What does it mean when an HTTP request returns status code 0?
Many of the answers here are wrong. It seems people figure out what was causing status==0 in their particular case and then generalize that as the answer.
Practically speaking, status==0 for a failed XmlHttpRequest should be considered an undefined error.
The actual W3C spec defines the conditions for which zero is returned here: https://fetch.spec.whatwg.org/#concept-network-error
As you can see from the spec (fetch or XmlHttpRequest) this code could be the result of an error that happened even before the server is contacted.
Some of the common situations that produce this status code are reflected in the other answers but it could be any or none of these problems:
- Illegal cross origin request (see CORS)
- Firewall block or filtering
- The request itself was cancelled in code
- An installed browser extension is mucking things up
What would be helpful would be for browsers to provide detailed error reporting for more of these status==0 scenarios. Indeed, sometimes status==0 will accompany a helpful console message, but in others there is no other information.
I believe the error code indicates that the response was empty, (as not even headers were returned). This means the connection was accepted and then closed gracefully (TCP FIN).There are a number of things which could cause this, but based off of your description, some form of firewall seems the most likely culprit.
For what it is worth, depending on the browser, jQuery-based AJAX calls will call your success callback with a HTTP status code of 0. We've found a status code of "0" usually means the user navigated to a different page before the AJAX call completed.
Not the same technology stack as you are using, but hopefully useful to somebody.