WIP

peer/multicast.go

@@ -0,0 +1,101 @@
+package peer
+
+import (
+	"encoding/binary"
+	"fmt"
+	"log"
+	"net"
+	"net/netip"
+	"time"
+
+	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
+)
+
+const (
+	mcBeaconLen          = 35 // 1 VPN IP byte + 32 WG pubkey + 2 WG listen port
+	mcBroadcastInterval  = 32 * time.Second
+	mcErrorRetryInterval = 16 * time.Second
+)
+
+var mcAddr = net.UDPAddrFromAddrPort(netip.AddrPortFrom(
+	netip.AddrFrom4([4]byte{224, 0, 0, 157}),
+	4560))
+
+// RunMCWriter broadcasts a beacon on the local multicast group every
+// mcBroadcastInterval so that LAN peers can discover our WireGuard endpoint.
+func RunMCWriter(selfVPNIP netip.Addr, pubKey wgtypes.Key, wgPort uint16) {
+	conn, err := net.ListenMulticastUDP("udp", nil, mcAddr)
+	if err != nil {
+		log.Fatalf("[MCWriter] bind: %v", err)
+	}
+
+	beacon := buildBeacon(selfVPNIP, pubKey, wgPort)
+
+	for range time.Tick(mcBroadcastInterval) {
+		if _, err := conn.WriteToUDP(beacon, mcAddr); err != nil {
+			log.Printf("[MCWriter] write: %v", err)
+		}
+	}
+}
+
+func buildBeacon(selfVPNIP netip.Addr, pubKey wgtypes.Key, wgPort uint16) []byte {
+	beacon := make([]byte, mcBeaconLen)
+	beacon[0] = selfVPNIP.As4()[3]
+	copy(beacon[1:33], pubKey[:])
+	binary.BigEndian.PutUint16(beacon[33:35], wgPort)
+	return beacon
+}
+
+// RunMCReader listens for multicast beacons from LAN peers and feeds
+// MulticastEvents to ch.
+func RunMCReader(vpnNet netip.Prefix, selfVPNIP netip.Addr, ch chan<- MulticastEvent) {
+	for {
+		if err := runMCReaderInner(vpnNet, selfVPNIP, ch); err != nil {
+			log.Printf("[MCReader] %v", err)
+		}
+		time.Sleep(mcErrorRetryInterval)
+	}
+}
+
+func runMCReaderInner(vpnNet netip.Prefix, selfVPNIP netip.Addr, ch chan<- MulticastEvent) error {
+	conn, err := net.ListenMulticastUDP("udp", nil, mcAddr)
+	if err != nil {
+		return fmt.Errorf("bind: %w", err)
+	}
+	defer conn.Close()
+
+	buf := make([]byte, 64)
+	netAddr := vpnNet.Addr().As4()
+
+	for {
+		conn.SetReadDeadline(time.Now().Add(32 * time.Second))
+		n, src, err := conn.ReadFromUDPAddrPort(buf)
+		if err != nil {
+			return fmt.Errorf("read: %w", err)
+		}
+		if n != mcBeaconLen {
+			continue
+		}
+
+		octets := netAddr
+		octets[3] = buf[0]
+		vpnIP := netip.AddrFrom4(octets)
+		if vpnIP == selfVPNIP {
+			continue
+		}
+
+		pubKey, err := wgtypes.NewKey(buf[1:33])
+		if err != nil {
+			continue
+		}
+
+		wgPort := binary.BigEndian.Uint16(buf[33:35])
+		endpoint := netip.AddrPortFrom(src.Addr().Unmap(), wgPort)
+
+		ch <- MulticastEvent{
+			pubKey:   pubKey,
+			vpnIP:    vpnIP,
+			endpoint: endpoint,
+		}
+	}
+}