UNIX网络编程：网络数据包检测

返回值：

成功返回0，

失败返回-1，

被中断返回-2.

过滤数据包

根据过滤器表达式生成过滤器

int * pcap_compile( pcap_t * p,//pcap_open_live返回的句柄

struct bpf_program * fp,//保存过滤器的结构体指针

const char * str,//要转换的过滤器表达式

int optimize,//过滤程序是否优化

bpf_u_int32 netmask )//子网掩码

返回值：

成功返回0，

失败返回-1。

为指定的捕捉句柄安装过滤器

int pcap_next ( pcap_t * p//pcap_open_live返回的句柄

struct bpf_program * fp);//指向要安装的过滤器的指针

返回值：

成功返回0；

失败返回-1.

捕获数据包流程

示例程序：

//ping.c

#include <stdio.h>   
#include <stdlib.h>  
#include <string.h>   
#include <signal.h>   
#include <arpa/inet.h>   
#include <sys/types.h>   
#include <sys/socket.h>   
#include <unistd.h>   
#include <netinet/in.h>   
#include <netinet/ip.h>   
#include <netinet/ip_icmp.h>   
#include <netdb.h>   
#include <setjmp.h>   
#include <errno.h>   
      
#define PACKET_SIZE 4096   
#define MAX_WAIT_TIME 5   
#define MAX_NO_PACKETS 3   
      
char sendpacket[PACKET_SIZE];   
char recvpacket[PACKET_SIZE];   
int sockfd,datalen=56;   
int nsend=0,nreceived=0;   
struct sockaddr_in dest_addr;   
pid_t pid;   
struct sockaddr_in from;   
struct timeval tvrecv;   
      
void f(int signo) {}  
void statistics(int signo);   
unsigned short cal_chksum(unsigned short *addr,int len);   
int pack(int pack_no);   
void send_packet(void);   
void recv_packet(void);   
int unpack(char *buf,int len);   
void tv_sub(struct timeval *out,struct timeval *in);   
      
void statistics(int signo)   
{   
   printf("n--------------------PING statistics-------------------n");   
   printf("%d packets transmitted, %d received , %%%.2f lostn",nsend,nreceived,(float)(nsend-nreceived)/nsend*100);   
   close(sockfd);   
   exit(1);   
}   
/*校验和算法*/
unsigned short cal_chksum(unsigned short *addr,int len)   
{   
   int nleft=len;   
   int sum=0;   
   unsigned short *w=addr;   
   unsigned short answer=0;   
      
   /*把ICMP报头二进制数据以2字节为单位累加起来*/
   while(nleft>1)   
   {   
     sum+=*w++;   
     nleft-=2;   
   }   
   /*若ICMP报头为奇数个字节，会剩下最后一字节。把最后一个字节视为一个2字节数据的高字节，  
   这个2字节数据的低字节为0，继续累加*/
   if( nleft==1)   
   {   
       *(unsigned char *)(&answer)=*(unsigned char *)w;   
       sum+=answer;   
   }   
   sum=(sum>>16)+(sum&0xffff);   
   sum+=(sum>>16);   
   answer=~sum;   
   return answer;   
}   
/*设置ICMP报头*/
int pack(int pack_no)   
{   
    int i,packsize;   
    struct icmp *icmp;   
    struct timeval *tval;   
    icmp=(struct icmp*)sendpacket; //char sendpacket[4096];  
    icmp->icmp_type=ICMP_ECHO;   
    icmp->icmp_code=0;   
#if 1  
    icmp->icmp_seq=pack_no;   
    icmp->icmp_cksum = 0;  
#endif  
    icmp->icmp_id=pid;   
    packsize=8+datalen;   
    tval= (struct timeval *)icmp->icmp_data;   
    gettimeofday(tval,NULL); /*记录发送时间*/
    icmp->icmp_cksum=cal_chksum( (unsigned short *)icmp,packsize); /*校验算法*/
    return packsize;   
}   
      
/*发送三个ICMP报文*/
void send_packet()   
{   
    int packetsize;   
    //while(nsend)   
    //{   
      nsend++;   
      packetsize=pack(nsend); /*设置ICMP报头*/
      if(sendto(sockfd,sendpacket,packetsize,0,(struct sockaddr *)&dest_addr,sizeof(dest_addr))<0 )   
      {   
        perror("sendto error");   
        return;  
      }   
     // sleep(1); /*每隔一秒发送一个ICMP报文*/   
//    }   
}   
      
