Install-Package Titanium.Web.Proxy

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            ProxyServer proxy = new ProxyServer();

            proxy.TrustRootCertificate = true;
            proxy.BeforeRequest -= OnRequest; // Подписываем методы запрос
            proxy.BeforeResponse -= OnResponse; // Подписываем методы ответа
            proxy.ServerCertificateValidationCallback -= OnCertificateValidation; // Получени сертификата для SSL
            proxy.ClientCertificateSelectionCallback -= OnCertificateSelection;

           // Создаем точки входа и исключения
            ExplicitProxyEndPoint explicitEndPoint = new ExplicitProxyEndPoint(IPAddress.Any, 8000, true);
            proxy.AddEndPoint(explicitEndPoint);
            proxy.Start();

            var transparentEndPoint = new TransparentProxyEndPoint(IPAddress.Any, 8001, true)
            {
                GenericCertificateName = "google.com"
            };

            proxy.AddEndPoint(transparentEndPoint);

            foreach (ProxyEndPoint endPoint in proxy.ProxyEndPoints)
                Console.WriteLine("Listening on '{0}' endpoint at Ip {1} and port: {2} ",
                    endPoint.GetType().Name, endPoint.IpAddress, endPoint.Port);

            // Устанавливаем прокси
            proxy.SetAsSystemHttpProxy(explicitEndPoint);
            proxy.SetAsSystemHttpsProxy(explicitEndPoint);

            Console.Read();

            proxy.BeforeRequest -= OnRequest;
            proxy.BeforeResponse -= OnResponse;
            proxy.ServerCertificateValidationCallback -= OnCertificateValidation;
            proxy.ClientCertificateSelectionCallback -= OnCertificateSelection;
            proxy.Stop();
        }

        public static async Task OnRequest(object sender, SessionEventArgs e)
        {
            Console.WriteLine(e.WebSession.Request.Url);

            requestHeaders = e.WebSession.Request.RequestHeaders;

            var method = e.WebSession.Request.Method.ToUpper();
            if ((method == "POST"))
            {
                byte[] bodyBytes = await e.GetRequestBody();
                await e.SetRequestBody(bodyBytes);
                string bodyString = await e.GetRequestBodyAsString();
                await e.SetRequestBodyString(bodyString);
            }
        }

        public static async Task OnResponse(object sender, SessionEventArgs e)
        {
            var responseHeaders = e.WebSession.Response.ResponseHeaders;
            if (e.WebSession.Request.Method == "GET" || e.WebSession.Request.Method == "POST")
            {
                // Если всё ок возвращаем ответ
                if (e.WebSession.Response.ResponseStatusCode == "200")
                {
                    if (e.WebSession.Response.ContentType != null && e.WebSession.Response.ContentType.Trim().ToLower().Contains("text/html"))
                    {
                        byte[] bodyBytes = await e.GetResponseBody();
                        await e.SetResponseBody(bodyBytes);
                        // если запрашиваем вконтакте
                        if (e.WebSession.Request.Url.Contains("vk.com"))
                        {
                            //то изменяем верстку
                            string body = e.GetResponseBodyAsString();
                            await e.SetResponseBodyString(body.Replace(@"W.E.L.C.O.M.E..", @"Специально для Codeby"));
                        }
                        else
                        {
                            string body = await e.GetResponseBodyAsString();
                            await e.SetResponseBodyString(body);
                        }
                    }
                }
            }
        }

        public static Task OnCertificateValidation(object sender, CertificateValidationEventArgs e)
        {
            if (e.SslPolicyErrors = System.Net.Security.SslPolicyErrors.None)
                e.IsValid = true;

            return Task.FromResult(0);
        }

        public static Task OnCertificateSelection(object sender, CertificateSelectionEventArgs e)
        {
            return Task.FromResult(0);
        }
    }
}