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tailscale/derp/derp_server.go

382 lines
9.1 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): revise protocol so unknown type packets have a predictable length for skipping.
// 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"
"crypto/rand"
"encoding/binary"
"encoding/json"
"fmt"
"io"
"math/big"
"net"
"sync"
"time"
"golang.org/x/crypto/curve25519"
"golang.org/x/crypto/nacl/box"
"tailscale.com/logger"
)
const magic = 0x44c55250 // "DERP" with a non-ASCII high-bit
const (
typeServerKey = 0x01
typeServerInfo = 0x02
typeSendPacket = 0x03
typeRecvPacket = 0x04
typeKeepAlive = 0x05
)
const keepAlive = 60 * time.Second
var bin = binary.BigEndian
const oneMB = 1 << 20
type Server struct {
privateKey [32]byte // TODO(crawshaw): make this wgcfg.PrivateKey?
publicKey [32]byte
logf logger.Logf
mu sync.Mutex
netConns map[net.Conn]chan struct{}
clients map[[32]byte]*client
}
func NewServer(privateKey [32]byte, logf logger.Logf) *Server {
s := &Server{
privateKey: privateKey,
logf: logf,
clients: make(map[[32]byte]*client),
netConns: make(map[net.Conn]chan struct{}),
}
curve25519.ScalarBaseMult(&s.publicKey, &s.privateKey)
return s
}
func (s *Server) Close() error {
var closedChs []chan struct{}
s.mu.Lock()
for netConn, closed := range s.netConns {
netConn.Close()
closedChs = append(closedChs, closed)
}
s.mu.Unlock()
for _, closed := range closedChs {
<-closed
}
return nil
}
func (s *Server) Accept(netConn net.Conn, conn *bufio.ReadWriter) {
closed := make(chan struct{})
s.mu.Lock()
s.netConns[netConn] = closed
s.mu.Unlock()
defer func() {
netConn.Close()
close(closed)
s.mu.Lock()
delete(s.netConns, netConn)
s.mu.Unlock()
}()
if err := s.accept(netConn, conn); err != nil {
s.logf("derp: %s: %v", netConn.RemoteAddr(), err)
}
}
func (s *Server) accept(netConn net.Conn, conn *bufio.ReadWriter) error {
netConn.SetDeadline(time.Now().Add(10 * time.Second))
if err := s.sendServerKey(conn); err != nil {
return fmt.Errorf("send server key: %v", err)
}
netConn.SetDeadline(time.Now().Add(10 * time.Second))
clientKey, clientInfo, err := s.recvClientKey(conn)
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.
netConn.SetDeadline(time.Time{})
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
c := &client{
key: clientKey,
netConn: netConn,
conn: conn,
}
if clientInfo != nil {
c.info = *clientInfo
}
go func() {
if err := c.keepAlive(ctx); err != nil {
s.logf("derp: %s: client %x: keep alive failed: %v", netConn.RemoteAddr(), c.key, err)
}
}()
defer func() {
s.mu.Lock()
curClient := s.clients[c.key]
if curClient != nil && curClient.conn == conn {
s.logf("derp: %s: client %x: removing connection", netConn.RemoteAddr(), c.key)
delete(s.clients, c.key)
}
s.mu.Unlock()
}()
// Hold mu while we add the new client to the clients list and under
// the same acquisition send server info. This ensure that both:
// 1. by the time the client receives the server info, it can be addressed.
// 2. the server info is the very first
c.mu.Lock()
s.mu.Lock()
oldClient := s.clients[c.key]
s.clients[c.key] = c
s.mu.Unlock()
if err := s.sendServerInfo(conn, clientKey); err != nil {
return fmt.Errorf("send server info: %v", err)
}
c.mu.Unlock()
if oldClient == nil {
s.logf("derp: %s: client %x: adding connection", netConn.RemoteAddr(), c.key)
} else {
oldClient.netConn.Close()
s.logf("derp: %s: client %x: adding connection, replacing %s", netConn.RemoteAddr(), c.key, oldClient.netConn.RemoteAddr())
}
for {
dstKey, contents, err := s.recvPacket(c.conn)
if err != nil {
return fmt.Errorf("client %x: recv: %v", c.key, err)
}
s.mu.Lock()
dst := s.clients[dstKey]
s.mu.Unlock()
if dst == nil {
s.logf("derp: %s: client %x: dropping packet for unknown %x", netConn.RemoteAddr(), c.key, dstKey)
continue
}
dst.mu.Lock()
err = s.sendPacket(dst.conn, c.key, contents)
dst.mu.Unlock()
if err != nil {
s.logf("derp: %s: client %x: dropping packet for %x: %v", netConn.RemoteAddr(), c.key, dstKey, err)
// If we cannot send to a destination, shut it down.
