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tailscale/disco/disco.go

273 lines
7.4 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package disco contains the discovery message types.
//
// A discovery message is:
//
// Header:
//
// magic [6]byte // “TS💬” (0x54 53 f0 9f 92 ac)
// senderDiscoPub [32]byte // nacl public key
// nonce [24]byte
//
// The recipient then decrypts the bytes following (the nacl box)
// and then the inner payload structure is:
//
// messageType byte (the MessageType constants below)
// messageVersion byte (0 for now; but always ignore bytes at the end)
// message-payload [...]byte
package disco
import (
"encoding/binary"
"errors"
"fmt"
"net"
"net/netip"
"go4.org/mem"
"tailscale.com/types/key"
)
// Magic is the 6 byte header of all discovery messages.
const Magic = "TS💬" // 6 bytes: 0x54 53 f0 9f 92 ac
const keyLen = 32
// NonceLen is the length of the nonces used by nacl box.
const NonceLen = 24
type MessageType byte
const (
TypePing = MessageType(0x01)
TypePong = MessageType(0x02)
TypeCallMeMaybe = MessageType(0x03)
)
const v0 = byte(0)
var errShort = errors.New("short message")
// LooksLikeDiscoWrapper reports whether p looks like it's a packet
// containing an encrypted disco message.
func LooksLikeDiscoWrapper(p []byte) bool {
if len(p) < len(Magic)+keyLen+NonceLen {
return false
}
return string(p[:len(Magic)]) == Magic
}
// Source returns the slice of p that represents the
// disco public key source, and whether p looks like
// a disco message.
func Source(p []byte) (src []byte, ok bool) {
if !LooksLikeDiscoWrapper(p) {
return nil, false
}
return p[len(Magic):][:keyLen], true
}
// Parse parses the encrypted part of the message from inside the
// nacl box.
func Parse(p []byte) (Message, error) {
if len(p) < 2 {
return nil, errShort
}
t, ver, p := MessageType(p[0]), p[1], p[2:]
switch t {
case TypePing:
return parsePing(ver, p)
case TypePong:
return parsePong(ver, p)
case TypeCallMeMaybe:
return parseCallMeMaybe(ver, p)
default:
return nil, fmt.Errorf("unknown message type 0x%02x", byte(t))
}
}
// Message a discovery message.
type Message interface {
// AppendMarshal appends the message's marshaled representation.
AppendMarshal([]byte) []byte
}
// MessageHeaderLen is the length of a message header, 2 bytes for type and version.
const MessageHeaderLen = 2
// appendMsgHeader appends two bytes (for t and ver) and then also
// dataLen bytes to b, returning the appended slice in all. The
// returned data slice is a subslice of all with just dataLen bytes of
// where the caller will fill in the data.
func appendMsgHeader(b []byte, t MessageType, ver uint8, dataLen int) (all, data []byte) {
// TODO: optimize this?
all = append(b, make([]byte, dataLen+2)...)
all[len(b)] = byte(t)
all[len(b)+1] = ver
data = all[len(b)+2:]
return
}
type Ping struct {
// TxID is a random client-generated per-ping transaction ID.
TxID [12]byte
// NodeKey is allegedly the ping sender's wireguard public key.
// Old clients (~1.16.0 and earlier) don't send this field.
// It shouldn't be trusted by itself, but can be combined with
// netmap data to reduce the discokey:nodekey relation from 1:N to
// 1:1.
NodeKey key.NodePublic
// Padding is the number of 0 bytes at the end of the
// message. (It's used to probe path MTU.)
Padding int
}
// PingLen is the length of a marshalled ping message, without the message
// header or padding.
const PingLen = 12 + key.NodePublicRawLen
func (m *Ping) AppendMarshal(b []byte) []byte {
dataLen := 12
hasKey := !m.NodeKey.IsZero()
if hasKey {
dataLen += key.NodePublicRawLen
}
ret, d := appendMsgHeader(b, TypePing, v0, dataLen+m.Padding)
n := copy(d, m.TxID[:])
if hasKey {
m.NodeKey.AppendTo(d[:n])
}
return ret
}
func parsePing(ver uint8, p []byte) (m *Ping, err error) {
if len(p) < 12 {
return nil, errShort
}
m = new(Ping)
m.Padding = len(p)
p = p[copy(m.TxID[:], p):]
m.Padding -= 12
// Deliberately lax on longer-than-expected messages, for future
// compatibility.
if len(p) >= key.NodePublicRawLen {
m.NodeKey = key.NodePublicFromRaw32(mem.B(p[:key.NodePublicRawLen]))
m.Padding -= key.NodePublicRawLen
}
return m, nil
}
// CallMeMaybe is a message sent only over DERP to request that the recipient try
// to open up a magicsock path back to the sender.
