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tailscale/derp/derp_server.go

635 lines
16 KiB
Go

// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package derp
// TODO(crawshaw): send srcKey with packets to clients?
// TODO(crawshaw): with predefined serverKey in clients and HMAC on packets we could skip TLS
import (
"bufio"
"context"
crand "crypto/rand"
"encoding/json"
"errors"
"expvar"
"fmt"
"io"
"io/ioutil"
"math/big"
"os"
"strconv"
"sync"
"time"
"golang.org/x/crypto/nacl/box"
"tailscale.com/metrics"
"tailscale.com/types/key"
"tailscale.com/types/logger"
)
var debug, _ = strconv.ParseBool(os.Getenv("DERP_DEBUG_LOGS"))
const perClientSendQueueDepth = 32 // packets buffered for sending
// Server is a DERP server.
type Server struct {
// WriteTimeout, if non-zero, specifies how long to wait
// before failing when writing to a client.
WriteTimeout time.Duration
privateKey key.Private
publicKey key.Public
logf logger.Logf
// Counters:
packetsSent, bytesSent expvar.Int
packetsRecv, bytesRecv expvar.Int
packetsDropped expvar.Int
accepts expvar.Int
curClients expvar.Int
curHomeClients expvar.Int // ones with preferred
clientsReplaced expvar.Int
unknownFrames expvar.Int
homeMovesIn expvar.Int // established clients announce home server moves in
homeMovesOut expvar.Int // established clients announce home server moves out
mu sync.Mutex
closed bool
netConns map[Conn]chan struct{} // chan is closed when conn closes
clients map[key.Public]*sclient
clientsEver map[key.Public]bool // never deleted from, for stats; fine for now
}
// Conn is the subset of the underlying net.Conn the DERP Server needs.
// It is a defined type so that non-net connections can be used.
type Conn interface {
io.Closer
// The *Deadline methods follow the semantics of net.Conn.
SetDeadline(time.Time) error
SetReadDeadline(time.Time) error
SetWriteDeadline(time.Time) error
}
// NewServer returns a new DERP server. It doesn't listen on its own.
// Connections are given to it via Server.Accept.
func NewServer(privateKey key.Private, logf logger.Logf) *Server {
s := &Server{
privateKey: privateKey,
publicKey: privateKey.Public(),
logf: logf,
clients: make(map[key.Public]*sclient),
clientsEver: make(map[key.Public]bool),
netConns: make(map[Conn]chan struct{}),
}
return s
}
// Close closes the server and waits for the connections to disconnect.
func (s *Server) Close() error {
s.mu.Lock()
wasClosed := s.closed
s.closed = true
s.mu.Unlock()
if wasClosed {
return nil
}
var closedChs []chan struct{}
s.mu.Lock()
for nc, closed := range s.netConns {
nc.Close()
closedChs = append(closedChs, closed)
}
s.mu.Unlock()
for _, closed := range closedChs {
<-closed
}
return nil
}
func (s *Server) isClosed() bool {
s.mu.Lock()
defer s.mu.Unlock()
return s.closed
}
// Accept adds a new connection to the server and serves it.
//
// The provided bufio ReadWriter must be already connected to nc.
// Accept blocks until the Server is closed or the connection closes
// on its own.
//
// Accept closes nc.
func (s *Server) Accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) {
closed := make(chan struct{})
s.accepts.Add(1)
s.mu.Lock()
s.netConns[nc] = closed
s.mu.Unlock()
defer func() {
nc.Close()
close(closed)
s.mu.Lock()
delete(s.netConns, nc)
s.mu.Unlock()
}()
if err := s.accept(nc, brw, remoteAddr); err != nil && !s.isClosed() {
s.logf("derp: %s: %v", remoteAddr, err)
}
}
// registerClient notes that client c is now authenticated and ready for packets.
