mirror of
https://github.com/tailscale/tailscale.git
synced 2024-11-26 11:35:35 +00:00
366 lines
8.8 KiB
Go
366 lines
8.8 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 packet
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import (
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"encoding/binary"
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"fmt"
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"log"
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"net"
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"strings"
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"tailscale.com/types/strbuilder"
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)
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type IPProto int
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const (
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Junk IPProto = iota
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Fragment
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ICMP
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UDP
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TCP
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)
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// RFC1858: prevent overlapping fragment attacks.
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const minFrag = 60 + 20 // max IPv4 header + basic TCP header
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func (p IPProto) String() string {
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switch p {
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case Fragment:
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return "Frag"
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case ICMP:
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return "ICMP"
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case UDP:
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return "UDP"
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case TCP:
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return "TCP"
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default:
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return "Junk"
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}
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}
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// IP is an IPv4 address.
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type IP uint32
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// NewIP converts a standard library IP address into an IP.
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// It panics if b is not an IPv4 address.
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func NewIP(b net.IP) IP {
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b4 := b.To4()
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if b4 == nil {
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panic(fmt.Sprintf("To4(%v) failed", b))
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}
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return IP(binary.BigEndian.Uint32(b4))
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}
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func (ip IP) String() string {
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return fmt.Sprintf("%d.%d.%d.%d", byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
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}
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// ICMP types.
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const (
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ICMPEchoReply = 0x00
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ICMPEchoRequest = 0x08
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ICMPUnreachable = 0x03
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ICMPTimeExceeded = 0x0b
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)
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const (
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TCPSyn = 0x02
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TCPAck = 0x10
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TCPSynAck = TCPSyn | TCPAck
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)
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type QDecode struct {
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b []byte // Packet buffer that this decodes
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subofs int // byte offset of IP subprotocol
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IPProto IPProto // IP subprotocol (UDP, TCP, etc)
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SrcIP IP // IP source address
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DstIP IP // IP destination address
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SrcPort uint16 // TCP/UDP source port
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DstPort uint16 // TCP/UDP destination port
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TCPFlags uint8 // TCP flags (SYN, ACK, etc)
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}
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func (q *QDecode) String() string {
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if q.IPProto == Junk {
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return "Junk{}"
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}
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sb := strbuilder.Get()
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sb.WriteString(q.IPProto.String())
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sb.WriteByte('{')
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writeIPPort(sb, q.SrcIP, q.SrcPort)
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sb.WriteString(" > ")
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writeIPPort(sb, q.DstIP, q.DstPort)
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sb.WriteByte('}')
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return sb.String()
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}
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func writeIPPort(sb *strbuilder.Builder, ip IP, port uint16) {
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sb.WriteUint(uint64(byte(ip >> 24)))
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sb.WriteByte('.')
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sb.WriteUint(uint64(byte(ip >> 16)))
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sb.WriteByte('.')
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sb.WriteUint(uint64(byte(ip >> 8)))
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sb.WriteByte('.')
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sb.WriteUint(uint64(byte(ip)))
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sb.WriteByte(':')
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sb.WriteUint(uint64(port))
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}
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// based on https://tools.ietf.org/html/rfc1071
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func ipChecksum(b []byte) uint16 {
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var ac uint32
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i := 0
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n := len(b)
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for n >= 2 {
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ac += uint32(binary.BigEndian.Uint16(b[i : i+2]))
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n -= 2
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i += 2
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}
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if n == 1 {
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ac += uint32(b[i]) << 8
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}
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for (ac >> 16) > 0 {
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ac = (ac >> 16) + (ac & 0xffff)
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}
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return uint16(^ac)
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}
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var put16 = binary.BigEndian.PutUint16
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var put32 = binary.BigEndian.PutUint32
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// GenICMP returns the bytes of an ICMP packet.
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// If payload is too short or too long, it returns nil.
