c# 知道我们使用哪个 IP 与另一个 IP 通信吗？

Question

通过自制的发现工具，我发现了我感兴趣的服务列表。

我有他们的 IP、服务名称、端口、主机……但要使用它们，我必须向客户端库指定我们将使用的 IP。

由于我有几块网卡，所以我需要检测使用哪个接口与我知道的目标 IP 进行通信，然后将这个IPAddress 提供给我的库。

但是我如何检测我应该使用哪个接口 IP？

我尝试在互联网上进行一些搜索，但我认为我没有正确的关键字，因为我没有找到任何相关的内容。

Answer 1

我已经从我开发的网络库networkComms.net中为你提取了相关方法：

/// <summary>
/// Determines the most appropriate local end point to contact the provided remote end point. 
/// Testing shows this method takes on average 1.6ms to return.
/// </summary>
/// <param name="remoteIPEndPoint">The remote end point</param>
/// <returns>The selected local end point</returns>
public static IPEndPoint BestLocalEndPoint(IPEndPoint remoteIPEndPoint)
{    
    Socket testSocket = new Socket(remoteIPEndPoint.AddressFamily, SocketType.Dgram, ProtocolType.Udp);
    testSocket.Connect(remoteIPEndPoint);
    return (IPEndPoint)testSocket.LocalEndPoint;
}

一旦您拥有正确的 IP，您就可以遍历 NetworkInterface.GetAllNetworkInterfaces() 以找到匹配的适配器。

Answer 2

以下通过查询路由表来工作。这与Socket.Connect 确定要使用的本地端点的方式相同。差异：

• 不能因为防火墙而失败
• 由于远程端点不存在而不能失败
• 不保留本地端口
• 更快

.

private static IPEndPoint QueryRoutingInterface(
          Socket socket,
          IPEndPoint remoteEndPoint)
{
    SocketAddress address = remoteEndPoint.Serialize();

    byte[] remoteAddrBytes = new byte[address.Size];
    for (int i = 0; i < address.Size; i++) {
        remoteAddrBytes[i] = address[i];
    }

    byte[] outBytes = new byte[remoteAddrBytes.Length];
    socket.IOControl(
                IOControlCode.RoutingInterfaceQuery, 
                remoteAddrBytes, 
                outBytes);
    for (int i = 0; i < address.Size; i++) {
        address[i] = outBytes[i];
    }

    EndPoint ep = remoteEndPoint.Create(address);
    return (IPEndPoint)ep;
}

使用如下：

IPAddress remoteIp = IPAddress.Parse("192.168.1.55");
IpEndPoint remoteEndPoint = new IPEndPoint(remoteIp, 0);
Socket socket = new Socket(
                      AddressFamily.InterNetwork, 
                      SocketType.Dgram, 
                      ProtocolType.Udp);
IPEndPoint localEndPoint = QueryRoutingInterface(socket, remoteEndPoint );
Console.WriteLine("Local EndPoint is: {0}", localEndPoint);

（源代码复制自here）

请注意，尽管使用端口指定IpEndPoint，但端口无关紧要。此外，返回的IpEndPoint.Port 始终为0。

Answer 3

我花了几个小时来找到这个问题的简短答案，感谢 BatteryBackupUnit，我已经做出了这个解决方案，希望它可以帮助其他程序员：

    [DllImport("ws2_32.dll", SetLastError = true)]
    private static extern SocketError WSAIoctl([In] IntPtr socketHandle, uint ioControlCode, [In] byte[] inBuffer, int inBufferSize, [Out] byte[] outBuffer, int outBufferSize, out int bytesTransferred, IntPtr overlapped, IntPtr completionRoutine);

    /// <summary>Get local IP-address for remote address</summary>
    /// <param name="remoteAddress">Remote Address</param>
    /// <returns></returns>
    public static IPAddress GetLocalAddressForRemote(IPAddress remoteAddress)
    {
        if (remoteAddress == null) return null;

        long rm = remoteAddress.Address;
        //Temporary socket only for handle of it
        Socket s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        byte[] src = new byte[16];

        src[0] = 2;
        src[4] = (byte)rm;
        src[5] = (byte)(rm >> 8);
        src[6] = (byte)(rm >> 16);
        src[7] = (byte)(rm >> 24);

        int transeferred = 0;
        if (WSAIoctl(s.Handle, 3355443220, src, 16, src, 16, out transeferred, IntPtr.Zero, IntPtr.Zero) == SocketError.Success)
        {
            s.Dispose();
            rm = src[4];
            rm |= ((long)src[5]) << 8;
            rm |= ((long)src[6]) << 16;
            rm |= ((long)src[7]) << 24;
            return new IPAddress(rm);
        }
        s.Dispose();
        return null;
    }