path: root/p2p/server.go

// Copyright 2014 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 p2p implements the Ethereum p2p network protocols.
package p2p

import (
    "bytes"
    "crypto/ecdsa"
    "encoding/hex"
    "errors"
    "fmt"
    "net"
    "sort"
    "sync"
    "sync/atomic"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/common/mclock"
    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/event"
    "github.com/ethereum/go-ethereum/log"
    "github.com/ethereum/go-ethereum/p2p/discover"
    "github.com/ethereum/go-ethereum/p2p/discv5"
    "github.com/ethereum/go-ethereum/p2p/enode"
    "github.com/ethereum/go-ethereum/p2p/enr"
    "github.com/ethereum/go-ethereum/p2p/nat"
    "github.com/ethereum/go-ethereum/p2p/netutil"
)

const (
    defaultDialTimeout = 15 * time.Second

    // Connectivity defaults.
    maxActiveDialTasks     = 16
    defaultMaxPendingPeers = 50
    defaultDialRatio       = 3

    // This time limits inbound connection attempts per source IP.
    inboundThrottleTime = 30 * time.Second

    // Maximum time allowed for reading a complete message.
    // This is effectively the amount of time a connection can be idle.
    frameReadTimeout = 30 * time.Second

    // Maximum amount of time allowed for writing a complete message.
    frameWriteTimeout = 20 * time.Second
)

var errServerStopped = errors.New("server stopped")

// Config holds Server options.
type Config struct {
    // This field must be set to a valid secp256k1 private key.
    PrivateKey *ecdsa.PrivateKey `toml:"-"`

    // MaxPeers is the maximum number of peers that can be
    // connected. It must be greater than zero.
    MaxPeers int

    // MaxPendingPeers is the maximum number of peers that can be pending in the
    // handshake phase, counted separately for inbound and outbound connections.
    // Zero defaults to preset values.
    MaxPendingPeers int `toml:",omitempty"`

    // DialRatio controls the ratio of inbound to dialed connections.
    // Example: a DialRatio of 2 allows 1/2 of connections to be dialed.
    // Setting DialRatio to zero defaults it to 3.
    DialRatio int `toml:",omitempty"`

    // NoDiscovery can be used to disable the peer discovery mechanism.
    // Disabling is useful for protocol debugging (manual topology).
    NoDiscovery bool

    // DiscoveryV5 specifies whether the new topic-discovery based V5 discovery
    // protocol should be started or not.
    DiscoveryV5 bool `toml:",omitempty"`

    // Name sets the node name of this server.
    // Use common.MakeName to create a name that follows existing conventions.
    Name string `toml:"-"`

    // BootstrapNodes are used to establish connectivity
    // with the rest of the network.
    BootstrapNodes []*enode.Node

    // BootstrapNodesV5 are used to establish connectivity
    // with the rest of the network using the V5 discovery
    // protocol.
    BootstrapNodesV5 []*discv5.Node `toml:",omitempty"`

    // Static nodes are used as pre-configured connections which are always
    // maintained and re-connected on disconnects.
    StaticNodes []*enode.Node

    // Trusted nodes are used as pre-configured connections which are always
    // allowed to connect, even above the peer limit.
    TrustedNodes []*enode.Node

    // Connectivity can be restricted to certain IP networks.
    // If this option is set to a non-nil value, only hosts which match one of the
    // IP networks contained in the list are considered.
    NetRestrict *netutil.Netlist `toml:",omitempty"`

    // NodeDatabase is the path to the database containing the previously seen
    // live nodes in the network.
    NodeDatabase string `toml:",omitempty"`

    // Protocols should contain the protocols supported
    // by the server. Matching protocols are launched for
    // each peer.
    Protocols []Protocol `toml:"-"`

    // If ListenAddr is set to a non-nil address, the server
    // will listen for incoming connections.
    //
    // If the port is zero, the operating system will pick a port. The
    // ListenAddr field will be updated with the actual address when
    // the server is started.
    ListenAddr string

    // If set to a non-nil value, the given NAT port mapper
    // is used to make the listening port available to the
    // Internet.
    NAT nat.Interface `toml:",omitempty"`

    // If Dialer is set to a non-nil value, the given Dialer
    // is used to dial outbound peer connections.
    Dialer NodeDialer `toml:"-"`

    // If NoDial is true, the server will not dial any peers.
    NoDial bool `toml:",omitempty"`

    // If EnableMsgEvents is set then the server will emit PeerEvents
    // whenever a message is sent to or received from a peer
    EnableMsgEvents bool

