Is there any reason why an -sS scan (or most others) would not report an open port that doesn't have a service attached to it?

For example, in a regular -sS scan, the following port will be missing, but when I scan that port specifically, it appears as open.


2509/tcp open unknown

1 Answer 1


Most likely, it's because port 2509 isn't in the 1000 most frequently used TCP ports. Unless you specify specific ports or a port ratio or top port number, nmap scans only the 1000 most commonly seen ports. See the nmap user guide for details on port selection.

  • Thanks, went back and tried a full port scan and it was successful. Previously had used -p"*" which was wrong syntax. Worked with -p-
    – Ben
    Commented Jun 23, 2014 at 20:37

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