discosrv: Source based rate limiting

cmd/discosrv/main.go

@@ -11,6 +11,8 @@ import (
 	"time"
 
 	"github.com/calmh/syncthing/discover"
+	"github.com/golang/groupcache/lru"
+	"github.com/juju/ratelimit"
 )
 
 type Node struct {
@@ -28,7 +30,9 @@ var (
 	lock     sync.Mutex
 	queries  = 0
 	answered = 0
+	limited  = 0
 	debug    = false
+	limiter  = lru.New(1024)
 )
 
 func main() {
@@ -57,9 +61,16 @@ func main() {
 	for {
 		buf = buf[:cap(buf)]
 		n, addr, err := conn.ReadFromUDP(buf)
+
+		if limit(addr) {
+			// Rate limit in effect for source
+			continue
+		}
+
 		if err != nil {
 			log.Fatal(err)
 		}
+
 		if n < 4 {
 			log.Printf("Received short packet (%d bytes)", n)
 			continue
@@ -84,6 +95,36 @@ func main() {
 	}
 }
 
+func limit(addr *net.UDPAddr) bool {
+	key := addr.IP.String()
+
+	lock.Lock()
+	defer lock.Unlock()
+
+	bkt, ok := limiter.Get(key)
+	if ok {
+		bkt := bkt.(*ratelimit.Bucket)
+		if bkt.TakeAvailable(1) != 1 {
+			// Rate limit exceeded; ignore packet
+			if debug {
+				log.Printf("Rate limit exceeded for", key)
+			}
+			limited++
+			return true
+		} else if debug {
+			log.Printf("Rate limit OK for", key)
+		}
+	} else {
+		if debug {
+			log.Printf("New limiter for", key)
+		}
+		// One packet per ten seconds average rate, burst ten packets
+		limiter.Add(key, ratelimit.NewBucket(10*time.Second, 10))
+	}
+
+	return false
+}
+
 func handleAnnounceV1(addr *net.UDPAddr, buf []byte) {
 	var pkt discover.AnnounceV1
 	err := pkt.UnmarshalXDR(buf)
@@ -246,9 +287,11 @@ func logStats() {
 				deleted++
 			}
 		}
-		log.Printf("Expired %d nodes; %d nodes in registry; %d queries (%d answered)", deleted, len(nodes), queries, answered)
+		log.Printf("Expired %d nodes; %d nodes in registry; %d queries (%d answered); %d entries in limiter cache", deleted, len(nodes), queries, answered, limiter.Len())
+		log.Printf("Limited %d queries; %d entries in limiter cache", limited, limiter.Len())
 		queries = 0
 		answered = 0
+		limited = 0
 
 		lock.Unlock()
 	}