tailscale/derp/derp_client.go

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// 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
import (
"bufio"
crand "crypto/rand"
"encoding/json"
"errors"
"fmt"
"io"
"net"
"sync"
"time"
"golang.org/x/crypto/nacl/box"
"tailscale.com/types/key"
"tailscale.com/types/logger"
)
type Client struct {
serverKey key.Public // of the DERP server; not a machine or node key
privateKey key.Private
publicKey key.Public // of privateKey
protoVersion int // min of server+client
logf logger.Logf
nc net.Conn
br *bufio.Reader
wmu sync.Mutex // hold while writing to bw
bw *bufio.Writer
readErr error // sticky read error
}
func NewClient(privateKey key.Private, nc net.Conn, brw *bufio.ReadWriter, logf logger.Logf) (*Client, error) {
c := &Client{
privateKey: privateKey,
publicKey: privateKey.Public(),
logf: logf,
nc: nc,
br: brw.Reader,
bw: brw.Writer,
}
if err := c.recvServerKey(); err != nil {
return nil, fmt.Errorf("derp.Client: failed to receive server key: %v", err)
}
if err := c.sendClientKey(); err != nil {
return nil, fmt.Errorf("derp.Client: failed to send client key: %v", err)
}
info, err := c.recvServerInfo()
if err != nil {
return nil, fmt.Errorf("derp.Client: failed to receive server info: %v", err)
}
c.protoVersion = minInt(protocolVersion, info.Version)
return c, nil
}
func (c *Client) recvServerKey() error {
var buf [40]byte
t, flen, err := readFrame(c.br, 1<<10, buf[:])
if err == io.ErrShortBuffer {
// For future-proofing, allow server to send more in its greeting.
err = nil
}
if err != nil {
return err
}
if flen < uint32(len(buf)) || t != frameServerKey || string(buf[:len(magic)]) != magic {
return errors.New("invalid server greeting")
}
copy(c.serverKey[:], buf[len(magic):])
return nil
}
func (c *Client) recvServerInfo() (*serverInfo, error) {
fl, err := readFrameTypeHeader(c.br, frameServerInfo)
if err != nil {
return nil, err
}
const maxLength = nonceLen + maxInfoLen
if fl < nonceLen {
return nil, fmt.Errorf("short serverInfo frame")
}
if fl > maxLength {
return nil, fmt.Errorf("long serverInfo frame")
}
// TODO: add a read-nonce-and-box helper
var nonce [nonceLen]byte
if _, err := io.ReadFull(c.br, nonce[:]); err != nil {
return nil, fmt.Errorf("nonce: %v", err)
}
msgLen := fl - nonceLen
msgbox := make([]byte, msgLen)
if _, err := io.ReadFull(c.br, msgbox); err != nil {
return nil, fmt.Errorf("msgbox: %v", err)
}
msg, ok := box.Open(nil, msgbox, &nonce, c.serverKey.B32(), c.privateKey.B32())
if !ok {
return nil, fmt.Errorf("msgbox: cannot open len=%d with server key %x", msgLen, c.serverKey[:])
}
info := new(serverInfo)
if err := json.Unmarshal(msg, info); err != nil {
return nil, fmt.Errorf("msg: %v", err)
}
return info, nil
}
type clientInfo struct {
Version int // `json:"version,omitempty"`
}
func (c *Client) sendClientKey() error {
var nonce [nonceLen]byte
if _, err := crand.Read(nonce[:]); err != nil {
return err
}
msg, err := json.Marshal(clientInfo{Version: protocolVersion})
if err != nil {
return err
}
msgbox := box.Seal(nil, msg, &nonce, c.serverKey.B32(), c.privateKey.B32())
buf := make([]byte, 0, nonceLen+keyLen+len(msgbox))
buf = append(buf, c.publicKey[:]...)
buf = append(buf, nonce[:]...)
buf = append(buf, msgbox...)
return writeFrame(c.bw, frameClientInfo, buf)
}
// Send sends a packet to the Tailscale node identified by dstKey.
//
// It is an error if the packet is larger than 64KB.
func (c *Client) Send(dstKey key.Public, pkt []byte) error { return c.send(dstKey, pkt) }
func (c *Client) send(dstKey key.Public, pkt []byte) (ret error) {
defer func() {
if ret != nil {
ret = fmt.Errorf("derp.Send: %v", ret)
}
}()
if len(pkt) > MaxPacketSize {
return fmt.Errorf("packet too big: %d", len(pkt))
}
c.wmu.Lock()
defer c.wmu.Unlock()
if err := writeFrameHeader(c.bw, frameSendPacket, uint32(len(dstKey)+len(pkt))); err != nil {
return err
}
if _, err := c.bw.Write(dstKey[:]); err != nil {
return err
}
if _, err := c.bw.Write(pkt); err != nil {
return err
}
return c.bw.Flush()
}
// NotePreferred sends a packet that tells the server whether this
// client is the user's preferred server. This is only used in the
// server for stats.
func (c *Client) NotePreferred(preferred bool) (err error) {
defer func() {
if err != nil {
err = fmt.Errorf("derp.NotePreferred: %v", err)
}
}()
c.wmu.Lock()
defer c.wmu.Unlock()
if err := writeFrameHeader(c.bw, frameNotePreferred, 1); err != nil {
return err
}
var b byte = 0x00
if preferred {
b = 0x01
}
if err := c.bw.WriteByte(b); err != nil {
return err
}
return c.bw.Flush()
}
// ReceivedMessage represents a type returned by Client.Recv. Unless
// otherwise documented, the returned message aliases the byte slice
// provided to Recv and thus the message is only as good as that
// buffer, which is up to the caller.
type ReceivedMessage interface {
msg()
}
// ReceivedPacket is a ReceivedMessage representing an incoming packet.
type ReceivedPacket struct {
Source key.Public
// Data is the received packet bytes. It aliases the memory
// passed to Client.Recv.
Data []byte
}
func (ReceivedPacket) msg() {}
// Recv reads a message from the DERP server.
// The provided buffer must be large enough to receive a complete packet,
// which in practice are are 1.5-4 KB, but can be up to 64 KB.
// Once Recv returns an error, the Client is dead forever.
func (c *Client) Recv(b []byte) (m ReceivedMessage, err error) {
if c.readErr != nil {
return nil, c.readErr
}
defer func() {
if err != nil {
err = fmt.Errorf("derp.Recv: %v", err)
c.readErr = err
}
}()
for {
c.nc.SetReadDeadline(time.Now().Add(120 * time.Second))
t, n, err := readFrame(c.br, 1<<20, b)
if err != nil {
return nil, err
}
switch t {
default:
continue
case frameKeepAlive:
// TODO: eventually we'll have server->client pings that
// require ack pongs.
continue
case frameRecvPacket:
var rp ReceivedPacket
if c.protoVersion < protocolSrcAddrs {
rp.Data = b[:n]
} else {
if n < keyLen {
c.logf("[unexpected] dropping short packet from DERP server")
continue
}
copy(rp.Source[:], b[:keyLen])
rp.Data = b[keyLen:n]
}
return rp, nil
}
}
}