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

In JavaScript, you have to work-around the asynchronous issue. Here is a proposal:

• The HTML page displays an animated image as a progress bar
• You invoke the checkURL
• After either receiving the callback or a defined timeout, you change display for an error message or do on with the job to do

Based on the following document with the use of XMLHttpRequest, here is a code example for checkURL:

var myrequest = new ajaxRequest();
var isAccessible = false;
myrequest._timeout = setTimeout(function() {
      myrequest.abort();
      displayErrorMessage();
    },
    1000
    ) //end setTimeout
myrequest.onreadystatechange = function() {
    if (myrequest.readyState == 4) { //if request has completed
      if (myrequest.status == 200) {
        isAccessible = false;
        goOnWithTheJob();
      } else {
        displayErrorMessage();
      }
    }
myrequest.open("GET", url, true);
myrequest.send(null); //send GET request
// do nothing - wait for either timeout or readystate callback 

This code lets 1 second to get the 200 response from a HTTP GET on a basic resource.

In your local test, you get an immediate answer because the system answers connection reset if the port is closed but a firewall just does not answer.

Even if the open method may be used synchronously, I recommend the use of a timer because the code is likely to wait for TCP timeouts and retries (3 x 1 minute ?) as a firewall usually just drops packets on closed ports and may reject ICMP packets, preventing you to test availability thanks to ping. And I imagine such a long wait is not expected for such a check.