Understanding CORS

I've been looking on the web regarding CORS, and I wanted to confirm if whatever I made of it is, what it actually is.

Mentioned below is a totally fictional scenario.

I'll take an example of a normal website. Say my html page has a form that takes a text field name. On submitting it, it sends the form data to myPage.php. Now, what happens internally is that, the server sends the request to www.mydomain.com/mydirectory/myPage.php along with the text fields. Now, the server sees that the request was fired off from the same domain/port/protocol

(Question 1. How does server know about all these details. Where does it extract all these details froms?)

Nonetheless, since the request is originated from same domain, it server the php script and returns whatever is required off it.

Now, for the sake of argument, let's say I don't want to manually fill the data in text field, but instead I want to do it programmatically. What I do is, I create a html page with javascript and fire off a POST request along with the parameters (i.e. values of textField). Now since my request is not from any domain as such, the server disregards the service to my request. and I get cross domain error?

Similarly, I could have written a Java program also, that makes use of HTTPClient/Post request and do the same thing.

Question 2 : Is this what the problem is?

Now, what CORS provide us is, that the server will say that 'anyone can access myPage.php'. From enable cors.org it says that

For simple CORS requests, the server only needs to add the following header to its response: Access-Control-Allow-Origin: *

Now, what exactly is the client going to do with this header. As in, the client anyway wanted to make call to the resources on server right? It should be upto server to just configure itself with whether it wants to accept or not, and act accordingly.

Question 3 : What's the use of sending a header back to client (who has already made a request to the server)?

And finally, what I don't get is that, say I am building some RESTful services for my android app. Now, say I have one POST service www.mydomain.com/rest/services/myPost. I've got my Tomcat server hosting these services on my local machine.

In my android app, I just call this service, and get the result back (if any). Where exactly did I use CORS in this case. Does this fall under a different category of server calls? If yes, then how exactly.

Furthermore, I checked Enable Cors for Tomcat and it says that I can add a filter in my web.xml of my dynamic web project, and then it will start accepting it.

Question 4 : Is that what is enabling the calls from my android device to my webservices?

Thanks

1. First of all, the cross domain check is performed by the browser, not the server. When the JavaScript makes an XmlHttpRequest to a server other than its origin, if the browser supports CORS it will initialize a CORS process. Or else, the request will result in an error (unless user has deliberately reduced browser security)

2. When the server encounters Origin HTTP header, server will decide if it is in the list of allowed domains. If it is not in the list, the request will fail (i.e. server will send an error response).

For number 3 and 4, I think you should ask separate questions. Otherwise this question will become too broad. And I think it will quickly get close if you do not remove it.

For an explanation of CORS, please see this answer from programmers: https://softwareengineering.stackexchange.com/a/253043/139479

NOTE: CORS is more of a convention. It does not guarantee security. You can write a malicious browser that disregards the same domain policy. And it will execute JavaScript fetched from any site. You can also create HTTP headers with arbitrary Origin headers, and get information from any third party server that implements CORS. CORS only works if you trust your browser.

For question 3, you need to understand the relationship between the two sites and the client's browser. As Krumia alluded to in their answer, it's more of a convention between the three participants in the request.

I recently posted an article which goes into a bit more detail about how CORS handshakes are designed to work.