    // Logger is a custom logger to use with the p2p.Server.
    Logger log.Logger `toml:",omitempty"`
}

// Server manages all peer connections.
type Server struct {
    // Config fields may not be modified while the server is running.
    Config

    // Hooks for testing. These are useful because we can inhibit
    // the whole protocol stack.
    newTransport func(net.Conn) transport
    newPeerHook  func(*Peer)
    listenFunc   func(network, addr string) (net.Listener, error)

    lock    sync.Mutex // protects running
    running bool

    nodedb       *enode.DB
    localnode    *enode.LocalNode
    ntab         discoverTable
    listener     net.Listener
    ourHandshake *protoHandshake
    DiscV5       *discv5.Network
    loopWG       sync.WaitGroup // loop, listenLoop
    peerFeed     event.Feed
    log          log.Logger

    // Channels into the run loop.
    quit                    chan struct{}
    addstatic               chan *enode.Node
    removestatic            chan *enode.Node
    addtrusted              chan *enode.Node
    removetrusted           chan *enode.Node
    peerOp                  chan peerOpFunc
    peerOpDone              chan struct{}
    delpeer                 chan peerDrop
    checkpointPostHandshake chan *conn
    checkpointAddPeer       chan *conn

    // State of run loop and listenLoop.
    lastLookup     time.Time
    inboundHistory expHeap
}

type peerOpFunc func(map[enode.ID]*Peer)

type peerDrop struct {
    *Peer
    err       error
    requested bool // true if signaled by the peer
}

type connFlag int32

const (
    dynDialedConn connFlag = 1 << iota
    staticDialedConn
    inboundConn
    trustedConn
)

// conn wraps a network connection with information gathered
// during the two handshakes.
type conn struct {
    fd net.Conn
    transport
    node  *enode.Node
    flags connFlag
    cont  chan error // The run loop uses cont to signal errors to SetupConn.
    caps  []Cap      // valid after the protocol handshake
    name  string     // valid after the protocol handshake
}

type transport interface {
    // The two handshakes.
    doEncHandshake(prv *ecdsa.PrivateKey, dialDest *ecdsa.PublicKey) (*ecdsa.PublicKey, error)
    doProtoHandshake(our *protoHandshake) (*protoHandshake, error)
    // The MsgReadWriter can only be used after the encryption
    // handshake has completed. The code uses conn.id to track this
    // by setting it to a non-nil value after the encryption handshake.
    MsgReadWriter
    // transports must provide Close because we use MsgPipe in some of
    // the tests. Closing the actual network connection doesn't do
    // anything in those tests because MsgPipe doesn't use it.
    close(err error)
}

func (c *conn) String() string {
    s := c.flags.String()
    if (c.node.ID() != enode.ID{}) {
        s += " " + c.node.ID().String()
    }
    s += " " + c.fd.RemoteAddr().String()
    return s
}

func (f connFlag) String() string {
    s := ""
    if f&trustedConn != 0 {
        s += "-trusted"
    }
    if f&dynDialedConn != 0 {
        s += "-dyndial"
    }
    if f&staticDialedConn != 0 {
        s += "-staticdial"
    }
    if f&inboundConn != 0 {
        s += "-inbound"
    }
    if s != "" {
        s = s[1:]
    }
    return s
}

func (c *conn) is(f connFlag) bool {
    flags := connFlag(atomic.LoadInt32((*int32)(&c.flags)))
    return flags&f != 0
}

func (c *conn) set(f connFlag, val bool) {
    for {
        oldFlags := connFlag(atomic.LoadInt32((*int32)(&c.flags)))
        flags := oldFlags
        if val {
            flags |= f
        } else {
            flags &= ^f
        }
        if atomic.CompareAndSwapInt32((*int32)(&c.flags), int32(oldFlags), int32(flags)) {
            return
        }
    }
}

// LocalNode returns the local node record.
func (srv *Server) LocalNode() *enode.LocalNode {
    return srv.localnode
}

// Peers returns all connected peers.
func (srv *Server) Peers() []*Peer {
    var ps []*Peer
    select {
    // Note: We'd love to put this function into a variable but
    // that seems to cause a weird compiler error in some
    // environments.
    case srv.peerOp <- func(peers map[enode.ID]*Peer) {
        for _, p := range peers {
            ps = append(ps, p)
        }
    }:
        <-srv.peerOpDone
    case <-srv.quit:
    }
    return ps
}

