java - UDP 打洞 Java 示例

Question

我想在具有静态 IP 的服务器的帮助下对两个客户端进行 UDP 打洞。服务器在端口 7070 和 7071 上等待两个客户端。之后，它向对方发送 IP 地址和端口。这部分工作正常。但是我无法在两个客户之间建立通信。我在不同的 Wifi 网络和 3G 移动网络中尝试了代码。客户端程序抛出 IO-Exception“No route to host”。客户端代码用于两个客户端。一次使用端口 7070 执行，一次使用 7071。

您认为我是否正确地实现了 UDP 打洞概念？有什么想法让它发挥作用吗？首先是服务器代码，然后是客户端代码。

谢谢你的帮助。

服务器代码:

public class UDPHolePunchingServer {

    public static void main(String args[]) throws Exception {

    // Waiting for Connection of Client1 on Port 7070
    // ////////////////////////////////////////////////

    // open serverSocket on Port 7070
    DatagramSocket serverSocket1 = new DatagramSocket(7070);

    System.out.println("Waiting for Client 1 on Port "
            + serverSocket1.getLocalPort());

    // receive Data
    DatagramPacket receivePacket = new DatagramPacket(new byte[1024], 1024);
    serverSocket1.receive(receivePacket);

    // Get IP-Address and Port of Client1
    InetAddress IPAddress1 = receivePacket.getAddress();
    int port1 = receivePacket.getPort();
    String msgInfoOfClient1 = IPAddress1 + "-" + port1 + "-";

    System.out.println("Client1: " + msgInfoOfClient1);

    // Waiting for Connection of Client2 on Port 7071
    // ////////////////////////////////////////////////

    // open serverSocket on Port 7071
    DatagramSocket serverSocket2 = new DatagramSocket(7071);

    System.out.println("Waiting for Client 2 on Port "
            + serverSocket2.getLocalPort());

    // receive Data
    receivePacket = new DatagramPacket(new byte[1024], 1024);
    serverSocket2.receive(receivePacket);

    // GetIP-Address and Port of Client1
    InetAddress IPAddress2 = receivePacket.getAddress();
    int port2 = receivePacket.getPort();
    String msgInfoOfClient2 = IPAddress2 + "-" + port2 + "-";

    System.out.println("Client2:" + msgInfoOfClient2);

    // Send the Information to the other Client
    // /////////////////////////////////////////////////

    // Send Information of Client2 to Client1
    serverSocket1.send(new DatagramPacket(msgInfoOfClient2.getBytes(),
            msgInfoOfClient2.getBytes().length, IPAddress1, port1));

    // Send Infos of Client1 to Client2
    serverSocket2.send(new DatagramPacket(msgInfoOfClient1.getBytes(),
            msgInfoOfClient1.getBytes().length, IPAddress2, port2));

    //close Sockets
    serverSocket1.close();
    serverSocket2.close();
}

客户端代码

public class UDPHolePunchingClient {

    public static void main(String[] args) throws Exception {
    // prepare Socket
    DatagramSocket clientSocket = new DatagramSocket();

    // prepare Data
    byte[] sendData = "Hello".getBytes();

    // send Data to Server with fix IP (X.X.X.X)
    // Client1 uses port 7070, Client2 uses port 7071
    DatagramPacket sendPacket = new DatagramPacket(sendData,
            sendData.length, InetAddress.getByName("X.X.X.X"), 7070);
    clientSocket.send(sendPacket);

    // receive Data ==> Format:"<IP of other Client>-<Port of other Client>"
    DatagramPacket receivePacket = new DatagramPacket(new byte[1024], 1024);
    clientSocket.receive(receivePacket);

    // Convert Response to IP and Port
    String response = new String(receivePacket.getData());
    String[] splitResponse = response.split("-");
    InetAddress ip = InetAddress.getByName(splitResponse[0].substring(1));

    int port = Integer.parseInt(splitResponse[1]);

    // output converted Data for check
    System.out.println("IP: " + ip + " PORT: " + port);

    // close socket and open new socket with SAME localport
    int localPort = clientSocket.getLocalPort();
    clientSocket.close();
    clientSocket = new DatagramSocket(localPort);

    // set Timeout for receiving Data
    clientSocket.setSoTimeout(1000);

    // send 5000 Messages for testing
    for (int i = 0; i < 5000; i++) {

        // send Message to other client
        sendData = ("Datapacket(" + i + ")").getBytes();
        sendPacket = new DatagramPacket(sendData, sendData.length, ip, port);
        clientSocket.send(sendPacket);

        // receive Message from other client
        try {
            receivePacket.setData(new byte[1024]);
            clientSocket.receive(receivePacket);
            System.out.println("REC: "
                    + new String(receivePacket.getData()));

        } catch (Exception e) {
            System.out.println("SERVER TIMED OUT");
        }
    }

    // close connection
    clientSocket.close();
}

更新该代码通常可以正常工作。我现在已经在两个不同的家庭网络中尝试过它并且可以正常工作。但它在我的 3G 或大学网络中不起作用。在 3G 中，我验证了 NAT 再次将两个端口(客户端端口和路由器分配的端口)映射在一起，即使在关闭和打开 clientSocket 之后也是如此。有谁知道为什么它不工作呢？

Answer 1

UDP 打洞并不是所有类型的 NAT 都可以实现的。没有为所有类型的 NAT 定义通用或可靠的方法。对称NAT更是难上加难。

根据 NAT 行为，发送 UDP 数据包的不同设备的端口映射可能不同。比如，如果 A 向 B 发送一个 UDP 数据包，它可能会得到一些端口，比如 50000。但是如果 A 向 C 发送一个 UDP 数据包，那么它可能会得到一个不同的映射，比如 50002。所以，在你的情况下，向服务器发送一个数据包可能给客户端一些端口，但向其他客户端发送数据包可能会提供其他端口。

您应在此处阅读有关 NAT 行为的更多信息:

https://www.rfc-editor.org/rfc/rfc4787

https://www.rfc-editor.org/rfc/rfc5128

UDP hole punching not going through on 3G