Arduino Ethernet Access-Control-Allow-Origin issues

Question

Hey all i am trying to post data to the arduino ethernet microprocesser but it seems to not work when i try to call the function within my app using Jquery AJAX.

Here is the ajax code:

function sendToArduinoEthernet(whatsBeingSent) {
    jQuery.support.cors = true;

    $.ajax({
        crossDomain: true,
        async : true,
        type: "POST",
        headers: {
            'Access-Control-Allow-Origin': '*'
        },
        url: "http://192.168.9.199:199/?r=" + whatsBeingSent,
        data: '{}',
        contentType: "text/html",
        dataType: "html",
        success: OnSuccessCallArduino,
        error: OnErrorCallArduino
    });      
}

And this is the Arduino code:

#include <Ethernet.h>
#include <SPI.h>

boolean reading = false;
char c;
EthernetClient client;
String readString = String(30); //string for fetching data from address

///////////////////////////////////////////////////////////////////////////////////////////
//Ethernet CONFIGURE
///////////////////////////////////////////////////////////////////////////////////////////
  byte ip[] = { 192, 168, 9, 199 };   //Manual setup only 421
  byte gateway[] = { 192, 168, 9, 1 }; //Manual setup only
  byte subnet[] = { 255, 255, 255, 0 }; //Manual setup only
  byte mac[] = { 0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED }; // if need to change the MAC address (Very Rare)
  EthernetServer server = EthernetServer(199); //port 199
///////////////////////////////////////////////////////////////////////////////////////////
//CONFIGURE for relays
///////////////////////////////////////////////////////////////////////////////////////////
  //(analog->digital output)
  int relay1 = 14; //Android phone USB power (turn on to shut off power to phone)
  int relay2 = 15; //Bluray eject
  int relay3 = 16; //PC power button
  int relay4 = 17; //Android Phone Power button

void setup(){
    Serial.begin(9600);

    pinMode(relay1, OUTPUT);
    pinMode(relay2, OUTPUT);
    pinMode(relay3, OUTPUT);
    pinMode(relay4, OUTPUT);
    pinMode(fan1, OUTPUT);
    pinMode(fan2, OUTPUT);

    analogWrite(fan1, pwmVal);
    analogWrite(fan2, pwmVal);
    digitalWrite(relay1, HIGH);
    digitalWrite(relay2, HIGH);
    digitalWrite(relay3, HIGH);
    digitalWrite(relay4, HIGH);

    Ethernet.begin(mac, ip, gateway, subnet); //for manual setup  
    server.begin();
    Serial.println(Ethernet.localIP());
}

void loop(){
    checkForClient();
}

void checkForClient(){
    client = server.available();

    if (client) {
      boolean currentLineIsBlank = true;
      boolean sentHeader = false;

      while (client.connected()) {
          if (client.available()) {  
              if(!sentHeader){
                // send a standard http response header
                client.println("HTTP/1.1 200 OK");
                client.println("Content-Type: text/html");
                sentHeader = true;
              }

              c = client.read();

              if (readString.length() < 16)
              {
                readString += String(c);
              }

              if(reading && c == ' ') reading = false;
              if(c == '?') reading = true; //found the ?, begin reading the info 
              if (c == '\n' && currentLineIsBlank) break;

              if (c == '\n') {
                currentLineIsBlank = true;          
              }else if (c != '\r') {
                currentLineIsBlank = false;
              }
          }
      }  
      delay(1);   
      switchStuff(readString);
      client.stop();
      readString = "";
      c = ' ';
      reading = false;
    }
}

void switchStuff(String stuff) {
  stuff.toLowerCase();
  //client.println(stuff);

  if (stuff.indexOf("r1on") > 0) {    
    digitalWrite(relay1, LOW); //Turn 5v USB power ON
  } else if (stuff.indexOf("r1off") > 0) {
    digitalWrite(relay1, HIGH); //Turn 5v USB power OFF
  } else if (stuff.indexOf("r2") > 0) {
    digitalWrite(relay2, LOW); //Eject bluray Disc
    delay(500); //pause for 1/2 second
    digitalWrite(relay2, HIGH);
  } else if (stuff.indexOf("r3") > 0) {
    digitalWrite(relay3, LOW); //"Push" the pc power button
    delay(500); //pause for 1/2 second
    digitalWrite(relay3, HIGH); //"Release" the pc power button
  } else if (stuff.indexOf("r4") > 0) {
    digitalWrite(relay4, LOW); //"Push" phone power button down
    delay(7000); //pause for 7 seconds
    digitalWrite(relay4, HIGH); //"Relese" phone power button
  } else if (stuff.indexOf("r5") > 0) {
    digitalWrite(relay4, LOW); //"Push" phone power button down
    delay(500); //pause for 1/2 second
    digitalWrite(relay4, HIGH); //"Relese" phone power button
  }
}

Answer 1

That's because the Access-Control-Allow-Origin header must be sent by the HTTP server.

The Cross-Origin Resource Sharing is a security feature embedded in every recent web browser which defines a way in which the browser and the server can interact to determine whether or not to allow the cross-origin request.

The access to the resources exposed by your server is dictated by the CORS policy he defines in its response to your request. So, you might want to return this header server-side rather than using it in your request client-side. Failing to do so will lead to an error throwed by your browser stating that you have no right to access the given resource.

You'll find further details and a straight-forward example of this here.