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Re^3: Proving a UDP port is closed

by Limbic~Region (Chancellor)
on Feb 01, 2011 at 15:00 UTC ( #885510=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Proving a UDP port is closed
in thread Proving a UDP port is closed

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.

Cheers - L~R


Comment on Re^3: Proving a UDP port is closed
Re^4: Proving a UDP port is closed
by davis (Vicar) on Feb 01, 2011 at 15:11 UTC
    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.

    davis

      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.

        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...

        davis

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