TWA-App/TWASys-App/Models/ServerInfo/NetUtils.cs

using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;

namespace TWASys_App.Models.ServerInfo
{
    public class NetUtils
    {
        public static string? GetPrimaryPrivateIPv4String()
        {
            if (!Socket.OSSupportsIPv4) return null;
            try
            {
                using var s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
                s.Connect(new IPEndPoint(IPAddress.Parse("8.8.8.8"), 53));
                return (s.LocalEndPoint as IPEndPoint)?.Address.ToString();
            }
            catch
            {
                // fallback: liệt kê NIC
                return GetAllPrivateIPsStrings().FirstOrDefault(ip => ip.Contains('.'));
            }
        }

        public static string? GetPrimaryPrivateIPv6String()
        {
            if (!Socket.OSSupportsIPv6) return null;
            try
            {
                using var s = new Socket(AddressFamily.InterNetworkV6, SocketType.Dgram, ProtocolType.Udp);
                s.Connect(new IPEndPoint(IPAddress.Parse("2001:4860:4860::8888"), 53));
                var ip = (s.LocalEndPoint as IPEndPoint)?.Address;
                return ip is null ? null : FormatIPv6WithScope(ip);
            }
            catch
            {
                var ip = FirstIPv6_ByEnum();
                return ip is null ? null : FormatIPv6WithScope(ip);
            }
        }

        public static IEnumerable<string> GetAllPrivateIPsStrings()
        {
            foreach (var ni in NetworkInterface.GetAllNetworkInterfaces())
            {
                if (ni.OperationalStatus != OperationalStatus.Up) continue;
                if (ni.NetworkInterfaceType is NetworkInterfaceType.Loopback or NetworkInterfaceType.Tunnel) continue;

                var idx = ni.GetIPProperties().GetIPv6Properties()?.Index;

                foreach (var ua in ni.GetIPProperties().UnicastAddresses)
                {
                    var ip = ua.Address;
                    if (IPAddress.IsLoopback(ip)) continue;

                    if (ip.AddressFamily == AddressFamily.InterNetwork)
                    {
                        if (IsPrivateIPv4(ip)) yield return ip.ToString();
                    }
                    else if (ip.AddressFamily == AddressFamily.InterNetworkV6)
                    {
                        if (IsPrivateIPv6(ip)) yield return FormatIPv6WithScope(ip, idx);
                    }
                }
            }
        }

        // ---------- PUBLIC IP (string) ----------
        public static async Task<string?> GetPublicIPv4Async(CancellationToken ct = default) =>
            await FetchFirstAsync(new[] { "https://api.ipify.org", "https://checkip.amazonaws.com", "https://ipv4.icanhazip.com" }, ct);

        public static async Task<string?> GetPublicIPv6Async(CancellationToken ct = default) =>
            await FetchFirstAsync(new[] { "https://api6.ipify.org", "https://ipv6.icanhazip.com", "https://ifconfig.co/ip" }, ct);

        public static async Task<string?> GetPublicIpAsync(bool preferIPv6 = false, CancellationToken ct = default) =>
            preferIPv6 ? await GetPublicIPv6Async(ct) ?? await GetPublicIPv4Async(ct)
                       : await GetPublicIPv4Async(ct) ?? await GetPublicIPv6Async(ct);

        // ---------- helpers ----------
        static async Task<string?> FetchFirstAsync(IEnumerable<string> urls, CancellationToken ct)
        {
            using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(3) };
            foreach (var u in urls)
                try
                {
                    var s = (await http.GetStringAsync(u, ct)).Trim();
                    if (!string.IsNullOrWhiteSpace(s)) return s;
                }
                catch { /* thử URL kế */ }
            return null;
        }

        static IPAddress? FirstIPv6_ByEnum() =>
            NetworkInterface.GetAllNetworkInterfaces()
                .Where(n => n.OperationalStatus == OperationalStatus.Up &&
                            n.NetworkInterfaceType != NetworkInterfaceType.Loopback &&
                            n.NetworkInterfaceType != NetworkInterfaceType.Tunnel)
                .SelectMany(n => n.GetIPProperties().UnicastAddresses)
                .Select(u => u.Address)
                .FirstOrDefault(ip => ip.AddressFamily == AddressFamily.InterNetworkV6);

        static bool IsPrivateIPv4(IPAddress ip)
        {
            var b = ip.GetAddressBytes();
            return b[0] == 10 || (b[0] == 172 && b[1] is >= 16 and <= 31) || (b[0] == 192 && b[1] == 168);
        }
        static bool IsPrivateIPv6(IPAddress ip)
        {
            var b = ip.GetAddressBytes();
            return ((b[0] & 0xFE) == 0xFC) || (b[0] == 0xFE && (b[1] & 0xC0) == 0x80); // fc00::/7, fe80::/10
        }

        // KHÔNG đụng IPAddress.ScopeId. Nếu link-local, thêm scope = index của NIC.
        static string FormatIPv6WithScope(IPAddress ip, int? nicIndex = null)
        {
            if (ip.AddressFamily != AddressFamily.InterNetworkV6) return ip.ToString();
            var b = ip.GetAddressBytes();
            bool linkLocal = b[0] == 0xFE && (b[1] & 0xC0) == 0x80;
            if (linkLocal && nicIndex is int idx) return $"{ip}%{idx}";
            return ip.ToString();
        }
    }
}