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stun.go
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stun.go
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// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package STUN generates STUN request packets and parses response packets.
package stun
import (
"bytes"
crand "crypto/rand"
"encoding/binary"
"errors"
"hash/crc32"
"net"
"net/netip"
)
const (
attrNumSoftware = 0x8022
attrNumFingerprint = 0x8028
attrMappedAddress = 0x0001
attrXorMappedAddress = 0x0020
// This alternative attribute type is not
// mentioned in the RFC, but the shift into
// the "comprehension-optional" range seems
// like an easy mistake for a server to make.
// And servers appear to send it.
attrXorMappedAddressAlt = 0x8020
software = "tailnode" // notably: 8 bytes long, so no padding
bindingRequest = "\x00\x01"
magicCookie = "\x21\x12\xa4\x42"
lenFingerprint = 8 // 2+byte header + 2-byte length + 4-byte crc32
headerLen = 20
)
// TxID is a transaction ID.
type TxID [12]byte
// NewTxID returns a new random TxID.
func NewTxID() TxID {
var tx TxID
if _, err := crand.Read(tx[:]); err != nil {
panic(err)
}
return tx
}
// Request generates a binding request STUN packet.
// The transaction ID, tID, should be a random sequence of bytes.
func Request(tID TxID) []byte {
// STUN header, RFC5389 Section 6.
const lenAttrSoftware = 4 + len(software)
b := make([]byte, 0, headerLen+lenAttrSoftware+lenFingerprint)
b = append(b, bindingRequest...)
b = appendU16(b, uint16(lenAttrSoftware+lenFingerprint)) // number of bytes following header
b = append(b, magicCookie...)
b = append(b, tID[:]...)
// Attribute SOFTWARE, RFC5389 Section 15.5.
b = appendU16(b, attrNumSoftware)
b = appendU16(b, uint16(len(software)))
b = append(b, software...)
// Attribute FINGERPRINT, RFC5389 Section 15.5.
fp := fingerPrint(b)
b = appendU16(b, attrNumFingerprint)
b = appendU16(b, 4)
b = appendU32(b, fp)
return b
}
func fingerPrint(b []byte) uint32 { return crc32.ChecksumIEEE(b) ^ 0x5354554e }
func appendU16(b []byte, v uint16) []byte {
return append(b, byte(v>>8), byte(v))
}
func appendU32(b []byte, v uint32) []byte {
return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
}
// ParseBindingRequest parses a STUN binding request.
//
// It returns an error unless it advertises that it came from
// Tailscale.
func ParseBindingRequest(b []byte) (TxID, error) {
if !Is(b) {
return TxID{}, ErrNotSTUN
}
if string(b[:len(bindingRequest)]) != bindingRequest {
return TxID{}, ErrNotBindingRequest
}
var txID TxID
copy(txID[:], b[8:8+len(txID)])
var softwareOK bool
var lastAttr uint16
var gotFP uint32
if err := foreachAttr(b[headerLen:], func(attrType uint16, a []byte) error {
lastAttr = attrType
if attrType == attrNumSoftware && string(a) == software {
softwareOK = true
}
if attrType == attrNumFingerprint && len(a) == 4 {
gotFP = binary.BigEndian.Uint32(a)
}
return nil
}); err != nil {
return TxID{}, err
}
if !softwareOK {
return TxID{}, ErrWrongSoftware
}
if lastAttr != attrNumFingerprint {
return TxID{}, ErrNoFingerprint
}
wantFP := fingerPrint(b[:len(b)-lenFingerprint])
if gotFP != wantFP {
return TxID{}, ErrWrongFingerprint
}
return txID, nil
}
var (
ErrNotSTUN = errors.New("response is not a STUN packet")
ErrNotSuccessResponse = errors.New("STUN packet is not a response")
ErrMalformedAttrs = errors.New("STUN response has malformed attributes")
ErrNotBindingRequest = errors.New("STUN request not a binding request")
ErrWrongSoftware = errors.New("STUN request came from non-Tailscale software")
ErrNoFingerprint = errors.New("STUN request didn't end in fingerprint")
ErrWrongFingerprint = errors.New("STUN request had bogus fingerprint")
)
func foreachAttr(b []byte, fn func(attrType uint16, a []byte) error) error {
for len(b) > 0 {
if len(b) < 4 {
return ErrMalformedAttrs
}
attrType := binary.BigEndian.Uint16(b[:2])
attrLen := int(binary.BigEndian.Uint16(b[2:4]))
attrLenWithPad := (attrLen + 3) &^ 3
b = b[4:]
if attrLenWithPad > len(b) {
return ErrMalformedAttrs
}
if err := fn(attrType, b[:attrLen]); err != nil {
return err
}
b = b[attrLenWithPad:]
}
return nil
}
// Response generates a binding response.
func Response(txID TxID, addrPort netip.AddrPort) []byte {
addr := addrPort.Addr()
var fam byte
if addr.Is4() {
fam = 1
} else if addr.Is6() {
fam = 2
} else {
return nil
}
attrsLen := 8 + addr.BitLen()/8
b := make([]byte, 0, headerLen+attrsLen)
// Header
b = append(b, 0x01, 0x01) // success
b = appendU16(b, uint16(attrsLen))
b = append(b, magicCookie...)
b = append(b, txID[:]...)
