Golang 写的 UDP-Client 无法接收来自服务器的消息

Question

我写了一个Java客户端，它向广播地址发送消息。

我也写了一个Java服务器，它接受所有发送的消息并将消息发送回客户端。

现在我想尝试在 Go 中做完全相同的事情，只是为了获得一些经验。服务器工作正常，正在接收消息并响应 Java 客户端。

但是我的 Go 客户端只向 Go/Java 服务器发送消息，但没有收到任何返回消息。根据 wireshark，消息被发送回正确的 IP 和端口，但显然该端口无法访问。

我的代码如下：围棋服务器：

package main

import (
    "fmt"
    "log"
    "net"
)

func main() {
    //Resolving address
    udpAddr, err := net.ResolveUDPAddr("udp4", "0.0.0.0:8888")

    if err != nil {
        log.Println("Error: ", err)
    }

    // Build listining connections
    conn, err := net.ListenUDP("udp", udpAddr)

    defer conn.Close()

    if err != nil {
        log.Println("Error: ", err)
    }

    // Interacting with one client at a time
    for {
        fmt.Println(">>>Ready to receive broadcast packets!")

        // Receiving a message
        recvBuff := make([]byte, 15000)
        _, rmAddr, err := conn.ReadFromUDP(recvBuff)

        if err != nil {
            panic(err)
        }

        fmt.Println(">>>Discovery packet received from: " + rmAddr.String())
        fmt.Println(">>>Packet received; data: " + string(recvBuff))

        // Sending the same message back to current client
        conn.WriteToUDP(recvBuff, rmAddr)

        fmt.Println(">>>Sent packet to: " + rmAddr.String())

} }

Go 客户端：

package main

import (
    "fmt"
    "log"
    "net"
    "os"
)

func main() {
    service := "158.129.239.255:8888"

    // Resolving Address
    RemoteAddr, err := net.ResolveUDPAddr("udp", service)

    // Make a connection
    conn, err := net.DialUDP("udp", nil, RemoteAddr)

    defer conn.Close()

    // Exit if some error occured
    if err != nil {
        log.Fatal(err)
        os.Exit(1)
    }

    // write a message to server
    message := []byte("message")

    _, err = conn.Write(message)
    fmt.Println(">>> Request packet sent to: 158.129.239.255 (DEFAULT)")

    if err != nil {
        log.Println(err)
    }

    // Receive response from server
    buf := make([]byte, 15000)
    amountByte, remAddr, err := conn.ReadFromUDP(buf)

    if err != nil {
        log.Println(err)
    } else {
        fmt.Println(amountByte, "bytes received from", remAddr)
    }

}

Java 客户：

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.InterfaceAddress;
import java.net.NetworkInterface;
import java.util.Enumeration;
import java.util.logging.Level;
import java.util.logging.Logger;

public class BroadcastUDPClient {

    public static void main(String[] args) {
        // Find the server using UDP broadcast
        try {
            //Open a random port to send the package
            DatagramSocket sendSD = new DatagramSocket();
            sendSD.setBroadcast(true);

                byte[] sendData = "message".getBytes();

            //Try the 255.255.255.255 first
            try {
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, InetAddress.getByName("158.129.239.255"), 8888);
                sendSD.send(sendPacket);
                System.out.println(">>> Request packet sent to: 158.129.239.255 (DEFAULT)");
            } catch (Exception e) {
            }

            //Wait for a response
            byte[] recvBuf = new byte[15000];
            DatagramPacket receivePacket = new DatagramPacket(recvBuf, recvBuf.length);
            sendSD.receive(receivePacket);

            //We have a response
            System.out.println(">>> Broadcast response from server: " + receivePacket.getAddress().getHostAddress());
            String message = new String(receivePacket.getData()).trim();
            System.out.println(">>> Message Body: " + message);

            //Close the port!
            sendSD.close();
        } catch (IOException ex) {
            Logger.getLogger(BroadcastUDPClient.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

}

我的 Go 客户端有什么问题？

Answer 1

如果您不忽略从 WriteToUDP 返回的错误，它实际上会给您一个错误："sendto: message to long"

在 OSX 上，最大 UDP 数据报大小默认设置为 9216 字节。您尝试发送 15000 个字节。

如果您只想写回收到的内容，请写

recvBuff[:n]

，其中n是之前接收到的字节数。

Answer 2

即使您只打算从连接发送 UDP 数据包，您通常也希望使用 ListenUDP 来创建连接，并使用 ReadFromUDP 和 WriteToUDP 方法。

当您使用 DialUDP 时，它会创建一个 "connected" UDP 套接字，带有一个隐式远程端点，它将过滤传入的数据包。来自 Linux connect 手册页：

If the socket sockfd is of type SOCK_DGRAM then addr is the address to which datagrams are sent by default, and the only address from which datagrams are received.

Golang 写的 UDP-Client 无法接收来自服务器的消息

UDP-Client written in Golang fails to receive Message from Server

udp

broadcast

go