Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response

Question

I am trying to make a login page from cross domain but I couldn't solve the problem, the error is:

XMLHttpRequest cannot load http://localhost/testing/resp.php. Request header field Access-Control-Allow-Headers is not allowed by Access-Control-Allow-Headers in preflight response.

My Javascript code is:

$('#login').click(function(){
		var username = $('#uname').val();
		var password = $('#pass').val();
		var result = $('.result');
		result.text('loading....');

		if (username != '' && password !=''){
			var urltopass = 'action=login&username='+username+'&password='+password;
			$.ajax({
				type: 'POST',
				data: urltopass,
				headers: {"Access-Control-Allow-Headers": "Content-Type"},
				url: 'http://localhost/testing/resp.php',
				crossDomain: true,
				cache: false,
				success: function(responseText){
					console.log(responseText);
					if(responseText== "0"){
						result.text('incorrect login information');
					} else if (responseText == "1"){
						window.location="http://localhost/testing/home.php";
					} else{
						alert('error in sql query \n' + responseText);
					}
				}
			});
		} else return false;
	});

The PHP code for http://localhost/testing/resp.php :

<?php
	include "db.php"; //Connecting to database

	if (!isset($_SERVER['HTTP_ORIGIN'])) {
		echo "This is not cross-domain request";
    exit;
}
	header("Access-Control-Allow-Origin: *");
	header("Access-Control-Allow-Credentials: true");
	header("Access-Control-Allow-Methods: POST, GET, OPTIONS");
	header("Access-Control-Allow-Headers: Content-Type, Authorization, X-Requested-With");
	header('P3P: CP="CAO PSA OUR"'); // Makes IE to support cookies
	header("Content-Type: application/json; charset=utf-8");

	if (isset($_POST['action']) && $_POST['action'] == 'login'){
		$uname = $_POST['username'];
		$pass = $_POST['password'];

		$sql = "SELECT * FROM loginajax WHERE username='$uname' AND password='$pass'";
	
		$rs=$conn->query($sql);

		if (mysqli_num_rows($rs) <= 0){
			echo "0";
		} else {
			echo "1";
		}
		
	} else echo "this is not Login";

?>

Answer 1

remove this:

headers: {"Access-Control-Allow-Headers": "Content-Type"},

from your jQuery.ajax call.

The server responds with a Access-Control-Allow-Headers header, the client doesn't send it to the server.

The client sends a Access-Control-Request-Headers to request allowing certain headers, the server responds back with with a Access-Control-Allow-Headers that lists the actual headers its going to allow. The client does not get to demand what headers are allowed.