How to check whether a port is open at client's network/firewall?

Question

This is solved at last with \"timeout\" attribute of jQuery AJAX (and JSONP). See my own answer !

Please see the updated part, I

Answer 1

Check this out:

http://blog.andlabs.org/2010/12/port-scanning-with-html5-and-js-recon.html

With JS-Recon, you can do port scanning with javascript. You can simply point it to your local IP address. I believe it works by making a web sockets/cors connection to an arbitrary desintation ip/socket and measuring the timeouts. It is not a perfect approach, but this may be the limit of javascript ultimately.

If you can do it in a java applet/flash application, that may be better ultimately as they have lower-level access.