// Copyright 2015 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
// Package discover implements the Node Discovery Protocol.
//
// The Node Discovery protocol provides a way to find RLPx nodes that
// can be connected to. It uses a Kademlia-like protocol to maintain a
// distributed database of the IDs and endpoints of all listening
// nodes.
package discover
import (
"crypto/rand"
"encoding/binary"
"errors"
"fmt"
"net"
"sort"
"sync"
"time"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/crypto"
"github.com/ethereum/go-ethereum/log"
)
const (
alpha = 3 // Kademlia concurrency factor
bucketSize = 16 // Kademlia bucket size
hashBits = len(common.Hash{}) * 8
nBuckets = hashBits + 1 // Number of buckets
maxBondingPingPongs = 16
maxFindnodeFailures = 5
autoRefreshInterval = 1 * time.Hour
seedCount = 30
seedMaxAge = 5 * 24 * time.Hour
)
type Table struct {
mutex sync.Mutex // protects buckets, their content, and nursery
buckets [nBuckets]*bucket // index of known nodes by distance
nursery []*Node // bootstrap nodes
db *nodeDB // database of known nodes
refreshReq chan chan struct{}
closeReq chan struct{}
closed chan struct{}
bondmu sync.Mutex
bonding map[NodeID]*bondproc
bondslots chan struct{} // limits total number of active bonding processes
nodeAddedHook func(*Node) // for testing
net transport
self *Node // metadata of the local node
}
type bondproc struct {
err error
n *Node
done chan struct{}
}
// transport is implemented by the UDP transport.
// it is an interface so we can test without opening lots of UDP
// sockets and without generating a private key.
type transport interface {
ping(NodeID, *net.UDPAddr) error
waitping(NodeID) error
findnode(toid NodeID, addr *net.UDPAddr, target NodeID) ([]*Node, error)
close()
}
// bucket contains nodes, ordered by their last activity. the entry
// that was most recently active is the first element in entries.
type bucket struct{ entries []*Node }
func newTable(t transport, ourID NodeID, ourAddr *net.UDPAddr, nodeDBPath string) (*Table, error) {
// If no node database was given, use an in-memory one
db, err := newNodeDB(nodeDBPath, Version, ourID)
if err != nil {
return nil, err
}
tab := &Table{
net: t,
db: db,
self: NewNode(ourID, ourAddr.IP, uint16(ourAddr.Port), uint16(ourAddr.Port)),
bonding: make(map[NodeID]*bondproc),
bondslots: make(chan struct{}, maxBondingPingPongs),
refreshReq: make(chan chan struct{}),
closeReq: make(chan struct{}),
closed: make(chan struct{}),
}
for i := 0; i < cap(tab.bondslots); i++ {
tab.bondslots <- struct{}{}
}
for i := range tab.buckets {
tab.buckets[i] = new(bucket)
}
go tab.refreshLoop()
return tab, nil
}
// Self returns the local node.
// The returned node should not be modified by the caller.
func (tab *Table) Self() *Node {
return tab.self
}
// ReadRandomNodes fills the given slice with random nodes from the
// table. It will not write the same node more than once. The nodes in
// the slice are copies and can be modified by the caller.
func (tab *Table) ReadRandomNodes(buf []*Node) (n int) {
tab.mutex.Lock()
defer tab.mutex.Unlock()
// TODO: tree-based buckets would help here
// Find all non-empty buckets and get a fresh slice of their entries.
var buckets [][]*Node
for _, b := range tab.buckets {
if len(b.entries) > 0 {
buckets = append(buckets, b.entries[:])
}
}
if len(buckets) == 0 {
return 0
}
// Shuffle the buckets.
for i := uint32(len(buckets)) - 1; i > 0; i-- {
j := randUint(i)
buckets[i], buckets[j] = buckets[j], buckets[i]
}
// Move head of each bucket into buf, removing buckets that become empty.
var i, j int
for ; i < len(buf); i, j = i+1, (j+1)%len(buckets) {
b := buckets[j]
buf[i] = &(*b[0])
buckets[j] = b[1:]
if len(b) == 1 {
buckets = append(buckets[:j], buckets[j+1:]...)
}
if len(buckets) == 0 {
break
}
}
return i + 1
}
func randUint(max uint32) uint32 {
if max == 0 {
return 0
}
var b [4]byte
rand.Read(b[:])
return binary.BigEndian.Uint32(b[:]) % max
}
// Close terminates the network listener and flushes the node database.
func (tab *Table) Close() {
select {
case <-tab.closed:
// already closed.
case tab.closeReq <- struct{}{}:
<-tab.closed // wait for refreshLoop to end.
