NetworkTool/windows/NetworkTool.Client/Services/DiscoveryService.cs

using System.Net;
using System.Net.Sockets;
using System.Runtime.InteropServices;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using NetworkTool.Client.Models;

namespace NetworkTool.Client.Services;

public sealed class DiscoveryService
{
    private const int DiscoveryPort = 50000;

    [DllImport("iphlpapi.dll", ExactSpelling = true)]
    private static extern int SendARP(int destinationIp, int sourceIp, byte[] macAddress, ref int physicalAddressLength);

    public async Task<IReadOnlyList<DiscoveredDevice>> DiscoverManyAsync(string localIPv4, CancellationToken cancellationToken = default)
    {
        if (string.IsNullOrWhiteSpace(localIPv4))
        {
            return [];
        }

        using var client = new UdpClient(AddressFamily.InterNetwork)
        {
            EnableBroadcast = true,
        };

        client.Client.Bind(new IPEndPoint(IPAddress.Parse(localIPv4), 0));
        var request = new
        {
            protocol_version = 1,
            message_type = "discover",
            request_id = Guid.NewGuid().ToString(),
            client_name = Environment.MachineName,
        };

        var payload = JsonSerializer.SerializeToUtf8Bytes(request);
        await client.SendAsync(payload, payload.Length, new IPEndPoint(IPAddress.Broadcast, DiscoveryPort));

        using var timeoutCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
        timeoutCts.CancelAfter(TimeSpan.FromSeconds(3));

        var devicesByIp = new Dictionary<string, DiscoveredDevice>(StringComparer.OrdinalIgnoreCase);
        while (!timeoutCts.IsCancellationRequested)
        {
            try
            {
                var result = await client.ReceiveAsync(timeoutCts.Token);
                var response = JsonSerializer.Deserialize<DiscoveryResponse>(result.Buffer);
                if (response is null
                    || response.MessageType != "discover_response"
                    || string.IsNullOrWhiteSpace(response.Lan2Ip)
                    || !response.Lan2Ip.StartsWith("169.254.", StringComparison.Ordinal))
                {
                    continue;
                }

                var mac = response.Mac;
                if (string.IsNullOrWhiteSpace(mac))
                {
                    mac = ResolveMacAddress(result.RemoteEndPoint.Address);
                }

                devicesByIp[response.Lan2Ip] = new DiscoveredDevice
                {
                    DeviceId = response.DeviceId ?? string.Empty,
                    Hostname = response.Hostname ?? string.Empty,
                    ServerVersion = response.ServerVersion ?? string.Empty,
                    Mac = mac ?? string.Empty,
                    Lan2Ip = response.Lan2Ip,
                    AuthRequired = response.AuthRequired,
                };
            }
            catch (OperationCanceledException)
            {
                break;
            }
        }

        return devicesByIp.Values.OrderBy(device => device.Lan2Ip).ToList();
    }

    private static string ResolveMacAddress(IPAddress ipAddress)
    {
        if (ipAddress.AddressFamily != AddressFamily.InterNetwork)
        {
            return string.Empty;
        }

        var macAddress = new byte[6];
        var macAddressLength = macAddress.Length;
        var destinationIp = BitConverter.ToInt32(ipAddress.GetAddressBytes(), 0);
        if (SendARP(destinationIp, 0, macAddress, ref macAddressLength) != 0 || macAddressLength <= 0)
        {
            return string.Empty;
        }

        return string.Join(":", macAddress.Take(macAddressLength).Select(value => value.ToString("X2")));
    }

    private sealed class DiscoveryResponse
    {
        [JsonPropertyName("message_type")]
        public string? MessageType { get; set; }

        [JsonPropertyName("device_id")]
        public string? DeviceId { get; set; }

        [JsonPropertyName("hostname")]
        public string? Hostname { get; set; }

        [JsonPropertyName("server_version")]
        public string? ServerVersion { get; set; }

        [JsonPropertyName("mac")]
        public string? Mac { get; set; }

        [JsonPropertyName("lan2_ip")]
        public string? Lan2Ip { get; set; }

        [JsonPropertyName("auth_required")]
        public bool AuthRequired { get; set; }
    }
}