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packet.go
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packet.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 packet
import (
"encoding/binary"
"fmt"
"net"
"net/netip"
"strings"
"github.com/qwenode/tailscale/net/netaddr"
"github.com/qwenode/tailscale/types/ipproto"
)
const unknown = ipproto.Unknown
// RFC1858: prevent overlapping fragment attacks.
const minFrag = 60 + 20 // max IPv4 header + basic TCP header
type TCPFlag uint8
const (
TCPFin TCPFlag = 0x01
TCPSyn TCPFlag = 0x02
TCPRst TCPFlag = 0x04
TCPPsh TCPFlag = 0x08
TCPAck TCPFlag = 0x10
TCPUrg TCPFlag = 0x20
TCPECNEcho TCPFlag = 0x40
TCPCWR TCPFlag = 0x80
TCPSynAck TCPFlag = TCPSyn | TCPAck
TCPECNBits TCPFlag = TCPECNEcho | TCPCWR
)
// Parsed is a minimal decoding of a packet suitable for use in filters.
type Parsed 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 is the IP protocol version of the packet (4 or
// 6), or 0 if the packet doesn't look like IPv4 or IPv6.
IPVersion uint8
// IPProto is the IP subprotocol (UDP, TCP, etc.). Valid iff IPVersion != 0.
IPProto ipproto.Proto
// SrcIP4 is the source address. Family matches IPVersion. Port is
// valid iff IPProto == TCP || IPProto == UDP.
Src netip.AddrPort
// DstIP4 is the destination address. Family matches IPVersion.
Dst netip.AddrPort
// TCPFlags is the packet's TCP flag bits. Valid iff IPProto == TCP.
TCPFlags TCPFlag
}
func (p *Parsed) String() string {
if p.IPVersion != 4 && p.IPVersion != 6 {
return "Unknown{???}"
}
// max is the maximum reasonable length of the string we are constructing.
// It's OK to overshoot, as the temp buffer is allocated on the stack.
const max = len("ICMPv6{[ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0]:65535 > [ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%enp5s0]:65535}")
b := make([]byte, 0, max)
b = append(b, p.IPProto.String()...)
b = append(b, '{')
b = p.Src.AppendTo(b)
b = append(b, ' ', '>', ' ')
b = p.Dst.AppendTo(b)
b = append(b, '}')
return string(b)
}
// Decode extracts data from the packet in b into q.
// It performs extremely simple packet decoding for basic IPv4 and IPv6 packet types.
// It extracts only the subprotocol id, IP addresses, and (if any) ports,
// and shouldn't need any memory allocation.
func (q *Parsed) Decode(b []byte) {
q.b = b
if len(b) < 1 {
q.IPVersion = 0
q.IPProto = unknown
return
}
q.IPVersion = b[0] >> 4
switch q.IPVersion {
case 4:
q.decode4(b)
case 6:
q.decode6(b)
default:
q.IPVersion = 0
q.IPProto = unknown
}
}
// StuffForTesting makes Parsed contain a len-bytes buffer. Used in
// tests to build up a synthetic parse result with a non-zero buffer.
func (q *Parsed) StuffForTesting(len int) {
q.b = make([]byte, len)
}
func (q *Parsed) decode4(b []byte) {
if len(b) < ip4HeaderLength {
q.IPVersion = 0
q.IPProto = unknown
return
}
// Check that it's IPv4.
q.IPProto = ipproto.Proto(b[9])
q.length = int(binary.BigEndian.Uint16(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.Src = withIP(q.Src, netaddr.IPv4(b[12], b[13], b[14], b[15]))
q.Dst = withIP(q.Dst, netaddr.IPv4(b[16], b[17], b[18], b[19]))
q.subofs = int((b[0] & 0x0F) << 2)
if q.subofs > q.length {
// next-proto starts beyond end of packet.
q.IPProto = unknown
return
}
sub := b[q.subofs:]
sub = sub[:len(sub):len(sub)] // help the compiler do bounds check elimination
// 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 := binary.BigEndian.Uint16(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 ipproto.ICMPv4:
if len(sub) < icmp4HeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, 0)
q.Dst = withPort(q.Dst, 0)
q.dataofs = q.subofs + icmp4HeaderLength
return
case ipproto.IGMP:
// Keep IPProto, but don't parse anything else
// out.
