Server logs showing GET request right before POST request

Question

When I look through my server logs, I see periodic GET requests coming in instantaneously before the POST request from the same ip with the same referrer. I expect the POST, but not the GET. Has anyone seen this before?

I'm dynamically creating a form inside of an iframe with javascript in order to send the POST request to a server. I can't use Ajax because the POST request is to a different domain. This works on about 95% of the time. 5% of the time I get a GET request just before the POST. It seems to happen repeatedly from the same ip.

Here is the server log:

10.160.42.113 - - [16/Sep/2010:04:33:08 +0000] "GET /pixel HTTP/1.1" 200 2 "url" "Mozilla/5.0 (Windows; U; Windows NT 5.1; hu; rv:1.9.2.9) Gecko/20100824 Firefox/3.6.9 ( .NET CLR 3.5.30729)"
10.160.42.113 - - [16/Sep/2010:04:33:08 +0000] "POST /pixel HTTP/1.1" 200 2 "url" "Mozilla/5.0 (Windows; U; Windows NT 5.1; hu; rv:1.9.2.9) Gecko/20100824 Firefox/3.6.9 ( .NET CLR 3.5.30729)"

Here is the js code:

var iframe = document.createElement("iframe");
iframe.height = "0";
iframe.width = "0";
iframe.frameBorder = "0";
document.getElementById('canvas').appendChild(iframe);


var iframeDocument = iframe.contentDocument || iframe.contentWindow.document || iframe.document;
iframeDocument.open();
iframeDocument.close();


var form = document.createElement("form");
form.setAttribute("action", 'url');
form.setAttribute("method", 'POST');

for (var key in params) {
    var hiddenField = document.createElement("input");
    hiddenField.setAttribute("type", "hidden");
    hiddenField.setAttribute("name", key);
    hiddenField.setAttribute("value", params[key]);

    form.appendChild(hiddenField);
}

iframeDocument.body.appendChild(form);
form.submit();

Answer 1

This is likely a Cross Origin Request Sharing 'preflight' request. See Additional GET requests before POST request that my code isn't making .

You'll need to handle the CORS preflight requests on your web server.

Answer 2

Just a thought - what happens if someone submits the form, then reloads the page, or uses the back/forward buttons on the browser? Might be the case that their browser is trying to "get" the target of the submit when they move backward through the history.