To be able to get my application (Umbraco CMS) to handle 404 errors, I need to have following setting in my web.config:
<httpErrors existingResponse="PassThrough" />
It works well for displaying a custom 404 error page from Umbraco, but it doesn't work for displaying a 404 error page when a static file cant be found.
For example http://www.example.com/non-existing-file.png returns a 404 status code, but respons is blank. That fits with this description of the PassThrough mode:
PassThrough – If existingResponse is seen as “PassThrough”, custom error module will always pass through responses from modules. This setting will make custom error module return blank response if modules don’t set any text. (http://blogs.iis.net/ksingla/archive/2008/02/18/what-to-expect-from-iis7-custom-error-module.aspx)
What is the "custom error module"? And how do I get it to return a non blank response?
Update
After stepping through the request handling routines in Umbraco (with a debugger), I have a better understanding of the problem space. As written in the citation above, when existingResponse="PassThrough" then all handling of http errors is done in the custom modules. So to answer my own question, a "custom error module" is in this instance the "UmbracoModule".
One way to solve this problem would be to create a new custom module that will check for the existence of the static files on the disk. It can be done quite nice, I read the list of static files from IIS and read the configuration of customErrors or httpErrors elements from the web.config to get a custom error page. But I kind of think this is crude solution. I would much rather pass the responsibility back to IIS.
Any ideas?
