Merge pull request #1314 from karalabe/handle-fetcher-attacks-2

eth/fetcher: handle and test various DOS attacks

2 files changed, 264 insertions, 60 deletions

diff --git a/eth/fetcher/fetcher.go b/eth/fetcher/fetcher.go
index b80182a45..90a202235 100644
--- a/eth/fetcher/fetcher.go
+++ b/eth/fetcher/fetcher.go
@@ -20,6 +20,8 @@ const (
 	fetchTimeout  = 5 * time.Second        // Maximum alloted time to return an explicitly requested block
 	maxUncleDist  = 7                      // Maximum allowed backward distance from the chain head
 	maxQueueDist  = 32                     // Maximum allowed distance from the chain head to queue
+	hashLimit     = 256                    // Maximum number of unique blocks a peer may have announced
+	blockLimit    = 64                     // Maximum number of unique blocks a per may have delivered
 )
 
 var (
@@ -74,12 +76,14 @@ type Fetcher struct {
 	quit   chan struct{}
 
 	// Announce states
+	announces map[string]int              // Per peer announce counts to prevent memory exhaustion
 	announced map[common.Hash][]*announce // Announced blocks, scheduled for fetching
 	fetching  map[common.Hash]*announce   // Announced blocks, currently fetching
 
 	// Block cache
-	queue  *prque.Prque             // Queue containing the import operations (block number sorted)
-	queued map[common.Hash]struct{} // Presence set of already queued blocks (to dedup imports)
+	queue  *prque.Prque            // Queue containing the import operations (block number sorted)
+	queues map[string]int          // Per peer block counts to prevent memory exhaustion
+	queued map[common.Hash]*inject // Set of already queued blocks (to dedup imports)
 
 	// Callbacks
 	getBlock       blockRetrievalFn   // Retrieves a block from the local chain
@@ -88,6 +92,10 @@ type Fetcher struct {
 	chainHeight    chainHeightFn      // Retrieves the current chain's height
 	insertChain    chainInsertFn      // Injects a batch of blocks into the chain
 	dropPeer       peerDropFn         // Drops a peer for misbehaving
+
+	// Testing hooks
+	fetchingHook func([]common.Hash) // Method to call upon starting a block fetch
+	importedHook func(*types.Block)  // Method to call upon successful block import
 }
 
 // New creates a block fetcher to retrieve blocks based on hash announcements.
@@ -98,10 +106,12 @@ func New(getBlock blockRetrievalFn, validateBlock blockValidatorFn, broadcastBlo
 		filter:         make(chan chan []*types.Block),
 		done:           make(chan common.Hash),
 		quit:           make(chan struct{}),
+		announces:      make(map[string]int),
 		announced:      make(map[common.Hash][]*announce),
 		fetching:       make(map[common.Hash]*announce),
 		queue:          prque.New(),
-		queued:         make(map[common.Hash]struct{}),
+		queues:         make(map[string]int),
+		queued:         make(map[common.Hash]*inject),
 		getBlock:       getBlock,
 		validateBlock:  validateBlock,
 		broadcastBlock: broadcastBlock,
@@ -189,23 +199,24 @@ func (f *Fetcher) loop() {
 		// Clean up any expired block fetches
 		for hash, announce := range f.fetching {
 			if time.Since(announce.time) > fetchTimeout {
-				delete(f.announced, hash)
-				delete(f.fetching, hash)
+				f.forgetHash(hash)
 			}
 		}
 		// Import any queued blocks that could potentially fit
 		height := f.chainHeight()
 		for !f.queue.Empty() {
 			op := f.queue.PopItem().(*inject)
-			number := op.block.NumberU64()
 
