Checking if a url exist, not working for some urls

Question

I am checking if a url exist, using HttpWebRequest. The problem, it's not working for some url's.
Example:
http://www.gkrs.no/
https://www.politi.no/kripos/statistikk/narkotika/
These url's exist. But show up in the code as not existing. It is working for most of the url's i am checking, its just these two that are causing some issues. Does anyone have any examples of what i still need to check for. Maybe there is something different in their headers.

I have tried both GET and HEAD request methods.
I am still new to programming and might need a simpler explanation. Sorry for bad English, not my first language. Any help would be appreciated.

internal static bool IsValidLenke(string url){

        if (String.IsNullOrEmpty(url))
            return false;
        try
        {
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
            request.Method = "HEAD";
            HttpWebResponse response = request.GetResponse() as HttpWebResponse;
            response.Close();
            return (response.StatusCode == HttpStatusCode.OK);
        }
        catch
        {
            //Any exception will returns false.
            return false;
        }
}

Answer 1

http://www.gkrs.no/ blocks you because you don't supply a valid user agent. https://www.politi.no/kripos/statistikk/narkotika/ does not accept "HEAD", and then sends you a wild ride of redirects, so you need a cookie container to avoid being caught in an infinite loop.

Do something like this:

HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
request.MaximumAutomaticRedirections = 100;
request.AllowAutoRedirect = true;
request.CookieContainer = new CookieContainer();
request.Method = "GET";                
request.UserAgent = " Mozilla/5.0 (Windows NT 10.0; Win64; x64)";
HttpWebResponse response = request.GetResponse() as HttpWebResponse;
response.Close();