当网络适配器关闭时如何检测客户端断开连接？

Question

我在检测客户端与主机的断开连接时遇到了问题。目前我的代码如下：

Task.Run(() => {
 // Create server
 Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp) {
  ReceiveTimeout = -1,
 };

 server.Bind(new IPEndPoint(IPAddress.Any, port));
 server.Listen(-1);

 // Start listening
 while (true) {
  Socket socket = server.Accept();

  new Thread(() => {
   try {
    Process(socket);
   } catch (Exception ex) {
    Trace.WriteLine("Socket connection processing error: " + ex.Message);
   }
  }).Start();
 }
});

// Host client process
void Process(Socket socket) {
  byte[] response;
  int received;
  var ip = IPAddress.Parse(((IPEndPoint) socket.RemoteEndPoint).Address.ToString());
  Events.OnNodeConnected(ip);

  while (true) {
   // Rceive data
   response = new byte[socket.ReceiveBufferSize];
   received = socket.Receive(response);

   // Check connection
   if (!socket.IsConnected()) {
    socket.Close();
    Events.OnNodeDisconnected(ip);
    return;
   }

   try {
    // Decode recieved data
    List < byte > respBytesList = new List < byte > (response);

还有IsConnected()扩展功能：

public static class SocketExtensions {
 public static bool IsConnected(this Socket socket) {
  try {
   return !(socket.Poll(1, SelectMode.SelectRead) && socket.Available == 0);
  } catch (SocketException) {
   return false;
  }
 }
}

在关闭应用程序时工作正常，但在关闭网络卡时不起作用。我正在VirtualBox上运行基于Debian虚拟机的测试。有没有办法在这种情况下检测到断开连接？

- tofu

Answer 1

关闭应用程序时可以使用，但关闭网络卡时不行。 关闭网络卡实际上并不是断开连接。如果网络卡再次打开，则现有的连接可以继续（如果接口仍具有相同的IP地址）-例如，在暂停笔记本电脑并稍后恢复它时，这不足为奇。 对于TCP，真正的断开连接只有在显式断开连接（发送FIN）时才会发生，该操作可以通过显式关闭套接字或随着应用程序退出或应用程序崩溃而由OS内核隐式完成。 您要求的不是明确的断开连接，而是检测对等方是否当前无法到达，例如当线路（临时）断开或系统崩溃时。这可以通过在应用程序级别或TCP级别上设置某种心跳来完成。后者称为TCP保持活动状态，通过发送空TCP数据包并检查是否发送回ACK来工作。请参见此处以了解如何使用此功能的示例。

- Steffen Ullrich

Answer 2

我有类似的问题，当我想知道我的网络接口是否已知时。

我正在使用这段代码来检查不同的网络接口：

文件NetworkMonitor.cs

using System.Collections;
using System.Diagnostics;
using System.Timers;

namespace NetWork.Plus
{
    /// <summary>
    /// The NetworkMonitor class monitors network speed for each network adapter on the computer,
    /// using classes for Performance counter in .NET library.
    /// </summary>
    public class NetworkMonitor
    {
        private Timer timer;                        // The timer event executes every second to refresh the values in adapters.
        private ArrayList adapters;                 // The list of adapters on the computer.
        private ArrayList monitoredAdapters;        // The list of currently monitored adapters.

        public NetworkMonitor()
        {
            this.adapters   =   new ArrayList();
            this.monitoredAdapters  =   new ArrayList();
            EnumerateNetworkAdapters();

            timer   =   new Timer(1000);
            timer.Elapsed   +=  new ElapsedEventHandler(timer_Elapsed);
        }

        /// <summary>
        /// Enumerates network adapters installed on the computer.
        /// </summary>
        private void EnumerateNetworkAdapters()
        {
            PerformanceCounterCategory category =   new PerformanceCounterCategory("Network Interface");

            foreach (string name in category.GetInstanceNames())
            {
                // This one exists on every computer.
                if (name == "MS TCP Loopback interface")
                    continue;
                // Create an instance of NetworkAdapter class, and create performance counters for it.
                NetworkAdapter adapter  =   new NetworkAdapter(name);
                adapter.dlCounter   =   new PerformanceCounter("Network Interface", "Bytes Received/sec", name);
                adapter.ulCounter   =   new PerformanceCounter("Network Interface", "Bytes Sent/sec", name);
                this.adapters.Add(adapter);         // Add it to ArrayList adapter
            }
        }

        private void timer_Elapsed(object sender, ElapsedEventArgs e)
        {
            foreach (NetworkAdapter adapter in this.monitoredAdapters)
                adapter.refresh();
        }

        /// <summary>
        /// Get instances of NetworkAdapter for installed adapters on this computer.
        /// </summary>
        public NetworkAdapter[] Adapters
        {
            get
            {
                return (NetworkAdapter[])this.adapters.ToArray(typeof(NetworkAdapter));
            }
        }