 			// If too high up the chain or phase, continue later
+			number := op.block.NumberU64()
 			if number > height+1 {
 				f.queue.Push(op, -float32(op.block.NumberU64()))
 				break
 			}
 			// Otherwise if fresh and still unknown, try and import
-			if number+maxUncleDist < height || f.getBlock(op.block.Hash()) != nil {
+			hash := op.block.Hash()
+			if number+maxUncleDist < height || f.getBlock(hash) != nil {
+				f.forgetBlock(hash)
 				continue
 			}
 			f.insert(op.origin, op.block)
@@ -217,10 +228,17 @@ func (f *Fetcher) loop() {
 			return
 
 		case notification := <-f.notify:
-			// A block was announced, schedule if it's not yet downloading
+			// A block was announced, make sure the peer isn't DOSing us
+			count := f.announces[notification.origin] + 1
+			if count > hashLimit {
+				glog.V(logger.Debug).Infof("Peer %s: exceeded outstanding announces (%d)", notification.origin, hashLimit)
+				break
+			}
+			// All is well, schedule the announce if block's not yet downloading
 			if _, ok := f.fetching[notification.hash]; ok {
 				break
 			}
+			f.announces[notification.origin] = count
 			f.announced[notification.hash] = append(f.announced[notification.hash], notification)
 			if len(f.announced) == 1 {
 				f.reschedule(fetch)
@@ -232,9 +250,8 @@ func (f *Fetcher) loop() {
 
 		case hash := <-f.done:
 			// A pending import finished, remove all traces of the notification
-			delete(f.announced, hash)
-			delete(f.fetching, hash)
-			delete(f.queued, hash)
+			f.forgetHash(hash)
+			f.forgetBlock(hash)
 
 		case <-fetch.C:
 			// At least one block's timer ran out, check for needing retrieval
@@ -242,12 +259,15 @@ func (f *Fetcher) loop() {
 
 			for hash, announces := range f.announced {
 				if time.Since(announces[0].time) > arriveTimeout-gatherSlack {
+					// Pick a random peer to retrieve from, reset all others
 					announce := announces[rand.Intn(len(announces))]
+					f.forgetHash(hash)
+
+					// If the block still didn't arrive, queue for fetching
 					if f.getBlock(hash) == nil {
 						request[announce.origin] = append(request[announce.origin], hash)
 						f.fetching[hash] = announce
 					}
-					delete(f.announced, hash)
 				}
 			}
 			// Send out all block requests
@@ -261,7 +281,14 @@ func (f *Fetcher) loop() {
 
 					glog.V(logger.Detail).Infof("Peer %s: fetching %s", peer, list)
 				}
-				go f.fetching[hashes[0]].fetch(hashes)
+				// Create a closure of the fetch and schedule in on a new thread
+				fetcher, hashes := f.fetching[hashes[0]].fetch, hashes
+				go func() {
+					if f.fetchingHook != nil {
+						f.fetchingHook(hashes)
+					}
+					fetcher(hashes)
+				}()
 			}
 			// Schedule the next fetch if blocks are still pending
 			f.reschedule(fetch)
@@ -285,7 +312,7 @@ func (f *Fetcher) loop() {
 					if f.getBlock(hash) == nil {
 						explicit = append(explicit, block)
 					} else {
-						delete(f.fetching, hash)
+						f.forgetHash(hash)
 					}
 				} else {
 					download = append(download, block)
@@ -328,6 +355,12 @@ func (f *Fetcher) reschedule(fetch *time.Timer) {
 func (f *Fetcher) enqueue(peer string, block *types.Block) {
 	hash := block.Hash()
 
+	// Ensure the peer isn't DOSing us
+	count := f.queues[peer] + 1
+	if count > blockLimit {
+		glog.V(logger.Debug).Infof("Peer %s: discarded block #%d [%x], exceeded allowance (%d)", peer, block.NumberU64(), hash.Bytes()[:4], blockLimit)
+		return
+	}
 	// Discard any past or too distant blocks
 	if dist := int64(block.NumberU64()) - int64(f.chainHeight()); dist < -maxUncleDist || dist > maxQueueDist {
 		glog.V(logger.Debug).Infof("Peer %s: discarded block #%d [%x], distance %d", peer, block.NumberU64(), hash.Bytes()[:4], dist)
@@ -335,8 +368,13 @@ func (f *Fetcher) enqueue(peer string, block *types.Block) {
 	}
 	// Schedule the block for future importing
 	if _, ok := f.queued[hash]; !ok {
-		f.queued[hash] = struct{}{}
-		f.queue.Push(&inject{origin: peer, block: block}, -float32(block.NumberU64()))
+		op := &inject{
+			origin: peer,
+			block:  block,
+		}
+		f.queues[peer] = count
+		f.queued[hash] = op
+		f.queue.Push(op, -float32(block.NumberU64()))
 
