pcap.h

#define TAM_ETHERNET 14

/* Estructura auxiliar para manejar el header de IP */
struct ip_header {
	u_char ip_vhl;     /* version << 4 | header length >> 2 */
    u_char ip_tos;     /* type of service */
    u_short ip_len;    /* total length */
    u_short ip_id;     /* identification */
    u_short ip_off;    /* fragment offset field */
    #define IP_RF 0x8000      /* reserved fragment flag */
    #define IP_DF 0x4000      /* dont fragment flag */
    #define IP_MF 0x2000      /* more fragments flag */
    #define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
    u_char ip_ttl;                  /* time to live */
    u_char ip_p;                    /* protocol */
    u_short ip_sum;                 /* checksum */
    struct in_addr ip_src, ip_dst;  /* source and dest address */
};
// Para facilitar la lectura de version y tamanio del header
#define IP_HL(ip)               (((ip)->ip_vhl) & 0x0f)
#define IP_V(ip)                (((ip)->ip_vhl) >> 4)

/* TCP header */
typedef u_int tcp_seq;
struct sniff_tcp {
        u_short th_sport;               /* source port */
        u_short th_dport;               /* destination port */
        tcp_seq th_seq;                 /* sequence number */
        tcp_seq th_ack;                 /* acknowledgement number */
        u_char  th_offx2;               /* data offset, rsvd */
        #define TH_OFF(th)              (((th)->th_offx2 & 0xf0) >> 4)
        u_char  th_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 th_win;                 /* window */
        u_short th_sum;                 /* checksum */
        u_short th_urp;                 /* urgent pointer */
};