Knocker is a simple and easy to use TCP security port scanner written in C to analyze hosts and all of the different services started on them. It is available for Linux, FreeBSD, HP-UX, and Windows9x/2000/NT and it is licensed under the GPL.

With knocker it is possible to limit port scan by a single port or by a range of ports or you could also output the result in a text file, yeah that’s about it. That why they call it a “simple” port scanner. None of those fancy stuff Nmap does for you.

That’s Knocker for you: http://knocker.sourceforge.net/