// If c's public key was already connected with a different connection, the prior one is closed.
func (s *Server) registerClient(c *sclient) {
s.mu.Lock()
defer s.mu.Unlock()
old := s.clients[c.key]
if old == nil {
c.logf("adding connection")
} else {
s.clientsReplaced.Add(1)
c.logf("adding connection, replacing %s", old.remoteAddr)
go old.nc.Close()
}
s.clients[c.key] = c
s.clientsEver[c.key] = true
s.curClients.Add(1)
}
// unregisterClient removes a client from the server.
func (s *Server) unregisterClient(c *sclient) {
s.mu.Lock()
defer s.mu.Unlock()
cur := s.clients[c.key]
if cur == c {
c.logf("removing connection")
delete(s.clients, c.key)
}
s.curClients.Add(-1)
if c.preferred {
s.curHomeClients.Add(-1)
}
}
func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string) error {
br, bw := brw.Reader, brw.Writer
nc.SetDeadline(time.Now().Add(10 * time.Second))
if err := s.sendServerKey(bw); err != nil {
return fmt.Errorf("send server key: %v", err)
}
nc.SetDeadline(time.Now().Add(10 * time.Second))
clientKey, clientInfo, err := s.recvClientKey(br)
if err != nil {
return fmt.Errorf("receive client key: %v", err)
}
if err := s.verifyClient(clientKey, clientInfo); err != nil {
return fmt.Errorf("client %x rejected: %v", clientKey, err)
}
// At this point we trust the client so we don't time out.
nc.SetDeadline(time.Time{})
c := &sclient{
s: s,
key: clientKey,
nc: nc,
br: br,
bw: bw,
logf: logger.WithPrefix(s.logf, fmt.Sprintf("derp client %v/%x: ", remoteAddr, clientKey)),
remoteAddr: remoteAddr,
connectedAt: time.Now(),
sendQueue: make(chan pkt, perClientSendQueueDepth),
}
if clientInfo != nil {
c.info = *clientInfo
}
s.registerClient(c)
err = s.sendServerInfo(bw, clientKey)
if err != nil {
return fmt.Errorf("send server info: %v", err)
}
defer s.unregisterClient(c)
return c.run()
}
func (c *sclient) run() error {
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
defer func() {
// Atomically close+remove send queue, so racing writers don't
// send to closed channel.
c.mu.Lock()
close(c.sendQueue)
c.sendQueue = nil
c.mu.Unlock()
}()
go c.sender(ctx)
for {
ft, fl, err := readFrameHeader(c.br)
if err != nil {
return fmt.Errorf("client %x: readFrameHeader: %v", c.key, err)
}
switch ft {
case frameNotePreferred:
err = c.handleFrameNotePreferred(ft, fl)
case frameSendPacket:
err = c.handleFrameSendPacket(ctx, ft, fl)
default:
err = c.handleUnknownFrame(ctx, ft, fl)
}
if err != nil {
return err
}
}
}
func (c *sclient) handleUnknownFrame(ctx context.Context, ft frameType, fl uint32) error {
_, err := io.CopyN(ioutil.Discard, c.br, int64(fl))
return err
}
func (c *sclient) handleFrameNotePreferred(ft frameType, fl uint32) error {
if fl != 1 {
return fmt.Errorf("frameNotePreferred wrong size")
}
v, err := c.br.ReadByte()
if err != nil {
return fmt.Errorf("frameNotePreferred ReadByte: %v", err)
}
c.setPreferred(v != 0)
return nil
}
func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl uint32) error {
s := c.s
dstKey, contents, err := s.recvPacket(ctx, c.br, fl)
if err != nil {
return fmt.Errorf("client %x: recvPacket: %v", c.key, err)
}
s.mu.Lock()
dst := s.clients[dstKey]
s.mu.Unlock()
if dst == nil {
s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet for unknown %x", dstKey)
}
return nil
}
dst.mu.RLock()
defer dst.mu.RUnlock()
if dst.sendQueue == nil {
s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet for shutdown client %x", dstKey)
}
}
p := pkt{
bs: contents,
}
if dst.info.Version >= protocolSrcAddrs {
p.src = c.key
}
// Attempt to queue for sending up to 3 times. On each attempt, if
// the queue is full, try to drop from queue head to prioritize
// fresher packets.
for attempt := 0; attempt < 3; attempt++ {
select {
case dst.sendQueue <- p:
return nil
default:
}
select {
case <-dst.sendQueue:
s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet from client %x queue head", dstKey)
}
default:
}
}
// Failed to make room for packet. This can happen in a heavily
// contended queue with racing writers. Give up and tail-drop in
// this case to keep reader unblocked.
s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet from client %x queue tail", dstKey)
}
return nil
}
func (s *Server) verifyClient(clientKey key.Public, info *clientInfo) error {
// TODO(crawshaw): implement policy constraints on who can use the DERP server
// TODO(bradfitz): ... and at what rate.
