p2p/discover: add support for EIP-868 (v4 ENR extension) (#19540)

This change implements EIP-868. The UDPv4 transport announces support
for the extension in ping/pong and handles enrRequest messages.

There are two uses of the extension: If a remote node announces support
for EIP-868 in their pong, node revalidation pulls the node's record.
The Resolve method requests the record unconditionally.

6 files changed, 406 insertions, 136 deletions

diff --git a/p2p/discover/table.go b/p2p/discover/table.go
index 61c62f187..3460e8377 100644
--- a/p2p/discover/table.go
+++ b/p2p/discover/table.go
@@ -53,15 +53,17 @@ const (
 	bucketIPLimit, bucketSubnet = 2, 24 // at most 2 addresses from the same /24
 	tableIPLimit, tableSubnet   = 10, 24
 
-	maxFindnodeFailures = 5 // Nodes exceeding this limit are dropped
-	refreshInterval     = 30 * time.Minute
-	revalidateInterval  = 10 * time.Second
-	copyNodesInterval   = 30 * time.Second
-	seedMinTableTime    = 5 * time.Minute
-	seedCount           = 30
-	seedMaxAge          = 5 * 24 * time.Hour
+	refreshInterval    = 30 * time.Minute
+	revalidateInterval = 10 * time.Second
+	copyNodesInterval  = 30 * time.Second
+	seedMinTableTime   = 5 * time.Minute
+	seedCount          = 30
+	seedMaxAge         = 5 * 24 * time.Hour
 )
 
+// Table is the 'node table', a Kademlia-like index of neighbor nodes. The table keeps
+// itself up-to-date by verifying the liveness of neighbors and requesting their node
+// records when announcements of a new record version are received.
 type Table struct {
 	mutex   sync.Mutex        // protects buckets, bucket content, nursery, rand
 	buckets [nBuckets]*bucket // index of known nodes by distance
@@ -80,12 +82,13 @@ type Table struct {
 	nodeAddedHook func(*node) // for testing
 }
 
-// transport is implemented by UDP transports.
+// transport is implemented by the UDP transports.
 type transport interface {
 	Self() *enode.Node
 	lookupRandom() []*enode.Node
 	lookupSelf() []*enode.Node
-	ping(*enode.Node) error
+	ping(*enode.Node) (seq uint64, err error)
+	requestENR(*enode.Node) (*enode.Node, error)
 }
 
 // bucket contains nodes, ordered by their last activity. the entry
@@ -175,14 +178,16 @@ func (tab *Table) ReadRandomNodes(buf []*enode.Node) (n int) {
 	return i + 1
 }
 
-// Resolve searches for a specific node with the given ID.
-// It returns nil if the node could not be found.
-func (tab *Table) Resolve(n *enode.Node) *enode.Node {
+// getNode returns the node with the given ID or nil if it isn't in the table.
+func (tab *Table) getNode(id enode.ID) *enode.Node {
 	tab.mutex.Lock()
-	cl := tab.closest(n.ID(), 1, false)
-	tab.mutex.Unlock()
-	if len(cl.entries) > 0 && cl.entries[0].ID() == n.ID() {
-		return unwrapNode(cl.entries[0])
+	defer tab.mutex.Unlock()
+
+	b := tab.bucket(id)
+	for _, e := range b.entries {
+		if e.ID() == id {
+			return unwrapNode(e)
+		}
 	}
 	return nil
 }
@@ -226,7 +231,7 @@ func (tab *Table) refresh() <-chan struct{} {
 	return done
 }
 
-// loop schedules refresh, revalidate runs and coordinates shutdown.
+// loop schedules runs of doRefresh, doRevalidate and copyLiveNodes.
 func (tab *Table) loop() {
 	var (
 		revalidate     = time.NewTimer(tab.nextRevalidateTime())
@@ -288,9 +293,8 @@ loop:
 	close(tab.closed)
 }
 
-// doRefresh performs a lookup for a random target to keep buckets
-// full. seed nodes are inserted if the table is empty (initial
-// bootstrap or discarded faulty peers).
+// doRefresh performs a lookup for a random target to keep buckets full. seed nodes are
+// inserted if the table is empty (initial bootstrap or discarded faulty peers).
 func (tab *Table) doRefresh(done chan struct{}) {
 	defer close(done)
 
@@ -324,8 +328,8 @@ func (tab *Table) loadSeedNodes() {
 	}
 }
 
-// doRevalidate checks that the last node in a random bucket is still live
-// and replaces or deletes the node if it isn't.
+// doRevalidate checks that the last node in a random bucket is still live and replaces or
+// deletes the node if it isn't.
 func (tab *Table) doRevalidate(done chan<- struct{}) {
 	defer func() { done <- struct{}{} }()
 
@@ -336,7 +340,17 @@ func (tab *Table) doRevalidate(done chan<- struct{}) {
 	}
 
 	// Ping the selected node and wait for a pong.
-	err := tab.net.ping(unwrapNode(last))
+	remoteSeq, err := tab.net.ping(unwrapNode(last))
+
+	// Also fetch record if the node replied and returned a higher sequence number.
+	if last.Seq() < remoteSeq {
+		n, err := tab.net.requestENR(unwrapNode(last))
+		if err != nil {
+			tab.log.Debug("ENR request failed", "id", last.ID(), "addr", last.addr(), "err", err)
+		} else {
+			last = &node{Node: *n, addedAt: last.addedAt, livenessChecks: last.livenessChecks}
+		}
+	}
 
 	tab.mutex.Lock()
 	defer tab.mutex.Unlock()
diff --git a/p2p/discover/table_test.go b/p2p/discover/table_test.go
index 81763e7fe..895c284b2 100644
--- a/p2p/discover/table_test.go
+++ b/p2p/discover/table_test.go
@@ -368,6 +368,34 @@ func TestTable_addSeenNode(t *testing.T) {
 	checkIPLimitInvariant(t, tab)
 }
 