 		if glog.V(logger.Debug) {
 			glog.Infof("Peer %s: queued block #%d [%x], total %v", peer, block.NumberU64(), hash.Bytes()[:4], f.queue.Size())
@@ -375,5 +413,44 @@ func (f *Fetcher) insert(peer string, block *types.Block) {
 		}
 		// If import succeeded, broadcast the block
 		go f.broadcastBlock(block, false)
+
+		// Invoke the testing hook if needed
+		if f.importedHook != nil {
+			f.importedHook(block)
+		}
 	}()
 }
+
+// forgetHash removes all traces of a block announcement from the fetcher's
+// internal state.
+func (f *Fetcher) forgetHash(hash common.Hash) {
+	// Remove all pending announces and decrement DOS counters
+	for _, announce := range f.announced[hash] {
+		f.announces[announce.origin]--
+		if f.announces[announce.origin] == 0 {
+			delete(f.announces, announce.origin)
+		}
+	}
+	delete(f.announced, hash)
+
+	// Remove any pending fetches and decrement the DOS counters
+	if announce := f.fetching[hash]; announce != nil {
+		f.announces[announce.origin]--
+		if f.announces[announce.origin] == 0 {
+			delete(f.announces, announce.origin)
+		}
+		delete(f.fetching, hash)
+	}
+}
+
+// forgetBlock removes all traces of a queued block frmo the fetcher's internal
+// state.
+func (f *Fetcher) forgetBlock(hash common.Hash) {
+	if insert := f.queued[hash]; insert != nil {
+		f.queues[insert.origin]--
+		if f.queues[insert.origin] == 0 {
+			delete(f.queues, insert.origin)
+		}
+		delete(f.queued, hash)
+	}
+}
diff --git a/eth/fetcher/fetcher_test.go b/eth/fetcher/fetcher_test.go
index 0d069ac65..80247d9d2 100644
--- a/eth/fetcher/fetcher_test.go
+++ b/eth/fetcher/fetcher_test.go
@@ -159,11 +159,42 @@ func (f *fetcherTester) makeFetcher(blocks map[common.Hash]*types.Block) blockRe
 	}
 }
 
+// verifyImportEvent verifies that one single event arrive on an import channel.
+func verifyImportEvent(t *testing.T, imported chan *types.Block) {
+	select {
+	case <-imported:
+	case <-time.After(time.Second):
+		t.Fatalf("import timeout")
+	}
+}
+
+// verifyImportCount verifies that exactly count number of events arrive on an
+// import hook channel.
+func verifyImportCount(t *testing.T, imported chan *types.Block, count int) {
+	for i := 0; i < count; i++ {
+		select {
+		case <-imported:
+		case <-time.After(time.Second):
+			t.Fatalf("block %d: import timeout", i)
+		}
+	}
+	verifyImportDone(t, imported)
+}
+
+// verifyImportDone verifies that no more events are arriving on an import channel.
+func verifyImportDone(t *testing.T, imported chan *types.Block) {
+	select {
+	case <-imported:
+		t.Fatalf("extra block imported")
+	case <-time.After(50 * time.Millisecond):
+	}
+}
+
 // Tests that a fetcher accepts block announcements and initiates retrievals for
 // them, successfully importing into the local chain.
 func TestSequentialAnnouncements(t *testing.T) {
 	// Create a chain of blocks to import
-	targetBlocks := 24
+	targetBlocks := 4 * hashLimit
 	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
@@ -171,20 +202,21 @@ func TestSequentialAnnouncements(t *testing.T) {
 	fetcher := tester.makeFetcher(blocks)
 
