github.com/20yyq/packet@v0.1.4-0.20231013092308-386a004a3baa/ipv4.go

// @@
// @ Author       	: Eacher
// @ Date         	: 2023-07-13 15:20:40
// @ LastEditTime   : 2023-10-13 17:17:40
// @ LastEditors    : Eacher
// @ --------------------------------------------------------------------------------<
// @ Description  	:
// @ --------------------------------------------------------------------------------<
// @ FilePath       : /20yyq/packet/ipv4.go
// @@
package packet

import (
	"encoding/binary"
	"fmt"
	"runtime"
	"unsafe"
)

const (
	SizeofIPv4Packet = 0x14
)

// NativeEndian is the machine native endian implementation of ByteOrder.
var ipv4NativeEndian = binary.ByteOrder(binary.LittleEndian)

func init() {
	b := [4]byte{}
	*(*uint32)(unsafe.Pointer(&b[0])) = 1
	if b[0] != 1 {
		ipv4NativeEndian = binary.ByteOrder(binary.BigEndian)
	}
}

/*
// 14.byte  EthernetPacket

3.1.  Internet Header Format

		A summary of the contents of the internet header follows:
	    0                   1                   2                   3
	    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |Version|  IHL  |Type of Service|          Total Length         |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |         Identification        |Flags|      Fragment Offset    |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |  Time to Live |    Protocol   |         Header Checksum       |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |                       Source Address                          |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |                    Destination Address                        |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	   |                    Options                    |    Padding    |
	   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

	                    Example Internet Datagram Header

	                               Figure 4.

	  Note that each tick mark represents one bit position.

	  Version:  		4 bits
	  IHL:  			4 bits
	  Type of Service:  8 bits
	  		its 0-2:  Precedence.
	  		Bit    3:  0 = Normal Delay,      1 = Low Delay.
	  		Bits   4:  0 = Normal Throughput, 1 = High Throughput.
	  		Bits   5:  0 = Normal Relibility, 1 = High Relibility.
	  		Bit  6-7:  Reserved for Future Use.

	  Total Length:  	16 bits
	  Identification:  	16 bits
	  Flags:  			3 bits
	  Fragment Offset:  13 bits
	  Time to Live:  	8 bits
	  Protocol:  		8 bits
	  Header Checksum:  16 bits
	  Source Address:  	32 bits
	  Destination Address:  32 bits
*/
type IPv4Packet struct {
	Version  uint8
	TOS      uint8
	TotalLen uint16
	ID       uint16
	FragOff  uint16
	TTL      uint8
	Protocol uint8
	checksum uint16
	Src      IPv4
	Dst      IPv4

	Flags   uint8
	IHL     uint8
	Options []byte
}

// 14.byte  EthernetPacket
// 返回负载下标起始位
func NewIPv4Packet(b []byte) (ipv4 IPv4Packet, next uint8) {
	if len(b) < SizeofIPv4Packet {
		return
	}
	check := make([]byte, SizeofIPv4Packet)
	copy(check, b[:SizeofIPv4Packet])
	ipv4, next = *(*IPv4Packet)(unsafe.Pointer(&check[0])), SizeofIPv4Packet
	if ipv4.IHL = ipv4.Version & 0b00001111 << 2; ipv4.IHL < SizeofIPv4Packet || len(b) < int(ipv4.IHL) {
		ipv4, next = IPv4Packet{}, 0
		return
	}
	if ipv4.IHL > SizeofIPv4Packet {
		check = append(check, b[SizeofTCPPacket:ipv4.IHL]...)
		ipv4.Options, next = check[SizeofTCPPacket:], ipv4.IHL
	}
	if *(*uint16)(unsafe.Pointer(&check[10])) = 0; ipv4.checksum != CheckSum(check) {
		ipv4, next = IPv4Packet{}, 0
		return
	}
	ipv4.Version >>= 4
	ipv4.TotalLen = binary.BigEndian.Uint16(b[2:4])
	ipv4.ID = binary.BigEndian.Uint16(b[4:6])
	ipv4.FragOff = binary.BigEndian.Uint16(b[6:8])
	ipv4.checksum = binary.BigEndian.Uint16(b[10:12])
	switch runtime.GOOS {
	case "darwin", "ios", "dragonfly", "netbsd":
		ipv4.TotalLen = ipv4NativeEndian.Uint16(b[2:4]) + uint16(ipv4.IHL)
		ipv4.FragOff = ipv4NativeEndian.Uint16(b[6:8])
	}
	ipv4.Flags = uint8(ipv4.FragOff & 0b1110000000000000 >> 13)
	ipv4.FragOff &= 0b0001111111111111
	return
}

func (ipv4 IPv4Packet) WireFormat() []byte {
	opLen := len(ipv4.Options)
	if opLen > 40 {
		return nil
	}
	b := make([]byte, SizeofIPv4Packet+opLen)
	if opLen > 0 {
		copy(b[SizeofIPv4Packet:], ipv4.Options)
		b = append(b, make([]byte, opLen%4)...)
	}
	b[0] = byte(ipv4.Version<<4 | uint8(((SizeofIPv4Packet + opLen) >> 2 & 0b00001111)))
	b[1], b[8], b[9] = ipv4.TOS, ipv4.TTL, ipv4.Protocol
	binary.BigEndian.PutUint16(b[2:4], ipv4.TotalLen)
	binary.BigEndian.PutUint16(b[4:6], ipv4.ID)
	binary.BigEndian.PutUint16(b[6:8], (ipv4.FragOff&0b0001111111111111)|(uint16(ipv4.Flags)<<13))
	switch runtime.GOOS {
	case "darwin", "ios", "dragonfly", "netbsd":
		ipv4NativeEndian.PutUint16(b[2:4], ipv4.TotalLen)
		ipv4NativeEndian.PutUint16(b[6:8], (ipv4.FragOff&0b0001111111111111)|(uint16(ipv4.Flags)<<13))
	}
	*(*IPv4)(b[12:16]) = ipv4.Src
	*(*IPv4)(b[16:20]) = ipv4.Dst
	binary.BigEndian.PutUint16(b[10:12], CheckSum(b))
	return b
}

func (ipv4 IPv4Packet) String() string {
	str := fmt.Sprintf(
		`V=%d IHL=%d TOS=%#x TotalLen=%d ID=%#x Flags=%#x FragOff=%#x TTL=%d Protocol=%d Checksum=%#x Src=%v Dst=%v`,
		ipv4.Version, ipv4.IHL, ipv4.TOS, ipv4.TotalLen, ipv4.ID, ipv4.Flags,
		ipv4.FragOff, ipv4.TTL, ipv4.Protocol, ipv4.checksum, ipv4.Src, ipv4.Dst,
	)
	return str
}