/*接收所有ICMP报文*/
void recv_packet()   
{   
     int n,fromlen;   
     extern int errno;   
          
     struct sigaction act;  
     sigaction(SIGALRM, NULL, &act);  
     act.sa_handler = f;  
     sigaction(SIGALRM, &act, NULL);  
      
     fromlen=sizeof(from);   
     while(1)   
     {   
       alarm(5);   
       if( (n=recvfrom(sockfd,recvpacket,sizeof(recvpacket),0,(struct sockaddr *)&from,&fromlen)) <0)   
       {   
           if (errno == EINTR)  
           {  
               printf("errno=%dn", errno);  
               break;  
           }  
           else
               exit(-1);  
       }   
       gettimeofday(&tvrecv,NULL); /*记录接收时间*/
      
       if(unpack(recvpacket,n)==-1)   
       {  
           continue;   
       }  
       else
       {  
           nreceived++;   
           break;  
       }  
   }   
}   
/*剥去ICMP报头*/
int unpack(char *buf,int len)   
{   
    int i,iphdrlen;   
    struct ip *ip;   
    struct icmp *icmp;   
    struct timeval *tvsend;   
    double rtt;   
    ip=(struct ip *)buf;   
    iphdrlen=ip->ip_hl<<2; /*求ip报头长度,即ip报头的长度标志乘4*/
    icmp=(struct icmp *)(buf+iphdrlen); /*越过ip报头,指向ICMP报头*/
    len-=iphdrlen; /*ICMP报头及ICMP数据报的总长度*/
    if(len<8) /*小于ICMP报头长度则不合理*/
    {   
      printf("ICMP packets's length is less than 8n");   
      return -1;   
    }   
    /*确保所接收的是我所发的的ICMP的回应*/
    if( (icmp->icmp_type==ICMP_ECHOREPLY) && (icmp->icmp_id==pid) )   
    {   
        tvsend=(struct timeval *)icmp->icmp_data;   
        tv_sub(&tvrecv,tvsend); /*接收和发送的时间差*/
        rtt=tvrecv.tv_sec*1000+(float)tvrecv.tv_usec/1000; /*以毫秒为单位计算rtt*/
        /*显示相关信息*/
        printf("%d byte from %s: icmp_seq=%u ttl=%d rtt=%.3f msn",   
        len,inet_ntoa(from.sin_addr),icmp->icmp_seq,ip->ip_ttl,rtt);   
    }   
    else return -1;   
}   
      
int main(int argc,char *argv[])   
{   
    struct hostent *host;   
    struct protoent *protocol;   
    unsigned long inaddr=0l;   
    int waittime=MAX_WAIT_TIME;   
    int size=50*1024;   
      
    if(argc<2)   
    {   
       printf("usage:%s hostname/IP addressn",argv[0]);   
       exit(1);   
    }   
      
    if( (protocol=getprotobyname("icmp") )==NULL)   
    {   
        perror("getprotobyname");   
        exit(1);   
    }   
    /*生成使用ICMP的原始套接字,这种套接字只有root才能生成*/
    if( (sockfd=socket(AF_INET,SOCK_RAW,protocol->p_proto) )<0)   
    {   
        perror("socket error");   
        exit(1);   
    }   
    /* 回收root权限,设置当前用户权限*/
    setuid(getuid());   
    /*扩大套接字接收缓冲区到50K这样做主要为了减小接收缓冲区溢出的  
    的可能性,若无意中ping一个广播地址或多播地址,将会引来大量应答*/
    setsockopt(sockfd,SOL_SOCKET,SO_RCVBUF,&size,sizeof(size) );   
    bzero(&dest_addr,sizeof(dest_addr));   
    dest_addr.sin_family=AF_INET;   
      
    /*判断是主机名还是ip地址*/
    if( (inaddr=inet_addr(argv[1]))==INADDR_NONE)   
    {   
        if((host=gethostbyname(argv[1]) )==NULL) /*是主机名*/
        {   
           perror("gethostbyname error");   
           exit(1);   
        }   
        memcpy( (char *)&dest_addr.sin_addr,host->h_addr,host->h_length);   
    }   
    else /*是ip地址*/
        dest_addr.sin_addr.s_addr = inaddr;  
    //memcpy( (char *)&dest_addr,(char *)&inaddr,host->h_length);   
    /*获取main的进程id,用于设置ICMP的标志符*/
    pid=getpid();   
    printf("PING %s(%s): %d bytes data in ICMP packets.n",argv[1],   
    inet_ntoa(dest_addr.sin_addr),datalen);   
      
    int i = 0;  
    bzero(sendpacket, PACKET_SIZE);   
    bzero(recvpacket, PACKET_SIZE);   
      
    while (i < 3)  
    {  
        send_packet(); /*发送所有ICMP报文*/
        recv_packet(); /*接收所有ICMP报文*/
        sleep(1);  
        i++;  
    }  
      
    statistics(SIGALRM); /*进行统计*/
      
    return 0;   
      
}   
/*两个timeval结构相减*/
void tv_sub(struct timeval *out,struct timeval *in)   
{   
     if( (out->tv_usec-=in->tv_usec)<0)   
     {   
         --out->tv_sec;   
         out->tv_usec+=1000000;   
     }   
     out->tv_sec-=in->tv_sec;   
}   
      
/*------------- The End -----------*/

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