+// This test checks that ENR updates happen during revalidation. If a node in the table
+// announces a new sequence number, the new record should be pulled.
+func TestTable_revalidateSyncRecord(t *testing.T) {
+	transport := newPingRecorder()
+	tab, db := newTestTable(transport)
+	<-tab.initDone
+	defer db.Close()
+	defer tab.close()
+
+	// Insert a node.
+	var r enr.Record
+	r.Set(enr.IP(net.IP{127, 0, 0, 1}))
+	id := enode.ID{1}
+	n1 := wrapNode(enode.SignNull(&r, id))
+	tab.addSeenNode(n1)
+
+	// Update the node record.
+	r.Set(enr.WithEntry("foo", "bar"))
+	n2 := enode.SignNull(&r, id)
+	transport.updateRecord(n2)
+
+	tab.doRevalidate(make(chan struct{}, 1))
+	intable := tab.getNode(id)
+	if !reflect.DeepEqual(intable, n2) {
+		t.Fatalf("table contains old record with seq %d, want seq %d", intable.Seq(), n2.Seq())
+	}
+}
+
 // gen wraps quick.Value so it's easier to use.
 // it generates a random value of the given value's type.
 func gen(typ interface{}, rand *rand.Rand) interface{} {
diff --git a/p2p/discover/table_util_test.go b/p2p/discover/table_util_test.go
index 71cb1895b..3075c4340 100644
--- a/p2p/discover/table_util_test.go
+++ b/p2p/discover/table_util_test.go
@@ -98,6 +98,7 @@ func fillTable(tab *Table, nodes []*node) {
 type pingRecorder struct {
 	mu           sync.Mutex
 	dead, pinged map[enode.ID]bool
+	records      map[enode.ID]*enode.Node
 	n            *enode.Node
 }
 
@@ -107,38 +108,53 @@ func newPingRecorder() *pingRecorder {
 	n := enode.SignNull(&r, enode.ID{})
 
 	return &pingRecorder{
-		dead:   make(map[enode.ID]bool),
-		pinged: make(map[enode.ID]bool),
-		n:      n,
+		dead:    make(map[enode.ID]bool),
+		pinged:  make(map[enode.ID]bool),
+		records: make(map[enode.ID]*enode.Node),
+		n:       n,
 	}
 }
 
-func (t *pingRecorder) Self() *enode.Node {
-	return nullNode
+// setRecord updates a node record. Future calls to ping and
+// requestENR will return this record.
+func (t *pingRecorder) updateRecord(n *enode.Node) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	t.records[n.ID()] = n
 }
 
-func (t *pingRecorder) ping(n *enode.Node) error {
+// Stubs to satisfy the transport interface.
+func (t *pingRecorder) Self() *enode.Node           { return nullNode }
+func (t *pingRecorder) lookupSelf() []*enode.Node   { return nil }
+func (t *pingRecorder) lookupRandom() []*enode.Node { return nil }
+func (t *pingRecorder) close()                      {}
+
+// ping simulates a ping request.
+func (t *pingRecorder) ping(n *enode.Node) (seq uint64, err error) {
 	t.mu.Lock()
 	defer t.mu.Unlock()
 
 	t.pinged[n.ID()] = true
 	if t.dead[n.ID()] {
-		return errTimeout
-	} else {
-		return nil
+		return 0, errTimeout
 	}
+	if t.records[n.ID()] != nil {
+		seq = t.records[n.ID()].Seq()
+	}
+	return seq, nil
 }
 
-func (t *pingRecorder) lookupSelf() []*enode.Node {
-	return nil
-}
+// requestENR simulates an ENR request.
+func (t *pingRecorder) requestENR(n *enode.Node) (*enode.Node, error) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
 
-func (t *pingRecorder) lookupRandom() []*enode.Node {
-	return nil
+	if t.dead[n.ID()] || t.records[n.ID()] == nil {
+		return nil, errTimeout
+	}
+	return t.records[n.ID()], nil
 }
 
-func (t *pingRecorder) close() {}
-
 func hasDuplicates(slice []*node) bool {
 	seen := make(map[enode.ID]bool)
 	for i, e := range slice {
diff --git a/p2p/discover/v4_udp.go b/p2p/discover/v4_udp.go
index 3c68beac1..b0b0053a7 100644
--- a/p2p/discover/v4_udp.go
+++ b/p2p/discover/v4_udp.go
@@ -31,6 +31,7 @@ import (
 	"github.com/ethereum/go-ethereum/crypto"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/p2p/netutil"
 	"github.com/ethereum/go-ethereum/rlp"
 )
@@ -47,12 +48,12 @@ var (
 	errClosed           = errors.New("socket closed")
 )
 
-// Timeouts
 const (
 	respTimeout    = 500 * time.Millisecond
 	expiration     = 20 * time.Second
 	bondExpiration = 24 * time.Hour
 
+	maxFindnodeFailures = 5                // nodes exceeding this limit are dropped
 	ntpFailureThreshold = 32               // Continuous timeouts after which to check NTP
 	ntpWarningCooldown  = 10 * time.Minute // Minimum amount of time to pass before repeating NTP warning
 	driftThreshold      = 10 * time.Second // Allowed clock drift before warning user
@@ -69,6 +70,8 @@ const (
 	p_pongV4
 	p_findnodeV4
 	p_neighborsV4
+	p_enrRequestV4
+	p_enrResponseV4
 )
 
 // RPC request structures
@@ -112,6 +115,21 @@ type (
 		Rest []rlp.RawValue `rlp:"tail"`
 	}
 