}
}
// SetFallbackNodes sets the initial points of contact. These nodes
// are used to connect to the network if the table is empty and there
// are no known nodes in the database.
func (tab *Table) SetFallbackNodes(nodes []*Node) error {
for _, n := range nodes {
if err := n.validateComplete(); err != nil {
return fmt.Errorf("bad bootstrap/fallback node %q (%v)", n, err)
}
}
tab.mutex.Lock()
tab.nursery = make([]*Node, 0, len(nodes))
for _, n := range nodes {
cpy := *n
// Recompute cpy.sha because the node might not have been
// created by NewNode or ParseNode.
cpy.sha = crypto.Keccak256Hash(n.ID[:])
tab.nursery = append(tab.nursery, &cpy)
}
tab.mutex.Unlock()
tab.refresh()
return nil
}
// Resolve searches for a specific node with the given ID.
// It returns nil if the node could not be found.
func (tab *Table) Resolve(targetID NodeID) *Node {
// If the node is present in the local table, no
// network interaction is required.
hash := crypto.Keccak256Hash(targetID[:])
tab.mutex.Lock()
cl := tab.closest(hash, 1)
tab.mutex.Unlock()
if len(cl.entries) > 0 && cl.entries[0].ID == targetID {
return cl.entries[0]
}
// Otherwise, do a network lookup.
result := tab.Lookup(targetID)
for _, n := range result {
if n.ID == targetID {
return n
}
}
return nil
}
// Lookup performs a network search for nodes close
// to the given target. It approaches the target by querying
// nodes that are closer to it on each iteration.
// The given target does not need to be an actual node
// identifier.
func (tab *Table) Lookup(targetID NodeID) []*Node {
return tab.lookup(targetID, true)
}
func (tab *Table) lookup(targetID NodeID, refreshIfEmpty bool) []*Node {
var (
target = crypto.Keccak256Hash(targetID[:])
asked = make(map[NodeID]bool)
seen = make(map[NodeID]bool)
reply = make(chan []*Node, alpha)
pendingQueries = 0
result *nodesByDistance
)
// don't query further if we hit ourself.
// unlikely to happen often in practice.
asked[tab.self.ID] = true
for {
tab.mutex.Lock()
// generate initial result set
result = tab.closest(target, bucketSize)
tab.mutex.Unlock()
if len(result.entries) > 0 || !refreshIfEmpty {
break
}
// The result set is empty, all nodes were dropped, refresh.
// We actually wait for the refresh to complete here. The very
// first query will hit this case and run the bootstrapping
// logic.
<-tab.refresh()
refreshIfEmpty = false
}
for {
// ask the alpha closest nodes that we haven't asked yet
for i := 0; i < len(result.entries) && pendingQueries < alpha; i++ {
n := result.entries[i]
if !asked[n.ID] {
asked[n.ID] = true
pendingQueries++
go func() {
// Find potential neighbors to bond with
r, err := tab.net.findnode(n.ID, n.addr(), targetID)
if err != nil {
// Bump the failure counter to detect and evacuate non-bonded entries
fails := tab.db.findFails(n.ID) + 1
tab.db.updateFindFails(n.ID, fails)
log.Trace("Bumping findnode failure counter", "id", n.ID, "failcount", fails)
if fails >= maxFindnodeFailures {
log.Trace("Too many findnode failures, dropping", "id", n.ID, "failcount", fails)
tab.delete(n)
}
}
reply <- tab.bondall(r)
}()
}
}
if pendingQueries == 0 {
// we have asked all closest nodes, stop the search
break
}
// wait for the next reply
for _, n := range <-reply {
if n != nil && !seen[n.ID] {
seen[n.ID] = true
result.push(n, bucketSize)
}
}
pendingQueries--
}
return result.entries
}
func (tab *Table) refresh() <-chan struct{} {
done := make(chan struct{})
select {
case tab.refreshReq <- done:
case <-tab.closed:
close(done)
}
return done
}
// refreshLoop schedules doRefresh runs and coordinates shutdown.
func (tab *Table) refreshLoop() {
var (
timer = time.NewTicker(autoRefreshInterval)
waiting []chan struct{} // accumulates waiting callers while doRefresh runs
done chan struct{} // where doRefresh reports completion
)
loop:
for {
select {
case <-timer.C:
if done == nil {
done = make(chan struct{})
go tab.doRefresh(done)
}
case req := <-tab.refreshReq:
waiting = append(waiting, req)
if done == nil {
done = make(chan struct{})
go tab.doRefresh(done)
}
case <-done:
for _, ch := range waiting {
close(ch)
}
waiting = nil
done = nil
case <-tab.closeReq:
break loop
}
}
if tab.net != nil {
tab.net.close()
}
if done != nil {
<-done
}
for _, ch := range waiting {
close(ch)
}
tab.db.close()
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).
func (tab *Table) doRefresh(done chan struct{}) {
defer close(done)
// The Kademlia paper specifies that the bucket refresh should
// perform a lookup in the least recently used bucket. We cannot
// adhere to this because the findnode target is a 512bit value
// (not hash-sized) and it is not easily possible to generate a
// sha3 preimage that falls into a chosen bucket.
