如何在Golang中在多播UDPConn上设置IP_MULTICAST_LOOP

Question

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.

Answer 1

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
", 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
", 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
", loop)
        if !loop {
            if err := pc.SetMulticastLoopback(true); err != nil {
            fmt.Printf("SetMulticastLoopback error:%v
", err)
            }
        }
    }

    if _, err := conn.WriteTo([]byte("hello"), ipv4Addr); err != nil {
        fmt.Printf("Write failed, %v
", 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
", string(buf[:n]), addr)
        }
    }

    return
}