return nil
}
func (s *Server) sendServerKey(bw *bufio.Writer) error {
buf := make([]byte, 0, len(magic)+len(s.publicKey))
buf = append(buf, magic...)
buf = append(buf, s.publicKey[:]...)
return writeFrame(bw, frameServerKey, buf)
}
type serverInfo struct {
Version int // `json:"version,omitempty"`
}
func (s *Server) sendServerInfo(bw *bufio.Writer, clientKey key.Public) error {
var nonce [24]byte
if _, err := crand.Read(nonce[:]); err != nil {
return err
}
msg, err := json.Marshal(serverInfo{Version: protocolVersion})
if err != nil {
return err
}
msgbox := box.Seal(nil, msg, &nonce, clientKey.B32(), s.privateKey.B32())
if err := writeFrameHeader(bw, frameServerInfo, nonceLen+uint32(len(msgbox))); err != nil {
return err
}
if _, err := bw.Write(nonce[:]); err != nil {
return err
}
if _, err := bw.Write(msgbox); err != nil {
return err
}
return bw.Flush()
}
// recvClientKey reads the frameClientInfo frame from the client (its
// proof of identity) upon its initial connection. It should be
// considered especially untrusted at this point.
func (s *Server) recvClientKey(br *bufio.Reader) (clientKey key.Public, info *clientInfo, err error) {
fl, err := readFrameTypeHeader(br, frameClientInfo)
if err != nil {
return key.Public{}, nil, err
}
const minLen = keyLen + nonceLen
if fl < minLen {
return key.Public{}, nil, errors.New("short client info")
}
// We don't trust the client at all yet, so limit its input size to limit
// things like JSON resource exhausting (http://github.com/golang/go/issues/31789).
if fl > 256<<10 {
return key.Public{}, nil, errors.New("long client info")
}
if _, err := io.ReadFull(br, clientKey[:]); err != nil {
return key.Public{}, nil, err
}
var nonce [24]byte
if _, err := io.ReadFull(br, nonce[:]); err != nil {
return key.Public{}, nil, fmt.Errorf("nonce: %v", err)
}
msgLen := int(fl - minLen)
msgbox := make([]byte, msgLen)
if _, err := io.ReadFull(br, msgbox); err != nil {
return key.Public{}, nil, fmt.Errorf("msgbox: %v", err)
}
msg, ok := box.Open(nil, msgbox, &nonce, (*[32]byte)(&clientKey), s.privateKey.B32())
if !ok {
return key.Public{}, nil, fmt.Errorf("msgbox: cannot open len=%d with client key %x", msgLen, clientKey[:])
}
info = new(clientInfo)
if err := json.Unmarshal(msg, info); err != nil {
return key.Public{}, nil, fmt.Errorf("msg: %v", err)
}
return clientKey, info, nil
}
func (s *Server) recvPacket(ctx context.Context, br *bufio.Reader, frameLen uint32) (dstKey key.Public, contents []byte, err error) {
if frameLen < keyLen {
return key.Public{}, nil, errors.New("short send packet frame")
}
if _, err := io.ReadFull(br, dstKey[:]); err != nil {
return key.Public{}, nil, err
}
packetLen := frameLen - keyLen
if packetLen > MaxPacketSize {
return key.Public{}, nil, fmt.Errorf("data packet longer (%d) than max of %v", packetLen, MaxPacketSize)
}
contents = make([]byte, packetLen)
if _, err := io.ReadFull(br, contents); err != nil {
return key.Public{}, nil, err
}
s.packetsRecv.Add(1)
s.bytesRecv.Add(int64(len(contents)))
return dstKey, contents, nil
}
// sclient is a client connection to the server.
//
// (The "s" prefix is to more explicitly distinguish it from Client in derp_client.go)
type sclient struct {
// Static after construction.
s *Server
nc Conn
key key.Public
info clientInfo
logf logger.Logf
remoteAddr string // usually ip:port from net.Conn.RemoteAddr().String()
// Owned by run, not thread-safe.
br *bufio.Reader
connectedAt time.Time
preferred bool
// Owned by sender, not thread-safe.
bw *bufio.Writer
mu sync.RWMutex // guards access to sendQueue for shutdown.
sendQueue chan pkt // packets queued to this client
}
type pkt struct {
src key.Public
bs []byte
// TODO(danderson): enqueue time, to measure queue latency?