//
// The sender should've already sent UDP packets to the peer to open
// up the stateful firewall mappings inbound.
//
// The recipient may choose to not open a path back, if it's already
// happy with its path. But usually it will.
type CallMeMaybe struct {
// MyNumber is what the peer believes its endpoints are.
//
// Prior to Tailscale 1.4, the endpoints were exchanged purely
// between nodes and the control server.
//
// Starting with Tailscale 1.4, clients advertise their endpoints.
// Older clients won't use this, but newer clients should
// use any endpoints in here that aren't included from control.
//
// Control might have sent stale endpoints if the client was idle
// before contacting us. In that case, the client likely did a STUN
// request immediately before sending the CallMeMaybe to recreate
// their NAT port mapping, and that new good endpoint is included
// in this field, but might not yet be in control's endpoints.
// (And in the future, control will stop distributing endpoints
// when clients are suitably new.)
MyNumber []netip.AddrPort
}
const epLength = 16 + 2 // 16 byte IP address + 2 byte port
func (m *CallMeMaybe) AppendMarshal(b []byte) []byte {
ret, p := appendMsgHeader(b, TypeCallMeMaybe, v0, epLength*len(m.MyNumber))
for _, ipp := range m.MyNumber {
a := ipp.Addr().As16()
copy(p[:], a[:])
binary.BigEndian.PutUint16(p[16:], ipp.Port())
p = p[epLength:]
}
return ret
}
func parseCallMeMaybe(ver uint8, p []byte) (m *CallMeMaybe, err error) {
m = new(CallMeMaybe)
if len(p)%epLength != 0 || ver != 0 || len(p) == 0 {
return m, nil
}
m.MyNumber = make([]netip.AddrPort, 0, len(p)/epLength)
for len(p) > 0 {
var a [16]byte
copy(a[:], p)
m.MyNumber = append(m.MyNumber, netip.AddrPortFrom(
netip.AddrFrom16(a).Unmap(),
binary.BigEndian.Uint16(p[16:18])))
p = p[epLength:]
}
return m, nil
}
// Pong is a response a Ping.
//
// It includes the sender's source IP + port, so it's effectively a
// STUN response.
type Pong struct {
TxID [12]byte
Src netip.AddrPort // 18 bytes (16+2) on the wire; v4-mapped ipv6 for IPv4
}
// pongLen is the length of a marshalled pong message, without the message
// header or padding.
const pongLen = 12 + 16 + 2
func (m *Pong) AppendMarshal(b []byte) []byte {
ret, d := appendMsgHeader(b, TypePong, v0, pongLen)
d = d[copy(d, m.TxID[:]):]
ip16 := m.Src.Addr().As16()
d = d[copy(d, ip16[:]):]
binary.BigEndian.PutUint16(d, m.Src.Port())
return ret
}
func parsePong(ver uint8, p []byte) (m *Pong, err error) {
if len(p) < pongLen {
return nil, errShort
}
m = new(Pong)
copy(m.TxID[:], p)
p = p[12:]
srcIP, _ := netip.AddrFromSlice(net.IP(p[:16]))
p = p[16:]
port := binary.BigEndian.Uint16(p)
m.Src = netip.AddrPortFrom(srcIP.Unmap(), port)
return m, nil
}
// MessageSummary returns a short summary of m for logging purposes.
func MessageSummary(m Message) string {
switch m := m.(type) {
case *Ping:
return fmt.Sprintf("ping tx=%x padding=%v", m.TxID[:6], m.Padding)
case *Pong:
return fmt.Sprintf("pong tx=%x", m.TxID[:6])
case *CallMeMaybe:
return "call-me-maybe"
default:
return fmt.Sprintf("%#v", m)
}
}