// Attributes (well, one)
b = appendU16(b, attrXorMappedAddress)
b = appendU16(b, uint16(4+addr.BitLen()/8))
b = append(b,
0, // unused byte
fam)
b = appendU16(b, addrPort.Port()^0x2112) // first half of magicCookie
ipa := addr.As16()
for i, o := range ipa[16-addr.BitLen()/8:] {
if i < 4 {
b = append(b, o^magicCookie[i])
} else {
b = append(b, o^txID[i-len(magicCookie)])
}
}
return b
}
// ParseResponse parses a successful binding response STUN packet.
// The IP address is extracted from the XOR-MAPPED-ADDRESS attribute.
func ParseResponse(b []byte) (tID TxID, addr netip.AddrPort, err error) {
if !Is(b) {
return tID, netip.AddrPort{}, ErrNotSTUN
}
copy(tID[:], b[8:8+len(tID)])
if b[0] != 0x01 || b[1] != 0x01 {
return tID, netip.AddrPort{}, ErrNotSuccessResponse
}
attrsLen := int(binary.BigEndian.Uint16(b[2:4]))
b = b[headerLen:] // remove STUN header
if attrsLen > len(b) {
return tID, netip.AddrPort{}, ErrMalformedAttrs
} else if len(b) > attrsLen {
b = b[:attrsLen] // trim trailing packet bytes
}
var fallbackAddr netip.AddrPort
// Read through the attributes.
// The the addr+port reported by XOR-MAPPED-ADDRESS
// as the canonical value. If the attribute is not
// present but the STUN server responds with
// MAPPED-ADDRESS we fall back to it.
if err := foreachAttr(b, func(attrType uint16, attr []byte) error {
switch attrType {
case attrXorMappedAddress, attrXorMappedAddressAlt:
ipSlice, port, err := xorMappedAddress(tID, attr)
if err != nil {
return err
}
if ip, ok := netip.AddrFromSlice(ipSlice); ok {
addr = netip.AddrPortFrom(ip.Unmap(), port)
}
case attrMappedAddress:
ipSlice, port, err := mappedAddress(attr)
if err != nil {
return ErrMalformedAttrs
}
if ip, ok := netip.AddrFromSlice(ipSlice); ok {
fallbackAddr = netip.AddrPortFrom(ip.Unmap(), port)
}
}
return nil
}); err != nil {
return TxID{}, netip.AddrPort{}, err
}
if addr.IsValid() {
return tID, addr, nil
}
if fallbackAddr.IsValid() {
return tID, fallbackAddr, nil
}
return tID, netip.AddrPort{}, ErrMalformedAttrs
}
func xorMappedAddress(tID TxID, b []byte) (addr []byte, port uint16, err error) {
// XOR-MAPPED-ADDRESS attribute, RFC5389 Section 15.2
if len(b) < 4 {
return nil, 0, ErrMalformedAttrs
}
xorPort := binary.BigEndian.Uint16(b[2:4])
addrField := b[4:]
port = xorPort ^ 0x2112 // first half of magicCookie
addrLen := familyAddrLen(b[1])
if addrLen == 0 {
return nil, 0, ErrMalformedAttrs
}
if len(addrField) < addrLen {
return nil, 0, ErrMalformedAttrs
}
xorAddr := addrField[:addrLen]
addr = make([]byte, addrLen)
for i := range xorAddr {
if i < len(magicCookie) {
addr[i] = xorAddr[i] ^ magicCookie[i]
} else {
addr[i] = xorAddr[i] ^ tID[i-len(magicCookie)]
}
}
return addr, port, nil
}
func familyAddrLen(fam byte) int {
switch fam {
case 0x01: // IPv4
return net.IPv4len
case 0x02: // IPv6
return net.IPv6len
default:
return 0
}
}
func mappedAddress(b []byte) (addr []byte, port uint16, err error) {
if len(b) < 4 {
return nil, 0, ErrMalformedAttrs
}
port = uint16(b[2])<<8 | uint16(b[3])
addrField := b[4:]
addrLen := familyAddrLen(b[1])
if addrLen == 0 {
return nil, 0, ErrMalformedAttrs
}
if len(addrField) < addrLen {
return nil, 0, ErrMalformedAttrs
}
return bytes.Clone(addrField[:addrLen]), port, nil
}
// Is reports whether b is a STUN message.
func Is(b []byte) bool {
return len(b) >= headerLen &&
b[0]&0b11000000 == 0 && // top two bits must be zero
string(b[4:8]) == magicCookie
}