}
func (c *sclient) setPreferred(v bool) {
if c.preferred == v {
return
}
c.preferred = v
var homeMove *expvar.Int
if v {
c.s.curHomeClients.Add(1)
homeMove = &c.s.homeMovesIn
} else {
c.s.curHomeClients.Add(-1)
homeMove = &c.s.homeMovesOut
}
// Keep track of varz for home serve moves in/out. But ignore
// the initial packet set when a client connects, which we
// assume happens within 5 seconds. In any case, just for
// graphs, so not important to miss a move. But it shouldn't:
// the netcheck/re-STUNs in magicsock only happen about every
// 30 seconds.
if time.Since(c.connectedAt) > 5*time.Second {
homeMove.Add(1)
}
}
func (c *sclient) sender(ctx context.Context) {
// If the sender shuts down unilaterally due to an error, close so
// that the receive loop unblocks and cleans up the rest.
defer c.nc.Close()
if err := c.sendLoop(ctx); err != nil {
c.logf("sender failed: %v", err)
}
}
func (c *sclient) sendLoop(ctx context.Context) error {
c.mu.RLock()
queue := c.sendQueue
c.mu.RUnlock()
if queue == nil {
// Uncommon race, sclient shut down before this loop ever
// started. Nothing to do here, move along.
return nil
}
defer func() {
// Drain the send queue to count dropped packets
for {
_, ok := <-queue
if !ok {
break
}
c.s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet for shutdown %x", c.key)
}
}
}()
jitterMs, err := crand.Int(crand.Reader, big.NewInt(5000))
if err != nil {
panic(err)
}
jitter := time.Duration(jitterMs.Int64()) * time.Millisecond
keepAliveTick := time.NewTicker(keepAlive + jitter)
defer keepAliveTick.Stop()
for {
select {
case <-ctx.Done():
return nil
case pkt, ok := <-queue:
if !ok {
return nil
}
if err := c.sendPacket(pkt.src, pkt.bs); err != nil {
return err
}
case <-keepAliveTick.C:
if err := c.sendKeepalive(); err != nil {
return err
}
}
}
}
func (c *sclient) sendKeepalive() error {
if c.s.WriteTimeout != 0 {
c.nc.SetWriteDeadline(time.Now().Add(c.s.WriteTimeout))
}
if err := writeFrame(c.bw, frameKeepAlive, nil); err != nil {
return err
}
return c.bw.Flush()
}
func (c *sclient) sendPacket(srcKey key.Public, contents []byte) (err error) {
defer func() {
// Stats update.
if err != nil {
c.s.packetsDropped.Add(1)
if debug {
c.logf("dropping packet to %x: %v", c.key, err)
}
} else {
c.s.packetsSent.Add(1)
c.s.bytesSent.Add(int64(len(contents)))
}
}()
if c.s.WriteTimeout != 0 {
c.nc.SetWriteDeadline(time.Now().Add(c.s.WriteTimeout))
}
withKey := !srcKey.IsZero()
pktLen := len(contents)
if withKey {
pktLen += len(srcKey)
}
if err = writeFrameHeader(c.bw, frameRecvPacket, uint32(pktLen)); err != nil {
return err
}
if withKey {
if _, err = c.bw.Write(srcKey[:]); err != nil {
return err
}
}
if _, err = c.bw.Write(contents); err != nil {
return err
}
return c.bw.Flush()
}
func (s *Server) expVarFunc(f func() interface{}) expvar.Func {
return expvar.Func(func() interface{} {
s.mu.Lock()
defer s.mu.Unlock()
return f()
})
}
// ExpVar returns an expvar variable suitable for registering with expvar.Publish.
func (s *Server) ExpVar() expvar.Var {
m := new(metrics.Set)
m.Set("counter_unique_clients_ever", s.expVarFunc(func() interface{} { return len(s.clientsEver) }))
m.Set("gauge_current_connnections", &s.curClients)
m.Set("gauge_current_home_connnections", &s.curHomeClients)
m.Set("accepts", &s.accepts)
m.Set("clients_replaced", &s.clientsReplaced)
m.Set("bytes_received", &s.bytesRecv)
m.Set("bytes_sent", &s.bytesSent)
m.Set("packets_dropped", &s.packetsDropped)
m.Set("packets_sent", &s.packetsSent)
m.Set("packets_received", &s.packetsRecv)
m.Set("unknown_frames", &s.unknownFrames)
m.Set("home_moves_in", &s.homeMovesIn)
m.Set("home_moves_out", &s.homeMovesOut)
return m
}