A Linux administrator is checking connectivity between two servers in separate subnets: Server A: 10.1.2.20 (source) Server B: 10.2.1.10 (destination on port 8080) The administrator tried to run the…

`tcpdump` captures network traffic but does not actively test or confirm the listening status of a port; it would only show the refused connection packets.

`ss -l -p dport=8080 dst 10.2.1.10` is not a valid way to check for listening services; `ss -l` lists all listening sockets, and filtering by `dport` or `dst` does not apply to a listening socket expecting incoming connections.

`nc -l -p 8080 10.2.1.10` would cause `netcat` to listen on port 8080, which would then allow the `curl` command from Server A to successfully connect, but it doesn't diagnose the original 'Connection refused' problem which was due to nothing listening.

The `nmap` command is a powerful network scanning tool that can determine the status of ports on a target host. Running `nmap -v -p 8080 10.2.1.10` on Server B (targeting itself on port 8080) would successfully scan the port and report its state, confirming that nothing is listening, which explains the 'Connection refused' error received by `curl` from Server A. This provides a successful diagnostic test.