anti-syktsu-proxy/Program.cs

using System;
using System.Buffers;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static string USER = "user";
    static string PASS = "pass";

    static async Task Main()
    {
        int port = 8888;

        Console.WriteLine($"Local proxy:");
        Console.WriteLine($"  http://127.0.0.1:{port}");
        Console.WriteLine($"  login: {USER}");
        Console.WriteLine($"  pass : {PASS}");

        var listener = new TcpListener(IPAddress.Any, port);
        listener.Start();

        while (true)
        {
            var client = await listener.AcceptTcpClientAsync();
            _ = Task.Run(() => HandleClient(client));
        }
    }

    static async Task HandleClient(TcpClient client)
    {
        using (client)
        {
            var stream = client.GetStream();

            var request = await ReadHeaders(stream);
            if (request == null) return;

            if (!CheckAuth(request.HeadersRaw))
            {
                await Write407(stream);
                return;
            }

            if (request.Method == "CONNECT")
            {
                await HandleConnect(stream, request.Target);
            }
            else
            {
                await HandleHttp(stream, request);
            }
        }
    }

    // ================= HTTP =================

    static async Task HandleHttp(NetworkStream clientStream, HttpRequestData req)
    {
        var handler = new SocketsHttpHandler
        {
            UseProxy = true,
            Proxy = new WinHttpProxy(),
            DefaultProxyCredentials = CredentialCache.DefaultCredentials,
            AllowAutoRedirect = false
        };

        using var http = new HttpClient(handler);

        var message = new HttpRequestMessage(new HttpMethod(req.Method), req.Url);

        foreach (var h in req.Headers)
        {
            if (!h.Key.Equals("Proxy-Authorization", StringComparison.OrdinalIgnoreCase))
                message.Headers.TryAddWithoutValidation(h.Key, h.Value);
        }

        if (req.Body != null)
            message.Content = new StreamContent(req.Body);

        var resp = await http.SendAsync(message, HttpCompletionOption.ResponseHeadersRead);

        // статус
        var statusLine = $"HTTP/1.1 {(int)resp.StatusCode} {resp.ReasonPhrase}\r\n";
        await clientStream.WriteAsync(Encoding.ASCII.GetBytes(statusLine));

        // заголовки
        foreach (var h in resp.Headers)
        {
            foreach (var v in h.Value)
                await WriteHeader(clientStream, h.Key, v);
        }

        foreach (var h in resp.Content.Headers)
        {
            foreach (var v in h.Value)
                await WriteHeader(clientStream, h.Key, v);
        }

        await clientStream.WriteAsync(Encoding.ASCII.GetBytes("\r\n"));

        // тело (стриминг)
        await resp.Content.CopyToAsync(clientStream);
    }

    // ================= CONNECT =================

    static async Task HandleConnect(NetworkStream clientStream, string target)
    {
        var parts = target.Split(':');
        string host = parts[0];
        int port = int.Parse(parts[1]);

        var proxy = new WinHttpProxy();
        var proxyUri = proxy.GetProxy(new Uri($"https://{host}:{port}"));

        using var upstream = new TcpClient();
        await upstream.ConnectAsync(proxyUri.Host, proxyUri.Port);

        var upstreamStream = upstream.GetStream();

        // CONNECT к upstream
        string connectReq =
            $"CONNECT {host}:{port} HTTP/1.1\r\nHost: {host}:{port}\r\n\r\n";

        await upstreamStream.WriteAsync(Encoding.ASCII.GetBytes(connectReq));

        // читаем ответ от upstream
        var buffer = new byte[8192];
        int read = await upstreamStream.ReadAsync(buffer);

        // отвечаем клиенту что туннель открыт
        await clientStream.WriteAsync(Encoding.ASCII.GetBytes("HTTP/1.1 200 Connection Established\r\n\r\n"));

        // двунаправленный туннель
        var t1 = upstreamStream.CopyToAsync(clientStream);
        var t2 = clientStream.CopyToAsync(upstreamStream);

        await Task.WhenAny(t1, t2);
    }

    // ================= AUTH =================

    static bool CheckAuth(string headers)
    {
        string expected = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{USER}:{PASS}"));
        return headers.Contains($"Proxy-Authorization: Basic {expected}");
    }

    static async Task Write407(NetworkStream stream)
    {
        string resp =
            "HTTP/1.1 407 Proxy Authentication Required\r\n" +
            "Proxy-Authenticate: Basic realm=\"proxy\"\r\n\r\n";

        await stream.WriteAsync(Encoding.ASCII.GetBytes(resp));
    }

    // ================= REQUEST PARSER =================

    class HttpRequestData
    {
        public string Method;
        public string Url;
        public string Target;
        public string HeadersRaw;
        public Dictionary<string, string> Headers = new();
        public Stream Body;
    }

    static async Task<HttpRequestData> ReadHeaders(NetworkStream stream)
    {
        var buffer = new byte[8192];
        int read = await stream.ReadAsync(buffer);

        if (read <= 0) return null;

        string text = Encoding.ASCII.GetString(buffer, 0, read);

        var lines = text.Split("\r\n");
        var first = lines[0].Split(' ');

        var data = new HttpRequestData
        {
            Method = first[0],
            Target = first[1],
            HeadersRaw = text
        };

        foreach (var line in lines.Skip(1))
        {
            if (string.IsNullOrWhiteSpace(line)) break;

            int idx = line.IndexOf(':');
            if (idx > 0)
                data.Headers[line[..idx]] = line[(idx + 1)..].Trim();
        }

        if (data.Method != "CONNECT")
        {
            data.Url = data.Target.StartsWith("http")
                ? data.Target
                : $"http://{data.Headers["Host"]}{data.Target}";
        }

        return data;
    }

    static async Task WriteHeader(NetworkStream stream, string key, string value)
    {
        string line = $"{key}: {value}\r\n";
        await stream.WriteAsync(Encoding.ASCII.GetBytes(line));
    }
}

// ================= WINHTTP PROXY =================

class WinHttpProxy : IWebProxy
{
    public ICredentials Credentials { get; set; }

    public Uri GetProxy(Uri destination)
    {
        return WinHttpHelper.GetProxyForUrl(destination);
    }

    public bool IsBypassed(Uri host) => false;
}

static class WinHttpHelper
{
    [DllImport("winhttp.dll", SetLastError = true)]
    static extern bool WinHttpGetIEProxyConfigForCurrentUser(out WINHTTP_CURRENT_USER_IE_PROXY_CONFIG config);

    public static Uri GetProxyForUrl(Uri url)
    {
        WinHttpGetIEProxyConfigForCurrentUser(out var cfg);

        if (!string.IsNullOrEmpty(cfg.lpszProxy))
        {
            var parts = cfg.lpszProxy.Split(':');
            return new Uri($"http://{parts[0]}:{parts[1]}");
        }

        return url; // direct
    }

    struct WINHTTP_CURRENT_USER_IE_PROXY_CONFIG
    {
        public bool fAutoDetect;
        public IntPtr lpszAutoConfigUrl;
        public IntPtr lpszProxy;
        public IntPtr lpszProxyBypass;
    }
}