无法建立连接，因为目标机器主动拒绝它 Golang

我有服务器:

func main() {
    ln, err := net.Listen("tcp", "localhost:12345")
    if err != nil {
        log.Fatal(err)
    }

    for {
        conn, err := ln.Accept()
        if err != nil {
            fmt.Println("Error during Accept")
            fmt.Println(err)
            return
        }

        _, err = ws.Upgrade(conn)
        if err != nil {
            fmt.Println("Error during Upgrade")
            fmt.Println(err)
            return
        }

        go func() {
           some code
            }
        }()
    }
}

和try 连接到我的计算机的客户端:

func main() {
    buf := new(bytes.Buffer)
    payload := []byte("Hello World!")

    err := binary.Write(buf, binary.LittleEndian, payload)
    if err != nil {
        fmt.Println("Error during writing into Binary")
        fmt.Println(err)
        return
    }

    conn, err := net.Dial("tcp", "37.57.79.119:12345")
    if err != nil {
        fmt.Println("Error during Dialing")
        fmt.Println(err)
        return
    }
    buf.WriteTo(conn)
    defer conn.Close()

    answer, _ := io.ReadAll(conn)
    fmt.Println(string(answer))
}

我的系统是Kubuntu 20.04，我编译了Windows客户端，并把它发给了我的朋友.在Friend机器上，他启动它，并收到错误:

Error during Dialing
dial tcp 37.57.79.119:12345: connectex: No connection could be made because the target machine actively refused it.

为什么？我的防火墙关闭了.