Default redirect for Error 404
I want to introduce a functionality in my ASP.net website that, whenever a request is received for an unknown URL on my domain, the user is redirected to my error_404.htm page in the root of the application.
For example, if the request is http://www.mydomain.com/blahblahblah
Then instead of returning the standard 404 error page, I want it to redirect the request to http://www.mydomain.com/error_404.htm
Update IIS Version 7.5 and .NET Framework Version 4
Update
/blah.aspx redirects but /blah does not
Answer
This is how you configure a custom 404 error page for both ASP.NET and non-ASP.NET requests:
<configuration>
<system.web>
<compilation targetFramework="4.0" />
<customErrors mode="On" redirectMode="ResponseRewrite">
<error statusCode="404" redirect="http404.aspx" />
</customErrors>
</system.web>
<system.webServer>
<httpErrors errorMode="Custom">
<remove statusCode="404"/>
<error statusCode="404" path="/http404.aspx" responseMode="ExecuteURL"/>
</httpErrors>
</system.webServer>
</configuration>
As others already pointed out, you should not use an HTTP redirection to send the user to the home page, this is not only confusing to users but also to machines (e.g. search engines). It is important to use the 404 status code and not a 3xx code.
You can achieve the desired functionality using meta refresh on HTML:
<%@ Page Language="C#" %>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Not Found</title>
<meta http-equiv="refresh" content="5;url=/"/>
</head>
<body>
<h1>Not Found</h1>
<p>Redirecting to Home...</p>
</body>
</html>