<configuration>
    <system.web>
                  <customErrors defaultRedirect="404.aspx" mode="RemoteOnly" />
    </system.web>
    <system.webServer>
        <httpErrors errorMode="DetailedLocalOnly">
            <remove statusCode="404" subStatusCode="-1" />
            <error statusCode="404" prefixLanguageFilePath="" path="/index.html" responseMode="ExecuteURL" />
        </httpErrors>
    </system.webServer>
</configuration>