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tailscale/wgengine/packet/packet.go

344 lines
8.7 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 packet
import (
"encoding/binary"
"fmt"
"strings"
"tailscale.com/types/strbuilder"
)
// RFC1858: prevent overlapping fragment attacks.
const minFrag = 60 + 20 // max IPv4 header + basic TCP header
const (
TCPSyn = 0x02
TCPAck = 0x10
TCPSynAck = TCPSyn | TCPAck
)
var (
get16 = binary.BigEndian.Uint16
get32 = binary.BigEndian.Uint32
put16 = binary.BigEndian.PutUint16
put32 = binary.BigEndian.PutUint32
)
// ParsedPacket is a minimal decoding of a packet suitable for use in filters.
//
// In general, it only supports IPv4. The IPv6 parsing is very minimal.
type ParsedPacket struct {
// b is the byte buffer that this decodes.
b []byte
// subofs is the offset of IP subprotocol.
subofs int
// dataofs is the offset of IP subprotocol payload.
dataofs int
// length is the total length of the packet.
// This is not the same as len(b) because b can have trailing zeros.
length int
IPVersion uint8 // 4, 6, or 0
IPProto IPProto // IP subprotocol (UDP, TCP, etc); the NextHeader field for IPv6
SrcIP IP // IP source address (not used for IPv6)
DstIP IP // IP destination address (not used for IPv6)
SrcPort uint16 // TCP/UDP source port
DstPort uint16 // TCP/UDP destination port
TCPFlags uint8 // TCP flags (SYN, ACK, etc)
}
// NextHeader
type NextHeader uint8
func (p *ParsedPacket) String() string {
if p.IPVersion == 6 {
return fmt.Sprintf("IPv6{Proto=%d}", p.IPProto)
}
switch p.IPProto {
case Unknown:
return "Unknown{???}"
}
sb := strbuilder.Get()
sb.WriteString(p.IPProto.String())
sb.WriteByte('{')
writeIPPort(sb, p.SrcIP, p.SrcPort)
sb.WriteString(" > ")
writeIPPort(sb, p.DstIP, p.DstPort)
sb.WriteByte('}')
return sb.String()
}
func writeIPPort(sb *strbuilder.Builder, ip IP, port uint16) {
sb.WriteUint(uint64(byte(ip >> 24)))
sb.WriteByte('.')
sb.WriteUint(uint64(byte(ip >> 16)))
sb.WriteByte('.')
sb.WriteUint(uint64(byte(ip >> 8)))
sb.WriteByte('.')
sb.WriteUint(uint64(byte(ip)))
sb.WriteByte(':')
sb.WriteUint(uint64(port))
}
// based on https://tools.ietf.org/html/rfc1071
func ipChecksum(b []byte) uint16 {
var ac uint32
i := 0
n := len(b)
for n >= 2 {
ac += uint32(get16(b[i : i+2]))
n -= 2
i += 2
}
if n == 1 {
ac += uint32(b[i]) << 8
}
for (ac >> 16) > 0 {
ac = (ac >> 16) + (ac & 0xffff)
}
return uint16(^ac)
}
// Decode extracts data from the packet in b into q.
// It performs extremely simple packet decoding for basic IPv4 packet types.
// It extracts only the subprotocol id, IP addresses, and (if any) ports,
// and shouldn't need any memory allocation.
func (q *ParsedPacket) Decode(b []byte) {
q.b = b
if len(b) < ipHeaderLength {
q.IPVersion = 0
q.IPProto = Unknown
return
}
// Check that it's IPv4.
// TODO(apenwarr): consider IPv6 support
q.IPVersion = (b[0] & 0xF0) >> 4
switch q.IPVersion {
case 4:
q.IPProto = IPProto(b[9])
case 6:
q.IPProto = IPProto(b[6]) // "Next Header" field
return
default:
q.IPVersion = 0
q.IPProto = Unknown
return
}
q.length = int(get16(b[2:4]))
if len(b) < q.length {
// Packet was cut off before full IPv4 length.
q.IPProto = Unknown
return
}
// If it's valid IPv4, then the IP addresses are valid
q.SrcIP = IP(get32(b[12:16]))
q.DstIP = IP(get32(b[16:20]))
q.subofs = int((b[0] & 0x0F) << 2)
sub := b[q.subofs:]
// We don't care much about IP fragmentation, except insofar as it's
// used for firewall bypass attacks. The trick is make the first
// fragment of a TCP or UDP packet so short that it doesn't fit
// the TCP or UDP header, so we can't read the port, in hope that
// it'll sneak past. Then subsequent fragments fill it in, but we're
// missing the first part of the header, so we can't read that either.