 	// Iteratively announce blocks until all are imported
-	for i := len(hashes) - 1; i >= 0; i-- {
+	imported := make(chan *types.Block)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
+	for i := len(hashes) - 2; i >= 0; i-- {
 		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
-		time.Sleep(50 * time.Millisecond)
-	}
-	if imported := len(tester.blocks); imported != targetBlocks+1 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+		verifyImportEvent(t, imported)
 	}
+	verifyImportDone(t, imported)
 }
 
 // Tests that if blocks are announced by multiple peers (or even the same buggy
 // peer), they will only get downloaded at most once.
 func TestConcurrentAnnouncements(t *testing.T) {
 	// Create a chain of blocks to import
-	targetBlocks := 24
+	targetBlocks := 4 * hashLimit
 	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
@@ -198,16 +230,18 @@ func TestConcurrentAnnouncements(t *testing.T) {
 		return fetcher(hashes)
 	}
 	// Iteratively announce blocks until all are imported
-	for i := len(hashes) - 1; i >= 0; i-- {
+	imported := make(chan *types.Block)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
+	for i := len(hashes) - 2; i >= 0; i-- {
 		tester.fetcher.Notify("first", hashes[i], time.Now().Add(-arriveTimeout), wrapper)
 		tester.fetcher.Notify("second", hashes[i], time.Now().Add(-arriveTimeout+time.Millisecond), wrapper)
 		tester.fetcher.Notify("second", hashes[i], time.Now().Add(-arriveTimeout-time.Millisecond), wrapper)
 
-		time.Sleep(50 * time.Millisecond)
-	}
-	if imported := len(tester.blocks); imported != targetBlocks+1 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+		verifyImportEvent(t, imported)
 	}
+	verifyImportDone(t, imported)
+
 	// Make sure no blocks were retrieved twice
 	if int(counter) != targetBlocks {
 		t.Fatalf("retrieval count mismatch: have %v, want %v", counter, targetBlocks)
@@ -218,7 +252,7 @@ func TestConcurrentAnnouncements(t *testing.T) {
 // results in a valid import.
 func TestOverlappingAnnouncements(t *testing.T) {
 	// Create a chain of blocks to import
-	targetBlocks := 24
+	targetBlocks := 4 * hashLimit
 	hashes := createHashes(targetBlocks, knownHash)
 	blocks := createBlocksFromHashes(hashes)
 
@@ -226,16 +260,21 @@ func TestOverlappingAnnouncements(t *testing.T) {
 	fetcher := tester.makeFetcher(blocks)
 
 	// Iteratively announce blocks, but overlap them continuously
-	delay, overlap := 50*time.Millisecond, time.Duration(5)
-	for i := len(hashes) - 1; i >= 0; i-- {
-		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout+overlap*delay), fetcher)
-		time.Sleep(delay)
-	}
-	time.Sleep(overlap * delay)
+	fetching := make(chan []common.Hash)
+	imported := make(chan *types.Block, len(hashes)-1)
+	tester.fetcher.fetchingHook = func(hashes []common.Hash) { fetching <- hashes }
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
 
-	if imported := len(tester.blocks); imported != targetBlocks+1 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
+	for i := len(hashes) - 2; i >= 0; i-- {
+		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
+		select {
+		case <-fetching:
+		case <-time.After(time.Second):
+			t.Fatalf("hash %d: announce timeout", len(hashes)-i)
+		}
 	}
+	// Wait for all the imports to complete and check count
+	verifyImportCount(t, imported, len(hashes)-1)
 }
 
 // Tests that announces already being retrieved will not be duplicated.
@@ -280,56 +319,52 @@ func TestPendingDeduplication(t *testing.T) {
 // imported when all the gaps are filled in.
 func TestRandomArrivalImport(t *testing.T) {
 	// Create a chain of blocks to import, and choose one to delay
-	targetBlocks := 24
-	hashes := createHashes(targetBlocks, knownHash)
+	hashes := createHashes(maxQueueDist, knownHash)
 	blocks := createBlocksFromHashes(hashes)
-	skip := targetBlocks / 2
+	skip := maxQueueDist / 2
 
 	tester := newTester()
 	fetcher := tester.makeFetcher(blocks)
 
