Use IPv4 multicast discovery

mc/beacon.go

@@ -0,0 +1,91 @@
+package mc
+
+import (
+	"log"
+	"net"
+)
+
+type recv struct {
+	data []byte
+	src  net.Addr
+}
+
+type Beacon struct {
+	group  string
+	port   int
+	conns  []*net.UDPConn
+	inbox  chan []byte
+	outbox chan recv
+}
+
+func NewBeacon(group string, port int) *Beacon {
+	b := &Beacon{
+		group:  group,
+		port:   port,
+		inbox:  make(chan []byte),
+		outbox: make(chan recv),
+	}
+	go b.run()
+	return b
+}
+
+func (b *Beacon) Send(data []byte) {
+	b.inbox <- data
+}
+
+func (b *Beacon) Recv() ([]byte, net.Addr) {
+	recv := <-b.outbox
+	return recv.data, recv.src
+}
+
+func (b *Beacon) run() {
+	group := &net.UDPAddr{IP: net.ParseIP(b.group), Port: b.port}
+
+	intfs, err := net.Interfaces()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, intf := range intfs {
+		intf := intf
+
+		if debug {
+			dlog.Printf("trying interface %q", intf.Name)
+		}
+		conn, err := net.ListenMulticastUDP("udp4", &intf, group)
+		if err != nil {
+			if debug {
+				dlog.Printf("listen for multicast group on %q: %v", intf.Name, err)
+			}
+		} else {
+			b.conns = append(b.conns, conn)
+		}
+	}
+
+	for _, conn := range b.conns {
+		conn := conn
+		go func() {
+			for {
+				var bs = make([]byte, 1500)
+				n, addr, err := conn.ReadFrom(bs)
+				if err != nil {
+					dlog.Println(err)
+					return
+				}
+				b.outbox <- recv{bs[:n], addr}
+			}
+		}()
+	}
+
+	go func() {
+		for bs := range b.inbox {
+			for _, conn := range b.conns {
+				_, err := conn.WriteTo(bs, group)
+				if err != nil {
+					dlog.Println(err)
+					return
+				}
+			}
+		}
+	}()
+}