+	// enrRequestV4 queries for the remote node's record.
+	enrRequestV4 struct {
+		Expiration uint64
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
+	// enrResponseV4 is the reply to enrRequestV4.
+	enrResponseV4 struct {
+		ReplyTok []byte // Hash of the enrRequest packet.
+		Record   enr.Record
+		// Ignore additional fields (for forward compatibility).
+		Rest []rlp.RawValue `rlp:"tail"`
+	}
+
 	rpcNode struct {
 		IP  net.IP // len 4 for IPv4 or 16 for IPv6
 		UDP uint16 // for discovery protocol
@@ -126,14 +144,15 @@ type (
 	}
 )
 
-// packet is implemented by all v4 protocol messages.
+// packetV4 is implemented by all v4 protocol messages.
 type packetV4 interface {
 	// preverify checks whether the packet is valid and should be handled at all.
 	preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error
 	// handle handles the packet.
 	handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []byte)
-	// name returns the name of the packet for logging purposes.
+	// packet name and type for logging purposes.
 	name() string
+	kind() byte
 }
 
 func makeEndpoint(addr *net.UDPAddr, tcpPort uint16) rpcEndpoint {
@@ -191,7 +210,7 @@ type UDPv4 struct {
 	closing         chan struct{}
 }
 
-// pending represents a pending reply.
+// replyMatcher represents a pending reply.
 //
 // Some implementations of the protocol wish to send more than one
 // reply packet to findnode. In general, any neighbors packet cannot
@@ -217,17 +236,20 @@ type replyMatcher struct {
 
 	// errc receives nil when the callback indicates completion or an
 	// error if no further reply is received within the timeout.
-	errc chan<- error
+	errc chan error
+
+	// reply contains the most recent reply. This field is safe for reading after errc has
+	// received a value.
+	reply packetV4
 }
 
 type replyMatchFunc func(interface{}) (matched bool, requestDone bool)
 
+// reply is a reply packet from a certain node.
 type reply struct {
-	from  enode.ID
-	ip    net.IP
-	ptype byte
-	data  packetV4
-
+	from enode.ID
+	ip   net.IP
+	data packetV4
 	// loop indicates whether there was
 	// a matching request by sending on this channel.
 	matched chan<- bool
@@ -377,7 +399,8 @@ func (t *UDPv4) lookupWorker(n *node, targetKey encPubkey, reply chan<- []*node)
 			t.tab.delete(n)
 		}
 	} else if fails > 0 {
-		t.db.UpdateFindFails(n.ID(), n.IP(), fails-1)
+		// Reset failure counter because it counts _consecutive_ failures.
+		t.db.UpdateFindFails(n.ID(), n.IP(), 0)
 	}
 
 	// Grab as many nodes as possible. Some of them might not be alive anymore, but we'll
@@ -388,23 +411,34 @@ func (t *UDPv4) lookupWorker(n *node, targetKey encPubkey, reply chan<- []*node)
 	reply <- r
 }
 
-// Resolve searches for a specific node with the given ID.
-// It returns nil if the node could not be found.
+// Resolve searches for a specific node with the given ID and tries to get the most recent
+// version of the node record for it. It returns n if the node could not be resolved.
 func (t *UDPv4) Resolve(n *enode.Node) *enode.Node {
-	// If the node is present in the local table, no
-	// network interaction is required.
-	if intab := t.tab.Resolve(n); intab != nil {
-		return intab
-	}
-	// Otherwise, do a network lookup.
-	hash := n.ID()
-	result := t.LookupPubkey(n.Pubkey())
-	for _, n := range result {
-		if n.ID() == hash {
-			return n
+	// Try asking directly. This works if the node is still responding on the endpoint we have.
+	if rn, err := t.requestENR(n); err == nil {
+		return rn
+	}
+	// Check table for the ID, we might have a newer version there.
+	if intable := t.tab.getNode(n.ID()); intable != nil && intable.Seq() > n.Seq() {
+		n = intable
+		if rn, err := t.requestENR(n); err == nil {
+			return rn
 		}
 	}
-	return nil
+	// Otherwise perform a network lookup.
+	var key *enode.Secp256k1
+	if n.Load(key) != nil {
+		return n // no secp256k1 key
+	}
+	result := t.LookupPubkey((*ecdsa.PublicKey)(key))
+	for _, rn := range result {
+		if rn.ID() == n.ID() {
+			if rn, err := t.requestENR(rn); err == nil {
+				return rn
+			}
+		}
+	}
+	return n
 }
 
 func (t *UDPv4) ourEndpoint() rpcEndpoint {
@@ -414,28 +448,27 @@ func (t *UDPv4) ourEndpoint() rpcEndpoint {
 }
 
 // ping sends a ping message to the given node and waits for a reply.
-func (t *UDPv4) ping(n *enode.Node) error {
-	return <-t.sendPing(n.ID(), &net.UDPAddr{IP: n.IP(), Port: n.UDP()}, nil)
+func (t *UDPv4) ping(n *enode.Node) (seq uint64, err error) {
+	rm := t.sendPing(n.ID(), &net.UDPAddr{IP: n.IP(), Port: n.UDP()}, nil)
+	if err = <-rm.errc; err == nil {
+		seq = seqFromTail(rm.reply.(*pongV4).Rest)
+	}
+	return seq, err
 }
 
