MVC5 custom error page

Add following <customErrors> tag in the web.config that helps you to redirect to the NotFound action method of Error controller if the system fails to find the requested url (status code 404) and redirects to ServerError action method of error controller if the system fires an internal server error (status code 500)

<!--<Redirect to error page>-->
<customErrors mode="On" defaultRedirect="~/Error/ServerError">
  <error redirect="~/Error/NotFound" statusCode="404" />
</customErrors>
<!--</Redirect to error page>-->

You have to create an Error controller that contains ServerError and NotFound action method that renders the related view to display the proper message to the user.

public class ErrorController : Controller
{
    public ActionResult NotFound()
    {
        return View();
    }
    public ActionResult Error()
    {
        return View();
    }
}

Check out this page by Ben Foster, very detailed explanation of the issues you will encounter and a solid solution. http://benfoster.io/blog/aspnet-mvc-custom-error-pages

if you prefer you can handle errors in globalasax Application_Error() method like this:

protected void Application_Error(object sender, EventArgs e) {
    var exception = Server.GetLastError();
    Response.Clear();

    string redirectUrl = "/Error"; // default
    if (exception is HttpException) {
        if (((HttpException)exception).GetHttpCode() == 404) {
            redirectUrl += "/NotFound";
        } else if (((HttpException)exception).GetHttpCode() == 401) {
            redirectUrl += "/NotAuthorized";
        } else { 
            redirectUrl += "?e=" + ((HttpException)exception).GetHttpCode();
        }
        Server.ClearError();
        Response.Clear();
    }
    Response.Redirect(redirectUrl);
}