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func GenICMP(srcIP, dstIP IP, ipid uint16, icmpType, icmpCode uint8, payload []byte) []byte {
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if len(payload) < 4 {
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return nil
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}
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if len(payload) > 65535-24 {
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return nil
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}
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sz := 24 + len(payload)
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out := make([]byte, 24+len(payload))
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out[0] = 0x45 // IPv4, 20-byte header
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out[1] = 0x00 // DHCP, ECN
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put16(out[2:4], uint16(sz))
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put16(out[4:6], ipid)
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put16(out[6:8], 0) // flags, offset
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out[8] = 64 // TTL
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out[9] = 0x01 // ICMPv4
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// out[10:12] = 0x00 // blank IP header checksum
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put32(out[12:16], uint32(srcIP))
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put32(out[16:20], uint32(dstIP))
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out[20] = icmpType
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out[21] = icmpCode
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//out[22:24] = 0x00 // blank ICMP checksum
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copy(out[24:], payload)
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put16(out[10:12], ipChecksum(out[0:20]))
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put16(out[22:24], ipChecksum(out))
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return out
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}
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// An extremely simple packet decoder for basic IPv4 packet types.
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// It extracts only the subprotocol id, IP addresses, and (if any) ports,
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// and shouldn't need any memory allocation.
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func (q *QDecode) Decode(b []byte) {
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q.b = nil
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if len(b) < 20 {
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q.IPProto = Junk
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return
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}
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// Check that it's IPv4.
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// TODO(apenwarr): consider IPv6 support
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if ((b[0] & 0xF0) >> 4) != 4 {
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q.IPProto = Junk
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return
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}
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n := int(binary.BigEndian.Uint16(b[2:4]))
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if len(b) < n {
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// Packet was cut off before full IPv4 length.
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q.IPProto = Junk
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return
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}
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// If it's valid IPv4, then the IP addresses are valid
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q.SrcIP = IP(binary.BigEndian.Uint32(b[12:16]))
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q.DstIP = IP(binary.BigEndian.Uint32(b[16:20]))
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q.subofs = int((b[0] & 0x0F) * 4)
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sub := b[q.subofs:]
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// We don't care much about IP fragmentation, except insofar as it's
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// used for firewall bypass attacks. The trick is make the first
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// fragment of a TCP or UDP packet so short that it doesn't fit
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// the TCP or UDP header, so we can't read the port, in hope that
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// it'll sneak past. Then subsequent fragments fill it in, but we're
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// missing the first part of the header, so we can't read that either.
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//
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// A "perfectly correct" implementation would have to reassemble
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// fragments before deciding what to do. But the truth is there's
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// zero reason to send such a short first fragment, so we can treat
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// it as Junk. We can also treat any subsequent fragment that starts
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// at such a low offset as Junk.
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fragFlags := binary.BigEndian.Uint16(b[6:8])
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moreFrags := (fragFlags & 0x20) != 0
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fragOfs := fragFlags & 0x1FFF
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if fragOfs == 0 {
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// This is the first fragment
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if moreFrags && len(sub) < minFrag {
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// Suspiciously short first fragment, dump it.
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log.Printf("junk1!\n")
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q.IPProto = Junk
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return
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}
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// otherwise, this is either non-fragmented (the usual case)
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// or a big enough initial fragment that we can read the
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// whole subprotocol header.
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proto := b[9]
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switch proto {
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case 1: // ICMPv4
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if len(sub) < 8 {
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q.IPProto = Junk
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return
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}
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q.IPProto = ICMP
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q.SrcPort = 0
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q.DstPort = 0
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q.b = b
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return
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case 6: // TCP
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if len(sub) < 20 {
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q.IPProto = Junk
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return
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}
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q.IPProto = TCP
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q.SrcPort = binary.BigEndian.Uint16(sub[0:2])
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q.DstPort = binary.BigEndian.Uint16(sub[2:4])
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q.TCPFlags = sub[13] & 0x3F
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q.b = b
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return
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case 17: // UDP
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if len(sub) < 8 {
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q.IPProto = Junk
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return
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}
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q.IPProto = UDP
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q.SrcPort = binary.BigEndian.Uint16(sub[0:2])
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q.DstPort = binary.BigEndian.Uint16(sub[2:4])
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q.b = b
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return
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default:
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q.IPProto = Junk
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return
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}
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} else {
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// This is a fragment other than the first one.
