How to check if a request if coming from the same server or different server?

Question

How can I check whether a request being received is sent from the same server??

Say, I've my domain at www.domain.com. Now I've php processing files which will process forms hosted through this domain. This processes will be executed only if the requests are sent from within the domain ie. www.domain.com and any other requests sent from other domains will be discarded.

Answer 1

Basically : you cannot.
With the HTTP protocol, each request is independent from the others.

A first idea would be to check the Referer HTTP header, but note that :

• It can be faked (it's sent by the browser)
• It is not always present.

So : not a reliable solution.

A possible, and far better than the Referer idea, solution could be to use a nonce :

• When displaying the form, put a hidden input field in it, containing a random value
• At the same time, store that random value into the session that correspond to the user.
• When the form is submitted, check that the hidden field has the same value as the one that's stored in session.

If those two values are not the same, refuse to use the submitted data.

Note : this idea is often used to help fight against CSRF -- and integrated in the "Form" component of some Frameworks (Zend Framework, for instance).