// PeerCount returns the number of connected peers.
func (srv *Server) PeerCount() int {
    var count int
    select {
    case srv.peerOp <- func(ps map[enode.ID]*Peer) { count = len(ps) }:
        <-srv.peerOpDone
    case <-srv.quit:
    }
    return count
}

// AddPeer connects to the given node and maintains the connection until the
// server is shut down. If the connection fails for any reason, the server will
// attempt to reconnect the peer.
func (srv *Server) AddPeer(node *enode.Node) {
    select {
    case srv.addstatic <- node:
    case <-srv.quit:
    }
}

// RemovePeer disconnects from the given node
func (srv *Server) RemovePeer(node *enode.Node) {
    select {
    case srv.removestatic <- node:
    case <-srv.quit:
    }
}

// AddTrustedPeer adds the given node to a reserved whitelist which allows the
// node to always connect, even if the slot are full.
func (srv *Server) AddTrustedPeer(node *enode.Node) {
    select {
    case srv.addtrusted <- node:
    case <-srv.quit:
    }
}

// RemoveTrustedPeer removes the given node from the trusted peer set.
func (srv *Server) RemoveTrustedPeer(node *enode.Node) {
    select {
    case srv.removetrusted <- node:
    case <-srv.quit:
    }
}

// SubscribePeers subscribes the given channel to peer events
func (srv *Server) SubscribeEvents(ch chan *PeerEvent) event.Subscription {
    return srv.peerFeed.Subscribe(ch)
}

// Self returns the local node's endpoint information.
func (srv *Server) Self() *enode.Node {
    srv.lock.Lock()
    ln := srv.localnode
    srv.lock.Unlock()

    if ln == nil {
        return enode.NewV4(&srv.PrivateKey.PublicKey, net.ParseIP("0.0.0.0"), 0, 0)
    }
    return ln.Node()
}

// Stop terminates the server and all active peer connections.
// It blocks until all active connections have been closed.
func (srv *Server) Stop() {
    srv.lock.Lock()
    if !srv.running {
        srv.lock.Unlock()
        return
    }
    srv.running = false
    if srv.listener != nil {
        // this unblocks listener Accept
        srv.listener.Close()
    }
    close(srv.quit)
    srv.lock.Unlock()
    srv.loopWG.Wait()
}

// sharedUDPConn implements a shared connection. Write sends messages to the underlying connection while read returns
// messages that were found unprocessable and sent to the unhandled channel by the primary listener.
type sharedUDPConn struct {
    *net.UDPConn
    unhandled chan discover.ReadPacket
}

// ReadFromUDP implements discv5.conn
func (s *sharedUDPConn) ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error) {
    packet, ok := <-s.unhandled
    if !ok {
        return 0, nil, errors.New("Connection was closed")
    }
    l := len(packet.Data)
    if l > len(b) {
        l = len(b)
    }
    copy(b[:l], packet.Data[:l])
    return l, packet.Addr, nil
}

// Close implements discv5.conn
func (s *sharedUDPConn) Close() error {
    return nil
}

// Start starts running the server.
// Servers can not be re-used after stopping.
func (srv *Server) Start() (err error) {
    srv.lock.Lock()
    defer srv.lock.Unlock()
    if srv.running {
        return errors.New("server already running")
    }
    srv.running = true
    srv.log = srv.Config.Logger
    if srv.log == nil {
        srv.log = log.Root()
    }
    if srv.NoDial && srv.ListenAddr == "" {
        srv.log.Warn("P2P server will be useless, neither dialing nor listening")
    }

    // static fields
    if srv.PrivateKey == nil {
        return errors.New("Server.PrivateKey must be set to a non-nil key")
    }
    if srv.newTransport == nil {
        srv.newTransport = newRLPX
    }
    if srv.listenFunc == nil {
        srv.listenFunc = net.Listen
    }
    if srv.Dialer == nil {
        srv.Dialer = TCPDialer{&net.Dialer{Timeout: defaultDialTimeout}}
    }
    srv.quit = make(chan struct{})
    srv.delpeer = make(chan peerDrop)
    srv.checkpointPostHandshake = make(chan *conn)
    srv.checkpointAddPeer = make(chan *conn)
    srv.addstatic = make(chan *enode.Node)
    srv.removestatic = make(chan *enode.Node)
    srv.addtrusted = make(chan *enode.Node)
    srv.removetrusted = make(chan *enode.Node)
    srv.peerOp = make(chan peerOpFunc)
    srv.peerOpDone = make(chan struct{})

    if err := srv.setupLocalNode(); err != nil {
        return err
    }
    if srv.ListenAddr != "" {
        if err := srv.setupListening(); err != nil {
            return err
        }
    }
    if err := srv.setupDiscovery(); err != nil {
        return err
    }

