mirror of
https://github.com/tailscale/tailscale.git
synced 2024-12-11 18:54:42 +00:00
325 lines
8.3 KiB
Go
325 lines
8.3 KiB
Go
// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package STUN generates STUN request packets and parses response packets.
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package stun
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import (
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crand "crypto/rand"
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"encoding/binary"
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"errors"
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"hash/crc32"
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"net"
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)
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const (
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attrNumSoftware = 0x8022
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attrNumFingerprint = 0x8028
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attrMappedAddress = 0x0001
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attrXorMappedAddress = 0x0020
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// This alternative attribute type is not
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// mentioned in the RFC, but the shift into
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// the "comprehension-optional" range seems
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// like an easy mistake for a server to make.
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// And servers appear to send it.
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attrXorMappedAddressAlt = 0x8020
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software = "tailnode" // notably: 8 bytes long, so no padding
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bindingRequest = "\x00\x01"
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magicCookie = "\x21\x12\xa4\x42"
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lenFingerprint = 8 // 2+byte header + 2-byte length + 4-byte crc32
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headerLen = 20
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)
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// TxID is a transaction ID.
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type TxID [12]byte
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// NewTxID returns a new random TxID.
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func NewTxID() TxID {
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var tx TxID
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if _, err := crand.Read(tx[:]); err != nil {
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panic(err)
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}
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return tx
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}
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// Request generates a binding request STUN packet.
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// The transaction ID, tID, should be a random sequence of bytes.
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func Request(tID TxID) []byte {
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// STUN header, RFC5389 Section 6.
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const lenAttrSoftware = 4 + len(software)
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b := make([]byte, 0, headerLen+lenAttrSoftware+lenFingerprint)
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b = append(b, bindingRequest...)
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b = appendU16(b, uint16(lenAttrSoftware+lenFingerprint)) // number of bytes following header
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b = append(b, magicCookie...)
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b = append(b, tID[:]...)
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// Attribute SOFTWARE, RFC5389 Section 15.5.
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b = appendU16(b, attrNumSoftware)
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b = appendU16(b, uint16(len(software)))
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b = append(b, software...)
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// Attribute FINGERPRINT, RFC5389 Section 15.5.
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fp := fingerPrint(b)
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b = appendU16(b, attrNumFingerprint)
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b = appendU16(b, 4)
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b = appendU32(b, fp)
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return b
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}
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func fingerPrint(b []byte) uint32 { return crc32.ChecksumIEEE(b) ^ 0x5354554e }
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func appendU16(b []byte, v uint16) []byte {
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return append(b, byte(v>>8), byte(v))
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}
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func appendU32(b []byte, v uint32) []byte {
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return append(b, byte(v>>24), byte(v>>16), byte(v>>8), byte(v))
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}
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// ParseBindingRequest parses a STUN binding request.
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//
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// It returns an error unless it advertises that it came from
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// Tailscale.
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func ParseBindingRequest(b []byte) (TxID, error) {
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if !Is(b) {
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return TxID{}, ErrNotSTUN
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}
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if string(b[:len(bindingRequest)]) != bindingRequest {
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return TxID{}, ErrNotBindingRequest
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}
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var txID TxID
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copy(txID[:], b[8:8+len(txID)])
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var softwareOK bool
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var lastAttr uint16
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var gotFP uint32
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if err := foreachAttr(b[headerLen:], func(attrType uint16, a []byte) error {
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lastAttr = attrType
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if attrType == attrNumSoftware && string(a) == software {
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softwareOK = true
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}
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if attrType == attrNumFingerprint && len(a) == 4 {
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gotFP = binary.BigEndian.Uint32(a)
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}
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return nil
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}); err != nil {
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return TxID{}, err
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}
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if !softwareOK {
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return TxID{}, ErrWrongSoftware
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}
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if lastAttr != attrNumFingerprint {
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return TxID{}, ErrNoFingerprint
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}
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wantFP := fingerPrint(b[:len(b)-lenFingerprint])
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if gotFP != wantFP {
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return TxID{}, ErrWrongFingerprint
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}
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return txID, nil
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}
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var (
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ErrNotSTUN = errors.New("response is not a STUN packet")
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ErrNotSuccessResponse = errors.New("STUN packet is not a response")
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ErrMalformedAttrs = errors.New("STUN response has malformed attributes")
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ErrNotBindingRequest = errors.New("STUN request not a binding request")
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ErrWrongSoftware = errors.New("STUN request came from non-Tailscale software")
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ErrNoFingerprint = errors.New("STUN request didn't end in fingerprint")
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ErrWrongFingerprint = errors.New("STUN request had bogus fingerprint")
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)
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func foreachAttr(b []byte, fn func(attrType uint16, a []byte) error) error {
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for len(b) > 0 {
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if len(b) < 4 {
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return ErrMalformedAttrs
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}
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attrType := binary.BigEndian.Uint16(b[:2])
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attrLen := int(binary.BigEndian.Uint16(b[2:4]))
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attrLenWithPad := (attrLen + 3) &^ 3
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b = b[4:]
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if attrLenWithPad > len(b) {
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return ErrMalformedAttrs
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}
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if err := fn(attrType, b[:attrLen]); err != nil {
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return err
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}
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b = b[attrLenWithPad:]
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}
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return nil
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}
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// Response generates a binding response.
