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Our Server Application Unavailable?

February 20, 2008

Recently we had a really strange error – one of those hard to pin-point, drive you crazy kind of problems that has you running around in circles.

A test website that had a few pages and web services running in the 2.0 framework was working very well until we upgraded it with some new functionality – a HttpHandler that runs in a few other sites.
When we called a url with a specific extension (an RSS feed) with something like “/feed/rss.xml” we suddenly got the dreaded “Server Application Unavailable” error:

“The web application you are attempting to access on this web server is currently unavailable. Please hit the “Refresh” button in your web browser to retry your request.
Administrator Note: An error message detailing the cause of this specific request failure can be found in the application event log of the web server. Please review this log entry to discover what caused this error to occur. “

How can we get such a disasterous error message for just one url?
The event log was no help as the framework did not report any issues – it just bombed out.

We checked the <HttpHandler> section of the web.config that relates all “*rss/xml” extensions to a custom handler – this was correct. We checked the settings on the server and again everything seemed fine and IIS had the “.xml” extension configured to point to the aspnet_isapi.dll.

After running around in decreasing circle after circle and just when we thought we might go mad we hit our event horizon and finally found that it was this aspnet_isapi.dll association that was the issue.

This test site was running as a virtual directory under a test web site; in fact a number of sites operate in this way for quick development testing and the IIS configuration for the “.xml” was set at the higher test web site level (so all virtual directories inherit this).

Here lay our problem – the IIS extension configuration pointed to the 1.1 framework aspnet_isapi.dll and not the 2.0 framework that the site was currently running under. D’oh!

Obvious in hindsight, but sometimes don’t these bugbears just eat up your afternoon?

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