path: root/p2p/discover/udp.go

package discover

import (
    "bytes"
    "crypto/ecdsa"
    "errors"
    "fmt"
    "net"
    "time"

    "github.com/ethereum/go-ethereum/crypto"
    "github.com/ethereum/go-ethereum/logger"
    "github.com/ethereum/go-ethereum/p2p/nat"
    "github.com/ethereum/go-ethereum/rlp"
)

var log = logger.NewLogger("P2P Discovery")

const Version = 3

// Errors
var (
    errPacketTooSmall   = errors.New("too small")
    errBadHash          = errors.New("bad hash")
    errExpired          = errors.New("expired")
    errBadVersion       = errors.New("version mismatch")
    errUnsolicitedReply = errors.New("unsolicited reply")
    errUnknownNode      = errors.New("unknown node")
    errTimeout          = errors.New("RPC timeout")
    errClosed           = errors.New("socket closed")
)

// Timeouts
const (
    respTimeout = 300 * time.Millisecond
    sendTimeout = 300 * time.Millisecond
    expiration  = 20 * time.Second

    refreshInterval = 1 * time.Hour
)

// RPC packet types
const (
    pingPacket = iota + 1 // zero is 'reserved'
    pongPacket
    findnodePacket
    neighborsPacket
)

// RPC request structures
type (
    ping struct {
        Version    uint   // must match Version
        IP         string // our IP
        Port       uint16 // our port
        Expiration uint64
    }

    // reply to Ping
    pong struct {
        ReplyTok   []byte
        Expiration uint64
    }

    findnode struct {
        // Id to look up. The responding node will send back nodes
        // closest to the target.
        Target     NodeID
        Expiration uint64
    }

    // reply to findnode
    neighbors struct {
        Nodes      []*Node
        Expiration uint64
    }
)

type rpcNode struct {
    IP   string
    Port uint16
    ID   NodeID
}

type packet interface {
    handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error
}

type conn interface {
    ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error)
    WriteToUDP(b []byte, addr *net.UDPAddr) (n int, err error)
    Close() error
    LocalAddr() net.Addr
}

// udp implements the RPC protocol.
type udp struct {
    conn conn
    priv *ecdsa.PrivateKey

    addpending chan *pending
    gotreply   chan reply

    closing chan struct{}
    nat     nat.Interface

    *Table
}

// pending 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
// be matched up with a specific findnode packet.
//
// our implementation handles this by storing a callback function for
// each pending reply. incoming packets from a node are dispatched
// to all the callback functions for that node.
type pending struct {
    // these fields must match in the reply.
    from  NodeID
    ptype byte

    // time when the request must complete
    deadline time.Time

    // callback is called when a matching reply arrives. if it returns
    // true, the callback is removed from the pending reply queue.
    // if it returns false, the reply is considered incomplete and
    // the callback will be invoked again for the next matching reply.
    callback func(resp interface{}) (done bool)

    // errc receives nil when the callback indicates completion or an
    // error if no further reply is received within the timeout.
    errc chan<- error
}

type reply struct {
    from  NodeID
    ptype byte
    data  interface{}
    // loop indicates whether there was
    // a matching request by sending on this channel.
    matched chan<- bool
}

// ListenUDP returns a new table that listens for UDP packets on laddr.
func ListenUDP(priv *ecdsa.PrivateKey, laddr string, natm nat.Interface) (*Table, error) {
    addr, err := net.ResolveUDPAddr("udp", laddr)
    if err != nil {
        return nil, err
    }
    conn, err := net.ListenUDP("udp", addr)
    if err != nil {
        return nil, err
    }
    tab, _ := newUDP(priv, conn, natm)
    log.Infoln("Listening,", tab.self)
    return tab, nil
}

func newUDP(priv *ecdsa.PrivateKey, c conn, natm nat.Interface) (*Table, *udp) {
    udp := &udp{
        conn:       c,
        priv:       priv,
        closing:    make(chan struct{}),
        gotreply:   make(chan reply),
        addpending: make(chan *pending),
    }
    realaddr := c.LocalAddr().(*net.UDPAddr)
    if natm != nil {
        if !realaddr.IP.IsLoopback() {
            go nat.Map(natm, udp.closing, "udp", realaddr.Port, realaddr.Port, "ethereum discovery")
        }
        // TODO: react to external IP changes over time.
        if ext, err := natm.ExternalIP(); err == nil {
            realaddr = &net.UDPAddr{IP: ext, Port: realaddr.Port}
        }
    }
    udp.Table = newTable(udp, PubkeyID(&priv.PublicKey), realaddr)
    go udp.loop()
    go udp.readLoop()
    return udp.Table, udp
}

func (t *udp) close() {
    close(t.closing)
    t.conn.Close()
    // TODO: wait for the loops to end.
}