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func Response(txID TxID, ip net.IP, port uint16) []byte {
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if ip4 := ip.To4(); ip4 != nil {
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ip = ip4
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}
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var fam byte
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switch len(ip) {
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case net.IPv4len:
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fam = 1
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case net.IPv6len:
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fam = 2
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default:
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return nil
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}
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attrsLen := 8 + len(ip)
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b := make([]byte, 0, headerLen+attrsLen)
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// Header
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b = append(b, 0x01, 0x01) // success
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b = appendU16(b, uint16(attrsLen))
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b = append(b, magicCookie...)
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b = append(b, txID[:]...)
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// Attributes (well, one)
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b = appendU16(b, attrXorMappedAddress)
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b = appendU16(b, uint16(4+len(ip)))
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b = append(b,
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0, // unused byte
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fam)
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b = appendU16(b, port^0x2112) // first half of magicCookie
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for i, o := range []byte(ip) {
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if i < 4 {
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b = append(b, o^magicCookie[i])
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} else {
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b = append(b, o^txID[i-len(magicCookie)])
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}
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}
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return b
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}
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// ParseResponse parses a successful binding response STUN packet.
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// The IP address is extracted from the XOR-MAPPED-ADDRESS attribute.
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// The returned addr slice is owned by the caller and does not alias b.
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func ParseResponse(b []byte) (tID TxID, addr []byte, port uint16, err error) {
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if !Is(b) {
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return tID, nil, 0, ErrNotSTUN
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}
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copy(tID[:], b[8:8+len(tID)])
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if b[0] != 0x01 || b[1] != 0x01 {
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return tID, nil, 0, ErrNotSuccessResponse
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}
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attrsLen := int(binary.BigEndian.Uint16(b[2:4]))
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b = b[headerLen:] // remove STUN header
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if attrsLen > len(b) {
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return tID, nil, 0, ErrMalformedAttrs
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} else if len(b) > attrsLen {
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b = b[:attrsLen] // trim trailing packet bytes
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}
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var addr6, fallbackAddr, fallbackAddr6 []byte
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var port6, fallbackPort, fallbackPort6 uint16
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// Read through the attributes.
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// The the addr+port reported by XOR-MAPPED-ADDRESS
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// as the canonical value. If the attribute is not
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// present but the STUN server responds with
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// MAPPED-ADDRESS we fall back to it.
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if err := foreachAttr(b, func(attrType uint16, attr []byte) error {
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switch attrType {
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case attrXorMappedAddress, attrXorMappedAddressAlt:
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a, p, err := xorMappedAddress(tID, attr)
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if err != nil {
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return err
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}
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if len(a) == 16 {
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addr6, port6 = a, p
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} else {
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addr, port = a, p
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}
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case attrMappedAddress:
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a, p, err := mappedAddress(attr)
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if err != nil {
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return ErrMalformedAttrs
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}
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if len(a) == 16 {
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fallbackAddr6, fallbackPort6 = a, p
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} else {
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fallbackAddr, fallbackPort = a, p
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}
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}
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return nil
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}); err != nil {
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return TxID{}, nil, 0, err
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}
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if addr != nil {
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return tID, addr, port, nil
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}
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if fallbackAddr != nil {
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return tID, append([]byte{}, fallbackAddr...), fallbackPort, nil
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}
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if addr6 != nil {
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return tID, addr6, port6, nil
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}
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if fallbackAddr6 != nil {
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return tID, append([]byte{}, fallbackAddr6...), fallbackPort6, nil
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}
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return tID, nil, 0, ErrMalformedAttrs
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}
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func xorMappedAddress(tID TxID, b []byte) (addr []byte, port uint16, err error) {
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// XOR-MAPPED-ADDRESS attribute, RFC5389 Section 15.2
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if len(b) < 4 {
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return nil, 0, ErrMalformedAttrs
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}
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xorPort := binary.BigEndian.Uint16(b[2:4])
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addrField := b[4:]
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port = xorPort ^ 0x2112 // first half of magicCookie
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addrLen := familyAddrLen(b[1])
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if addrLen == 0 {
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return nil, 0, ErrMalformedAttrs
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}
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if len(addrField) < addrLen {
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return nil, 0, ErrMalformedAttrs
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}
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xorAddr := addrField[:addrLen]
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addr = make([]byte, addrLen)
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for i := range xorAddr {
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if i < len(magicCookie) {
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addr[i] = xorAddr[i] ^ magicCookie[i]
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} else {
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addr[i] = xorAddr[i] ^ tID[i-len(magicCookie)]
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}
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}
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return addr, port, nil
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}
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func familyAddrLen(fam byte) int {
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switch fam {
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case 0x01: // IPv4
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return net.IPv4len
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case 0x02: // IPv6
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return net.IPv6len
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default:
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return 0
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}
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}
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func mappedAddress(b []byte) (addr []byte, port uint16, err error) {
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if len(b) < 4 {
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return nil, 0, ErrMalformedAttrs
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}
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port = uint16(b[2])<<8 | uint16(b[3])
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addrField := b[4:]
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addrLen := familyAddrLen(b[1])
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if addrLen == 0 {
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return nil, 0, ErrMalformedAttrs
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}
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if len(addrField) < addrLen {
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return nil, 0, ErrMalformedAttrs
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}
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return append([]byte(nil), addrField[:addrLen]...), port, nil
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}
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// Is reports whether b is a STUN message.
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func Is(b []byte) bool {
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return len(b) >= headerLen &&
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b[0]&0b11000000 == 0 && // top two bits must be zero
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string(b[4:8]) == magicCookie
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}
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