无法使用UdpClient捕获收到的数据报

Posted 2021-04-14

我正在尝试向设备发送UDP命令并从同一设备接收UDP响应。发送工作正常。我可以看到数据报离开（通过WireShark）。我还可以看到数据报从设备返回（再次通过WireShark）。命令离开和响应接收之间的周转时间约为15毫秒。

码

Byte[] button_click(Byte[] command) 
{
    // Device exists at a particular IP address and listens for UDP commands on a particular port
    IPEndPoint SendingEndpoint = new IPEndPoint(DEVICE_IP, DEVICE_PORT);

    // Device always sends from port 32795 to whatever port the command originated from on my machine
    IPEndPoint ReceivingEndpoint = new IPEndPoint(DEVICE_IP, 32795);

    // Sending client
    sendingClient = new UdpClient();
    sendingClient.Connect(SendingEndpoint);

    // Receiving client
    receivingClient = new UdpClient();
    receivingClient.Client.ReceiveTimeout = RECEIVE_TIMEOUT; // timeout after 4 seconds
    receivingClient.Connect(receivingEndpoint);

    // Send command and wait for response
    Byte[] response = null;
    try
    {
        sendingClient.Connect(DEVICE_IP, DEVICE_PORT);
        sendingClient.Send(command, command.Length);
        response = receivingClient.Receive(ref receivingEndpoint);
    }
    catch (SocketException e)
    {
        // If we timeout, discard SocketException and return null response
    }

    return response;
}

问题

我无法在我的应用程序中捕获收到的数据报。当我运行上面的代码时，我得到以下异常：

“连接尝试失败，因为连接方在一段时间后没有正确响应，或者建立的连接失败，因为连接的主机无法响应。”

StackOverflow上有类似的帖子，但它们似乎都没有解决我的情况。我已经验证我的数据包没有被防火墙扫除。

我究竟做错了什么？

答案

我解决了这个问题。解决方案需要两件事：

1. 发送和接收客户端必须使用相同的本地端口
2. 发送客户端必须使用IPEndPoint声明的IPAddress.Any，接收客户端必须使用我本地机器的确切IP地址声明的IPEndPoint

码

// Create endpoints
IPEndPoint DeviceEndPoint = new IPEndPoint(DEVICE_IP, DEVICE_PORT);
IPEndPoint localEndPointAny = new IPEndPoint(IPAddress.Any, LOCAL_PORT); // helps satisfy point 2
IPEndPoint localEndPointExplicit = new IPEndPoint(IPAddress.Parse(GetLocalIPAddress()), LOCAL_PORT);  // helps satisfy point 2
IPEndPoint incomingEndPoint = null; // Later populated with remote sender's info

// Create sending client
UdpClient sendingClient = new UdpClient();
sendingClient.ExclusiveAddressUse = false; // Going to use same port for outgoing and incoming (helps satisfy point 1)
sendingClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true); // helps satisfy point 1
sendingClient.Client.Bind(localEndPointAny); // Any outgoing IP address will do

// Create receiving client
UdpClient receivingClient = new UdpClient();
receivingClient.Client.ReceiveTimeout = RECEIVE_TIMEOUT; // 4000 milliseconds
receivingClient.ExclusiveAddressUse = false; // Going to use same port for outgoing and incoming (helps satisfy point 1)
receivingClient.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true); // helps satisfy point 1
receivingClient.Client.Bind(localEndPointExplicit); // Must explicitly give machine's outgoing IP address

获取本地IP地址can be found here的代码。

另一答案

如果您使用sendingClient接收，那么您可以获得正确的消息。原因是IP由Host + Port + Protocol组成，当发送点连接到设备并发送消息时，设备接收端点，UDP与发送端点配对。当接收客户端尝试接收消息时，由于UDP是对等端，并且接收客户端的端口必须与发送客户端不同，因此接收客户端什么也得不到，因此没有任何事情发生。以下是我的示例代码供您参考。

        IPAddress address;
        IPAddress.TryParse("127.0.0.1", out address);
        IPEndPoint recPoint = new IPEndPoint(address, 13154);
        // IPEndPoint sendPoint = new IPEndPoint(address, 9999);
        UdpClient send = new UdpClient(9999);
        send.Connect(recPoint);
        Byte[] response = null;
        Byte[] command = System.Text.Encoding.Default.GetBytes("NO one");
        try
        {
            send.Send(command, command.Length);
            response = send.Receive(ref recPoint);
        }
        catch(Exception ex) {
            Console.WriteLine(ex.ToString());
        }

根据Alex的回答，我更新了一个完整的示例代码供参考。

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.IO;
using System.Net.Sockets;
using System.Text;
using System.Net;

namespace console
{
    class Program
    {
        static void Main(string[] args)
        {
            IPAddress address;
            IPAddress.TryParse("192.168.14.173", out address);
            IPEndPoint recPoint = new IPEndPoint(address, 13154);
            IPEndPoint recAnyPoint = new IPEndPoint(IPAddress.Any, 13154);
            IPEndPoint ipPoint = new IPEndPoint(IPAddress.Parse("192.168.14.174"), 13154);

            // IPEndPoint sendPoint = new IPEndPoint(address, 9999);
            UdpClient send = new UdpClient();
            send.ExclusiveAddressUse = false;
            // no need to use the low level socketoption 
            // send.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            send.Client.Bind(recAnyPoint);
            send.Connect(ipPoint);
            UdpClient receive = new UdpClient();

            receive.ExclusiveAddressUse = false;
            // receive.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
            receive.Client.Bind(recPoint);
            receive.Connect(ipPoint);
            Byte[] response = null;
            Byte[] command = System.Text.Encoding.Default.GetBytes("NO one");
            try
            {
                send.Send(command, command.Length);
                response = receive.Receive(ref ipPoint);
                Console.WriteLine(System.Text.Encoding.Default.GetString(response));
            }
            catch(Exception ex) {
                Console.WriteLine(ex.ToString());
            }
        }
    }
}