// ping sends a ping message to the given node and waits for a reply.
func (t *udp) ping(toid NodeID, toaddr *net.UDPAddr) error {
    // TODO: maybe check for ReplyTo field in callback to measure RTT
    errc := t.pending(toid, pongPacket, func(interface{}) bool { return true })
    t.send(toaddr, pingPacket, ping{
        Version:    Version,
        IP:         t.self.IP.String(),
        Port:       uint16(t.self.TCPPort),
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    return <-errc
}

func (t *udp) waitping(from NodeID) error {
    return <-t.pending(from, pingPacket, func(interface{}) bool { return true })
}

// findnode sends a findnode request to the given node and waits until
// the node has sent up to k neighbors.
func (t *udp) findnode(toid NodeID, toaddr *net.UDPAddr, target NodeID) ([]*Node, error) {
    nodes := make([]*Node, 0, bucketSize)
    nreceived := 0
    errc := t.pending(toid, neighborsPacket, func(r interface{}) bool {
        reply := r.(*neighbors)
        for _, n := range reply.Nodes {
            nreceived++
            if n.isValid() {
                nodes = append(nodes, n)
            }
        }
        return nreceived >= bucketSize
    })
    t.send(toaddr, findnodePacket, findnode{
        Target:     target,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    err := <-errc
    return nodes, err
}

// pending adds a reply callback to the pending reply queue.
// see the documentation of type pending for a detailed explanation.
func (t *udp) pending(id NodeID, ptype byte, callback func(interface{}) bool) <-chan error {
    ch := make(chan error, 1)
    p := &pending{from: id, ptype: ptype, callback: callback, errc: ch}
    select {
    case t.addpending <- p:
        // loop will handle it
    case <-t.closing:
        ch <- errClosed
    }
    return ch
}

func (t *udp) handleReply(from NodeID, ptype byte, req packet) bool {
    matched := make(chan bool)
    select {
    case t.gotreply <- reply{from, ptype, req, matched}:
        // loop will handle it
        return <-matched
    case <-t.closing:
        return false
    }
}

// loop runs in its own goroutin. it keeps track of
// the refresh timer and the pending reply queue.
func (t *udp) loop() {
    var (
        pending      []*pending
        nextDeadline time.Time
        timeout      = time.NewTimer(0)
        refresh      = time.NewTicker(refreshInterval)
    )
    <-timeout.C // ignore first timeout
    defer refresh.Stop()
    defer timeout.Stop()

    rearmTimeout := func() {
        if len(pending) == 0 || nextDeadline == pending[0].deadline {
            return
        }
        nextDeadline = pending[0].deadline
        timeout.Reset(nextDeadline.Sub(time.Now()))
    }

    for {
        select {
        case <-refresh.C:
            go t.refresh()

        case <-t.closing:
            for _, p := range pending {
                p.errc <- errClosed
            }
            pending = nil
            return

        case p := <-t.addpending:
            p.deadline = time.Now().Add(respTimeout)
            pending = append(pending, p)
            rearmTimeout()

        case r := <-t.gotreply:
            var matched bool
            for i := 0; i < len(pending); i++ {
                if p := pending[i]; p.from == r.from && p.ptype == r.ptype {
                    matched = true
                    if p.callback(r.data) {
                        // callback indicates the request is done, remove it.
                        p.errc <- nil
                        copy(pending[i:], pending[i+1:])
                        pending = pending[:len(pending)-1]
                        i--
                    }
                }
            }
            r.matched <- matched

        case now := <-timeout.C:
            // notify and remove callbacks whose deadline is in the past.
            i := 0
            for ; i < len(pending) && now.After(pending[i].deadline); i++ {
                pending[i].errc <- errTimeout
            }
            if i > 0 {
                copy(pending, pending[i:])
                pending = pending[:len(pending)-i]
            }
            rearmTimeout()
        }
    }
}

const (
    macSize  = 256 / 8
    sigSize  = 520 / 8
    headSize = macSize + sigSize // space of packet frame data
)

var headSpace = make([]byte, headSize)

