pkthdr.h

#ifndef _PKTHEADER_H
#define _PKTHEADER_H
#include <stdlib.h>
#include <stdint.h>

/*----------------------------------------------*/
/*                Link Layer                    */
/*----------------------------------------------*/
#define ETH_ALEN 6
#define ETHERNET_HDR_LEN 14

#ifndef u_char
typedef unsigned char u_char;
#endif

#ifndef u_short
typedef unsigned short u_short;
#endif

#ifndef u_int
typedef unsigned int u_int;
#endif

typedef struct ether_header
{
  u_char ether_dhost[ETH_ALEN]; /* destination eth addr        */
  u_char ether_shost[ETH_ALEN]; /* source ether addr        */
  u_short ether_type;           /* packet type ID field        */
} __attribute__((__packed__)) ether_header;

/*----------------------------------------------*/
/*                  Network Layer              */
/*----------------------------------------------*/
#define ARP_HDR_LEN 8
#define IPV4_HDR_STATIC_LEN 20

// 4字节的IP地址
typedef struct ip_address
{
  u_char byte1;
  u_char byte2;
  u_char byte3;
  u_char byte4;
} ip_address;

// IPv4 首部
typedef struct ip_header
{
  u_char ver_ihl;         // 版本 (4 bits) + 首部长度 (4 bits)
  u_char tos;             // 服务类型(Type of service)
  u_short tlen;           // 总长(Total length)
  u_short identification; // 标识(Identification)
  u_short flags_fo;       // 标志位(Flags) (3 bits) + 段偏移量(Fragment offset) (13 bits)
  u_char ttl;             // 存活时间(Time to live)
  u_char proto;           // 协议(Protocol)
  u_short crc;            // 首部校验和(Header checksum)
  ip_address saddr;       // 源地址(Source address)
  ip_address daddr;       // 目的地址(Destination address)
  u_int op_pad;           // 选项与填充(Option + Padding)
} __attribute__((packed)) ip_header;

typedef struct arp_hdr
{
  uint16_t ar_hrd; /* format of hardware address */
  uint16_t ar_pro; /* format of protocol address */
  uint8_t ar_hln;  /* length of hardware address (ETH_ADDR_LEN) */
  uint8_t ar_pln;  /* length of protocol address (IP_ADDR_LEN) */
  uint16_t ar_op;  /* operation */
} __attribute__((packed)) arp_header;

/*----------------------------------------------*/
/*                  Transport Layer             */
/*----------------------------------------------*/
// UDP
#define UDP_HDR_LEN 8
#define TCP_HDR_STATIC_LEN 20
#define ICMP_HDR_LEN 8

typedef struct udp_header
{
  u_short sport; // 源端口(Source port)
  u_short dport; // 目的端口(Destination port)
  u_short len;   // UDP数据包长度(Datagram length)
  u_short crc;   // 校验和(Checksum)
} __attribute__((packed)) udp_header;

// TCP
typedef struct tcphdr
{
  u_int16_t source;
  u_int16_t dest;
  u_int32_t seq;
  u_int32_t ack_seq;
  // little endian to parse flags
  u_int16_t res1 : 4;
  u_int16_t doff : 4;
  u_int16_t fin : 1;
  u_int16_t syn : 1;
  u_int16_t rst : 1;
  u_int16_t psh : 1;
  u_int16_t ack : 1;
  u_int16_t urg : 1;
  u_int16_t res2 : 2;
  // end

  u_int16_t window;
  u_int16_t check;
  u_int16_t urg_ptr;
} __attribute__((packed)) tcp_header;

// ICMP
typedef struct icmphdr
{
  u_int8_t type; /* message type */
  u_int8_t code; /* type sub-code */
  u_int16_t checksum;
  union
  {
    struct
    {
      u_int16_t id;
      u_int16_t sequence;
    } echo;            /* echo datagram */
    u_int32_t gateway; /* gateway address */
    struct
    {
      u_int16_t __unused;
      u_int16_t mtu;
    } frag; /* path mtu discovery */
  } un;
} __attribute__((packed)) icmp_header;

#endif