c# - How can I get html from page with cloudflare ddos portection?

Question

I use htmlagility to get webpage data but I tried everything with page using www.cloudflare.com protection for ddos. The redirect page is not possible to handle in htmlagility because they don't redirect with meta nor js I guess, they check if you have already being checked with a cookie that I failed to simulate with c#. When I get the page, the html code is from the landing cloadflare page.

Answer 1

I also encountered this problem some time ago. The real solution would be solve the challenge the cloudflare websites gives you (you need to compute a correct answer using javascript, send it back, and then you receive a cookie / your token with which you can continue to view the website). So all you would get normally is a page like

In the end, I just called a python-script with a shell-execute. I used the modules provided within this github fork. This could serve as a starting point to implement the circumvention of the cloudflare anti-dDoS page in C# aswell.

FYI, the python script I wrote for my personal usage just wrote the cookie in a file. I read that later again using C# and store it in a CookieJar to continue browsing the page within C#.

#!/usr/bin/env python
import cfscrape
import sys

scraper = cfscrape.create_scraper() # returns a requests.Session object
fd = open("cookie.txt", "w")
c = cfscrape.get_cookie_string(sys.argv[1])
fd.write(str(c))
fd.close()  
print(c)

EDIT: To repeat this, this has only LITTLE to do with cookies! Cloudflare forces you to solve a REAL challenge using javascript commands. It's not as easy as accepting a cookie and using it later on. Look at https://github.com/Anorov/cloudflare-scrape/blob/master/cfscrape/init.py and the ~40 lines of javascript emulation to solve the challenge.

Edit2: Instead of writing something to circumvent the protection, I've also seen people using a fully-fledged browser-object (this is not a headless browser) to go to the website and subscribe to certain events when the page is loaded. Use the WebBrowser class to create an infinetly small browser window and subscribe to the appropiate events.

Edit3: Alright, I actually implemented the C# way to do this. This uses the JavaScript Engine Jint for .NET, available via https://www.nuget.org/packages/Jint

The cookie-handling code is ugly because sometimes the HttpResponse class won't pick up the cookies, although the header contains a Set-Cookie section.

using System;
using System.Net;
using System.IO;
using System.Text.RegularExpressions;
using System.Web;
using System.Collections;
using System.Threading;

namespace Cloudflare_Evader
{
    public class CloudflareEvader
    {
        /// <summary>
        /// Tries to return a webclient with the neccessary cookies installed to do requests for a cloudflare protected website.
        /// </summary>
        /// <param name="url">The page which is behind cloudflare's anti-dDoS protection</param>
        /// <returns>A WebClient object or null on failure</returns>
        public static WebClient CreateBypassedWebClient(string url)
        {
            var JSEngine = new Jint.Engine(); //Use this JavaScript engine to compute the result.

            //Download the original page
            var uri = new Uri(url);
            HttpWebRequest req =(HttpWebRequest) WebRequest.Create(url);
            req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0";
            //Try to make the usual request first. If this fails with a 503, the page is behind cloudflare.
            try
            {
                var res = req.GetResponse();
                string html = "";
                using (var reader = new StreamReader(res.GetResponseStream()))
                    html = reader.ReadToEnd();
                return new WebClient();
            }
            catch (WebException ex) //We usually get this because of a 503 service not available.
            {
                string html = "";
                using (var reader = new StreamReader(ex.Response.GetResponseStream()))
                    html = reader.ReadToEnd();
                //If we get on the landing page, Cloudflare gives us a User-ID token with the cookie. We need to save that and use it in the next request.
                var cookie_container = new CookieContainer();
                //using a custom function because ex.Response.Cookies returns an empty set ALTHOUGH cookies were sent back.
                var initial_cookies = GetAllCookiesFromHeader(ex.Response.Headers["Set-Cookie"], uri.Host); 
                foreach (Cookie init_cookie in initial_cookies)
                    cookie_container.Add(init_cookie);

                /* solve the actual challenge with a bunch of RegEx's. Copy-Pasted from the python scrapper version.*/
                var challenge = Regex.Match(html, "name="jschl_vc" value="(\w+)"").Groups[1].Value;
                var challenge_pass = Regex.Match(html, "name="pass" value="(.+?)"").Groups[1].Value;

                var builder = Regex.Match(html, @"setTimeout(function(){s+(var t,r,a,f.+?
?
[sS]+?a.value =.+?)
?
").Groups[1].Value;
                builder = Regex.Replace(builder, @"a.value =(.+?) + .+?;", "$1");
                builder = Regex.Replace(builder, @"s{3,}[a-z](?: = |.).+", "");

                //Format the javascript..
                builder = Regex.Replace(builder, @"[
\']", "");

                //Execute it. 
                long solved = long.Parse(JSEngine.Execute(builder).GetCompletionValue().ToObject().ToString());
                solved += uri.Host.Length; //add the length of the domain to it.

