UDP套接字未从Go中的服务器读取

Question

I'm developing a fast dns client in go just to mess around with But I'm facing troubles at the time of reading from server responses cause it never arrives and I know it actually did because I have WireShark open and it read the packet.

Here is the code sample(8.8.8.8 is Google DNS and the hex msg is a valid DNS query):

package main

import (
    "fmt"
    "net"
    "encoding/hex"
    "bufio"
)

func CheckError(err error) {
    if err  != nil {
        fmt.Println("Error: " , err)
    }
}

func main() {

    Conn, err := net.Dial("udp", "8.8.8.8:53")
    CheckError(err)

    defer Conn.Close()
    msg, _ := hex.DecodeString("5ab9010000010000000000001072312d2d2d736e2d68357137646e65650b676f6f676c65766964656f03636f6d0000010001")
    scanner := bufio.NewScanner(Conn)
    buf := []byte(msg)
    _, err1 := Conn.Write(buf)
    if err1 != nil {
        fmt.Println(msg, err1)
    }
    for scanner.Scan() {
        fmt.Println(scanner.Bytes())
    }
}

Here you have the proof that it actually arrives: WireShark Screen Capture

I've testes reading directly from conn with:

func main() {

    Conn, err := net.Dial("udp", "8.8.8.8:53")
    CheckError(err)

    defer Conn.Close()
    msg, _ := hex.DecodeString("5ab9010000010000000000001072312d2d2d736e2d68357137646e65650b676f6f676c65766964656f03636f6d0000010001")
    buf := []byte(msg)
    _, err1 := Conn.Write(buf)
    if err1 != nil {
        fmt.Println(msg, err1)
    }
    Reader(Conn)
}

func Reader(conn net.Conn) {
    var buf []byte
    for {
        conn.Read(buf)
        fmt.Println(buf)
    }
}

Answer 1

You can't use bufio around a UDP connection. UDP is not a stream oriented protocol, so you need to differentiate the individual datagrams yourself, and avoid partial reads to prevent data loss.

In order to read from an io.Reader, you must have space allocated to read into, and you need to use the bytes read value returned from the Read operation. Your example could be reduced to:

conn, err := net.Dial("udp", "8.8.8.8:53")
if err != nil {
    log.Fatal(err)
}
defer conn.Close()

msg, _ := base64.RawStdEncoding.DecodeString("WrkBAAABAAAAAAAAEHIxLS0tc24taDVxN2RuZWULZ29vZ2xldmlkZW8DY29tAAABAAE")
resp := make([]byte, 512)

conn.Write(msg)

n, err := conn.Read(resp)
if err != nil {
    log.Fatal(err)
}

fmt.Printf("%q
", resp[:n])