Webclient / HttpWebRequest with Basic Authentication returns 404 not found for valid URL

//BEWARE
//This works ONLY if the server returns 401 first
//The client DOES NOT send credentials on first request
//ONLY after a 401
client.Credentials = new NetworkCredential(userName, passWord); //doesnt work

//So use THIS instead to send credentials RIGHT AWAY
string credentials = Convert.ToBase64String(
    Encoding.ASCII.GetBytes(userName + ":" + password));
client.Headers[HttpRequestHeader.Authorization] = string.Format(
    "Basic {0}", credentials);

Try changing the Web Client request authentication part to:

NetworkCredential myCreds = new NetworkCredential(userName, passWord);
client.Credentials = myCreds;

Then make your call, seems to work fine for me.

This part of code worked fine for me:

        WebRequest request = WebRequest.Create(url);
        request.Method = WebRequestMethods.Http.Get;
        NetworkCredential networkCredential = new NetworkCredential(logon, password); // logon in format "domain\username"
        CredentialCache myCredentialCache = new CredentialCache {{new Uri(url), "Basic", networkCredential}};
        request.PreAuthenticate = true;
        request.Credentials = myCredentialCache;
        using (WebResponse response = request.GetResponse())
        {
            Console.WriteLine(((HttpWebResponse)response).StatusDescription);

            using (Stream dataStream = response.GetResponseStream())
            {
                using (StreamReader reader = new StreamReader(dataStream))
                {
                    string responseFromServer = reader.ReadToEnd();
                    Console.WriteLine(responseFromServer);
                }
            }
        }

If its working when you are using a browser and then passing on your username and password for the first time - then this means that once authentication is done Request header of your browser is set with required authentication values, which is then passed on each time a request is made to hosting server.

So start with inspecting Request Header (this could be done using Web Developers tools), Once you established whats required in header then you could pass this within your HttpWebRequest Header.

Example with Digest Authentication:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Security.Cryptography;
using System.Text.RegularExpressions;
using System.Net;
using System.IO;

namespace NUI
{
    public class DigestAuthFixer
    {
        private static string _host;
        private static string _user;
        private static string _password;
        private static string _realm;
        private static string _nonce;
        private static string _qop;
        private static string _cnonce;
        private static DateTime _cnonceDate;
        private static int _nc;

public DigestAuthFixer(string host, string user, string password)
{
    // TODO: Complete member initialization
    _host = host;
    _user = user;
    _password = password;
}

private string CalculateMd5Hash(
    string input)
{
    var inputBytes = Encoding.ASCII.GetBytes(input);
    var hash = MD5.Create().ComputeHash(inputBytes);
    var sb = new StringBuilder();
    foreach (var b in hash)
        sb.Append(b.ToString("x2"));
    return sb.ToString();
}

private string GrabHeaderVar(
    string varName,
    string header)
{
    var regHeader = new Regex(string.Format(@"{0}=""([^""]*)""", varName));
    var matchHeader = regHeader.Match(header);
    if (matchHeader.Success)
        return matchHeader.Groups[1].Value;
    throw new ApplicationException(string.Format("Header {0} not found", varName));
}

private string GetDigestHeader(
    string dir)
{
    _nc = _nc + 1;

    var ha1 = CalculateMd5Hash(string.Format("{0}:{1}:{2}", _user, _realm, _password));
    var ha2 = CalculateMd5Hash(string.Format("{0}:{1}", "GET", dir));
    var digestResponse =
        CalculateMd5Hash(string.Format("{0}:{1}:{2:00000000}:{3}:{4}:{5}", ha1, _nonce, _nc, _cnonce, _qop, ha2));

    return string.Format("Digest username=\"{0}\", realm=\"{1}\", nonce=\"{2}\", uri=\"{3}\", " +
        "algorithm=MD5, response=\"{4}\", qop={5}, nc={6:00000000}, cnonce=\"{7}\"",
        _user, _realm, _nonce, dir, digestResponse, _qop, _nc, _cnonce);
}

public string GrabResponse(
    string dir)
{
    var url = _host + dir;
    var uri = new Uri(url);

    var request = (HttpWebRequest)WebRequest.Create(uri);

    // If we've got a recent Auth header, re-use it!
    if (!string.IsNullOrEmpty(_cnonce) &&
        DateTime.Now.Subtract(_cnonceDate).TotalHours < 1.0)
    {
        request.Headers.Add("Authorization", GetDigestHeader(dir));
    }

    HttpWebResponse response;
    try
    {
        response = (HttpWebResponse)request.GetResponse();
    }
    catch (WebException ex)
    {
        // Try to fix a 401 exception by adding a Authorization header
        if (ex.Response == null || ((HttpWebResponse)ex.Response).StatusCode != HttpStatusCode.Unauthorized)
            throw;

        var wwwAuthenticateHeader = ex.Response.Headers["WWW-Authenticate"];
        _realm = GrabHeaderVar("realm", wwwAuthenticateHeader);
        _nonce = GrabHeaderVar("nonce", wwwAuthenticateHeader);
        _qop = GrabHeaderVar("qop", wwwAuthenticateHeader);

        _nc = 0;
        _cnonce = new Random().Next(123400, 9999999).ToString();
        _cnonceDate = DateTime.Now;

        var request2 = (HttpWebRequest)WebRequest.Create(uri);
        request2.Headers.Add("Authorization", GetDigestHeader(dir));
        response = (HttpWebResponse)request2.GetResponse();
    }
    var reader = new StreamReader(response.GetResponseStream());
    return reader.ReadToEnd();
}

}

Then you could call it:

DigestAuthFixer digest = new DigestAuthFixer(domain, username, password);
string strReturn = digest.GrabResponse(dir);

if Url is: http://xyz.rss.com/folder/rss then domain: http://xyz.rss.com (domain part) dir: /folder/rss (rest of the url)

you could also return it as stream and use XmlDocument Load() method.