 // sendPing sends a ping message to the given node and invokes the callback
 // when the reply arrives.
-func (t *UDPv4) sendPing(toid enode.ID, toaddr *net.UDPAddr, callback func()) <-chan error {
-	req := &pingV4{
-		Version:    4,
-		From:       t.ourEndpoint(),
-		To:         makeEndpoint(toaddr, 0), // TODO: maybe use known TCP port from DB
-		Expiration: uint64(time.Now().Add(expiration).Unix()),
-	}
-	packet, hash, err := t.encode(t.priv, p_pingV4, req)
+func (t *UDPv4) sendPing(toid enode.ID, toaddr *net.UDPAddr, callback func()) *replyMatcher {
+	req := t.makePing(toaddr)
+	packet, hash, err := t.encode(t.priv, req)
 	if err != nil {
 		errc := make(chan error, 1)
 		errc <- err
-		return errc
+		return &replyMatcher{errc: errc}
 	}
 	// Add a matcher for the reply to the pending reply queue. Pongs are matched if they
 	// reference the ping we're about to send.
-	errc := t.pending(toid, toaddr.IP, p_pongV4, func(p interface{}) (matched bool, requestDone bool) {
+	rm := t.pending(toid, toaddr.IP, p_pongV4, func(p interface{}) (matched bool, requestDone bool) {
 		matched = bytes.Equal(p.(*pongV4).ReplyTok, hash)
 		if matched && callback != nil {
 			callback()
@@ -445,25 +478,30 @@ func (t *UDPv4) sendPing(toid enode.ID, toaddr *net.UDPAddr, callback func()) <-
 	// Send the packet.
 	t.localNode.UDPContact(toaddr)
 	t.write(toaddr, toid, req.name(), packet)
-	return errc
+	return rm
+}
+
+func (t *UDPv4) makePing(toaddr *net.UDPAddr) *pingV4 {
+	seq, _ := rlp.EncodeToBytes(t.localNode.Node().Seq())
+	return &pingV4{
+		Version:    4,
+		From:       t.ourEndpoint(),
+		To:         makeEndpoint(toaddr, 0),
+		Expiration: uint64(time.Now().Add(expiration).Unix()),
+		Rest:       []rlp.RawValue{seq},
+	}
 }
 
 // findnode sends a findnode request to the given node and waits until
 // the node has sent up to k neighbors.
 func (t *UDPv4) findnode(toid enode.ID, toaddr *net.UDPAddr, target encPubkey) ([]*node, error) {
-	// If we haven't seen a ping from the destination node for a while, it won't remember
-	// our endpoint proof and reject findnode. Solicit a ping first.
-	if time.Since(t.db.LastPingReceived(toid, toaddr.IP)) > bondExpiration {
-		<-t.sendPing(toid, toaddr, nil)
-		// Wait for them to ping back and process our pong.
-		time.Sleep(respTimeout)
-	}
+	t.ensureBond(toid, toaddr)
 
 	// Add a matcher for 'neighbours' replies to the pending reply queue. The matcher is
 	// active until enough nodes have been received.
 	nodes := make([]*node, 0, bucketSize)
 	nreceived := 0
-	errc := t.pending(toid, toaddr.IP, p_neighborsV4, func(r interface{}) (matched bool, requestDone bool) {
+	rm := t.pending(toid, toaddr.IP, p_neighborsV4, func(r interface{}) (matched bool, requestDone bool) {
 		reply := r.(*neighborsV4)
 		for _, rn := range reply.Nodes {
 			nreceived++
@@ -476,16 +514,56 @@ func (t *UDPv4) findnode(toid enode.ID, toaddr *net.UDPAddr, target encPubkey) (
 		}
 		return true, nreceived >= bucketSize
 	})
-	t.send(toaddr, toid, p_findnodeV4, &findnodeV4{
+	t.send(toaddr, toid, &findnodeV4{
 		Target:     target,
 		Expiration: uint64(time.Now().Add(expiration).Unix()),
 	})
-	return nodes, <-errc
+	return nodes, <-rm.errc
+}
+
+// requestENR sends enrRequest to the given node and waits for a response.
+func (t *UDPv4) requestENR(n *enode.Node) (*enode.Node, error) {
+	addr := &net.UDPAddr{IP: n.IP(), Port: n.UDP()}
+	t.ensureBond(n.ID(), addr)
+
+	req := &enrRequestV4{
+		Expiration: uint64(time.Now().Add(expiration).Unix()),
+	}
+	packet, hash, err := t.encode(t.priv, req)
+	if err != nil {
+		return nil, err
+	}
+	// Add a matcher for the reply to the pending reply queue. Responses are matched if
+	// they reference the request we're about to send.
+	rm := t.pending(n.ID(), addr.IP, p_enrResponseV4, func(r interface{}) (matched bool, requestDone bool) {
+		matched = bytes.Equal(r.(*enrResponseV4).ReplyTok, hash)
+		return matched, matched
+	})
+	// Send the packet and wait for the reply.
+	t.write(addr, n.ID(), req.name(), packet)
+	if err := <-rm.errc; err != nil {
+		return nil, err
+	}
+	// Verify the response record.
+	respN, err := enode.New(enode.ValidSchemes, &rm.reply.(*enrResponseV4).Record)
+	if err != nil {
+		return nil, err
+	}
+	if respN.ID() != n.ID() {
+		return nil, fmt.Errorf("invalid ID in response record")
+	}
+	if respN.Seq() < n.Seq() {
+		return n, nil // response record is older
+	}
+	if err := netutil.CheckRelayIP(addr.IP, respN.IP()); err != nil {
+		return nil, fmt.Errorf("invalid IP in response record: %v", err)
+	}
+	return respN, nil
 }
 
 // pending adds a reply matcher to the pending reply queue.
 // see the documentation of type replyMatcher for a detailed explanation.
-func (t *UDPv4) pending(id enode.ID, ip net.IP, ptype byte, callback replyMatchFunc) <-chan error {
+func (t *UDPv4) pending(id enode.ID, ip net.IP, ptype byte, callback replyMatchFunc) *replyMatcher {
 	ch := make(chan error, 1)
 	p := &replyMatcher{from: id, ip: ip, ptype: ptype, callback: callback, errc: ch}
 	select {
@@ -494,15 +572,15 @@ func (t *UDPv4) pending(id enode.ID, ip net.IP, ptype byte, callback replyMatchF
 	case <-t.closing:
 		ch <- errClosed
 	}
-	return ch
+	return p
 }
 
