tailscale/wgengine/packet/ip.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 (
	"fmt"
	"net"
)

// IP is an IPv4 address.
type IP uint32

// NewIP converts a standard library IP address into an IP.
// It panics if b is not an IPv4 address.
func NewIP(b net.IP) IP {
	b4 := b.To4()
	if b4 == nil {
		panic(fmt.Sprintf("To4(%v) failed", b))
	}
	return IP(get32(b4))
}

func (ip IP) String() string {
	return fmt.Sprintf("%d.%d.%d.%d", byte(ip>>24), byte(ip>>16), byte(ip>>8), byte(ip))
}

// IPProto is either a real IP protocol (ITCP, UDP, ...) or an special value like Unknown.
// If it is a real IP protocol, its value corresponds to its IP protocol number.
type IPProto uint8

const (
	// Unknown represents an unknown or unsupported protocol; it's deliberately the zero value.
	Unknown IPProto = 0x00
	ICMP    IPProto = 0x01
	TCP     IPProto = 0x06
	UDP     IPProto = 0x11
	// IPv6 and Fragment are special values. They're not really IPProto values
	// so we're using the unassigned 0xFE and 0xFF values for them.
	// TODO(dmytro): special values should be taken out of here.
	IPv6     IPProto = 0xFE
	Fragment IPProto = 0xFF
)

func (p IPProto) String() string {
	switch p {
	case Fragment:
		return "Frag"
	case ICMP:
		return "ICMP"
	case UDP:
		return "UDP"
	case TCP:
		return "TCP"
	case IPv6:
		return "IPv6"
	default:
		return "Unknown"
	}
}

// IPHeader represents an IP packet header.
type IPHeader struct {
	IPProto IPProto
	IPID    uint16
	SrcIP   IP
	DstIP   IP
}

const ipHeaderLength = 20

func (IPHeader) Len() int {
	return ipHeaderLength
}

func (h IPHeader) Marshal(buf []byte) error {
	if len(buf) < ipHeaderLength {
		return errSmallBuffer
	}
	if len(buf) > maxPacketLength {
		return errLargePacket
	}

	buf[0] = 0x40 | (ipHeaderLength >> 2) // IPv4
	buf[1] = 0x00                         // DHCP, ECN
	put16(buf[2:4], uint16(len(buf)))
	put16(buf[4:6], h.IPID)
	put16(buf[6:8], 0) // flags, offset
	buf[8] = 64        // TTL
	buf[9] = uint8(h.IPProto)
	put16(buf[10:12], 0) // blank IP header checksum
	put32(buf[12:16], uint32(h.SrcIP))
	put32(buf[16:20], uint32(h.DstIP))

	put16(buf[10:12], ipChecksum(buf[0:20]))

	return nil
}

// MarshalPseudo serializes the header into buf in pseudo format.
// It clobbers the header region, which is the first h.Length() bytes of buf.
// It explicitly initializes every byte of the header region,
// so pre-zeroing it on reuse is not required. It does not allocate memory.
func (h IPHeader) MarshalPseudo(buf []byte) error {
	if len(buf) < ipHeaderLength {
		return errSmallBuffer
	}
	if len(buf) > maxPacketLength {
		return errLargePacket
	}

	length := len(buf) - ipHeaderLength
	put32(buf[8:12], uint32(h.SrcIP))
	put32(buf[12:16], uint32(h.DstIP))
	buf[16] = 0x0
	buf[17] = uint8(h.IPProto)
	put16(buf[18:20], uint16(length))

	return nil
}

func (h *IPHeader) ToResponse() {
	h.SrcIP, h.DstIP = h.DstIP, h.SrcIP
	// Flip the bits in the IPID. If incoming IPIDs are distinct, so are these.
	h.IPID = ^h.IPID
}