自己用libpcap来抓包

#define APP_NAME        "sniffex"
#define APP_DESC        "Sniffer example using libpcap"
#define APP_COPYRIGHT    "Copyright (c) 2005"
#define APP_DISCLAIMER    "THERE IS ABSOLUTELY NO WARRANTY FOR THIS PROGRAM."

#include <pcap.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

/* default snap length (maximum bytes per packet to capture) */
#define SNAP_LEN 1518

/* ethernet headers are always exactly 14 bytes [1] */
#define SIZE_ETHERNET 14

/* Ethernet addresses are 6 bytes */
#define ETHER_ADDR_LEN    6

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

/* IP header */
struct sniff_ip {
        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 */
};
#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 */
};

void
got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet);

void
print_payload(const u_char *payload, int len);

void
print_hex_ascii_line(const u_char *payload, int len, int offset);

void
print_app_banner(void);

void
print_app_usage(void);

/*
 * app name/banner
 */
void
print_app_banner(void)
{

    printf("%s - %s\n", APP_NAME, APP_DESC);
    printf("%s\n", APP_COPYRIGHT);
    printf("%s\n", APP_DISCLAIMER);
    printf("\n");

return;
}

/*
 * print help text
 */
void
print_app_usage(void)
{

    printf("Usage: %s [interface]\n", APP_NAME);
    printf("\n");
    printf("Options:\n");
    printf(" interface Listen on <interface> for packets.\n");
    printf("\n");

return;
}

/*
 * print data in rows of 16 bytes: offset hex ascii
 *
 * 00000 47 45 54 20 2f 20 48 54 54 50 2f 31 2e 31 0d 0a GET / HTTP/1.1..
 */
void
print_hex_ascii_line(const u_char *payload, int len, int offset)
{

    int i;
    int gap;
    const u_char *ch;

    /* offset */
    printf("%05d ", offset);
    
    /* hex */
    ch = payload;
    for(i = 0; i < len; i++) {
        printf("%02x ", *ch);
        ch++;
        /* print extra space after 8th byte for visual aid */
        if (i == 7)
            printf(" ");
    }
    /* print space to handle line less than 8 bytes */
    if (len < 8)
        printf(" ");
    
    /* fill hex gap with spaces if not full line */
    if (len < 16) {
        gap = 16 - len;
        for (i = 0; i < gap; i++) {
            printf(" ");
        }
    }
    printf(" ");
    
    /* ascii (if printable) */
    ch = payload;
    for(i = 0; i < len; i++) {
        if (isprint(*ch))
            printf("%c", *ch);
        else
            printf(".");
        ch++;
    }

    printf("\n");

return;
}

/*
 * print packet payload data (avoid printing binary data)
 */
void
print_payload(const u_char *payload, int len)
{

    int len_rem = len;
    int line_width = 16;            /* number of bytes per line */
    int line_len;
    int offset = 0;                    /* zero-based offset counter */
    const u_char *ch = payload;

    if (len <= 0)
        return;

    /* data fits on one line */
    if (len <= line_width) {
        print_hex_ascii_line(ch, len, offset);
        return;
    }

    /* data spans multiple lines */
    for ( ;; ) {
        /* compute current line length */
        line_len = line_width % len_rem;
        /* print line */
        print_hex_ascii_line(ch, line_len, offset);
        /* compute total remaining */
        len_rem = len_rem - line_len;
        /* shift pointer to remaining bytes to print */
        ch = ch + line_len;
        /* add offset */
        offset = offset + line_width;
        /* check if we have line width chars or less */
        if (len_rem <= line_width) {
            /* print last line and get out */
            print_hex_ascii_line(ch, len_rem, offset);
            break;
        }
    }

return;
}

/*
 * dissect/print packet
 */
void
got_packet(u_char *args, const struct pcap_pkthdr *header, const u_char *packet)
{

    static int count = 1; /* packet counter */
    
    /* declare pointers to packet headers */
    const struct sniff_ethernet *ethernet; /* The ethernet header [1] */
    const struct sniff_ip *ip; /* The IP header */
    const struct sniff_tcp *tcp; /* The TCP header */
    const u_char *payload; /* Packet payload */

    int size_ip;
    int size_tcp;
    int size_payload;
    
    printf("\nPacket number %d:\n", count);
    count++;
    
    /* define ethernet header */
    ethernet = (struct sniff_ethernet*)(packet);
    
    /* define/compute ip header offset */
    ip = (struct sniff_ip*)(packet + SIZE_ETHERNET);
    size_ip = IP_HL(ip)*4;
    if (size_ip < 20) {
        printf(" * Invalid IP header length: %u bytes\n", size_ip);
        return;
    }

