add multicast.InterfaceForSource (#414)

pkg/multicast/multicast.go

@@ -2,6 +2,7 @@
 package multicast
 
 import (
+	"fmt"
 	"net"
 )
 
@@ -10,3 +11,33 @@ type Conn interface {
 	net.PacketConn
 	SetReadBuffer(int) error
 }
+
+// InterfaceForSource returns a multicast-capable interface that can communicate with given IP.
+func InterfaceForSource(ip net.IP) (*net.Interface, error) {
+	if ip.Equal(net.ParseIP("127.0.0.1")) {
+		return nil, fmt.Errorf("IP 127.0.0.1 can't be used as source of a multicast stream. Use the LAN IP of your PC")
+	}
+
+	intfs, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	for _, intf := range intfs {
+		if (intf.Flags & net.FlagMulticast) == 0 {
+			continue
+		}
+
+		addrs, err := intf.Addrs()
+		if err == nil {
+			for _, addr := range addrs {
+				_, ipnet, err := net.ParseCIDR(addr.String())
+				if err == nil && ipnet.Contains(ip) {
+					return &intf, nil
+				}
+			}
+		}
+	}
+
+	return nil, fmt.Errorf("found no interface that is multicast-capable and can communicate with IP %v", ip)
+}