利用libpcap分析网络上的数据包（入门级）

)，
    struct ether_header *eptr;//以太网字头
    u_char *ptr;
    int i;
 
    if (packet == NULL)//packet里面有内容，可以证明上面的猜想，
    {
        printf ("Didn't grab packet!/n");
        exit (1);
    }
    printf ("/n$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$/n");
    printf ("Grabbed packet of length %d/n", hdr.len);
    printf ("Received at : %s/n", ctime((const time_t*)&hdr.ts.tv_sec));
    printf ("Ethernet address length is %d/n", ETHER_HDR_LEN);
    
    eptr = (struct ether_header*)packet;//得到以太网字头
    
    if (ntohs(eptr->ether_type) == ETHERTYPE_IP)
    {
        printf ("Ethernet type hex:%x dec:%d is an IP packet/n",
                    ntohs(eptr->ether_type), ntohs(eptr->ether_type));
    }
    else 
    {
        if (ntohs(eptr->ether_type) == ETHERTYPE_ARP)
        {
            printf ("Ethernet type hex:%x dec:%d is an ARP packet/n",
                        ntohs(eptr->ether_type), ntohs(eptr->ether_type));
        }
        else
        {
            printf ("Ethernet type %x not IP/n", ntohs(eptr->ether_type));
            exit (1);
        }
    }
        
    ptr = eptr->ether_dhost;
    i = ETHER_ADDR_LEN;
    printf ("i=%d/n", i);
    printf ("Destination Address: ");
    do
    {
        printf ("%s%x", (i == ETHER_ADDR_LEN)?"":":", *ptr++);
    }while(--i>0);
    printf ("/n");
    //printf ("%x/n",ptr);
    
    ptr = eptr->ether_shost;
    i = ETHER_ADDR_LEN;
    printf ("Source Address: ");
    do
    {
        printf ("%s%x", (i == ETHER_ADDR_LEN)?"":":", *ptr++);
    }while(--i>0);
    printf ("/n");
    printf ("Now decoding the IP packet./n");
    ipptr = (struct iphdr*)    (packet+sizeof(struct ether_header));//得到ip包头
    
    printf ("the IP packets total_length is :%d/n", ipptr->tot_len);
    printf ("the IP protocol is %d/n", ipptr->protocol);
    addr.s_addr = ipptr->daddr;
    printf ("Destination IP: %s/n", inet_ntoa(addr));    
    addr.s_addr = ipptr->saddr;
    printf ("Source IP: %s/n", inet_ntoa(addr));
    
    printf ("Now decoding the TCP packet./n");
    tcpptr = (struct iphdr*)(packet+sizeof(struct ether_header)
                                    +sizeof(struct iphdr));//得到tcp包头
    printf ("Destination port : %d/n", tcpptr->dest);
    printf ("Source port : %d/n", tcpptr->source);
    printf ("the seq of packet is %d/n", tcpptr->seq);
//以上关于ip、tcp的结构信息请查询/usr/include/linux/ip.h | tcp.h
    
    data = (char*)(packet+sizeof(struct ether_header)+sizeof(struct iphdr)
                                    +sizeof(struct tcphdr));//得到数据包里内容，不过一般为乱码。
    
    printf ("the content of packets is /n%s/n",data);
}
int main(int argc, char **argv)
{
    int i;
    char *dev;
    char errbuf[PCAP_ERRBUF_SIZE];
    pcap_t *descr;
    const u_char *packet;
    struct pcap_pkthdr hdr;
    struct ether_header *eptr;
    
    if (argc != 2)
    {
        fprintf (stdout, "Usage: %s numpackets/n", argv[0]);
        return 0;
    }
    
    dev = pcap_lookupdev (errbuf);
    if (dev == NULL)
    {
        printf ("%s/n", errbuf);
        exit (1);
    }
    
    descr = pcap_open_live (dev, BUFSIZ, 1, -1, errbuf);
    //第三个参数，1为混杂模式;0为非混杂模式
    //BUFSIZ同PCAP_ERRBUF_SIZE一样，均为库文件已经定义好的，不推荐使用
    if (descr == NULL)
    {
        printf ("pcap_open_live(): %s/n", errbuf);
        exit (1);
    }
    pcap_loop (descr, atoi(argv[1]), my_callback, NULL);//调用回调函数
    
 
        
    printf("Hello world/n");
    return (0);
}
关于过滤机制，以后再写