    /* print source and destination IP addresses */
    printf(" From: %s\n", inet_ntoa(ip->ip_src));
    printf(" To: %s\n", inet_ntoa(ip->ip_dst));
    
    /* determine protocol */    
    switch(ip->ip_p) {
        case IPPROTO_TCP:
            printf(" Protocol: TCP\n");
            break;
        case IPPROTO_UDP:
            printf(" Protocol: UDP\n");
            return;
        case IPPROTO_ICMP:
            printf(" Protocol: ICMP\n");
            return;
        case IPPROTO_IP:
            printf(" Protocol: IP\n");
            return;
        default:
            printf(" Protocol: unknown\n");
            return;
    }
    
    /*
     * OK, this packet is TCP.
     */
    
    /* define/compute tcp header offset */
    tcp = (struct sniff_tcp*)(packet + SIZE_ETHERNET + size_ip);
    size_tcp = TH_OFF(tcp)*4;
    if (size_tcp < 20) {
        printf(" * Invalid TCP header length: %u bytes\n", size_tcp);
        return;
    }
    
    printf(" Src port: %d\n", ntohs(tcp->th_sport));
    printf(" Dst port: %d\n", ntohs(tcp->th_dport));
    
    /* define/compute tcp payload (segment) offset */
    payload = (u_char *)(packet + SIZE_ETHERNET + size_ip + size_tcp);
    
    /* compute tcp payload (segment) size */
    size_payload = ntohs(ip->ip_len) - (size_ip + size_tcp);
    
    /*
     * Print payload data; it might be binary, so don't just
     * treat it as a string.
     */
    if (size_payload > 0) {
        printf(" Payload (%d bytes):\n", size_payload);
        print_payload(payload, size_payload);
    }

return;
}

int main(int argc, char **argv)
{

    char *dev = NULL;            /* capture device name */
    char errbuf[PCAP_ERRBUF_SIZE];        /* error buffer */
    pcap_t *handle;                /* packet capture handle */

    char filter_exp[] = "dst 202.114.85.31 and tcp";        /* filter expression [3] */
    struct bpf_program fp;            /* compiled filter program (expression) */
    bpf_u_int32 mask;            /* subnet mask */
    bpf_u_int32 net;            /* ip */
    int num_packets = 10;            /* number of packets to capture */

    print_app_banner();

    /* check for capture device name on command-line */
    if (argc == 2) {
        dev = argv[1];
    }
    else if (argc > 2) {
        fprintf(stderr, "error: unrecognized command-line options\n\n");
        print_app_usage();
        exit(EXIT_FAILURE);
    }
    else {
        /* find a capture device if not specified on command-line */
        dev = pcap_lookupdev(errbuf);
        if (dev == NULL) {
            fprintf(stderr, "Couldn't find default device: %s\n",
             errbuf);
            exit(EXIT_FAILURE);
        }
    }
    
    /* get network number and mask associated with capture device */
    if (pcap_lookupnet(dev, &net, &mask, errbuf) == -1) {
        fprintf(stderr, "Couldn't get netmask for device %s: %s\n",
         dev, errbuf);
        net = 0;
        mask = 0;
    }

    /* print capture info */
    printf("Device: %s\n", dev);
    printf("Number of packets: %d\n", num_packets);
    printf("Filter expression: %s\n", filter_exp);

    /* open capture device */
    handle = pcap_open_live(dev, SNAP_LEN, 1, 1000, errbuf);
    if (handle == NULL) {
        fprintf(stderr, "Couldn't open device %s: %s\n", dev, errbuf);
        exit(EXIT_FAILURE);
    }

    /* make sure we're capturing on an Ethernet device [2] */
    if (pcap_datalink(handle) != DLT_EN10MB) {
        fprintf(stderr, "%s is not an Ethernet\n", dev);
        exit(EXIT_FAILURE);
    }

    /* compile the filter expression */
    if (pcap_compile(handle, &fp, filter_exp, 0, net) == -1) {
        fprintf(stderr, "Couldn't parse filter %s: %s\n",
         filter_exp, pcap_geterr(handle));
        exit(EXIT_FAILURE);
    }

    /* apply the compiled filter */
    if (pcap_setfilter(handle, &fp) == -1) {
        fprintf(stderr, "Couldn't install filter %s: %s\n",
         filter_exp, pcap_geterr(handle));
        exit(EXIT_FAILURE);
    }

    /* now we can set our callback function */
    pcap_loop(handle, num_packets, got_packet, NULL);

    /* cleanup */
    pcap_freecode(&fp);
    pcap_close(handle);

    printf("\nCapture complete.\n");

return 0;
}