I'll paste my custom 404 handler after this. Basicallly it's working fine, except that when I send urls like mysite.com/blablabla.php, I get the IIS 404 and not my custom 404. I breakpoint and debug on my custom handler, basically it's never called.

I feel this has something todo with IIS configuration (running 8.5) but I don't know what exactly.

If I add <httpErrors existingResponse="PassThrough"/> to my web.config,I instead get an empty response, but still no hit on my handler.

Thanks for the help!

using System.Web;
using umbraco.interfaces;
using Umbraco.Core;
using Umbraco.Core.Logging;
using Umbraco.Web;
using Umbraco.Web.Routing;

namespace BTSPromo.Code
{
    public class Umbraco404Handler : INotFoundHandler
    {
        private int redirectId;

        public bool Execute(string url)
        {

            // Use the url to figure out the culture
            var lang = UmbracoContext.Current.ContentCache.GetByRoute(HttpContext.Current.Request.Path.Substring(0, 4));
            var site = UmbracoContext.Current.ContentCache.GetByRoute("/");

            if (lang == null)
            {
                int notFoundNodeId = site.AsDynamic().page404; // Unset content picker, cast to int, will be 0
                if (notFoundNodeId != 0)
                {
                    redirectId = notFoundNodeId;
                    return true;
                }
                else
                {
                    // No specified 404
                    return false;
                }
            }
            else
            {
                int notFoundNodeId = lang.AsDynamic().page404; // Unset content picker, cast to int, will be 0
                if (notFoundNodeId != 0)
                {
                    redirectId = notFoundNodeId;
                    return true;
                }
                else
                {
                    // No specified 404 on language node, try site root
                    notFoundNodeId = site.AsDynamic().page404; // Unset content picker, cast to int, will be 0
                    if (notFoundNodeId != 0)
                    {
                        redirectId = notFoundNodeId;
                        return true;
                    }
                    else
                    {
                        // No specified 404
                        return false;
                    }
                }
            }

            return false;
        }

        public bool CacheUrl
        {
            get { return false; }
        }

        public int redirectID
        {
            get { return redirectId; }
        }
    }
}