// We perform a lookup with a random target instead.
var target NodeID
rand.Read(target[:])
result := tab.lookup(target, false)
if len(result) > 0 {
return
}
// The table is empty. Load nodes from the database and insert
// them. This should yield a few previously seen nodes that are
// (hopefully) still alive.
seeds := tab.db.querySeeds(seedCount, seedMaxAge)
seeds = tab.bondall(append(seeds, tab.nursery...))
if len(seeds) == 0 {
log.Debug("No discv4 seed nodes found")
}
for _, n := range seeds {
age := log.Lazy{Fn: func() time.Duration { return time.Since(tab.db.lastPong(n.ID)) }}
log.Trace("Found seed node in database", "id", n.ID, "addr", n.addr(), "age", age)
}
tab.mutex.Lock()
tab.stuff(seeds)
tab.mutex.Unlock()
// Finally, do a self lookup to fill up the buckets.
tab.lookup(tab.self.ID, false)
}
// closest returns the n nodes in the table that are closest to the
// given id. The caller must hold tab.mutex.
func (tab *Table) closest(target common.Hash, nresults int) *nodesByDistance {
// This is a very wasteful way to find the closest nodes but
// obviously correct. I believe that tree-based buckets would make
// this easier to implement efficiently.
close := &nodesByDistance{target: target}
for _, b := range tab.buckets {
for _, n := range b.entries {
close.push(n, nresults)
}
}
return close
}
func (tab *Table) len() (n int) {
for _, b := range tab.buckets {
n += len(b.entries)
}
return n
}
// bondall bonds with all given nodes concurrently and returns
// those nodes for which bonding has probably succeeded.
func (tab *Table) bondall(nodes []*Node) (result []*Node) {
rc := make(chan *Node, len(nodes))
for i := range nodes {
go func(n *Node) {
nn, _ := tab.bond(false, n.ID, n.addr(), n.TCP)
rc <- nn
}(nodes[i])
}
for range nodes {
if n := <-rc; n != nil {
result = append(result, n)
}
}
return result
}
// bond ensures the local node has a bond with the given remote node.
// It also attempts to insert the node into the table if bonding succeeds.
// The caller must not hold tab.mutex.
//
// A bond is must be established before sending findnode requests.
// Both sides must have completed a ping/pong exchange for a bond to
// exist. The total number of active bonding processes is limited in
// order to restrain network use.
//
// bond is meant to operate idempotently in that bonding with a remote
// node which still remembers a previously established bond will work.
// The remote node will simply not send a ping back, causing waitping
// to time out.
//
// If pinged is true, the remote node has just pinged us and one half
// of the process can be skipped.
func (tab *Table) bond(pinged bool, id NodeID, addr *net.UDPAddr, tcpPort uint16) (*Node, error) {
if id == tab.self.ID {
return nil, errors.New("is self")
}
// Retrieve a previously known node and any recent findnode failures
node, fails := tab.db.node(id), 0
if node != nil {
fails = tab.db.findFails(id)
}
// If the node is unknown (non-bonded) or failed (remotely unknown), bond from scratch
var result error
age := time.Since(tab.db.lastPong(id))
if node == nil || fails > 0 || age > nodeDBNodeExpiration {
log.Trace("Starting bonding ping/pong", "id", id, "known", node != nil, "failcount", fails, "age", age)
tab.bondmu.Lock()
w := tab.bonding[id]
if w != nil {
// Wait for an existing bonding process to complete.
tab.bondmu.Unlock()
<-w.done
} else {
// Register a new bonding process.
w = &bondproc{done: make(chan struct{})}
tab.bonding[id] = w
tab.bondmu.Unlock()
// Do the ping/pong. The result goes into w.
tab.pingpong(w, pinged, id, addr, tcpPort)
// Unregister the process after it's done.
tab.bondmu.Lock()
delete(tab.bonding, id)
tab.bondmu.Unlock()
}
// Retrieve the bonding results
result = w.err
if result == nil {
node = w.n
}
}
if node != nil {
// Add the node to the table even if the bonding ping/pong
// fails. It will be relaced quickly if it continues to be
// unresponsive.
tab.add(node)
tab.db.updateFindFails(id, 0)
}
return node, result
}
func (tab *Table) pingpong(w *bondproc, pinged bool, id NodeID, addr *net.UDPAddr, tcpPort uint16) {
// Request a bonding slot to limit network usage
<-tab.bondslots
defer func() { tab.bondslots <- struct{}{} }()
// Ping the remote side and wait for a pong.