 	// Iteratively announce blocks, skipping one entry
+	imported := make(chan *types.Block, len(hashes)-1)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
 	for i := len(hashes) - 1; i >= 0; i-- {
 		if i != skip {
 			tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
-			time.Sleep(50 * time.Millisecond)
+			time.Sleep(time.Millisecond)
 		}
 	}
 	// Finally announce the skipped entry and check full import
 	tester.fetcher.Notify("valid", hashes[skip], time.Now().Add(-arriveTimeout), fetcher)
-	time.Sleep(50 * time.Millisecond)
-
-	if imported := len(tester.blocks); imported != targetBlocks+1 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
-	}
+	verifyImportCount(t, imported, len(hashes)-1)
 }
 
 // Tests that direct block enqueues (due to block propagation vs. hash announce)
 // are correctly schedule, filling and import queue gaps.
 func TestQueueGapFill(t *testing.T) {
 	// Create a chain of blocks to import, and choose one to not announce at all
-	targetBlocks := 24
-	hashes := createHashes(targetBlocks, knownHash)
+	hashes := createHashes(maxQueueDist, knownHash)
 	blocks := createBlocksFromHashes(hashes)
-	skip := targetBlocks / 2
+	skip := maxQueueDist / 2
 
 	tester := newTester()
 	fetcher := tester.makeFetcher(blocks)
 
 	// Iteratively announce blocks, skipping one entry
+	imported := make(chan *types.Block, len(hashes)-1)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
 	for i := len(hashes) - 1; i >= 0; i-- {
 		if i != skip {
 			tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), fetcher)
-			time.Sleep(50 * time.Millisecond)
+			time.Sleep(time.Millisecond)
 		}
 	}
 	// Fill the missing block directly as if propagated
 	tester.fetcher.Enqueue("valid", blocks[hashes[skip]])
-	time.Sleep(50 * time.Millisecond)
-
-	if imported := len(tester.blocks); imported != targetBlocks+1 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, targetBlocks+1)
-	}
+	verifyImportCount(t, imported, len(hashes)-1)
 }
 
 // Tests that blocks arriving from various sources (multiple propagations, hash
@@ -348,9 +383,15 @@ func TestImportDeduplication(t *testing.T) {
 		atomic.AddUint32(&counter, uint32(len(blocks)))
 		return tester.insertChain(blocks)
 	}
+	// Instrument the fetching and imported events
+	fetching := make(chan []common.Hash)
+	imported := make(chan *types.Block, len(hashes)-1)
+	tester.fetcher.fetchingHook = func(hashes []common.Hash) { fetching <- hashes }
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
 	// Announce the duplicating block, wait for retrieval, and also propagate directly
 	tester.fetcher.Notify("valid", hashes[0], time.Now().Add(-arriveTimeout), fetcher)
-	time.Sleep(50 * time.Millisecond)
+	<-fetching
 
 	tester.fetcher.Enqueue("valid", blocks[hashes[0]])
 	tester.fetcher.Enqueue("valid", blocks[hashes[0]])
@@ -358,11 +399,8 @@ func TestImportDeduplication(t *testing.T) {
 
 	// Fill the missing block directly as if propagated, and check import uniqueness
 	tester.fetcher.Enqueue("valid", blocks[hashes[1]])
-	time.Sleep(50 * time.Millisecond)
+	verifyImportCount(t, imported, 2)
 