//
// A "perfectly correct" implementation would have to reassemble
// fragments before deciding what to do. But the truth is there's
// zero reason to send such a short first fragment, so we can treat
// it as Unknown. We can also treat any subsequent fragment that starts
// at such a low offset as Unknown.
fragFlags := get16(b[6:8])
moreFrags := (fragFlags & 0x20) != 0
fragOfs := fragFlags & 0x1FFF
if fragOfs == 0 {
// This is the first fragment
if moreFrags && len(sub) < minFrag {
// Suspiciously short first fragment, dump it.
q.IPProto = Unknown
return
}
// otherwise, this is either non-fragmented (the usual case)
// or a big enough initial fragment that we can read the
// whole subprotocol header.
switch q.IPProto {
case ICMP:
if len(sub) < icmpHeaderLength {
q.IPProto = Unknown
return
}
q.SrcPort = 0
q.DstPort = 0
q.dataofs = q.subofs + icmpHeaderLength
return
case TCP:
if len(sub) < tcpHeaderLength {
q.IPProto = Unknown
return
}
q.SrcPort = get16(sub[0:2])
q.DstPort = get16(sub[2:4])
q.TCPFlags = sub[13] & 0x3F
headerLength := (sub[12] & 0xF0) >> 2
q.dataofs = q.subofs + int(headerLength)
return
case UDP:
if len(sub) < udpHeaderLength {
q.IPProto = Unknown
return
}
q.SrcPort = get16(sub[0:2])
q.DstPort = get16(sub[2:4])
q.dataofs = q.subofs + udpHeaderLength
return
default:
q.IPProto = Unknown
return
}
} else {
// This is a fragment other than the first one.
if fragOfs < minFrag {
// First frag was suspiciously short, so we can't
// trust the followup either.
q.IPProto = Unknown
return
}
// otherwise, we have to permit the fragment to slide through.
// Second and later fragments don't have sub-headers.
// Ideally, we would drop fragments that we can't identify,
// but that would require statefulness. Anyway, receivers'
// kernels know to drop fragments where the initial fragment
// doesn't arrive.
q.IPProto = Fragment
return
}
}
func (q *ParsedPacket) IPHeader() IPHeader {
ipid := get16(q.b[4:6])
return IPHeader{
IPID: ipid,
IPProto: q.IPProto,
SrcIP: q.SrcIP,
DstIP: q.DstIP,
}
}
func (q *ParsedPacket) ICMPHeader() ICMPHeader {
return ICMPHeader{
IPHeader: q.IPHeader(),
Type: ICMPType(q.b[q.subofs+0]),
Code: ICMPCode(q.b[q.subofs+1]),
}
}
func (q *ParsedPacket) UDPHeader() UDPHeader {
return UDPHeader{
IPHeader: q.IPHeader(),
SrcPort: q.SrcPort,
DstPort: q.DstPort,
}
}
// Buffer returns the entire packet buffer.
// This is a read-only view; that is, q retains the ownership of the buffer.
func (q *ParsedPacket) Buffer() []byte {
return q.b
}
// Sub returns the IP subprotocol section.
// This is a read-only view; that is, q retains the ownership of the buffer.
func (q *ParsedPacket) Sub(begin, n int) []byte {
return q.b[q.subofs+begin : q.subofs+begin+n]
}
// Payload returns the payload of the IP subprotocol section.
// This is a read-only view; that is, q retains the ownership of the buffer.
func (q *ParsedPacket) Payload() []byte {
return q.b[q.dataofs:q.length]
}
// Trim trims the buffer to its IPv4 length.
// Sometimes packets arrive from an interface with extra bytes on the end.
// This removes them.
func (q *ParsedPacket) Trim() []byte {
return q.b[:q.length]
}
// IsTCPSyn reports whether q is a TCP SYN packet
// (i.e. the first packet in a new connection).
func (q *ParsedPacket) IsTCPSyn() bool {
return (q.TCPFlags & TCPSynAck) == TCPSyn
}
// IsError reports whether q is an IPv4 ICMP "Error" packet.
func (q *ParsedPacket) IsError() bool {
if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
switch ICMPType(q.b[q.subofs]) {
case ICMPUnreachable, ICMPTimeExceeded:
return true
}
}
return false
}
// IsEchoRequest reports whether q is an IPv4 ICMP Echo Request.
func (q *ParsedPacket) IsEchoRequest() bool {
if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
return ICMPType(q.b[q.subofs]) == ICMPEchoRequest &&
ICMPCode(q.b[q.subofs+1]) == ICMPNoCode
}
return false
}
// IsEchoRequest reports whether q is an IPv4 ICMP Echo Response.
func (q *ParsedPacket) IsEchoResponse() bool {
if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
return ICMPType(q.b[q.subofs]) == ICMPEchoReply &&
ICMPCode(q.b[q.subofs+1]) == ICMPNoCode
}
return false
}
func Hexdump(b []byte) string {
out := new(strings.Builder)
for i := 0; i < len(b); i += 16 {
if i > 0 {
fmt.Fprintf(out, "\n")
}
fmt.Fprintf(out, " %04x ", i)
j := 0
for ; j < 16 && i+j < len(b); j++ {
if j == 8 {
fmt.Fprintf(out, " ")
}
fmt.Fprintf(out, "%02x ", b[i+j])
}
for ; j < 16; j++ {
if j == 8 {
fmt.Fprintf(out, " ")
}
fmt.Fprintf(out, " ")
}
fmt.Fprintf(out, " ")
for j = 0; j < 16 && i+j < len(b); j++ {
if b[i+j] >= 32 && b[i+j] < 128 {
fmt.Fprintf(out, "%c", b[i+j])
} else {
fmt.Fprintf(out, ".")
}
}
}
return out.String()
}