    dynPeers := srv.maxDialedConns()
    dialer := newDialState(srv.localnode.ID(), srv.ntab, dynPeers, &srv.Config)
    srv.loopWG.Add(1)
    go srv.run(dialer)
    return nil
}

func (srv *Server) setupLocalNode() error {
    // Create the devp2p handshake.
    pubkey := crypto.FromECDSAPub(&srv.PrivateKey.PublicKey)
    srv.ourHandshake = &protoHandshake{Version: baseProtocolVersion, Name: srv.Name, ID: pubkey[1:]}
    for _, p := range srv.Protocols {
        srv.ourHandshake.Caps = append(srv.ourHandshake.Caps, p.cap())
    }
    sort.Sort(capsByNameAndVersion(srv.ourHandshake.Caps))

    // Create the local node.
    db, err := enode.OpenDB(srv.Config.NodeDatabase)
    if err != nil {
        return err
    }
    srv.nodedb = db
    srv.localnode = enode.NewLocalNode(db, srv.PrivateKey)
    srv.localnode.SetFallbackIP(net.IP{127, 0, 0, 1})
    // TODO: check conflicts
    for _, p := range srv.Protocols {
        for _, e := range p.Attributes {
            srv.localnode.Set(e)
        }
    }
    switch srv.NAT.(type) {
    case nil:
        // No NAT interface, do nothing.
    case nat.ExtIP:
        // ExtIP doesn't block, set the IP right away.
        ip, _ := srv.NAT.ExternalIP()
        srv.localnode.SetStaticIP(ip)
    default:
        // Ask the router about the IP. This takes a while and blocks startup,
        // do it in the background.
        srv.loopWG.Add(1)
        go func() {
            defer srv.loopWG.Done()
            if ip, err := srv.NAT.ExternalIP(); err == nil {
                srv.localnode.SetStaticIP(ip)
            }
        }()
    }
    return nil
}

func (srv *Server) setupDiscovery() error {
    if srv.NoDiscovery && !srv.DiscoveryV5 {
        return nil
    }

    addr, err := net.ResolveUDPAddr("udp", srv.ListenAddr)
    if err != nil {
        return err
    }
    conn, err := net.ListenUDP("udp", addr)
    if err != nil {
        return err
    }
    realaddr := conn.LocalAddr().(*net.UDPAddr)
    srv.log.Debug("UDP listener up", "addr", realaddr)
    if srv.NAT != nil {
        if !realaddr.IP.IsLoopback() {
            go nat.Map(srv.NAT, srv.quit, "udp", realaddr.Port, realaddr.Port, "ethereum discovery")
        }
    }
    srv.localnode.SetFallbackUDP(realaddr.Port)

    // Discovery V4
    var unhandled chan discover.ReadPacket
    var sconn *sharedUDPConn
    if !srv.NoDiscovery {
        if srv.DiscoveryV5 {
            unhandled = make(chan discover.ReadPacket, 100)
            sconn = &sharedUDPConn{conn, unhandled}
        }
        cfg := discover.Config{
            PrivateKey:  srv.PrivateKey,
            NetRestrict: srv.NetRestrict,
            Bootnodes:   srv.BootstrapNodes,
            Unhandled:   unhandled,
            Log:         srv.log,
        }
        ntab, err := discover.ListenUDP(conn, srv.localnode, cfg)
        if err != nil {
            return err
        }
        srv.ntab = ntab
    }
    // Discovery V5
    if srv.DiscoveryV5 {
        var ntab *discv5.Network
        var err error
        if sconn != nil {
            ntab, err = discv5.ListenUDP(srv.PrivateKey, sconn, "", srv.NetRestrict)
        } else {
            ntab, err = discv5.ListenUDP(srv.PrivateKey, conn, "", srv.NetRestrict)
        }
        if err != nil {
            return err
        }
        if err := ntab.SetFallbackNodes(srv.BootstrapNodesV5); err != nil {
            return err
        }
        srv.DiscV5 = ntab
    }
    return nil
}

func (srv *Server) setupListening() error {
    // Launch the listener.
    listener, err := srv.listenFunc("tcp", srv.ListenAddr)
    if err != nil {
        return err
    }
    srv.listener = listener
    srv.ListenAddr = listener.Addr().String()