func (t *udp) send(toaddr *net.UDPAddr, ptype byte, req interface{}) error {
    packet, err := encodePacket(t.priv, ptype, req)
    if err != nil {
        return err
    }
    log.DebugDetailf(">>> %v %T %v\n", toaddr, req, req)
    if _, err = t.conn.WriteToUDP(packet, toaddr); err != nil {
        log.DebugDetailln("UDP send failed:", err)
    }
    return err
}

func encodePacket(priv *ecdsa.PrivateKey, ptype byte, req interface{}) ([]byte, error) {
    b := new(bytes.Buffer)
    b.Write(headSpace)
    b.WriteByte(ptype)
    if err := rlp.Encode(b, req); err != nil {
        log.Errorln("error encoding packet:", err)
        return nil, err
    }
    packet := b.Bytes()
    sig, err := crypto.Sign(crypto.Sha3(packet[headSize:]), priv)
    if err != nil {
        log.Errorln("could not sign packet:", err)
        return nil, err
    }
    copy(packet[macSize:], sig)
    // add the hash to the front. Note: this doesn't protect the
    // packet in any way. Our public key will be part of this hash in
    // The future.
    copy(packet, crypto.Sha3(packet[macSize:]))
    return packet, nil
}

// readLoop runs in its own goroutine. it handles incoming UDP packets.
func (t *udp) readLoop() {
    defer t.conn.Close()
    buf := make([]byte, 4096) // TODO: good buffer size
    for {
        nbytes, from, err := t.conn.ReadFromUDP(buf)
        if err != nil {
            return
        }
        packet, fromID, hash, err := decodePacket(buf[:nbytes])
        if err != nil {
            log.Debugf("Bad packet from %v: %v\n", from, err)
            continue
        }
        log.DebugDetailf("<<< %v %T %v\n", from, packet, packet)
        go func() {
            if err := packet.handle(t, from, fromID, hash); err != nil {
                log.Debugf("error handling %T from %v: %v", packet, from, err)
            }
        }()
    }
}

func decodePacket(buf []byte) (packet, NodeID, []byte, error) {
    if len(buf) < headSize+1 {
        return nil, NodeID{}, nil, errPacketTooSmall
    }
    hash, sig, sigdata := buf[:macSize], buf[macSize:headSize], buf[headSize:]
    shouldhash := crypto.Sha3(buf[macSize:])
    if !bytes.Equal(hash, shouldhash) {
        return nil, NodeID{}, nil, errBadHash
    }
    fromID, err := recoverNodeID(crypto.Sha3(buf[headSize:]), sig)
    if err != nil {
        return nil, NodeID{}, hash, err
    }
    var req packet
    switch ptype := sigdata[0]; ptype {
    case pingPacket:
        req = new(ping)
    case pongPacket:
        req = new(pong)
    case findnodePacket:
        req = new(findnode)
    case neighborsPacket:
        req = new(neighbors)
    default:
        return nil, fromID, hash, fmt.Errorf("unknown type: %d", ptype)
    }
    err = rlp.Decode(bytes.NewReader(sigdata[1:]), req)
    return req, fromID, hash, err
}

func (req *ping) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    if req.Version != Version {
        return errBadVersion
    }
    t.send(from, pongPacket, pong{
        ReplyTok:   mac,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    if !t.handleReply(fromID, pingPacket, req) {
        // Note: we're ignoring the provided IP address right now
        t.bond(true, fromID, from, req.Port)
    }
    return nil
}

func (req *pong) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    if !t.handleReply(fromID, pongPacket, req) {
        return errUnsolicitedReply
    }
    return nil
}

func (req *findnode) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    if t.db.get(fromID) == nil {
        // No bond 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 and UDP
        // port of the target as the source address. The recipient of
        // the findnode packet would then send a neighbors packet
        // (which is a much bigger packet than findnode) to the victim.
        return errUnknownNode
    }
    t.mutex.Lock()
    closest := t.closest(req.Target, bucketSize).entries
    t.mutex.Unlock()

    t.send(from, neighborsPacket, neighbors{
        Nodes:      closest,
        Expiration: uint64(time.Now().Add(expiration).Unix()),
    })
    return nil
}

func (req *neighbors) handle(t *udp, from *net.UDPAddr, fromID NodeID, mac []byte) error {
    if expired(req.Expiration) {
        return errExpired
    }
    if !t.handleReply(fromID, neighborsPacket, req) {
        return errUnsolicitedReply
    }
    return nil
}

func expired(ts uint64) bool {
    return time.Unix(int64(ts), 0).Before(time.Now())
}