header.h

// Ethernet header
struct ethheader
{
    u_char ether_dhost[6]; // destination host address
    u_char ether_shost[6]; // source host address
    u_short ether_type;    // IP? ARP? RARP? etc
};

// IP Header
struct ipheader
{
    unsigned char iph_ihl : 4,       // IP header length
        iph_ver : 4;                 // IP version
    unsigned char iph_tos;           // Type of service
    unsigned short int iph_len;      // IP Packet length (data + header)
    unsigned short int iph_ident;    // Identification
    unsigned short int iph_flag : 3, // Fragmentation flags
        iph_offset : 13;             // Flags offset
    unsigned char iph_ttl;           // Time to Live
    unsigned char iph_protocol;      // Protocol type
    unsigned short int iph_chksum;   // IP datagram checksum
    struct in_addr iph_sourceip;     // Source IP address
    struct in_addr iph_destip;       // Destination IP address
};

// ICMP Header
struct icmpheader
{
    unsigned char icmp_type;        // ICMP message type
    unsigned char icmp_code;        // Error code
    unsigned short int icmp_chksum; // Checksum for ICMP Header and data
    unsigned short int icmp_id;     // Used for identifying request
    unsigned short int icmp_seq;    // Sequence number
};

// UDP Header
struct udpheader
{
    u_int16_t udp_sport; // source port
    u_int16_t udp_dport; // destination port
    u_int16_t udp_ulen;  // udp length
    u_int16_t udp_sum;   // udp checksum
};

// TCP Header
struct tcpheader
{
    u_short tcp_sport; // source port
    u_short tcp_dport; // destination port
    u_int tcp_seq;     // sequence number
    u_int tcp_ack;     // acknowledgement number
    u_char tcp_offx2;  // data offset, rsvd
#define TH_OFF(th) (((th)->tcp_offx2 & 0xf0) >> 4)
    u_char tcp_flags;
#define TH_FIN 0x01
#define TH_SYN 0x02
#define TH_RST 0x04
#define TH_PUSH 0x08
#define TH_ACK 0x10
#define TH_URG 0x20
#define TH_ECE 0x40
#define TH_CWR 0x80
#define TH_FLAGS (TH_FIN | TH_SYN | TH_RST | TH_ACK | TH_URG | TH_ECE | TH_CWR)
    u_short tcp_win; // window
    u_short tcp_sum; // checksum
    u_short tcp_urp; // urgent pointer
};

// Psuedo TCP header
struct pseudo_tcp
{
    unsigned saddr, daddr;
    unsigned char mbz;
    unsigned char ptcl;
    unsigned short tcpl;
    struct tcpheader tcp;
    char payload[1500];
};