        // Enable the timer and add all adapters to the monitoredAdapters list, unless the adapters list is empty.
        public void StartMonitoring()
        {
            if (this.adapters.Count > 0)
            {
                foreach(NetworkAdapter adapter in this.adapters)
                    if (!this.monitoredAdapters.Contains(adapter))
                    {
                        this.monitoredAdapters.Add(adapter);
                        adapter.init();
                    }

                timer.Enabled   =   true;
            }
        }

        // Enable the timer, and add the specified adapter to the monitoredAdapters list
        public void StartMonitoring(NetworkAdapter adapter)
        {
            if (!this.monitoredAdapters.Contains(adapter))
            {
                this.monitoredAdapters.Add(adapter);
                adapter.init();
            }
            timer.Enabled   =   true;
        }

        // Disable the timer, and clear the monitoredAdapters list.
        public void StopMonitoring()
        {
            this.monitoredAdapters.Clear();
            timer.Enabled   =   false;
        }

        // Remove the specified adapter from the monitoredAdapters list, and disable the timer if the monitoredAdapters list is empty.
        public void StopMonitoring(NetworkAdapter adapter)
        {
            if (this.monitoredAdapters.Contains(adapter))
                this.monitoredAdapters.Remove(adapter); 
            if(this.monitoredAdapters.Count == 0)
                timer.Enabled   =   false;
        }
    }
}



file NetworkAdapter.cs

using System.Diagnostics;

namespace NetWork.Plus
{
    /// <summary>
    /// Represents a network adapter installed on the machine.
    /// Properties of this class can be used to obtain current network speed.
    /// </summary>
    public class NetworkAdapter
    {
        /// <summary>
        /// Instances of this class are supposed to be created only in an NetworkMonitor.
        /// </summary>
        internal NetworkAdapter(string name)
        {
            this.name   =   name;
        }

        private long dlSpeed, ulSpeed;              // Download\Upload speed in bytes per second.
        private long dlValue, ulValue;              // Download\Upload counter value in bytes.
        private long dlValueOld, ulValueOld;        // Download\Upload counter value one second earlier, in bytes.

        internal string name;                               // The name of the adapter.
        internal PerformanceCounter dlCounter, ulCounter;   // Performance counters to monitor download and upload speed.

        /// <summary>
        /// Preparations for monitoring.
        /// </summary>
        internal void init()
        {
            // Since dlValueOld and ulValueOld are used in method refresh() to calculate network speed, they must have be initialized.
            this.dlValueOld =   this.dlCounter.NextSample().RawValue;
            this.ulValueOld =   this.ulCounter.NextSample().RawValue;
        }

        /// <summary>
        /// Obtain new sample from performance counters, and refresh the values saved in dlSpeed, ulSpeed, etc.
        /// This method is supposed to be called only in NetworkMonitor, one time every second.
        /// </summary>
        internal void refresh()
        {
            this.dlValue    =   this.dlCounter.NextSample().RawValue;
            this.ulValue    =   this.ulCounter.NextSample().RawValue;

            // Calculates download and upload speed.
            this.dlSpeed    =   this.dlValue - this.dlValueOld;
            this.ulSpeed    =   this.ulValue - this.ulValueOld;

            this.dlValueOld =   this.dlValue;
            this.ulValueOld =   this.ulValue;
        }

        /// <summary>
        /// Overrides method to return the name of the adapter.
        /// </summary>
        /// <returns>The name of the adapter.</returns>
        public override string ToString()
        {
            return this.name;
        }

        /// <summary>
        /// The name of the network adapter.
        /// </summary>
        public string Name
        {
            get
            {
                return this.name;
            }
        }
        /// <summary>
        /// Current download speed in bytes per second.
        /// </summary>
        public long DownloadSpeed
        {
            get
            {
                return this.dlSpeed;
            }
        }
        /// <summary>
        /// Current upload speed in bytes per second.
        /// </summary>
        public long UploadSpeed
        {
            get
            {
                return this.ulSpeed;
            }
        }
        /// <summary>
        /// Current download speed in kbytes per second.
        /// </summary>
        public double DownloadSpeedKbps
        {
            get
            {
                return this.dlSpeed/1024.0;
            }
        }
        /// <summary>
        /// Current upload speed in kbytes per second.
        /// </summary>
        public double UploadSpeedKbps
        {
            get
            {
                return this.ulSpeed/1024.0;
            }
        }
    }
}

例如，您可以像这样使用这些文件：

    private NetworkAdapter[] adapters;
    private NetworkMonitor monitor;   

你可以捕获网络设备列表，以便检查你想要的网络设备是已知还是未知的。

    monitor =   new NetworkMonitor();
    this.adapters   =   monitor.Adapters;

如果你想的话，你可以测量下载和/或上传速度。

根据框架的版本，你需要在exe.config文件中包含以下代码：（为了避免错误 -> InvalidOperation: 实例“XXX”不存在于指定的类别中）

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <system.net>
    <settings>
      <performanceCounters enabled="true" />
    </settings>
  </system.net>
</configuration>

- Frenchy