 // handleReply dispatches a reply packet, invoking reply matchers. It returns
 // whether any matcher considered the packet acceptable.
-func (t *UDPv4) handleReply(from enode.ID, fromIP net.IP, ptype byte, req packetV4) bool {
+func (t *UDPv4) handleReply(from enode.ID, fromIP net.IP, req packetV4) bool {
 	matched := make(chan bool, 1)
 	select {
-	case t.gotreply <- reply{from, fromIP, ptype, req, matched}:
+	case t.gotreply <- reply{from, fromIP, req, matched}:
 		// loop will handle it
 		return <-matched
 	case <-t.closing:
@@ -565,11 +643,12 @@ func (t *UDPv4) loop() {
 			var matched bool // whether any replyMatcher considered the reply acceptable.
 			for el := plist.Front(); el != nil; el = el.Next() {
 				p := el.Value.(*replyMatcher)
-				if p.from == r.from && p.ptype == r.ptype && p.ip.Equal(r.ip) {
+				if p.from == r.from && p.ptype == r.data.kind() && p.ip.Equal(r.ip) {
 					ok, requestDone := p.callback(r.data)
 					matched = matched || ok
 					// Remove the matcher if callback indicates that all replies have been received.
 					if requestDone {
+						p.reply = r.data
 						p.errc <- nil
 						plist.Remove(el)
 					}
@@ -635,8 +714,8 @@ func init() {
 	}
 }
 
-func (t *UDPv4) send(toaddr *net.UDPAddr, toid enode.ID, ptype byte, req packetV4) ([]byte, error) {
-	packet, hash, err := t.encode(t.priv, ptype, req)
+func (t *UDPv4) send(toaddr *net.UDPAddr, toid enode.ID, req packetV4) ([]byte, error) {
+	packet, hash, err := t.encode(t.priv, req)
 	if err != nil {
 		return hash, err
 	}
@@ -649,18 +728,19 @@ func (t *UDPv4) write(toaddr *net.UDPAddr, toid enode.ID, what string, packet []
 	return err
 }
 
-func (t *UDPv4) encode(priv *ecdsa.PrivateKey, ptype byte, req interface{}) (packet, hash []byte, err error) {
+func (t *UDPv4) encode(priv *ecdsa.PrivateKey, req packetV4) (packet, hash []byte, err error) {
+	name := req.name()
 	b := new(bytes.Buffer)
 	b.Write(headSpace)
-	b.WriteByte(ptype)
+	b.WriteByte(req.kind())
 	if err := rlp.Encode(b, req); err != nil {
-		t.log.Error("Can't encode discv4 packet", "err", err)
+		t.log.Error(fmt.Sprintf("Can't encode %s packet", name), "err", err)
 		return nil, nil, err
 	}
 	packet = b.Bytes()
 	sig, err := crypto.Sign(crypto.Keccak256(packet[headSize:]), priv)
 	if err != nil {
-		t.log.Error("Can't sign discv4 packet", "err", err)
+		t.log.Error(fmt.Sprintf("Can't sign %s packet", name), "err", err)
 		return nil, nil, err
 	}
 	copy(packet[macSize:], sig)
@@ -743,6 +823,10 @@ func decodeV4(buf []byte) (packetV4, encPubkey, []byte, error) {
 		req = new(findnodeV4)
 	case p_neighborsV4:
 		req = new(neighborsV4)
+	case p_enrRequestV4:
+		req = new(enrRequestV4)
+	case p_enrResponseV4:
+		req = new(enrResponseV4)
 	default:
 		return nil, fromKey, hash, fmt.Errorf("unknown type: %d", ptype)
 	}
@@ -751,7 +835,41 @@ func decodeV4(buf []byte) (packetV4, encPubkey, []byte, error) {
 	return req, fromKey, hash, err
 }
 
-// Packet Handlers
+// checkBond checks if the given node has a recent enough endpoint proof.
+func (t *UDPv4) checkBond(id enode.ID, ip net.IP) bool {
+	return time.Since(t.db.LastPongReceived(id, ip)) < bondExpiration
+}
+
+// ensureBond solicits a ping from a node if we haven't seen a ping from it for a while.
+// This ensures there is a valid endpoint proof on the remote end.
+func (t *UDPv4) ensureBond(toid enode.ID, toaddr *net.UDPAddr) {
+	tooOld := time.Since(t.db.LastPingReceived(toid, toaddr.IP)) > bondExpiration
+	if tooOld || t.db.FindFails(toid, toaddr.IP) > maxFindnodeFailures {
+		rm := t.sendPing(toid, toaddr, nil)
+		<-rm.errc
+		// Wait for them to ping back and process our pong.
+		time.Sleep(respTimeout)
+	}
+}
+
+// expired checks whether the given UNIX time stamp is in the past.
+func expired(ts uint64) bool {
+	return time.Unix(int64(ts), 0).Before(time.Now())
+}
+
+func seqFromTail(tail []rlp.RawValue) uint64 {
+	if len(tail) == 0 {
+		return 0
+	}
+	var seq uint64
+	rlp.DecodeBytes(tail[0], &seq)
+	return seq
+}
+
+// PING/v4
+
+func (req *pingV4) name() string { return "PING/v4" }
+func (req *pingV4) kind() byte   { return p_pingV4 }
 
 func (req *pingV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
 	if expired(req.Expiration) {
@@ -767,10 +885,12 @@ func (req *pingV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromK
 
 func (req *pingV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []byte) {
 	// Reply.
-	t.send(from, fromID, p_pongV4, &pongV4{
+	seq, _ := rlp.EncodeToBytes(t.localNode.Node().Seq())
+	t.send(from, fromID, &pongV4{
 		To:         makeEndpoint(from, req.From.TCP),
 		ReplyTok:   mac,
 		Expiration: uint64(time.Now().Add(expiration).Unix()),
+		Rest:       []rlp.RawValue{seq},
 	})
 
