nmap/netcat differentiate between open port with no service listening and open ports with a service listening

Question

I'm pretty new with networks scanning and test tools (nmap and netcat). I want to know how to differentiate :

• if a port is opened in a host with a service listening on this port
• If a port is opened and in host without no service listening on this port

How to differentiate these two situations on results of nmap and/or nc commands ?

Thanks

Answer 1

If the service is not listening you'll get a connection refused message.

Example

# no service listening
$ nc -vz 127.0.0.1 22
Ncat: Version 7.80 ( https://nmap.org/ncat )
Ncat: Connection refused.

# service listening
$ nc -vz 127.0.0.53 53
Ncat: Version 7.80 ( https://nmap.org/ncat )
Ncat: Connected to 127.0.0.53:53.
Ncat: 0 bytes sent, 0 bytes received in 0.03 seconds.