validation for connectivity

**JShor** · 04-22-2007, 05:38 AM

Hi,

This will sound silly but I need a script that tells if you are connected to the internet, and if the user is not, then it will show an error message. It is a long story why this script (I know it doesn't make sense) but I am in DIRE need for this script, and I don't know what to do

Can anyone help?

Thanks,
-JShor

**tech_support** · 04-22-2007, 06:36 AM

You could do this with PHP quite easily.

PHP Code:


<?php
if (file_exists('http://www.google.com.au')) {
echo 'It works!';
}
else {
echo 'Error! You cannot view Google!';
}
?>

But I'm not sure if you can do this with JavaScript.

**jscheuer1** · 04-22-2007, 07:24 AM

This works but, you cannot simply navigate to the page unless your cache is disabled or cleared (arrow2.gif or whatever you use there must exist locally):

Code:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd">
<html>
<head>
<title></title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body>
<img src="arrow2.gif" onerror="alert('not connected!');">
<script type="text/javascript">
document.images[0].src='http://www.google.com/intl/en_ALL/images/logo.gif';
</script>
</body>
</html>

Or, what you can do though is, once you have navigated to it, hit refresh.

**Bob90** · 04-22-2007, 08:30 AM

Ajax.
Send a request for conformation that you are online.

Cookies.
I'm sure there is a way to test. Maybe use a domain URL check.
IE domain must be "http://www.example.com"

Easily hacked though, but so is the php, unless it writes some encrypted code.

**djr33** · 04-22-2007, 08:40 AM

This is very useful if you have, for example, a slideshow, so you don't keep running it if you're disconnected... it should know, then stop.
Sure, it could be hacked, but this isn't security.... it's functionality.

**Twey** · 04-22-2007, 12:16 PM

Code:

<script type="text/javascript">
  function checkConnection(yesfunc, nofunc) {
    var e = document.createElement("img");
    e.onerror = function() {
      nofunc();
      e = null;
    };
    e.onload = function() {
      yesfunc();
      e = null;
    };
    e.src = "http://www.google.com/images/google_sm.gif?c=" + (new Date().getTime());
  }

  checkConnection(
    function() {
      alert("Internet connection succeeded.");
    },
    function() {
      alert("Internet connection failed.");
    }
  );
</script>

This is more reliable than AJAX: there are less reasons it could give a false negative (although there are still some).

**jscheuer1** · 04-22-2007, 01:49 PM

Twey, I had tried something like that at first, using new Date().valueOf(). However, either due to something at Google or a broader problem with the concept, it didn't work. Your code doesn't either. With a remote hosted image on a different server (preferably one that you control), it could work.

In any case, the code I eventually came up with works as I described in my post. It may be able to be improved so as to still use the ubiquitous Google logo while not requiring user intervention to refresh the page.

I wonder though, how the folks at Google feel about something like this?

**Twey** · 04-22-2007, 02:29 PM

Twey, I had tried something like that at first, using new Date().valueOf(). However, either due to something at Google or a broader problem with the concept, it didn't work. Your code doesn't either. With a remote hosted image on a different server (preferably one that you control), it could work.

"You" as in me, or "you" as in the OP? I was trying to avoid depending upon a remote service that might change or be discontinued in the future. This might work if the OP has a server with the appropriate facilities.

I wonder though, how the folks at Google feel about something like this?

I doubt they'd mind terribly, if they even noticed it. It would probably account for something like one in a billion of their requests, even if it were to be checked regularly. In any case, the Google logo was just an example of an image I thought was reasonably likely to stay there for, if not the life of the Internet, a darned long time.

**jscheuer1** · 04-22-2007, 02:38 PM

I meant you as in anybody trying to do this. I subsequently tested this concept on an image on a site I master and it worked fine, as I suspected that it would.

Apparently Google has something going on that prevents it from working with their images. Most likely, whatever that is isn't intended to break our scripts, it just does so as a side effect. Other servers over which one has inadequate control could also break a script like these, intentionally or otherwise.

**Bob90** · 04-23-2007, 09:36 AM

Doesn't

Code:

if(document.location.toString().lastIndexOf(/http/) > 0)
 {online}
else
 {offline}

Quicker

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