 	// Ping back if our last pong on file is too far in the past.
@@ -788,13 +908,16 @@ func (req *pingV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []by
 	t.localNode.UDPEndpointStatement(from, &net.UDPAddr{IP: req.To.IP, Port: int(req.To.UDP)})
 }
 
-func (req *pingV4) name() string { return "PING/v4" }
+// PONG/v4
+
+func (req *pongV4) name() string { return "PONG/v4" }
+func (req *pongV4) kind() byte   { return p_pongV4 }
 
 func (req *pongV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
 	if expired(req.Expiration) {
 		return errExpired
 	}
-	if !t.handleReply(fromID, from.IP, p_pongV4, req) {
+	if !t.handleReply(fromID, from.IP, req) {
 		return errUnsolicitedReply
 	}
 	return nil
@@ -805,13 +928,16 @@ func (req *pongV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []by
 	t.db.UpdateLastPongReceived(fromID, from.IP, time.Now())
 }
 
-func (req *pongV4) name() string { return "PONG/v4" }
+// FINDNODE/v4
+
+func (req *findnodeV4) name() string { return "FINDNODE/v4" }
+func (req *findnodeV4) kind() byte   { return p_findnodeV4 }
 
 func (req *findnodeV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
 	if expired(req.Expiration) {
 		return errExpired
 	}
-	if time.Since(t.db.LastPongReceived(fromID, from.IP)) > bondExpiration {
+	if !t.checkBond(fromID, from.IP) {
 		// No endpoint proof pong exists, we don't process the packet. This prevents an
 		// attack vector where the discovery protocol could be used to amplify traffic in a
 		// DDOS attack. A malicious actor would send a findnode request with the IP address
@@ -839,23 +965,26 @@ func (req *findnodeV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac
 			p.Nodes = append(p.Nodes, nodeToRPC(n))
 		}
 		if len(p.Nodes) == maxNeighbors {
-			t.send(from, fromID, p_neighborsV4, &p)
+			t.send(from, fromID, &p)
 			p.Nodes = p.Nodes[:0]
 			sent = true
 		}
 	}
 	if len(p.Nodes) > 0 || !sent {
-		t.send(from, fromID, p_neighborsV4, &p)
+		t.send(from, fromID, &p)
 	}
 }
 
-func (req *findnodeV4) name() string { return "FINDNODE/v4" }
+// NEIGHBORS/v4
+
+func (req *neighborsV4) name() string { return "NEIGHBORS/v4" }
+func (req *neighborsV4) kind() byte   { return p_neighborsV4 }
 
 func (req *neighborsV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
 	if expired(req.Expiration) {
 		return errExpired
 	}
-	if !t.handleReply(fromID, from.IP, p_neighborsV4, req) {
+	if !t.handleReply(fromID, from.IP, req) {
 		return errUnsolicitedReply
 	}
 	return nil
@@ -864,8 +993,39 @@ func (req *neighborsV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID,
 func (req *neighborsV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []byte) {
 }
 
-func (req *neighborsV4) name() string { return "NEIGHBORS/v4" }
+// ENRREQUEST/v4
 
-func expired(ts uint64) bool {
-	return time.Unix(int64(ts), 0).Before(time.Now())
+func (req *enrRequestV4) name() string { return "ENRREQUEST/v4" }
+func (req *enrRequestV4) kind() byte   { return p_enrRequestV4 }
+
+func (req *enrRequestV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
+	if expired(req.Expiration) {
+		return errExpired
+	}
+	if !t.checkBond(fromID, from.IP) {
+		return errUnknownNode
+	}
+	return nil
+}
+
+func (req *enrRequestV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []byte) {
+	t.send(from, fromID, &enrResponseV4{
+		ReplyTok: mac,
+		Record:   *t.localNode.Node().Record(),
+	})
+}
+
+// ENRRESPONSE/v4
+
+func (req *enrResponseV4) name() string { return "ENRRESPONSE/v4" }
+func (req *enrResponseV4) kind() byte   { return p_enrResponseV4 }
+
+func (req *enrResponseV4) preverify(t *UDPv4, from *net.UDPAddr, fromID enode.ID, fromKey encPubkey) error {
+	if !t.handleReply(fromID, from.IP, req) {
+		return errUnsolicitedReply
+	}
+	return nil
+}
+
+func (req *enrResponseV4) handle(t *UDPv4, from *net.UDPAddr, fromID enode.ID, mac []byte) {
 }
diff --git a/p2p/discover/v4_udp_lookup_test.go b/p2p/discover/v4_udp_lookup_test.go
index 7e12aa498..bc1cdfb08 100644
--- a/p2p/discover/v4_udp_lookup_test.go
+++ b/p2p/discover/v4_udp_lookup_test.go
@@ -54,11 +54,11 @@ func TestUDPv4_Lookup(t *testing.T) {
 			n, key := lookupTestnet.nodeByAddr(to)
 			switch p.(type) {
 			case *pingV4:
-				test.packetInFrom(nil, key, to, p_pongV4, &pongV4{Expiration: futureExp, ReplyTok: hash})
+				test.packetInFrom(nil, key, to, &pongV4{Expiration: futureExp, ReplyTok: hash})
 			case *findnodeV4:
 				dist := enode.LogDist(n.ID(), lookupTestnet.target.id())
 				nodes := lookupTestnet.nodesAtDistance(dist - 1)
-				test.packetInFrom(nil, key, to, p_neighborsV4, &neighborsV4{Expiration: futureExp, Nodes: nodes})
+				test.packetInFrom(nil, key, to, &neighborsV4{Expiration: futureExp, Nodes: nodes})
 			}
 		})
 	}
diff --git a/p2p/discover/v4_udp_test.go b/p2p/discover/v4_udp_test.go
index 0aa4c01e5..9d7badea1 100644
--- a/p2p/discover/v4_udp_test.go
+++ b/p2p/discover/v4_udp_test.go
@@ -37,6 +37,7 @@ import (
 	"github.com/ethereum/go-ethereum/internal/testlog"
 	"github.com/ethereum/go-ethereum/log"
 	"github.com/ethereum/go-ethereum/p2p/enode"
+	"github.com/ethereum/go-ethereum/p2p/enr"
 	"github.com/ethereum/go-ethereum/rlp"
 )
 