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if fragOfs < minFrag {
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// First frag was suspiciously short, so we can't
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// trust the followup either.
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q.IPProto = Junk
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return
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}
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// otherwise, we have to permit the fragment to slide through.
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// Second and later fragments don't have sub-headers.
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// Ideally, we would drop fragments that we can't identify,
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// but that would require statefulness. Anyway, receivers'
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// kernels know to drop fragments where the initial fragment
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// doesn't arrive.
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q.IPProto = Fragment
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return
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}
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}
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// Returns a subset of the IP subprotocol section.
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func (q *QDecode) Sub(begin, n int) []byte {
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return q.b[q.subofs+begin : q.subofs+begin+n]
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}
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// Trim trims the buffer to its IPv4 length.
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// Sometimes packets arrive from an interface with extra bytes on the end.
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// This removes them.
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func (q *QDecode) Trim() []byte {
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n := binary.BigEndian.Uint16(q.b[2:4])
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return q.b[:n]
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}
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// IsTCPSyn reports whether q is a TCP SYN packet (i.e. the
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// first packet in a new connection).
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func (q *QDecode) IsTCPSyn() bool {
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return (q.TCPFlags & TCPSynAck) == TCPSyn
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}
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// IsError reports whether q is an IPv4 ICMP "Error" packet.
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func (q *QDecode) IsError() bool {
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if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
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switch q.b[q.subofs] {
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case ICMPUnreachable, ICMPTimeExceeded:
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return true
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}
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}
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return false
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}
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// IsEchoRequest reports whether q is an IPv4 ICMP Echo Request.
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func (q *QDecode) IsEchoRequest() bool {
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if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
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return q.b[q.subofs] == ICMPEchoRequest && q.b[q.subofs+1] == 0
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}
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return false
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}
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// IsEchoRequest reports whether q is an IPv4 ICMP Echo Response.
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func (q *QDecode) IsEchoResponse() bool {
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if q.IPProto == ICMP && len(q.b) >= q.subofs+8 {
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return q.b[q.subofs] == ICMPEchoReply && q.b[q.subofs+1] == 0
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}
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return false
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}
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// EchoResponse returns an IPv4 ICMP echo reply to the request in q.
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func (q *QDecode) EchoRespond() []byte {
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icmpid := binary.BigEndian.Uint16(q.Sub(4, 2))
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b := q.Trim()
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return GenICMP(q.DstIP, q.SrcIP, icmpid, ICMPEchoReply, 0, b[q.subofs+4:])
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}
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func Hexdump(b []byte) string {
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out := new(strings.Builder)
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for i := 0; i < len(b); i += 16 {
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if i > 0 {
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fmt.Fprintf(out, "\n")
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}
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fmt.Fprintf(out, " %04x ", i)
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j := 0
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for ; j < 16 && i+j < len(b); j++ {
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if j == 8 {
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fmt.Fprintf(out, " ")
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}
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fmt.Fprintf(out, "%02x ", b[i+j])
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}
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for ; j < 16; j++ {
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if j == 8 {
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fmt.Fprintf(out, " ")
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}
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fmt.Fprintf(out, " ")
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}
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fmt.Fprintf(out, " ")
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for j = 0; j < 16 && i+j < len(b); j++ {
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if b[i+j] >= 32 && b[i+j] < 128 {
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fmt.Fprintf(out, "%c", b[i+j])
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} else {
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fmt.Fprintf(out, ".")
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}
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}
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}
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return out.String()
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}
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