Cannot receive response packets from multicast server in golang - go

Question

I setup a udp server listening multicast trafic, and create a client to sent test packet. When the server receives the package, it will send a response. Everything is ok, except the client cannot receive the packet from the server. Why?
package main
import (
"log"
"net"
"os"
"time"
"golang.org/x/net/ipv4"
)
func SetupMulticast(ifn, addr string) {
ifi, err := net.InterfaceByName(ifn)
if err != nil {
log.Fatal(err)
}
//server
c, err := net.ListenPacket("udp4", addr)
if err != nil {
log.Fatal(err)
}
log.Printf("listen ad:%s\n", addr)
p := ipv4.NewPacketConn(c)
gAddr, err2 := net.ResolveUDPAddr("udp4", addr)
if err2 != nil {
log.Fatal(err2)
}
if err := p.JoinGroup(ifi, gAddr); err != nil {
log.Fatal(err)
}
if err := p.SetControlMessage(ipv4.FlagDst|ipv4.FlagSrc, true); err != nil {
log.Fatal(err)
}
go func() {
b := make([]byte, 1500)
for {
n, cm, src, err := p.ReadFrom(b)
if err != nil {
break
}
log.Println("receive:", string(b[:n]), cm.Dst.IsMulticast(), cm.Dst)
if n2, err := p.WriteTo([]byte("world!"), nil, src); err != nil {
log.Printf("fail to write back:%v\n", err)
} else {
log.Printf("write back addr: %s length:%d\n", src, n2)
}
}
}()
//client
if conn, err2 := net.DialUDP("udp4", nil /*src*/, gAddr); err2 != nil {
log.Fatal(err)
} else {
go func() {
for {
log.Println("write hello...")
conn.Write([]byte("hello"))
time.Sleep(time.Second * 2)
}
}()
go func() {
b2 := make([]byte, 1500)
for {
n, err := conn.Read(b2)
if err != nil {
log.Panic(err)
}
log.Printf("sender received response:%s\n", string(b2[:n]))
}
}()
}
}
func main() {
If := os.Args[1] //ens33
Addr := os.Args[2] //224.0.0.248:1025
SetupMulticast(If, Addr)
for {
}
}
And the output:
2022/08/17 22:53:53 listen ad:224.0.0.248:1025
2022/08/17 22:53:53 write hello...
2022/08/17 22:53:53 receive: hello true 224.0.0.248
2022/08/17 22:53:53 write back addr: 192.168.19.131:43925 length:6
2022/08/17 22:53:55 write hello...
2022/08/17 22:53:55 receive: hello true 224.0.0.248
2022/08/17 22:53:55 write back addr: 192.168.19.131:43925 length:6
From the logs, there are no any "sender received response" record. I don't know why?