@@ -91,19 +92,19 @@ func (test *udpTest) close() {
 }
 
 // handles a packet as if it had been sent to the transport.
-func (test *udpTest) packetIn(wantError error, ptype byte, data packetV4) {
+func (test *udpTest) packetIn(wantError error, data packetV4) {
 	test.t.Helper()
 
-	test.packetInFrom(wantError, test.remotekey, test.remoteaddr, ptype, data)
+	test.packetInFrom(wantError, test.remotekey, test.remoteaddr, data)
 }
 
 // handles a packet as if it had been sent to the transport by the key/endpoint.
-func (test *udpTest) packetInFrom(wantError error, key *ecdsa.PrivateKey, addr *net.UDPAddr, ptype byte, data packetV4) {
+func (test *udpTest) packetInFrom(wantError error, key *ecdsa.PrivateKey, addr *net.UDPAddr, data packetV4) {
 	test.t.Helper()
 
-	enc, _, err := test.udp.encode(key, ptype, data)
+	enc, _, err := test.udp.encode(key, data)
 	if err != nil {
-		test.t.Errorf("packet (%d) encode error: %v", ptype, err)
+		test.t.Errorf("%s encode error: %v", data.name(), err)
 	}
 	test.sent = append(test.sent, enc)
 	if err = test.udp.handlePacket(addr, enc); err != wantError {
@@ -139,10 +140,10 @@ func TestUDPv4_packetErrors(t *testing.T) {
 	test := newUDPTest(t)
 	defer test.close()
 
-	test.packetIn(errExpired, p_pingV4, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4})
-	test.packetIn(errUnsolicitedReply, p_pongV4, &pongV4{ReplyTok: []byte{}, Expiration: futureExp})
-	test.packetIn(errUnknownNode, p_findnodeV4, &findnodeV4{Expiration: futureExp})
-	test.packetIn(errUnsolicitedReply, p_neighborsV4, &neighborsV4{Expiration: futureExp})
+	test.packetIn(errExpired, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4})
+	test.packetIn(errUnsolicitedReply, &pongV4{ReplyTok: []byte{}, Expiration: futureExp})
+	test.packetIn(errUnknownNode, &findnodeV4{Expiration: futureExp})
+	test.packetIn(errUnsolicitedReply, &neighborsV4{Expiration: futureExp})
 }
 
 func TestUDPv4_pingTimeout(t *testing.T) {
@@ -153,11 +154,21 @@ func TestUDPv4_pingTimeout(t *testing.T) {
 	key := newkey()
 	toaddr := &net.UDPAddr{IP: net.ParseIP("1.2.3.4"), Port: 2222}
 	node := enode.NewV4(&key.PublicKey, toaddr.IP, 0, toaddr.Port)
-	if err := test.udp.ping(node); err != errTimeout {
+	if _, err := test.udp.ping(node); err != errTimeout {
 		t.Error("expected timeout error, got", err)
 	}
 }
 
+type testPacket byte
+
+func (req testPacket) kind() byte   { return byte(req) }
+func (req testPacket) name() string { return "" }
+func (req testPacket) preverify(*UDPv4, *net.UDPAddr, enode.ID, encPubkey) error {
+	return nil
+}
+func (req testPacket) handle(*UDPv4, *net.UDPAddr, enode.ID, []byte) {
+}
+
 func TestUDPv4_responseTimeouts(t *testing.T) {
 	t.Parallel()
 	test := newUDPTest(t)
@@ -192,7 +203,7 @@ func TestUDPv4_responseTimeouts(t *testing.T) {
 			p.errc = nilErr
 			test.udp.addReplyMatcher <- p
 			time.AfterFunc(randomDuration(60*time.Millisecond), func() {
-				if !test.udp.handleReply(p.from, p.ip, p.ptype, nil) {
+				if !test.udp.handleReply(p.from, p.ip, testPacket(p.ptype)) {
 					t.Logf("not matched: %v", p)
 				}
 			})
@@ -277,7 +288,7 @@ func TestUDPv4_findnode(t *testing.T) {
 
 	// check that closest neighbors are returned.
 	expected := test.table.closest(testTarget.id(), bucketSize, true)
-	test.packetIn(nil, p_findnodeV4, &findnodeV4{Target: testTarget, Expiration: futureExp})
+	test.packetIn(nil, &findnodeV4{Target: testTarget, Expiration: futureExp})
 	waitNeighbors := func(want []*node) {
 		test.waitPacketOut(func(p *neighborsV4, to *net.UDPAddr, hash []byte) {
 			if len(p.Nodes) != len(want) {
@@ -340,8 +351,8 @@ func TestUDPv4_findnodeMultiReply(t *testing.T) {
 	for i := range list {
 		rpclist[i] = nodeToRPC(list[i])
 	}
-	test.packetIn(nil, p_neighborsV4, &neighborsV4{Expiration: futureExp, Nodes: rpclist[:2]})
-	test.packetIn(nil, p_neighborsV4, &neighborsV4{Expiration: futureExp, Nodes: rpclist[2:]})
+	test.packetIn(nil, &neighborsV4{Expiration: futureExp, Nodes: rpclist[:2]})
+	test.packetIn(nil, &neighborsV4{Expiration: futureExp, Nodes: rpclist[2:]})
 
 	// check that the sent neighbors are all returned by findnode
 	select {
@@ -357,6 +368,7 @@ func TestUDPv4_findnodeMultiReply(t *testing.T) {
 	}
 }
 
