How to set custom "Host" header in HttpWebRequest?

Necromancing.
For those still on .NET 2.0
It is in fact quite easy, if you know how.

Problem is, you can't set the host header, because the framework won't let you change the value at runtime. (.net framework 4.0+ will let you override host in a httpwebrequest).

Next attempt will be setting the header with reflection, to get around it, which will let you change the header value. But at runtime, it will overwrite this value with the host part of the url, which means reflection will bring you nothing.

If the dns-name doesn't exist, which is quite frankly the only case in which you want to do this in the first place, you can't set it, because .NET can't resolve it, and you can't override the .NET DNS resolver.

But what you can do, is setting a webproxy with the exact same IP as the destination server.

So, if your server IP is 28.14.88.71:

public class myweb : System.Net.WebClient
{
    protected override System.Net.WebRequest GetWebRequest(System.Uri address)
    {
        System.Net.WebRequest request = (System.Net.WebRequest)base.GetWebRequest(address);
        //string host = "redmine.nonexistantdomain.com";

        //request.Headers.GetType().InvokeMember("ChangeInternal",
        //    System.Reflection.BindingFlags.NonPublic |
        //    System.Reflection.BindingFlags.Instance |
        //    System.Reflection.BindingFlags.InvokeMethod, null,
        //    request.Headers, new object[] { "Host", host }
        //);

        //server IP and port
        request.Proxy = new System.Net.WebProxy("http://28.14.88.71:80");

        // .NET 4.0 only
        System.Net.HttpWebRequest foo = (System.Net.HttpWebRequest)request;
        //foo.Host = host;

        // The below reflection-based operation is not necessary, 
        // if the server speaks HTTP 1.1 correctly
        // and the firewall doesn't interfere
        // https://yoursunny.com/t/2009/HttpWebRequest-IP/
        System.Reflection.FieldInfo horribleProxyServicePoint = (typeof(System.Net.ServicePoint))
            .GetField("m_ProxyServicePoint", System.Reflection.BindingFlags.NonPublic |
            System.Reflection.BindingFlags.Instance);

        horribleProxyServicePoint.SetValue(foo.ServicePoint, false);
        return foo;



        return request;
    }


    }

and voila, now

myweb wc = new myweb();
string str = wc.DownloadString("http://redmine.non-existant-domain.com");

and you get the correct page back, if 28.14.88.71 is a webserver with virtual name-based hosting (based on http-host-header).

You can use this hack, designed for solve this problem in .Net 3.5 .

using System;
using System.Collections.Generic;
using System.IO;
using System.Net;
using System.Reflection;


namespace ConsoleApplication6
{
    class Program
    {
        static void Main(string[] args)
        {
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create("http://198.252.206.16");

            FieldInfo headersFieldInfo =  request.GetType().GetField("_HttpRequestHeaders", System.Reflection.BindingFlags.NonPublic
                                                    | System.Reflection.BindingFlags.Instance
                                                    | System.Reflection.BindingFlags.GetField);

            ClusteredHeaderCollection headers = new ClusteredHeaderCollection("stackoverflow.com");

            headersFieldInfo.SetValue(request, headers);

            request.Proxy = null;
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        
            StreamReader sr = new StreamReader(response.GetResponseStream());
            string result = sr.ReadToEnd();
            Console.WriteLine(result);
            Console.ReadLine();

        }
        public class ClusteredHeaderCollection : WebHeaderCollection
        {
            public bool HostHeaderValueReplaced { get;private  set; }

            public string ClusterUrl { get; private set; }

            public ClusteredHeaderCollection(string commonClusterUrl) : base()
            {
                if (string.IsNullOrEmpty("commonClusterUrl"))
                    throw new ArgumentNullException("commonClusterUrl");

                this.ClusterUrl = commonClusterUrl;
            }

            public override string ToString()
            {
                this["Host"] = this.ClusterUrl;
                string tmp =  base.ToString();
                this.HostHeaderValueReplaced = true;
                return tmp;
            }
        }
    }
}

WebClient allows it.

var client = new WebClient();
client.Headers.Add( "Host", WebHeader );

I couldn't tell you why. The documentation clearly states that Host is a system header.

you can use proxy, see my answer at: Request Web Page in c# spoofing the Host