return
case ipproto.TCP:
if len(sub) < tcpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
q.TCPFlags = TCPFlag(sub[13])
headerLength := (sub[12] & 0xF0) >> 2
q.dataofs = q.subofs + int(headerLength)
return
case ipproto.UDP:
if len(sub) < udpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
q.dataofs = q.subofs + udpHeaderLength
return
case ipproto.SCTP:
if len(sub) < sctpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
return
case ipproto.TSMP:
// Inter-tailscale messages.
q.dataofs = q.subofs
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 = ipproto.Fragment
return
}
}
func (q *Parsed) decode6(b []byte) {
if len(b) < ip6HeaderLength {
q.IPVersion = 0
q.IPProto = unknown
return
}
q.IPProto = ipproto.Proto(b[6])
q.length = int(binary.BigEndian.Uint16(b[4:6])) + ip6HeaderLength
if len(b) < q.length {
// Packet was cut off before the full IPv6 length.
q.IPProto = unknown
return
}
// okay to ignore `ok` here, because IPs pulled from packets are
// always well-formed stdlib IPs.
srcIP, _ := netip.AddrFromSlice(net.IP(b[8:24]))
dstIP, _ := netip.AddrFromSlice(net.IP(b[24:40]))
q.Src = withIP(q.Src, srcIP)
q.Dst = withIP(q.Dst, dstIP)
// We don't support any IPv6 extension headers. Don't try to
// be clever. Therefore, the IP subprotocol always starts at
// byte 40.
//
// Note that this means we don't support fragmentation in
// IPv6. This is fine, because IPv6 strongly mandates that you
// should not fragment, which makes fragmentation on the open
// internet extremely uncommon.
//
// This also means we don't support IPSec headers (AH/ESP), or
// IPv6 jumbo frames. Those will get marked Unknown and
// dropped.
q.subofs = 40
sub := b[q.subofs:]
sub = sub[:len(sub):len(sub)] // help the compiler do bounds check elimination
switch q.IPProto {
case ipproto.ICMPv6:
if len(sub) < icmp6HeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, 0)
q.Dst = withPort(q.Dst, 0)
q.dataofs = q.subofs + icmp6HeaderLength
case ipproto.TCP:
if len(sub) < tcpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
q.TCPFlags = TCPFlag(sub[13])
headerLength := (sub[12] & 0xF0) >> 2
q.dataofs = q.subofs + int(headerLength)
return
case ipproto.UDP:
if len(sub) < udpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
q.dataofs = q.subofs + udpHeaderLength
case ipproto.SCTP:
if len(sub) < sctpHeaderLength {
q.IPProto = unknown
return
}
q.Src = withPort(q.Src, binary.BigEndian.Uint16(sub[0:2]))
q.Dst = withPort(q.Dst, binary.BigEndian.Uint16(sub[2:4]))
return
case ipproto.TSMP:
// Inter-tailscale messages.
q.dataofs = q.subofs
return
default:
q.IPProto = unknown
return
}
}
func (q *Parsed) IP4Header() IP4Header {
if q.IPVersion != 4 {
panic("IP4Header called on non-IPv4 Parsed")
}
ipid := binary.BigEndian.Uint16(q.b[4:6])
return IP4Header{
IPID: ipid,
IPProto: q.IPProto,
Src: q.Src.Addr(),
Dst: q.Dst.Addr(),
}
}
func (q *Parsed) IP6Header() IP6Header {
if q.IPVersion != 6 {
panic("IP6Header called on non-IPv6 Parsed")
}
ipid := (binary.BigEndian.Uint32(q.b[:4]) << 12) >> 12
return IP6Header{
IPID: ipid,
IPProto: q.IPProto,
Src: q.Src.Addr(),
Dst: q.Dst.Addr(),
}
}
func (q *Parsed) ICMP4Header() ICMP4Header {
return ICMP4Header{
IP4Header: q.IP4Header(),
Type: ICMP4Type(q.b[q.subofs+0]),
Code: ICMP4Code(q.b[q.subofs+1]),
}
}
func (q *Parsed) ICMP6Header() ICMP6Header {
return ICMP6Header{
IP6Header: q.IP6Header(),
Type: ICMP6Type(q.b[q.subofs+0]),
Code: ICMP6Code(q.b[q.subofs+1]),
}
}
func (q *Parsed) UDP4Header() UDP4Header {
return UDP4Header{
IP4Header: q.IP4Header(),
SrcPort: q.Src.Port(),
DstPort: q.Dst.Port(),
}
}
// Buffer returns the entire packet buffer.
