feat: add VRRP unicast peer support for macvlan and restricted network environments

- Add unicast_src_ip and unicast_peers config fields
- NewUnicastSocket binds to source IP for proper packet reception
- SendTo() sends VRRP advertisements directly to peer IPs
- Receive loop filters packets from non-peer sources in unicast mode
- Backward compatible: multicast mode used when unicast_peers is empty
- Add unicast config example (etc/config.unicast.yaml)
- Update README with unicast documentation and examples

internal/vrrp/instance.go

@@ -21,6 +21,8 @@ type Instance struct {
 	AuthType        uint8
 	AuthPass        string
 	TrackScripts    []string
+	UnicastSrcIP    net.IP
+	UnicastPeers    []net.IP
 
 	state           *StateMachine
 	priorityCalc    *PriorityCalculator
@@ -54,6 +56,8 @@ func NewInstance(
 	authType string,
 	authPass string,
 	trackScripts []string,
+	unicastSrcIP string,
+	unicastPeers []string,
 	log *logger.Logger,
 ) (*Instance, error) {
 	if vrID < 1 || vrID > 255 {
@@ -90,6 +94,28 @@ func NewInstance(
 		return nil, fmt.Errorf("failed to get interface: %w", err)
 	}
 
+	var parsedSrcIP net.IP
+	var parsedPeers []net.IP
+
+	if len(unicastPeers) > 0 {
+		parsedPeers = make([]net.IP, 0, len(unicastPeers))
+		for _, peer := range unicastPeers {
+			ip := net.ParseIP(peer)
+			if ip == nil {
+				return nil, fmt.Errorf("invalid unicast peer IP: %s", peer)
+			}
+			parsedPeers = append(parsedPeers, ip.To4())
+		}
+
+		if unicastSrcIP != "" {
+			parsedSrcIP = net.ParseIP(unicastSrcIP)
+			if parsedSrcIP == nil {
+				return nil, fmt.Errorf("invalid unicast src IP: %s", unicastSrcIP)
+			}
+			parsedSrcIP = parsedSrcIP.To4()
+		}
+	}
+
 	inst := &Instance{
 		Name:            name,
 		VirtualRouterID: vrID,
@@ -101,6 +127,8 @@ func NewInstance(
 		AuthType:        authTypeNum,
 		AuthPass:        authPass,
 		TrackScripts:    trackScripts,
+		UnicastSrcIP:    parsedSrcIP,
+		UnicastPeers:    parsedPeers,
 		state:           NewStateMachine(StateInit),
 		priorityCalc:    NewPriorityCalculator(priority),
 		history:         NewStateHistory(100),
@@ -131,7 +159,15 @@ func (inst *Instance) Start() error {
 	inst.mu.Unlock()
 
 	var err error
-	inst.socket, err = NewSocket(inst.Interface)
+	if len(inst.UnicastPeers) > 0 {
+		if inst.UnicastSrcIP != nil {
+			inst.socket, err = NewUnicastSocket(inst.Interface, inst.UnicastSrcIP)
+		} else {
+			inst.socket, err = NewSocket(inst.Interface)
+		}
+	} else {
+		inst.socket, err = NewSocket(inst.Interface)
+	}
 	if err != nil {
 		return fmt.Errorf("failed to create socket: %w", err)
 	}
@@ -142,8 +178,13 @@ func (inst *Instance) Start() error {
 		return fmt.Errorf("failed to create ARP sender: %w", err)
 	}
 
-	inst.log.Info("[%s] starting VRRP instance (VRID=%d, Priority=%d, Interface=%s)",
-		inst.Name, inst.VirtualRouterID, inst.Priority, inst.Interface)
+	if len(inst.UnicastPeers) > 0 {
+		inst.log.Info("[%s] starting VRRP instance (VRID=%d, Priority=%d, Interface=%s, Mode=unicast, SrcIP=%s, Peers=%v)",
+			inst.Name, inst.VirtualRouterID, inst.Priority, inst.Interface, inst.socket.localIP, inst.UnicastPeers)
+	} else {
+		inst.log.Info("[%s] starting VRRP instance (VRID=%d, Priority=%d, Interface=%s, Mode=multicast)",
+			inst.Name, inst.VirtualRouterID, inst.Priority, inst.Interface)
+	}
 
 	inst.state.SetState(StateBackup)
 	inst.masterDownTimer.Start()
@@ -213,6 +254,13 @@ func (inst *Instance) receiveLoop() {
 			continue
 		}
 
+		if len(inst.UnicastPeers) > 0 {
+			if !inst.isUnicastPeer(srcIP) {
+				inst.log.Debug("[%s] ignoring packet from non-peer %s", inst.Name, srcIP)
+				continue
+			}
+		}
+
 		if err := pkt.Validate(inst.AuthPass); err != nil {
 			inst.log.Warn("[%s] packet validation failed: %v", inst.Name, err)
 			continue
@@ -222,6 +270,15 @@ func (inst *Instance) receiveLoop() {
 	}
 }
 
+func (inst *Instance) isUnicastPeer(ip net.IP) bool {
+	for _, peer := range inst.UnicastPeers {
+		if peer.Equal(ip) {
+			return true
+		}
+	}
+	return false
+}
+
 func (inst *Instance) handleAdvertisement(pkt *VRRPPacket, srcIP net.IP) {
 	currentState := inst.state.GetState()
 	localPriority := inst.priorityCalc.GetPriority()
@@ -272,12 +329,22 @@ func (inst *Instance) sendAdvertisement() error {
 		inst.AuthPass,
 	)
 
-	if err := inst.socket.Send(pkt); err != nil {
-		inst.log.Error("[%s] failed to send advertisement: %v", inst.Name, err)
-		return err
+	if len(inst.UnicastPeers) > 0 {
+		for _, peer := range inst.UnicastPeers {
+			if err := inst.socket.SendTo(pkt, peer); err != nil {
+				inst.log.Error("[%s] failed to send unicast advertisement to %s: %v", inst.Name, peer, err)
+				return err
+			}
+		}
+		inst.log.Debug("[%s] sent unicast advertisement (priority=%d, peers=%d)", inst.Name, priority, len(inst.UnicastPeers))
+	} else {
+		if err := inst.socket.Send(pkt); err != nil {
+			inst.log.Error("[%s] failed to send advertisement: %v", inst.Name, err)
+			return err
+		}
+		inst.log.Debug("[%s] sent multicast advertisement (priority=%d)", inst.Name, priority)
 	}
 
-	inst.log.Debug("[%s] sent advertisement (priority=%d)", inst.Name, priority)
 	return nil
 }