ASP.NET Custom 404 Returning 200 OK Instead of 404 Not Found

Solution:

The problem, it turned out, was the use of the master page. I got it to work by setting the status code later in the pages lifecycle, obviously the rendering of the master page was resetting it, so I overrode the render method and set it after the render was complete.

protected override void Render(HtmlTextWriter writer)
{
    base.Render(writer);
    Response.StatusCode = 404;
}

More work could be done to find out exactly when the master page is setting the status, but I’ll leave that to you.

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Original Post:

I was able to get a test web app to work fine, well it at least displayed the custom error page and returned a 404 status code. I can’t tell you what is wrong with your app, but I can tell you what I did:

1) Edited the web.config for custom errors:

<customErrors mode="On">
  <error statusCode="404" redirect="404.aspx"/>
</customErrors>

2) Added a 404.aspx page and set the status code to 404.

public partial class _04 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Response.StatusCode = 404;
    }
}

Thats about it, if I go to any page extension that is processed by Asp.Net and does not exist, my fiddler log clearly shows a 404, here is the header:

HTTP/1.1 404 Not Found
Server: Microsoft-IIS/5.1
Date: Sun, 07 Dec 2008 06:04:13 GMT
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 533

Now if I go to a page that is not processed by Asp.Net, like a htm file, the custom page does not show and the 404 that is configured by IIS is displayed.

Here is a post that goes into some more details that may be of use to you and your problem, my test does do a redirect to the new page so the url of the requested file is pretty much lost (except its in the query string).

Google 404 and .NET Custom Error Pages

Header Spy Response:

HTTP/1.1 404 Not Found
Date: Sun, 07 Dec 2008 06:21:20 GMT