multicast loopbacks - go

Question

I need to write multicast listener on Go. I faces the problem of twicing packets when I read it. It seems that I need to set IP_MULTICAST_LOOP to false. It is not easy to do in Go.
I found this post. It seems that it should work. But I still get copies of the same host. How should it be done?
ipAddr, err := net.ResolveUDPAddr("udp", groupAddress)
if err != nil {...}
iface, err := net.InterfaceByName("en0")
if err != nil {...}
conn, err := net.ListenPacket("udp4", groupAddress)
if err != nil {...}
pc := ipv4.NewPacketConn(conn)
if err := pc.JoinGroup(iface, ipAddr); err != nil {...}
if err := pc.SetMulticastLoopback(false); err != nil {...}
if loop, err := pc.MulticastLoopback(); err == nil {...}
buf := make([]byte, 1024)
for {
n, _, addr, err := pc.ReadFrom(buf)
if err != nil {...}
fmt.Printf("recv from %v: [%s] \n", addr, buf[:n])
}

Answer 1

The simplest way is to use the ListenMulticastUDP wrapper in the net package, as actually already explained in the other SO answer you point to, How to set IP_MULTICAST_LOOP on multicast UDPConn in Golang.
If you follow the implementation of ListenMulticastUDP(), you will see that at a certain point it calls setIPv4MulticastLoopback(fd, false).
If you need something more advanced, the documentation of ListenMulticastUDP() suggests to look at https://godoc.org/golang.org/x/net/ipv4 and https://godoc.org/golang.org/x/net/ipv6, which document extensively how to do multicast in Go.
Here is some minimal code (tested on MacOS, but platform-independent) that shows how to use ListenMulticastUDP():
func main() {
// MDNS (https://en.wikipedia.org/wiki/Multicast_DNS)
groupAddress := "224.0.0.251:5353"
ifaceName := "en0"
if err := run(groupAddress, ifaceName); err != nil {
fmt.Fprintln(os.Stderr, "error:", err)
os.Exit(1)
}
}
func run(groupAddr string, ifaceName string) error {
iface, err := net.InterfaceByName(ifaceName)
if err != nil {
return err
}
gaddr, err := net.ResolveUDPAddr("udp", groupAddr)
if err != nil {
return err
}
conn, err := net.ListenMulticastUDP("udp", iface, gaddr)
if err != nil {
return err
}
buf := make([]byte, 1024)
for {
n, addr, err := conn.ReadFromUDP(buf)
if err != nil {
return err
}
fmt.Printf("recv %4d bytes from %v\n", n, addr)
}
}