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分类: LINUX

2010-01-11 14:11:04

先介绍几个它的函数:
char *pcap_lookupdev(char *errbuf)
这个函数主要是用来返回一个网络设备名的指针,一般在你的机器上大多情况是eth0(linux下),参数errbuf是存放相关错误消息的.

int pcap_lookupnet(const char *device, bpf_u_int32 *netp,
bpf_u_int32 *maskp, char *errbuf)
这个函数是用来获得指定的网络设备的IP地址和掩码,其中netp和maskp就分别放IP和掩码用的,errbuf是出错消息存放处.

pcap_t *pcap_open_live(const char *device, int snaplen,
int promisc, int to_ms, char *errbuf)
这个函数用于获得数据包捕获描述符字,也就是说从参数device指定打开该设备以获得捕获包描述符字.snaplen 表示数据包捕获时的最大字节数,promisc表示设置网卡模式,设置为混杂模式或者为普通模式.to_ms 表示超时时间,errbuf出错信息存储

int pcap_compile(pcap_t *p, struct bpf_program *fp,
char *str, int optimize, bpf_u_int32 netmask)
该函数用于编译过滤规则,str是指过滤规则,fp就是规则结构,主要是根据str串中的内容来给fp这个结构初始话,optimize用来设置是否对结果进行优化,netmask表示掩码.

int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
该函数用来指定一个过滤规则,fp是前面使用pcap_compile后得到的结果,在程序运行的时候就会按照指定的这个规则来捕获符合要求的数据包,不服和的就丢弃

int pcap_loop(pcap_t *p, int cnt,
pcap_handler callback, u_char *user)
该函数就是用来捕获数据包时用的,当捕获了符合要求的数据包之后就交给回调函数callback来进行下一步处理.其中p就是前面获得的数据包捕获描述符字,cnt表示捕获数据包的个数,-1表示无限,callback就是回调函数,user通常设置为空.

 

 

 


#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 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;
}


编译的时候gcc -Wall -o sniffex sniffex.c -lpcap就可以了....这里的

char filter_exp[] = "dst 202.114.85.31 and tcp"; 可以修改为任何tcpdump所支持的pattern,也就是传说中的BSD Packet Filter  具体的pattern你可以man下tcpdump

文件: ethereal-tcpdump.pdf
大小: 76KB
下载: 下载

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