+// This test checks that reply matching of pong verifies the ping hash.
 func TestUDPv4_pingMatch(t *testing.T) {
 	test := newUDPTest(t)
 	defer test.close()
@@ -364,22 +376,23 @@ func TestUDPv4_pingMatch(t *testing.T) {
 	randToken := make([]byte, 32)
 	crand.Read(randToken)
 
-	test.packetIn(nil, p_pingV4, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
+	test.packetIn(nil, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
 	test.waitPacketOut(func(*pongV4, *net.UDPAddr, []byte) {})
 	test.waitPacketOut(func(*pingV4, *net.UDPAddr, []byte) {})
-	test.packetIn(errUnsolicitedReply, p_pongV4, &pongV4{ReplyTok: randToken, To: testLocalAnnounced, Expiration: futureExp})
+	test.packetIn(errUnsolicitedReply, &pongV4{ReplyTok: randToken, To: testLocalAnnounced, Expiration: futureExp})
 }
 
+// This test checks that reply matching of pong verifies the sender IP address.
 func TestUDPv4_pingMatchIP(t *testing.T) {
 	test := newUDPTest(t)
 	defer test.close()
 
-	test.packetIn(nil, p_pingV4, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
+	test.packetIn(nil, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
 	test.waitPacketOut(func(*pongV4, *net.UDPAddr, []byte) {})
 
 	test.waitPacketOut(func(p *pingV4, to *net.UDPAddr, hash []byte) {
 		wrongAddr := &net.UDPAddr{IP: net.IP{33, 44, 1, 2}, Port: 30000}
-		test.packetInFrom(errUnsolicitedReply, test.remotekey, wrongAddr, p_pongV4, &pongV4{
+		test.packetInFrom(errUnsolicitedReply, test.remotekey, wrongAddr, &pongV4{
 			ReplyTok:   hash,
 			To:         testLocalAnnounced,
 			Expiration: futureExp,
@@ -394,9 +407,9 @@ func TestUDPv4_successfulPing(t *testing.T) {
 	defer test.close()
 
 	// The remote side sends a ping packet to initiate the exchange.
-	go test.packetIn(nil, p_pingV4, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
+	go test.packetIn(nil, &pingV4{From: testRemote, To: testLocalAnnounced, Version: 4, Expiration: futureExp})
 
-	// the ping is replied to.
+	// The ping is replied to.
 	test.waitPacketOut(func(p *pongV4, to *net.UDPAddr, hash []byte) {
 		pinghash := test.sent[0][:macSize]
 		if !bytes.Equal(p.ReplyTok, pinghash) {
@@ -413,7 +426,7 @@ func TestUDPv4_successfulPing(t *testing.T) {
 		}
 	})
 
-	// remote is unknown, the table pings back.
+	// Remote is unknown, the table pings back.
 	test.waitPacketOut(func(p *pingV4, to *net.UDPAddr, hash []byte) {
 		if !reflect.DeepEqual(p.From, test.udp.ourEndpoint()) {
 			t.Errorf("got ping.From %#v, want %#v", p.From, test.udp.ourEndpoint())
@@ -427,10 +440,10 @@ func TestUDPv4_successfulPing(t *testing.T) {
 		if !reflect.DeepEqual(p.To, wantTo) {
 			t.Errorf("got ping.To %v, want %v", p.To, wantTo)
 		}
-		test.packetIn(nil, p_pongV4, &pongV4{ReplyTok: hash, Expiration: futureExp})
+		test.packetIn(nil, &pongV4{ReplyTok: hash, Expiration: futureExp})
 	})
 
-	// the node should be added to the table shortly after getting the
+	// The node should be added to the table shortly after getting the
 	// pong packet.
 	select {
 	case n := <-added:
@@ -452,6 +465,45 @@ func TestUDPv4_successfulPing(t *testing.T) {
 	}
 }
 
+// This test checks that EIP-868 requests work.
+func TestUDPv4_EIP868(t *testing.T) {
+	test := newUDPTest(t)
+	defer test.close()
+
+	test.udp.localNode.Set(enr.WithEntry("foo", "bar"))
+	wantNode := test.udp.localNode.Node()
+
+	// ENR requests aren't allowed before endpoint proof.
+	test.packetIn(errUnknownNode, &enrRequestV4{Expiration: futureExp})
+
+	// Perform endpoint proof and check for sequence number in packet tail.
+	test.packetIn(nil, &pingV4{Expiration: futureExp})
+	test.waitPacketOut(func(p *pongV4, addr *net.UDPAddr, hash []byte) {
+		if seq := seqFromTail(p.Rest); seq != wantNode.Seq() {
+			t.Errorf("wrong sequence number in pong: %d, want %d", seq, wantNode.Seq())
+		}
+	})
+	test.waitPacketOut(func(p *pingV4, addr *net.UDPAddr, hash []byte) {
+		if seq := seqFromTail(p.Rest); seq != wantNode.Seq() {
+			t.Errorf("wrong sequence number in ping: %d, want %d", seq, wantNode.Seq())
+		}
+		test.packetIn(nil, &pongV4{Expiration: futureExp, ReplyTok: hash})
+	})
+
+	// Request should work now.
+	test.packetIn(nil, &enrRequestV4{Expiration: futureExp})
+	test.waitPacketOut(func(p *enrResponseV4, addr *net.UDPAddr, hash []byte) {
+		n, err := enode.New(enode.ValidSchemes, &p.Record)
+		if err != nil {
+			t.Fatalf("invalid record: %v", err)
+		}
+		if !reflect.DeepEqual(n, wantNode) {
+			t.Fatalf("wrong node in enrResponse: %v", n)
+		}
+	})
+}
+
+// EIP-8 test vectors.
 var testPackets = []struct {
 	input      string
 	wantPacket interface{}