How to make postMessage applicable to all subdomains

Question

In window.postMessage second attribute specifies domain to which my message can be sent. Is there any way to specify that it is applicable to all subdomains.

Things tried:

iframe.contentWindow.postMessage('The message to send.','http://*.wordpress.com');
iframe.contentWindow.postMessage('The message to send.','http://wordpress.com');

Answer 1

It is possible without knowing all domains names. Just get a referrer URL and you actually get the origin from that:

var parentOrigin = document.referrer.match(/^.+:\/\/[^\/]+/)[0];

Now, the only thing is to check whether the URL matches the criteria (ends with wordpress.com) and if yes, allow the message to this specific domain.

Works only until the user navigates inside the iframe somewhere else: the referrer is changed. However, the referrer can be saved in an iframe's localStorage so you have a list of possible domains and can send the message to every domain from the list a proposed by Ivan Zuzak

Answer 2

Nope, not possible.

The only scenario in which you can help yourself is if you know that the target iframe is from a known, finite set of origins (e.g. "http://a.wordpress.com", "http://b.wordpress.com" and "http://c.wordpress.com"). In this case, just make a postMessage request for each of the origins, since only one of those will succeed and the other ones will fail.