How to set IP_MULTICAST_LOOP on multicast UDPConn in Golang

I need to set IP_MULTICAST_LOOP on a multicast UDP connection/socket so I can send/receive multicast packets on my local machine. This is the call that I've found which should work:

l, err := net.ListenMulticastUDP("udp4", nil, addr)
file, err := l.File()
fd := syscall.Handle(file.Fd())

err = syscall.SetsockoptInt(fd, syscall.IPPROTO_IP, syscall.IP_MULTICAST_LOOP, 1)

However it fails with "not supported by windows". I'm pretty sure this is supported by sockets in windows, just not in the Go net package. Any idea on how I can set this flag on my connection? (I'm new to the Go language, I may have overlooked something obvious). This is on Windows, I haven't had a chance to test on Linux.

As the doc says, net.ListenMulticastUDP is just for convenience of simple small applications. You can use golang.org/x/net/ipv4 for general purpose uses, this package offers you more options about multicast.. Actually, the source code of net.ListenMulticastUDP() sets IP_MULTICAST_LOOP to false:

func listenIPv4MulticastUDP(c *UDPConn, ifi *Interface, ip IP) error {
if ifi != nil {
if err := setIPv4MulticastInterface(c.fd, ifi); err != nil {
return err 
}   
}   
if err := setIPv4MulticastLoopback(c.fd, false); err != nil {
return err 
}   
if err := joinIPv4Group(c.fd, ifi, ip); err != nil {
return err 
}   
return nil 
}

setIPv4MulticastLoopback() is implemented for different OSes, and it's not exported. For Windows, it's in sockoptip_windows.go:

func setIPv4MulticastLoopback(fd *netFD, v bool) error {
if err := fd.incref(); err != nil {
return err 
}   
defer fd.decref()
return os.NewSyscallError("setsockopt", syscall.SetsockoptInt(fd.sysfd, syscall.IPPROTO_IP, syscall.IP_MULTICAST_LOOP, boolint(v)))
}

Below is an example with golang.org/x/net/ipv4, you can get/set MulticastLoopback

package main
import (
"fmt"
"net"
"golang.org/x/net/ipv4"
)
func main() {
ipv4Addr := &net.UDPAddr{IP: net.IPv4(224, 0, 0, 251), Port: 5352}
conn, err := net.ListenUDP("udp4", ipv4Addr)
if err != nil {
fmt.Printf("ListenUDP error %v\n", err)
return
}
pc := ipv4.NewPacketConn(conn)
// assume your have a interface named wlan
iface, err := net.InterfaceByName("wlan")
if err != nil {
fmt.Printf("can't find specified interface %v\n", err)
return
}
if err := pc.JoinGroup(iface, &net.UDPAddr{IP: net.IPv4(224, 0, 0, 251)}); err != nil {
return
}
// test
if loop, err := pc.MulticastLoopback(); err == nil {
fmt.Printf("MulticastLoopback status:%v\n", loop)
if !loop {
if err := pc.SetMulticastLoopback(true); err != nil {
fmt.Printf("SetMulticastLoopback error:%v\n", err)
}
}
}
if _, err := conn.WriteTo([]byte("hello"), ipv4Addr); err != nil {
fmt.Printf("Write failed, %v\n", err)
}
buf := make([]byte, 1024)
for {
if n, addr, err := conn.ReadFrom(buf); err != nil {
fmt.Printf("error %v", err)
} else {
fmt.Printf("recv %s from %v\n", string(buf[:n]), addr)
}
}
return
}

I think it's kinda stupid that the stdlib ListenMulticastUDP disables loopback by default because it renders it completely useless for testing purposes (ie, running more than one node on a test machine) and it doesn't say clearly on the documentation that it specifically disables loopback, it's unjustified and annoying.

https://github.com/p9c/pod/blob/master/pkg/comm/multicast/channel.go is a simple function that returns a standard 224.0.0.1 with an arbitrary port with loopback enabled. I suppose it would be even nicer if it let you pick the loopback address but for building LAN multicast IPv4 systems this is all you need.