// Let its receive loop do the cleanup.
s.mu.Lock()
if s.clients[dstKey].conn == dst.conn {
s.clients[dstKey].netConn.Close()
}
s.mu.Unlock()
}
}
}
func (s *Server) verifyClient(clientKey [32]byte, info *clientInfo) error {
// TODO(crawshaw): implement policy constraints on who can use the DERP server
return nil
}
func (s *Server) sendServerKey(conn *bufio.ReadWriter) error {
if err := putUint32(conn, magic); err != nil {
return err
}
if err := conn.WriteByte(typeServerKey); err != nil {
return err
}
if _, err := conn.Write(s.publicKey[:]); err != nil {
return err
}
return conn.Flush()
}
func (s *Server) sendServerInfo(conn *bufio.ReadWriter, clientKey [32]byte) error {
var nonce [24]byte
if _, err := rand.Read(nonce[:]); err != nil {
return err
}
msg := []byte("{}") // no serverInfo for now
msgbox := box.Seal(nil, msg, &nonce, &clientKey, &s.privateKey)
if err := conn.WriteByte(typeServerInfo); err != nil {
return err
}
if _, err := conn.Write(nonce[:]); err != nil {
return err
}
if err := putUint32(conn, uint32(len(msgbox))); err != nil {
return err
}
if _, err := conn.Write(msgbox); err != nil {
return err
}
return conn.Flush()
}
func (s *Server) recvClientKey(conn *bufio.ReadWriter) (clientKey [32]byte, info *clientInfo, err error) {
if _, err := io.ReadFull(conn, clientKey[:]); err != nil {
return [32]byte{}, nil, err
}
var nonce [24]byte
if _, err := io.ReadFull(conn, nonce[:]); err != nil {
return [32]byte{}, nil, fmt.Errorf("nonce: %v", err)
}
msgLen, err := readUint32(conn, oneMB)
if err != nil {
return [32]byte{}, nil, fmt.Errorf("msglen: %v", err)
}
msgbox := make([]byte, msgLen)
if _, err := io.ReadFull(conn, msgbox); err != nil {
return [32]byte{}, nil, fmt.Errorf("msgbox: %v", err)
}
msg, ok := box.Open(nil, msgbox, &nonce, &clientKey, &s.privateKey)
if !ok {
return [32]byte{}, 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 [32]byte{}, nil, fmt.Errorf("msg: %v", err)
}
return clientKey, info, nil
}
func (s *Server) sendPacket(conn *bufio.ReadWriter, srcKey [32]byte, contents []byte) error {
if err := conn.WriteByte(typeRecvPacket); err != nil {
return err
}
if err := putUint32(conn.Writer, uint32(len(contents))); err != nil {
return err
}
if _, err := conn.Write(contents); err != nil {
return err
}
return conn.Flush()
}
func (s *Server) recvPacket(conn *bufio.ReadWriter) (dstKey [32]byte, contents []byte, err error) {
if err := readType(conn.Reader, typeSendPacket); err != nil {
return [32]byte{}, nil, err
}
if _, err := io.ReadFull(conn, dstKey[:]); err != nil {
return [32]byte{}, nil, err
}
packetLen, err := readUint32(conn.Reader, oneMB)
if err != nil {
return [32]byte{}, nil, err
}
contents = make([]byte, packetLen)
if _, err := io.ReadFull(conn, contents); err != nil {
return [32]byte{}, nil, err
}
return dstKey, contents, nil
}
type client struct {
netConn net.Conn
key [32]byte
info clientInfo
keepAliveTimer *time.Timer
keepAliveReset chan struct{}
mu sync.Mutex
conn *bufio.ReadWriter
}
func (c *client) keepAlive(ctx context.Context) error {
jitterMs, err := rand.Int(rand.Reader, big.NewInt(5000))
if err != nil {
panic(err)
}
jitter := time.Duration(jitterMs.Int64()) * time.Millisecond
c.keepAliveTimer = time.NewTimer(keepAlive + jitter)
for {
select {
case <-ctx.Done():
return nil
case <-c.keepAliveReset:
if c.keepAliveTimer.Stop() {
<-c.keepAliveTimer.C
}
c.keepAliveTimer.Reset(keepAlive + jitter)
case <-c.keepAliveTimer.C:
c.mu.Lock()
err := c.conn.WriteByte(typeKeepAlive)
if err == nil {
err = c.conn.Flush()
}
c.mu.Unlock()
if err != nil {
// TODO log
c.netConn.Close()
return err
}
}
}
}
type clientInfo struct {
}
type serverInfo struct {
}
func readType(r *bufio.Reader, t uint8) error {
packetType, err := r.ReadByte()
if err != nil {
return err
}
if packetType != t {
return fmt.Errorf("bad packet type 0x%X, want 0x%X", packetType, t)
}
return nil
}
func putUint32(w io.Writer, v uint32) error {
var b [4]byte
bin.PutUint32(b[:], v)
_, err := w.Write(b[:])
return err
}
func readUint32(r io.Reader, maxVal uint32) (uint32, error) {
b := make([]byte, 4)
if _, err := io.ReadFull(r, b); err != nil {
return 0, err
}
val := bin.Uint32(b)
if val > maxVal {
return 0, fmt.Errorf("uint32 %d exceeds limit %d", val, maxVal)
}
return val, nil
}