-	if imported := len(tester.blocks); imported != 3 {
-		t.Fatalf("synchronised block mismatch: have %v, want %v", imported, 3)
-	}
 	if counter != 2 {
 		t.Fatalf("import invocation count mismatch: have %v, want %v", counter, 2)
 	}
@@ -395,3 +433,92 @@ func TestDistantDiscarding(t *testing.T) {
 		t.Fatalf("fetcher queued future block")
 	}
 }
+
+// Tests that a peer is unable to use unbounded memory with sending infinite
+// block announcements to a node, but that even in the face of such an attack,
+// the fetcher remains operational.
+func TestHashMemoryExhaustionAttack(t *testing.T) {
+	// Create a tester with instrumented import hooks
+	tester := newTester()
+
+	imported := make(chan *types.Block)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
+	// Create a valid chain and an infinite junk chain
+	hashes := createHashes(hashLimit+2*maxQueueDist, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+	valid := tester.makeFetcher(blocks)
+
+	attack := createHashes(hashLimit+2*maxQueueDist, unknownHash)
+	attacker := tester.makeFetcher(nil)
+
+	// Feed the tester a huge hashset from the attacker, and a limited from the valid peer
+	for i := 0; i < len(attack); i++ {
+		if i < maxQueueDist {
+			tester.fetcher.Notify("valid", hashes[len(hashes)-2-i], time.Now(), valid)
+		}
+		tester.fetcher.Notify("attacker", attack[i], time.Now(), attacker)
+	}
+	if len(tester.fetcher.announced) != hashLimit+maxQueueDist {
+		t.Fatalf("queued announce count mismatch: have %d, want %d", len(tester.fetcher.announced), hashLimit+maxQueueDist)
+	}
+	// Wait for fetches to complete
+	verifyImportCount(t, imported, maxQueueDist)
+
+	// Feed the remaining valid hashes to ensure DOS protection state remains clean
+	for i := len(hashes) - maxQueueDist - 2; i >= 0; i-- {
+		tester.fetcher.Notify("valid", hashes[i], time.Now().Add(-arriveTimeout), valid)
+		verifyImportEvent(t, imported)
+	}
+	verifyImportDone(t, imported)
+}
+
+// Tests that blocks sent to the fetcher (either through propagation or via hash
+// announces and retrievals) don't pile up indefinitely, exhausting available
+// system memory.
+func TestBlockMemoryExhaustionAttack(t *testing.T) {
+	// Create a tester with instrumented import hooks
+	tester := newTester()
+
+	imported := make(chan *types.Block)
+	tester.fetcher.importedHook = func(block *types.Block) { imported <- block }
+
+	// Create a valid chain and a batch of dangling (but in range) blocks
+	hashes := createHashes(blockLimit+2*maxQueueDist, knownHash)
+	blocks := createBlocksFromHashes(hashes)
+
+	attack := make(map[common.Hash]*types.Block)
+	for len(attack) < blockLimit+2*maxQueueDist {
+		hashes := createHashes(maxQueueDist-1, unknownHash)
+		blocks := createBlocksFromHashes(hashes)
+		for _, hash := range hashes[:maxQueueDist-2] {
+			attack[hash] = blocks[hash]
+		}
+	}
+	// Try to feed all the attacker blocks make sure only a limited batch is accepted
+	for _, block := range attack {
+		tester.fetcher.Enqueue("attacker", block)
+	}
+	time.Sleep(100 * time.Millisecond)
+	if queued := tester.fetcher.queue.Size(); queued != blockLimit {
+		t.Fatalf("queued block count mismatch: have %d, want %d", queued, blockLimit)
+	}
+	// Queue up a batch of valid blocks, and check that a new peer is allowed to do so
+	for i := 0; i < maxQueueDist-1; i++ {
+		tester.fetcher.Enqueue("valid", blocks[hashes[len(hashes)-3-i]])
+	}
+	time.Sleep(100 * time.Millisecond)
+	if queued := tester.fetcher.queue.Size(); queued != blockLimit+maxQueueDist-1 {
+		t.Fatalf("queued block count mismatch: have %d, want %d", queued, blockLimit+maxQueueDist-1)
+	}
+	// Insert the missing piece (and sanity check the import)
+	tester.fetcher.Enqueue("valid", blocks[hashes[len(hashes)-2]])
+	verifyImportCount(t, imported, maxQueueDist)
+
+	// Insert the remaining blocks in chunks to ensure clean DOS protection
+	for i := maxQueueDist; i < len(hashes)-1; i++ {
+		tester.fetcher.Enqueue("valid", blocks[hashes[len(hashes)-2-i]])
+		verifyImportEvent(t, imported)
+	}
+	verifyImportDone(t, imported)
+}