Here are the main parts (with some headers and the body omitted) of the response to a broken request for Raymond Chen’s blog – I deliberately misspelled “default” as “defult” to trigger a 404:

HTTP/1.1 302 Found
Date: Wed, 10 Dec 2008 17:46:51 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
CommunityServer: 2.1.61025.2
Location: /error-notfound.aspx?aspxerrorpath=/oldnewthing/defult.aspx
Content-Type: text/html; charset=utf-8
Content-Length: 176

So, a request for something that is not found returns 302 Found. Nice.

What happens when the browser follows the redirection to /error-notfound.aspx?aspxerrorpath=/oldnewthing/defult.aspx? This happens:

HTTP/1.1 404 Not Found
Date: Wed, 10 Dec 2008 17:46:52 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
CommunityServer: 2.1.61025.2
Content-Type: text/html; charset=utf-8
Content-Length: 11769

So, just to get this straight: when something is not found, ASP.NET returns a 302 Found response redirecting the client elsewhere. Then, when the redirect is followed, it returns a 404 Not Found response, despite the fact that this page clearly has been found, and it was the page that redirected us here that was not found.

Weird. Just plain weird.

Update: an online acquaintance (I don’t have a URL for him) has pointed me to a page on MSDN where the rationale for this behaviour is presented:

Redirect creates a new Context, Transfer does not. Redirect requires a round-trip to the browser, Transfer does not. As a result of this round-trip, Redirect rewrites the URL to reflect the location of the error page, Transfer does not.

If this seems to be an argument in favor of Transfer, it isn’t. The rationale of the ASP.NET development team is that Redirect accurately displays the URL of the custom error page…

However, at no point is it explained why displaying such an implementation detail to clients is preferable. Given that the article starts with the words:

The quality of a site should be measured not only by how well it works, but by how gracefully it fails.

it seems strange that it then recommends a technique which causes sites to fail in a singularly graceless manner.

My suggestion would be to modify your ASP.NET application configurations so that the customError capability uses the Transfer technique instead of Redirect. If that

…breaks a tenet of the design philosophy of ASP.NET…

then so be it.

But a redirect to the absolute URL of an error page? That’s just annoying. It means that I can’t try to reload the page to see if the server has recovered from whatever issues are affecting it – or, more accurately, I can try to reload, but as I’m now at the absolute URL of the error page, I just keep reloading that.

Unsurprisingly, the HTTP 1.1 standard has already provided for this kind of situation by means of the 503 Service Unavailable status code:

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay.

If the blogs.msdn.com people (who, by the way, Raymond is not responsible for) would use this status code to serve their error page for the post on Raymond’s blog, I could simply come back after a few minutes and try to reload the page, as my browser would still be at the same location.

It’s also possible that they do send this status code, but with a Location: header to cause the redirect. (The server seems to be back on its feet again now, so I can’t inspect the HTTP cycle to check.)

Either way, what they do is wrong and broken. Don’t send people somewhere else to explain a 5xx Server Error status; simply serve the explanation directly using the appropriate 5xx status code, leaving your visitors ready to try a reload when the server has recovered. Otherwise, you’ll serve a lot of 200 OK responses from your error page, most of them to people who haven’t noticed that their location bar now points there as they hit [Control | Command]-R in the hope of seeing what they originally came for.

Note: A more recent post examines this aspect of ASP.NET in greater depth, and suggests the use of a configuration option that resolves the problem.