    // Update the local node record and map the TCP listening port if NAT is configured.
    if tcp, ok := listener.Addr().(*net.TCPAddr); ok {
        srv.localnode.Set(enr.TCP(tcp.Port))
        if !tcp.IP.IsLoopback() && srv.NAT != nil {
            srv.loopWG.Add(1)
            go func() {
                nat.Map(srv.NAT, srv.quit, "tcp", tcp.Port, tcp.Port, "ethereum p2p")
                srv.loopWG.Done()
            }()
        }
    }

    srv.loopWG.Add(1)
    go srv.listenLoop()
    return nil
}

type dialer interface {
    newTasks(running int, peers map[enode.ID]*Peer, now time.Time) []task
    taskDone(task, time.Time)
    addStatic(*enode.Node)
    removeStatic(*enode.Node)
}

func (srv *Server) run(dialstate dialer) {
    srv.log.Info("Started P2P networking", "self", srv.localnode.Node().URLv4())
    defer srv.loopWG.Done()
    defer srv.nodedb.Close()

    var (
        peers        = make(map[enode.ID]*Peer)
        inboundCount = 0
        trusted      = make(map[enode.ID]bool, len(srv.TrustedNodes))
        taskdone     = make(chan task, maxActiveDialTasks)
        runningTasks []task
        queuedTasks  []task // tasks that can't run yet
    )
    // Put trusted nodes into a map to speed up checks.
    // Trusted peers are loaded on startup or added via AddTrustedPeer RPC.
    for _, n := range srv.TrustedNodes {
        trusted[n.ID()] = true
    }

    // removes t from runningTasks
    delTask := func(t task) {
        for i := range runningTasks {
            if runningTasks[i] == t {
                runningTasks = append(runningTasks[:i], runningTasks[i+1:]...)
                break
            }
        }
    }
    // starts until max number of active tasks is satisfied
    startTasks := func(ts []task) (rest []task) {
        i := 0
        for ; len(runningTasks) < maxActiveDialTasks && i < len(ts); i++ {
            t := ts[i]
            srv.log.Trace("New dial task", "task", t)
            go func() { t.Do(srv); taskdone <- t }()
            runningTasks = append(runningTasks, t)
        }
        return ts[i:]
    }
    scheduleTasks := func() {
        // Start from queue first.
        queuedTasks = append(queuedTasks[:0], startTasks(queuedTasks)...)
        // Query dialer for new tasks and start as many as possible now.
        if len(runningTasks) < maxActiveDialTasks {
            nt := dialstate.newTasks(len(runningTasks)+len(queuedTasks), peers, time.Now())
            queuedTasks = append(queuedTasks, startTasks(nt)...)
        }
    }

running:
    for {
        scheduleTasks()

        select {
        case <-srv.quit:
            // The server was stopped. Run the cleanup logic.
            break running

        case n := <-srv.addstatic:
            // This channel is used by AddPeer to add to the
            // ephemeral static peer list. Add it to the dialer,
            // it will keep the node connected.
            srv.log.Trace("Adding static node", "node", n)
            dialstate.addStatic(n)

        case n := <-srv.removestatic:
            // This channel is used by RemovePeer to send a
            // disconnect request to a peer and begin the
            // stop keeping the node connected.
            srv.log.Trace("Removing static node", "node", n)
            dialstate.removeStatic(n)
            if p, ok := peers[n.ID()]; ok {
                p.Disconnect(DiscRequested)
            }

        case n := <-srv.addtrusted:
            // This channel is used by AddTrustedPeer to add an enode
            // to the trusted node set.
            srv.log.Trace("Adding trusted node", "node", n)
            trusted[n.ID()] = true
            // Mark any already-connected peer as trusted
            if p, ok := peers[n.ID()]; ok {
                p.rw.set(trustedConn, true)
            }

        case n := <-srv.removetrusted:
            // This channel is used by RemoveTrustedPeer to remove an enode
            // from the trusted node set.
            srv.log.Trace("Removing trusted node", "node", n)
            delete(trusted, n.ID())

