Why Do I Get Unexpected 404 Errors in Visual Web Developer 2005 Express Edition?
I was just about to get very angry, after trying to change about every property on my project, web server and firewall when I noticed an additional file in the root of my site called app_offline.htm. It turned out this file was the culprit. I have no idea how the file got there, but I think it has been put there by Visual Web Developer somehow
There is hardly any documentation about this file. In fact, the only reference Google found was in the ASP.NET 2 Quickstart Tutorials. From a bit of experimenting, I found out that when this file is located in the root of your site and is completely empty, all files in your site (including images and other resources) return a 404 error. This can be useful if you temporarily need to block access to all of your pages.
To customize the error message your users get, you can simply add some markup to this app_offline.htm. Whenever this file contains any content, that content will be shown instead of the nasty 404 error message.
Note that this is different from the option "Take application offline" that you find in the ASP.NET Web Site Administration Tool. Whenever you use that option to take your application offline, the following key is added to the <system.web> node in your web.config file:
<httpRuntime enable="false" />
For more information about the app_offline.htm file, check out this blog post by Scott Guthrie.
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