Add handshakes.max_rate to limit new handshakes per second

Nebula is vulnerable to DoS via handshake flooding since each incoming
  handshake performs expensive DH operations. This adds a token bucket
  rate limiter to the handshake manager that caps both inbound and
  outbound new handshakes per second. When the limit is reached, new
  handshakes are silently dropped and counted via the
  handshake_manager.rate_limited metric.

  Configured via handshakes.max_rate (default 0 = unlimited).

  Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

handshake_manager.go

@@ -23,22 +23,25 @@ const (
 	DefaultHandshakeRetries       = 10
 	DefaultHandshakeTriggerBuffer = 64
 	DefaultUseRelays              = true
+	DefaultMaxHandshakeRate       = 0 // 0 means unlimited
 )
 
 var (
 	defaultHandshakeConfig = HandshakeConfig{
-		tryInterval:   DefaultHandshakeTryInterval,
-		retries:       DefaultHandshakeRetries,
-		triggerBuffer: DefaultHandshakeTriggerBuffer,
-		useRelays:     DefaultUseRelays,
+		tryInterval:      DefaultHandshakeTryInterval,
+		retries:          DefaultHandshakeRetries,
+		triggerBuffer:    DefaultHandshakeTriggerBuffer,
+		useRelays:        DefaultUseRelays,
+		maxHandshakeRate: DefaultMaxHandshakeRate,
 	}
 )
 
 type HandshakeConfig struct {
-	tryInterval   time.Duration
-	retries       int64
-	triggerBuffer int
-	useRelays     bool
+	tryInterval      time.Duration
+	retries          int64
+	triggerBuffer    int
+	useRelays        bool
+	maxHandshakeRate int
 
 	messageMetrics *MessageMetrics
 }
@@ -58,9 +61,15 @@ type HandshakeManager struct {
 	messageMetrics         *MessageMetrics
 	metricInitiated        metrics.Counter
 	metricTimedOut         metrics.Counter
+	metricRateLimited      metrics.Counter
 	f                      *Interface
 	l                      *logrus.Logger
 
+	// Rate limiting for new handshakes (token bucket)
+	rateBucket   int // tokens currently available
+	rateMax      int // max tokens (== max handshakes per second), 0 means unlimited
+	rateLastTick time.Time
+
 	// can be used to trigger outbound handshake for the given vpnIp
 	trigger chan netip.Addr
 }
@@ -116,10 +125,41 @@ func NewHandshakeManager(l *logrus.Logger, mainHostMap *HostMap, lightHouse *Lig
 		messageMetrics:         config.messageMetrics,
 		metricInitiated:        metrics.GetOrRegisterCounter("handshake_manager.initiated", nil),
 		metricTimedOut:         metrics.GetOrRegisterCounter("handshake_manager.timed_out", nil),
+		metricRateLimited:      metrics.GetOrRegisterCounter("handshake_manager.rate_limited", nil),
+		rateBucket:             config.maxHandshakeRate,
+		rateMax:                config.maxHandshakeRate,
+		rateLastTick:           time.Now(),
 		l:                      l,
 	}
 }
 
+// handshakeRateAllow checks the token bucket rate limiter and returns true if a
+// new handshake is allowed. Must be called with hm.Lock held.
+func (hm *HandshakeManager) handshakeRateAllow(now time.Time) bool {
+	if hm.rateMax == 0 {
+		return true
+	}
+
+	// Refill tokens based on elapsed time
+	elapsed := now.Sub(hm.rateLastTick)
+	if elapsed >= time.Second {
+		// Add tokens for full seconds elapsed
+		tokens := int(elapsed/time.Second) * hm.rateMax
+		hm.rateBucket += tokens
+		if hm.rateBucket > hm.rateMax {
+			hm.rateBucket = hm.rateMax
+		}
+		hm.rateLastTick = now
+	}
+
+	if hm.rateBucket > 0 {
+		hm.rateBucket--
+		return true
+	}
+
+	return false
+}
+
 func (hm *HandshakeManager) Run(ctx context.Context) {
 	clockSource := time.NewTicker(hm.config.tryInterval)
 	defer clockSource.Stop()
@@ -149,6 +189,15 @@ func (hm *HandshakeManager) HandleIncoming(via ViaSender, packet []byte, h *head
 	case header.HandshakeIXPSK0:
 		switch h.MessageCounter {
 		case 1:
+			// Check rate limit for new incoming handshakes
+			hm.Lock()
+			allowed := hm.handshakeRateAllow(time.Now())
+			hm.Unlock()
+			if !allowed {
+				hm.metricRateLimited.Inc(1)
+				hm.l.WithField("from", via).Debug("Handshake rate limit reached, dropping incoming handshake")
+				return
+			}
 			ixHandshakeStage1(hm.f, via, packet, h)
 
 		case 2:
@@ -447,6 +496,14 @@ func (hm *HandshakeManager) StartHandshake(vpnAddr netip.Addr, cacheCb func(*Han
 		return hh.hostinfo
 	}
 
+	// Check rate limit for new outbound handshakes
+	if !hm.handshakeRateAllow(time.Now()) {
+		hm.metricRateLimited.Inc(1)
+		hm.l.WithField("vpnAddr", vpnAddr).Debug("Handshake rate limit reached, dropping outbound handshake")
+		hm.Unlock()
+		return nil
+	}
+
 	hostinfo := &HostInfo{
 		vpnAddrs:        []netip.Addr{vpnAddr},
 		HandshakePacket: make(map[uint8][]byte, 0),