            // Unmark any already-connected peer as trusted
            if p, ok := peers[n.ID()]; ok {
                p.rw.set(trustedConn, false)
            }

        case op := <-srv.peerOp:
            // This channel is used by Peers and PeerCount.
            op(peers)
            srv.peerOpDone <- struct{}{}

        case t := <-taskdone:
            // A task got done. Tell dialstate about it so it
            // can update its state and remove it from the active
            // tasks list.
            srv.log.Trace("Dial task done", "task", t)
            dialstate.taskDone(t, time.Now())
            delTask(t)

        case c := <-srv.checkpointPostHandshake:
            // A connection has passed the encryption handshake so
            // the remote identity is known (but hasn't been verified yet).
            if trusted[c.node.ID()] {
                // Ensure that the trusted flag is set before checking against MaxPeers.
                c.flags |= trustedConn
            }
            // TODO: track in-progress inbound node IDs (pre-Peer) to avoid dialing them.
            c.cont <- srv.postHandshakeChecks(peers, inboundCount, c)

        case c := <-srv.checkpointAddPeer:
            // At this point the connection is past the protocol handshake.
            // Its capabilities are known and the remote identity is verified.
            err := srv.addPeerChecks(peers, inboundCount, c)
            if err == nil {
                // The handshakes are done and it passed all checks.
                p := newPeer(srv.log, c, srv.Protocols)
                // If message events are enabled, pass the peerFeed
                // to the peer
                if srv.EnableMsgEvents {
                    p.events = &srv.peerFeed
                }
                name := truncateName(c.name)
                p.log.Debug("Adding p2p peer", "addr", p.RemoteAddr(), "peers", len(peers)+1, "name", name)
                go srv.runPeer(p)
                peers[c.node.ID()] = p
                if p.Inbound() {
                    inboundCount++
                }
            }
            // The dialer logic relies on the assumption that
            // dial tasks complete after the peer has been added or
            // discarded. Unblock the task last.
            c.cont <- err

        case pd := <-srv.delpeer:
            // A peer disconnected.
            d := common.PrettyDuration(mclock.Now() - pd.created)
            pd.log.Debug("Removing p2p peer", "addr", pd.RemoteAddr(), "peers", len(peers)-1, "duration", d, "req", pd.requested, "err", pd.err)
            delete(peers, pd.ID())
            if pd.Inbound() {
                inboundCount--
            }
        }
    }

    srv.log.Trace("P2P networking is spinning down")

    // Terminate discovery. If there is a running lookup it will terminate soon.
    if srv.ntab != nil {
        srv.ntab.Close()
    }
    if srv.DiscV5 != nil {
        srv.DiscV5.Close()
    }
    // Disconnect all peers.
    for _, p := range peers {
        p.Disconnect(DiscQuitting)
    }
    // Wait for peers to shut down. Pending connections and tasks are
    // not handled here and will terminate soon-ish because srv.quit
    // is closed.
    for len(peers) > 0 {
        p := <-srv.delpeer
        p.log.Trace("<-delpeer (spindown)", "remainingTasks", len(runningTasks))
        delete(peers, p.ID())
    }
}

func (srv *Server) postHandshakeChecks(peers map[enode.ID]*Peer, inboundCount int, c *conn) error {
    switch {
    case !c.is(trustedConn|staticDialedConn) && len(peers) >= srv.MaxPeers:
        return DiscTooManyPeers
    case !c.is(trustedConn) && c.is(inboundConn) && inboundCount >= srv.maxInboundConns():
        return DiscTooManyPeers
    case peers[c.node.ID()] != nil:
        return DiscAlreadyConnected
    case c.node.ID() == srv.localnode.ID():
        return DiscSelf
    default:
        return nil
    }
}

func (srv *Server) addPeerChecks(peers map[enode.ID]*Peer, inboundCount int, c *conn) error {
    // Drop connections with no matching protocols.
    if len(srv.Protocols) > 0 && countMatchingProtocols(srv.Protocols, c.caps) == 0 {
        return DiscUselessPeer
    }
    // Repeat the post-handshake checks because the
    // peer set might have changed since those checks were performed.
    return srv.postHandshakeChecks(peers, inboundCount, c)
}

func (srv *Server) maxInboundConns() int {
    return srv.MaxPeers - srv.maxDialedConns()
}

func (srv *Server) maxDialedConns() int {
    if srv.NoDiscovery || srv.NoDial {
        return 0
    }
    r := srv.DialRatio
    if r == 0 {
        r = defaultDialRatio
    }
    return srv.MaxPeers / r
}

