network connections, you can use the -t option to show TCP connections and the -u option to show UDP connections:

netstat -t
netstat -u

To view a list of network services that are listening for incoming connections, you can use the -l option:

netstat -l

You can also use the -p option to show the process ID (PID) and program name for each connection or service:

netstat -lp

By default, the netstat command shows information for all network interfaces. You can use the -n option to show numerical addresses instead of hostnames, and the -4 or -6 option to show only IPv4 or IPv6 connections, respectively.

netstat -n
netstat -4
netstat -6

You can use these options in combination to customize the output of the netstat command. For example, to view a list of all active TCP connections and the PID and program name for each connection, you can use the following command:

netstat -t -p

You can also use the lsof command to view a list of open files and the processes that are using them. You can use the -i option to show network connections:

lsof -i

This will show a list of all open network connections, along with the PID and program name for each connection. You can use the -i TCP or -i UDP options to show only TCP or UDP connections, respectively.

lsof -i TCP
lsof -i UDP