application cache: don't serve file from appcache on 404

Question

I'm using the application cache to make my website also work offline.
My manifest looks something like this and is embedded using an <iframe>:

CACHE:
favicon.ico
assets/css/style.css
assets/js/libs/zepto.js
assets/js/app.js

NETWORK:
*

FALLBACK:
/ offline.html

it works as expected when I cut the internet connection -> everything is served via the offline.html page.
but just now I found out that every 404 (e.g. an URL that never existed on the server) also uses the FALLBACK section in the manifest on displays the offline.html.

is there a way to distinguish between being offline (in which case the offline.html is the right choice) and the server returning a 404 (in which case I just want to display the 404 error)?

Answer 1

Appcache will FALLBACK if the original request...

...results in a redirect to a resource with another origin (indicative of a captive portal), or a 4xx or 5xx status code or equivalent, or if there were network errors

(from http://www.whatwg.org/specs/web-apps/current-work/multipage/offline.html#changesToNetworkingModel)

Unfortunately, when a fallback occurs, there's no why to find out why. You can't differentiate between a 500, 404, redirect or failed connection.

Here's a hacky workaround:

FALLBACK:
/connection-test.js connection-failed.js
/ offline.html

You could embed connection-test.js in your offline.html page. connection-test.js loading would indicate the user has a connection, connection-failed.js would indicate there's no connection.