// listenLoop runs in its own goroutine and accepts
// inbound connections.
func (srv *Server) listenLoop() {
    defer srv.loopWG.Done()
    srv.log.Debug("TCP listener up", "addr", srv.listener.Addr())

    tokens := defaultMaxPendingPeers
    if srv.MaxPendingPeers > 0 {
        tokens = srv.MaxPendingPeers
    }
    slots := make(chan struct{}, tokens)
    for i := 0; i < tokens; i++ {
        slots <- struct{}{}
    }

    for {
        // Wait for a free slot before accepting.
        <-slots

        var (
            fd  net.Conn
            err error
        )
        for {
            fd, err = srv.listener.Accept()
            if netutil.IsTemporaryError(err) {
                srv.log.Debug("Temporary read error", "err", err)
                continue
            } else if err != nil {
                srv.log.Debug("Read error", "err", err)
                return
            }
            break
        }

        remoteIP := netutil.AddrIP(fd.RemoteAddr())
        if err := srv.checkInboundConn(fd, remoteIP); err != nil {
            srv.log.Debug("Rejected inbound connnection", "addr", fd.RemoteAddr(), "err", err)
            fd.Close()
            slots <- struct{}{}
            continue
        }
        if remoteIP != nil {
            fd = newMeteredConn(fd, true, remoteIP)
        }
        srv.log.Trace("Accepted connection", "addr", fd.RemoteAddr())
        go func() {
            srv.SetupConn(fd, inboundConn, nil)
            slots <- struct{}{}
        }()
    }
}

func (srv *Server) checkInboundConn(fd net.Conn, remoteIP net.IP) error {
    if remoteIP != nil {
        // Reject connections that do not match NetRestrict.
        if srv.NetRestrict != nil && !srv.NetRestrict.Contains(remoteIP) {
            return fmt.Errorf("not whitelisted in NetRestrict")
        }
        // Reject Internet peers that try too often.
        srv.inboundHistory.expire(time.Now())
        if !netutil.IsLAN(remoteIP) && srv.inboundHistory.contains(remoteIP.String()) {
            return fmt.Errorf("too many attempts")
        }
        srv.inboundHistory.add(remoteIP.String(), time.Now().Add(inboundThrottleTime))
    }
    return nil
}

// SetupConn runs the handshakes and attempts to add the connection
// as a peer. It returns when the connection has been added as a peer
// or the handshakes have failed.
func (srv *Server) SetupConn(fd net.Conn, flags connFlag, dialDest *enode.Node) error {
    c := &conn{fd: fd, transport: srv.newTransport(fd), flags: flags, cont: make(chan error)}
    err := srv.setupConn(c, flags, dialDest)
    if err != nil {
        c.close(err)
        srv.log.Trace("Setting up connection failed", "addr", fd.RemoteAddr(), "err", err)
    }
    return err
}

func (srv *Server) setupConn(c *conn, flags connFlag, dialDest *enode.Node) error {
    // Prevent leftover pending conns from entering the handshake.
    srv.lock.Lock()
    running := srv.running
    srv.lock.Unlock()
    if !running {
        return errServerStopped
    }

    // If dialing, figure out the remote public key.
    var dialPubkey *ecdsa.PublicKey
    if dialDest != nil {
        dialPubkey = new(ecdsa.PublicKey)
        if err := dialDest.Load((*enode.Secp256k1)(dialPubkey)); err != nil {
            return errors.New("dial destination doesn't have a secp256k1 public key")
        }
    }

    // Run the RLPx handshake.
    remotePubkey, err := c.doEncHandshake(srv.PrivateKey, dialPubkey)
    if err != nil {
        srv.log.Trace("Failed RLPx handshake", "addr", c.fd.RemoteAddr(), "conn", c.flags, "err", err)
        return err
    }
    if dialDest != nil {
        // For dialed connections, check that the remote public key matches.
        if dialPubkey.X.Cmp(remotePubkey.X) != 0 || dialPubkey.Y.Cmp(remotePubkey.Y) != 0 {
            return DiscUnexpectedIdentity
        }
        c.node = dialDest
    } else {
        c.node = nodeFromConn(remotePubkey, c.fd)
    }
    if conn, ok := c.fd.(*meteredConn); ok {
        conn.handshakeDone(c.node.ID())
    }
    clog := srv.log.New("id", c.node.ID(), "addr", c.fd.RemoteAddr(), "conn", c.flags)
    err = srv.checkpoint(c, srv.checkpointPostHandshake)
    if err != nil {
        clog.Trace("Rejected peer", "err", err)
        return err
    }

    // Run the capability negotiation handshake.
    phs, err := c.doProtoHandshake(srv.ourHandshake)
    if err != nil {
        clog.Trace("Failed proto handshake", "err", err)
        return err
    }
    if id := c.node.ID(); !bytes.Equal(crypto.Keccak256(phs.ID), id[:]) {
        clog.Trace("Wrong devp2p handshake identity", "phsid", hex.EncodeToString(phs.ID))
        return DiscUnexpectedIdentity
    }
    c.caps, c.name = phs.Caps, phs.Name
    err = srv.checkpoint(c, srv.checkpointAddPeer)
    if err != nil {
        clog.Trace("Rejected peer", "err", err)
        return err
    }

