Re^3: Proving a UDP port is closed

[Limbic~Region]

davis,
The netstat command shows network information and is not concerned with what services/processes are running. Also, inetd isn't magical and can't establish a connection without a listener. In order for the server to accept connections on a port then there has to be a listener for that port. The netstat command can be used to show what listeners are out there.

As far as proving it by attempting to connect to it, I would have no way of testing presently.

[davis]

Sorry, I realised the stupidity of what I'd said (the implied magic) fairly quickly! Yeah, grokking the "netstat -af inet" output would almost certainly work for me for both TCP and UDP listeners, but there's still something somehow... cleaner about the attempted connection method, at least in my head.

[Anonymous Monk]

Is it possible for a listener to black-hole the port, and soak up messages (possibly acting on them) with no replies or indication that it has happened to the outside world?

Such a case would require checking for listener processes rather than just shouting at the computer and waiting for echoes.

[davis]

Another good point. I suppose I could have "nc -lu 111" or similar running, and that might appear to be the same as "unreachable".

I think this is nudging me down the netstat/lsof route...