// This is a read-only view; that is, q retains the ownership of the buffer.
func (q *Parsed) Buffer() []byte {
return q.b
}
// 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 *Parsed) Payload() []byte {
return q.b[q.dataofs:q.length]
}
// Transport returns the transport header and payload (IP subprotocol, such as TCP or UDP).
// This is a read-only view; that is, p retains the ownership of the buffer.
func (p *Parsed) Transport() []byte {
return p.b[p.subofs:]
}
// IsTCPSyn reports whether q is a TCP SYN packet,
// without ACK set. (i.e. the first packet in a new connection)
func (q *Parsed) IsTCPSyn() bool {
return (q.TCPFlags & TCPSynAck) == TCPSyn
}
// IsError reports whether q is an ICMP "Error" packet.
func (q *Parsed) IsError() bool {
switch q.IPProto {
case ipproto.ICMPv4:
if len(q.b) < q.subofs+8 {
return false
}
t := ICMP4Type(q.b[q.subofs])
return t == ICMP4Unreachable || t == ICMP4TimeExceeded
case ipproto.ICMPv6:
if len(q.b) < q.subofs+8 {
return false
}
t := ICMP6Type(q.b[q.subofs])
return t == ICMP6Unreachable || t == ICMP6TimeExceeded
default:
return false
}
}
// IsEchoRequest reports whether q is an ICMP Echo Request.
func (q *Parsed) IsEchoRequest() bool {
switch q.IPProto {
case ipproto.ICMPv4:
return len(q.b) >= q.subofs+8 && ICMP4Type(q.b[q.subofs]) == ICMP4EchoRequest && ICMP4Code(q.b[q.subofs+1]) == ICMP4NoCode
case ipproto.ICMPv6:
return len(q.b) >= q.subofs+8 && ICMP6Type(q.b[q.subofs]) == ICMP6EchoRequest && ICMP6Code(q.b[q.subofs+1]) == ICMP6NoCode
default:
return false
}
}
// IsEchoResponse reports whether q is an IPv4 ICMP Echo Response.
func (q *Parsed) IsEchoResponse() bool {
switch q.IPProto {
case ipproto.ICMPv4:
return len(q.b) >= q.subofs+8 && ICMP4Type(q.b[q.subofs]) == ICMP4EchoReply && ICMP4Code(q.b[q.subofs+1]) == ICMP4NoCode
case ipproto.ICMPv6:
return len(q.b) >= q.subofs+8 && ICMP6Type(q.b[q.subofs]) == ICMP6EchoReply && ICMP6Code(q.b[q.subofs+1]) == ICMP6NoCode
default:
return false
}
}
// EchoIDSeq extracts the identifier/sequence bytes from an ICMP Echo response,
// and returns them as a uint32, used to lookup internally routed ICMP echo
// responses. This function is intentionally lightweight as it is called on
// every incoming ICMP packet.
func (q *Parsed) EchoIDSeq() uint32 {
switch q.IPProto {
case ipproto.ICMPv4:
offset := ip4HeaderLength + icmp4HeaderLength
if len(q.b) < offset+4 {
return 0
}
return binary.LittleEndian.Uint32(q.b[offset:])
case ipproto.ICMPv6:
offset := ip6HeaderLength + icmp6HeaderLength
if len(q.b) < offset+4 {
return 0
}
return binary.LittleEndian.Uint32(q.b[offset:])
default:
return 0
}
}
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()
}
func withIP(ap netip.AddrPort, ip netip.Addr) netip.AddrPort {
return netip.AddrPortFrom(ip, ap.Port())
}
func withPort(ap netip.AddrPort, port uint16) netip.AddrPort {
return netip.AddrPortFrom(ap.Addr(), port)
}