    // If the checks completed successfully, the connection has been added as a peer and
    // runPeer has been launched.
    clog.Trace("Connection set up", "inbound", dialDest == nil)
    return nil
}

func nodeFromConn(pubkey *ecdsa.PublicKey, conn net.Conn) *enode.Node {
    var ip net.IP
    var port int
    if tcp, ok := conn.RemoteAddr().(*net.TCPAddr); ok {
        ip = tcp.IP
        port = tcp.Port
    }
    return enode.NewV4(pubkey, ip, port, port)
}

func truncateName(s string) string {
    if len(s) > 20 {
        return s[:20] + "..."
    }
    return s
}

// checkpoint sends the conn to run, which performs the
// post-handshake checks for the stage (posthandshake, addpeer).
func (srv *Server) checkpoint(c *conn, stage chan<- *conn) error {
    select {
    case stage <- c:
    case <-srv.quit:
        return errServerStopped
    }
    return <-c.cont
}

// runPeer runs in its own goroutine for each peer.
// it waits until the Peer logic returns and removes
// the peer.
func (srv *Server) runPeer(p *Peer) {
    if srv.newPeerHook != nil {
        srv.newPeerHook(p)
    }

    // broadcast peer add
    srv.peerFeed.Send(&PeerEvent{
        Type:          PeerEventTypeAdd,
        Peer:          p.ID(),
        RemoteAddress: p.RemoteAddr().String(),
        LocalAddress:  p.LocalAddr().String(),
    })

    // run the protocol
    remoteRequested, err := p.run()

    // broadcast peer drop
    srv.peerFeed.Send(&PeerEvent{
        Type:          PeerEventTypeDrop,
        Peer:          p.ID(),
        Error:         err.Error(),
        RemoteAddress: p.RemoteAddr().String(),
        LocalAddress:  p.LocalAddr().String(),
    })

    // Note: run waits for existing peers to be sent on srv.delpeer
    // before returning, so this send should not select on srv.quit.
    srv.delpeer <- peerDrop{p, err, remoteRequested}
}

// NodeInfo represents a short summary of the information known about the host.
type NodeInfo struct {
    ID    string `json:"id"`    // Unique node identifier (also the encryption key)
    Name  string `json:"name"`  // Name of the node, including client type, version, OS, custom data
    Enode string `json:"enode"` // Enode URL for adding this peer from remote peers
    ENR   string `json:"enr"`   // Ethereum Node Record
    IP    string `json:"ip"`    // IP address of the node
    Ports struct {
        Discovery int `json:"discovery"` // UDP listening port for discovery protocol
        Listener  int `json:"listener"`  // TCP listening port for RLPx
    } `json:"ports"`
    ListenAddr string                 `json:"listenAddr"`
    Protocols  map[string]interface{} `json:"protocols"`
}

// NodeInfo gathers and returns a collection of metadata known about the host.
func (srv *Server) NodeInfo() *NodeInfo {
    // Gather and assemble the generic node infos
    node := srv.Self()
    info := &NodeInfo{
        Name:       srv.Name,
        Enode:      node.URLv4(),
        ID:         node.ID().String(),
        IP:         node.IP().String(),
        ListenAddr: srv.ListenAddr,
        Protocols:  make(map[string]interface{}),
    }
    info.Ports.Discovery = node.UDP()
    info.Ports.Listener = node.TCP()
    info.ENR = node.String()

    // Gather all the running protocol infos (only once per protocol type)
    for _, proto := range srv.Protocols {
        if _, ok := info.Protocols[proto.Name]; !ok {
            nodeInfo := interface{}("unknown")
            if query := proto.NodeInfo; query != nil {
                nodeInfo = proto.NodeInfo()
            }
            info.Protocols[proto.Name] = nodeInfo
        }
    }
    return info
}

// PeersInfo returns an array of metadata objects describing connected peers.
func (srv *Server) PeersInfo() []*PeerInfo {
    // Gather all the generic and sub-protocol specific infos
    infos := make([]*PeerInfo, 0, srv.PeerCount())
    for _, peer := range srv.Peers() {
        if peer != nil {
            infos = append(infos, peer.Info())
        }
    }
    // Sort the result array alphabetically by node identifier
    for i := 0; i < len(infos); i++ {
        for j := i + 1; j < len(infos); j++ {
            if infos[i].ID > infos[j].ID {
                infos[i], infos[j] = infos[j], infos[i]
            }
        }
    }
    return infos
}