In Golang, how to receive multicast packets with socket bound to specific address/port?

Task at hand is to bind a socket specifically to address 1.0.0.2:520 (assigned to eth2), then read multicast UDP packets addressed to 224.0.0.9:520.

I am trying the code below, based on https://godoc.org/golang.org/x/net/ipv4

Unfortunately, result is this debbuging message is never reached: log.Printf("udpReader: recv %d bytes from %s to %s on %s", n, cm.Src, cm.Dst, ifname)

I know eth2 is receiving the desired packets because I have this packet sniffer running on it:

sudo tcpdump -n -i eth2
18:40:28.571456 IP 1.0.0.1.520 > 224.0.0.9.520: RIPv2, Request, length: 24
18:40:29.556503 IP 1.0.0.1.520 > 224.0.0.9.520: RIPv2, Response, length: 64

This is the sample code. Can you spot why doesn't it work?

package main
import (
"fmt"
"log"
"net"
"golang.org/x/net/ipv4"
)
func main() {
if err := interfaceAdd("eth2"); err != nil {
log.Printf("main: error: %v", err)
}
log.Printf("main: waiting forever")
<-make(chan int)
}
func interfaceAdd(s string) error {
iface, err1 := net.InterfaceByName(s)
if err1 != nil {
return err1
}
addrList, err2 := iface.Addrs()
if err2 != nil {
return err2
}
for _, a := range addrList {
addr, _, err3 := net.ParseCIDR(a.String())
if err3 != nil {
log.Printf("interfaceAdd: parse CIDR error for '%s' on '%s': %v", addr, s, err3)
continue
}
if err := join(iface, addr); err != nil {
log.Printf("interfaceAdd: join error for '%s' on '%s': %v", addr, s, err)
}
}
return nil
}
func join(iface *net.Interface, addr net.IP) error {
proto := "udp"
var a string
if addr.To4() == nil {
// IPv6
a = fmt.Sprintf("[%s]", addr.String())
} else {
// IPv4
a = addr.String()
}
hostPort := fmt.Sprintf("%s:520", a) // rip multicast port
// open socket (connection)
conn, err2 := net.ListenPacket(proto, hostPort)
if err2 != nil {
return fmt.Errorf("join: %s/%s listen error: %v", proto, hostPort, err2)
}
// join multicast address
pc := ipv4.NewPacketConn(conn)
if err := pc.JoinGroup(iface, &net.UDPAddr{IP: net.IPv4(224, 0, 0, 9)}); err != nil {
conn.Close()
return fmt.Errorf("join: join error: %v", err)
}
// request control messages
/*
if err := pc.SetControlMessage(ipv4.FlagTTL|ipv4.FlagSrc|ipv4.FlagDst|ipv4.FlagInterface, true); err != nil {
// warning only
log.Printf("join: control message flags error: %v", err)
}
*/
go udpReader(pc, iface.Name, addr.String())
return nil
}
func udpReader(c *ipv4.PacketConn, ifname, ifaddr string) {
log.Printf("udpReader: reading from '%s' on '%s'", ifaddr, ifname)
defer c.Close()
buf := make([]byte, 10000)
for {
n, cm, _, err := c.ReadFrom(buf)
if err != nil {
log.Printf("udpReader: ReadFrom: error %v", err)
break
}
// make a copy because we will overwrite buf
b := make([]byte, n)
copy(b, buf)
log.Printf("udpReader: recv %d bytes from %s to %s on %s", n, cm.Src, cm.Dst, ifname)
}
log.Printf("udpReader: exiting '%s'", ifname)
}

Output:

2016/02/09 18:44:20 interfaceAdd: join error for 'fe80::a00:27ff:fe52:9575' on 'eth2': join: udp/[fe80::a00:27ff:fe52:9575]:520 listen error: listen udp [fe80::a00:27ff:fe52:9575]:520: bind: invalid argument
2016/02/09 18:44:20 main: waiting forever
2016/02/09 18:44:20 udpReader: reading from '1.0.0.2' on 'eth2'

In order to create multiple interface-specific sockets to receive packets addressed to the same multicast address (224.0.0.9:520), my original Go code was missing three major issues:

1. In order to bind multiple sockets to same UDP port, set syscall.SO_REUSEADDR
2. In order to restrict socket to specific interface, set syscall.SO_BINDTODEVICE
3. Bind the UDP sockets to 0.0.0.0:520

Find full sample code here: http://play.golang.org/p/NprsZPHQmj