if w.err = tab.ping(id, addr); w.err != nil {
close(w.done)
return
}
if !pinged {
// Give the remote node a chance to ping us before we start
// sending findnode requests. If they still remember us,
// waitping will simply time out.
tab.net.waitping(id)
}
// Bonding succeeded, update the node database.
w.n = NewNode(id, addr.IP, uint16(addr.Port), tcpPort)
tab.db.updateNode(w.n)
close(w.done)
}
// ping a remote endpoint and wait for a reply, also updating the node
// database accordingly.
func (tab *Table) ping(id NodeID, addr *net.UDPAddr) error {
tab.db.updateLastPing(id, time.Now())
if err := tab.net.ping(id, addr); err != nil {
return err
}
tab.db.updateLastPong(id, time.Now())
// Start the background expiration goroutine after the first
// successful communication. Subsequent calls have no effect if it
// is already running. We do this here instead of somewhere else
// so that the search for seed nodes also considers older nodes
// that would otherwise be removed by the expiration.
tab.db.ensureExpirer()
return nil
}
// add attempts to add the given node its corresponding bucket. If the
// bucket has space available, adding the node succeeds immediately.
// Otherwise, the node is added if the least recently active node in
// the bucket does not respond to a ping packet.
//
// The caller must not hold tab.mutex.
func (tab *Table) add(new *Node) {
b := tab.buckets[logdist(tab.self.sha, new.sha)]
tab.mutex.Lock()
defer tab.mutex.Unlock()
if b.bump(new) {
return
}
var oldest *Node
if len(b.entries) == bucketSize {
oldest = b.entries[bucketSize-1]
if oldest.contested {
// The node is already being replaced, don't attempt
// to replace it.
return
}
oldest.contested = true
// Let go of the mutex so other goroutines can access
// the table while we ping the least recently active node.
tab.mutex.Unlock()
err := tab.ping(oldest.ID, oldest.addr())
tab.mutex.Lock()
oldest.contested = false
if err == nil {
// The node responded, don't replace it.
return
}
}
added := b.replace(new, oldest)
if added && tab.nodeAddedHook != nil {
tab.nodeAddedHook(new)
}
}
// stuff adds nodes the table to the end of their corresponding bucket
// if the bucket is not full. The caller must hold tab.mutex.
func (tab *Table) stuff(nodes []*Node) {
outer:
for _, n := range nodes {
if n.ID == tab.self.ID {
continue // don't add self
}
bucket := tab.buckets[logdist(tab.self.sha, n.sha)]
for i := range bucket.entries {
if bucket.entries[i].ID == n.ID {
continue outer // already in bucket
}
}
if len(bucket.entries) < bucketSize {
bucket.entries = append(bucket.entries, n)
if tab.nodeAddedHook != nil {
tab.nodeAddedHook(n)
}
}
}
}
// delete removes an entry from the node table (used to evacuate
// failed/non-bonded discovery peers).
func (tab *Table) delete(node *Node) {
tab.mutex.Lock()
defer tab.mutex.Unlock()
bucket := tab.buckets[logdist(tab.self.sha, node.sha)]
for i := range bucket.entries {
if bucket.entries[i].ID == node.ID {
bucket.entries = append(bucket.entries[:i], bucket.entries[i+1:]...)
return
}
}
}
func (b *bucket) replace(n *Node, last *Node) bool {
// Don't add if b already contains n.
for i := range b.entries {
if b.entries[i].ID == n.ID {
return false
}
}
// Replace last if it is still the last entry or just add n if b
// isn't full. If is no longer the last entry, it has either been
// replaced with someone else or became active.
if len(b.entries) == bucketSize && (last == nil || b.entries[bucketSize-1].ID != last.ID) {
return false
}
if len(b.entries) < bucketSize {
b.entries = append(b.entries, nil)
}
copy(b.entries[1:], b.entries)
b.entries[0] = n
return true
}
func (b *bucket) bump(n *Node) bool {
for i := range b.entries {
if b.entries[i].ID == n.ID {
// move it to the front
copy(b.entries[1:], b.entries[:i])
b.entries[0] = n
return true
}
}
return false
}
// nodesByDistance is a list of nodes, ordered by
// distance to target.
type nodesByDistance struct {
entries []*Node
target common.Hash
}
// push adds the given node to the list, keeping the total size below maxElems.
func (h *nodesByDistance) push(n *Node, maxElems int) {
ix := sort.Search(len(h.entries), func(i int) bool {
return distcmp(h.target, h.entries[i].sha, n.sha) > 0
})
if len(h.entries) < maxElems {
h.entries = append(h.entries, n)
}
if ix == len(h.entries) {
// farther away than all nodes we already have.
// if there was room for it, the node is now the last element.
} else {
// slide existing entries down to make room
// this will overwrite the entry we just appended.
copy(h.entries[ix+1:], h.entries[ix:])
h.entries[ix] = n
}
}