                Console.WriteLine("***** SOLVED CHALLENGE ******: " + solved);
                Thread.Sleep(3000); //This sleeping IS requiered or cloudflare will not give you the token!!

                //Retreive the cookies. Prepare the URL for cookie exfiltration.
                string cookie_url = string.Format("{0}://{1}/cdn-cgi/l/chk_jschl", uri.Scheme, uri.Host);
                var uri_builder = new UriBuilder(cookie_url);
                var query = HttpUtility.ParseQueryString(uri_builder.Query);
                //Add our answers to the GET query
                query["jschl_vc"] = challenge;
                query["jschl_answer"] = solved.ToString();
                query["pass"] = challenge_pass;
                uri_builder.Query = query.ToString();

                //Create the actual request to get the security clearance cookie
                HttpWebRequest cookie_req = (HttpWebRequest) WebRequest.Create(uri_builder.Uri);
                cookie_req.AllowAutoRedirect = false;
                cookie_req.CookieContainer = cookie_container;
                cookie_req.Referer = url;
                cookie_req.UserAgent = "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0";
                //We assume that this request goes through well, so no try-catch
                var cookie_resp = (HttpWebResponse)cookie_req.GetResponse();
                //The response *should* contain the security clearance cookie!
                if (cookie_resp.Cookies.Count != 0) //first check if the HttpWebResponse has picked up the cookie.
                    foreach (Cookie cookie in cookie_resp.Cookies)
                        cookie_container.Add(cookie);
                else //otherwise, use the custom function again
                {
                    //the cookie we *hopefully* received here is the cloudflare security clearance token.
                    if (cookie_resp.Headers["Set-Cookie"] != null)
                    {
                        var cookies_parsed = GetAllCookiesFromHeader(cookie_resp.Headers["Set-Cookie"], uri.Host);
                        foreach (Cookie cookie in cookies_parsed)
                            cookie_container.Add(cookie);
                    }
                    else
                    {
                        //No security clearence? something went wrong.. return null.
                        //Console.WriteLine("MASSIVE ERROR: COULDN'T GET CLOUDFLARE CLEARANCE!");
                        return null;
                    }
                }
                //Create a custom webclient with the two cookies we already acquired.
                WebClient modedWebClient = new WebClientEx(cookie_container);
                modedWebClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0");
                modedWebClient.Headers.Add("Referer", url);
                return modedWebClient;
            }
        }

        /* Credit goes to https://stackoverflow.com/questions/15103513/httpwebresponse-cookies-empty-despite-set-cookie-header-no-redirect 
           (user https://stackoverflow.com/users/541404/cameron-tinker) for these functions 
        */
        public static CookieCollection GetAllCookiesFromHeader(string strHeader, string strHost)
        {
            ArrayList al = new ArrayList();
            CookieCollection cc = new CookieCollection();
            if (strHeader != string.Empty)
            {
                al = ConvertCookieHeaderToArrayList(strHeader);
                cc = ConvertCookieArraysToCookieCollection(al, strHost);
            }
            return cc;
        }

        private static ArrayList ConvertCookieHeaderToArrayList(string strCookHeader)
        {
            strCookHeader = strCookHeader.Replace("
", "");
            strCookHeader = strCookHeader.Replace("
", "");
            string[] strCookTemp = strCookHeader.Split(',');
            ArrayList al = new ArrayList();
            int i = 0;
            int n = strCookTemp.Length;
            while (i < n)
            {
                if (strCookTemp[i].IndexOf("expires=", StringComparison.OrdinalIgnoreCase) > 0)
                {
                    al.Add(strCookTemp[i] + "," + strCookTemp[i + 1]);
                    i = i + 1;
                }
                else
                    al.Add(strCookTemp[i]);
                i = i + 1;
            }
            return al;
        }

        private static CookieCollection ConvertCookieArraysToCookieCollection(ArrayList al, string strHost)
        {
            CookieCollection cc = new CookieCollection();

            int alcount = al.Count;
            string strEachCook;
            string[] strEachCookParts;
            for (int i = 0; i < alcount; i++)
            {
                strEachCook = al[i].ToString();
                strEachCookParts = strEachCook.Split(';');
                int intEachCookPartsCount = strEachCookParts.Length;
                string strCNameAndCValue = string.Empty;
                string strPNameAndPValue = string.Empty;
                string strDNameAndDValue = string.Empty;
                string[] NameValuePairTemp;
                Cookie cookTemp = new Cookie();

                for (int j = 0; j < intEachCookPartsCount; j++)
                {
                    if (j == 0)
                    {
                        strCNameAndCValue = strEachCookParts[j];
                        if (strCNameAndCValue != string.Empty)
                        {
                            int firstEqual = strCNameAndCValue.IndexOf("=");
                            string firstName = strCNameAndCValue.Substring(0, firstEqual);
                            string allValue = strCNameAndCValue.Substring(firstEqual + 1, strCNameAndCValue.Length - (firstEqual + 1));
                            cookTemp.Name = firstName;
                            cookTemp.Value = allValue;