linux下判断网络是否连接

原文地址::http://houjixin.blog.163.com/blog/static/356284102013324115246630/

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1、Linux网络编程--服务端判断客户端断开的经验方法 ----http://blog.csdn.net/hnlyyk/article/details/50819823?ref=myread

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本文改写自网上的一个程序，原始程序中为阻塞式调用，而且有现成创建的过程，非常不利于集成到自己程序中，因此对原始程序进行改造，使其可以完成发送一个imcp包的方式来判断网络连通，只需要调用改进后的

 bool NetIsOK()

函数即可，该函数返回true即表示网络状态良好，否则表示网络状态不连同，本程序中只发送了一个icmp包，在实际应用中可以根据需要改进为发送多个imcp包。修改之后的程序为：只需要调用函数NetIsOK()即可。源码如下所示：

#include <stdio.h>#include <signal.h>#include <stdlib.h>#include <fcntl.h>#include <sys/types.h>#include <sys/wait.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 <errno.h>#define MAX_WAIT_TIME   1#define MAX_NO_PACKETS  1#define ICMP_HEADSIZE 8 #define PACKET_SIZE     4096struct timeval tvsend,tvrecv;struct sockaddr_in dest_addr,recv_addr;int sockfd;pid_t pid;char sendpacket[PACKET_SIZE];char recvpacket[PACKET_SIZE];//函数定义void timeout(int signo);unsigned short cal_chksum(unsigned short *addr,int len);int pack(int pkt_no,char *sendpacket);int send_packet(int pkt_no,char *sendpacket);int recv_packet(int pkt_no,char *recvpacket);int unpack(int cur_seq,char *buf,int len);void tv_sub(struct timeval *out,struct timeval *in);void _CloseSocket();bool NetIsOk(){     double rtt;struct hostent *host;struct protoent *protocol;int i,recv_status;#ifdef _USE_DNS //如果定义该宏，则可以使用域名进行判断网络连接，例如www.baidu.com/* 设置目的地址信息 */char hostname[32];sprintf(hostname,"%s","www.baidu.com")bzero(&dest_addr, sizeof(dest_addr));dest_addr.sin_family = AF_INET;if((host=gethostbyname(hostname))==NULL) {printf("[NetStatus]  error : Can't get serverhost info!\n");return false;}bcopy((char*)host->h_addr,(char*)&dest_addr.sin_addr,host->h_length);#else //如果不使用域名，则只能用ip地址直接发送icmp包，例如谷歌的地址：8.8.8.8dest_addr.sin_addr.s_addr = inet_addr("8.8.8.8");#endifif ((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0) {/* 创建原始ICMP套接字 */printf("[NetStatus]  error : socket");return false;}int iFlag;if(iFlag = fcntl(sockfd,F_GETFL,0)<0){printf("[NetStatus]  error : fcntl(sockfd,F_GETFL,0)");_CloseSocket();return false;}iFlag |= O_NONBLOCK;if(iFlag = fcntl(sockfd,F_SETFL,iFlag)<0){printf("[NetStatus]  error : fcntl(sockfd,F_SETFL,iFlag )");_CloseSocket();return false;}pid=getpid();for(i=0;i<MAX_NO_PACKETS;i++){if(send_packet(i,sendpacket)<0){printf("[NetStatus]  error : send_packet");_CloseSocket();return false;}if(recv_packet(i,recvpacket)>0){_CloseSocket();return true;}} _CloseSocket();     return false;}int send_packet(int pkt_no,char *sendpacket){    int packetsize;       packetsize=pack(pkt_no,sendpacket); gettimeofday(&tvsend,NULL);    if(sendto(sockfd,sendpacket,packetsize,0,(struct sockaddr *)&dest_addr,sizeof(dest_addr) )<0){      printf("[NetStatus]  error : sendto error");return -1;}return 1;}int pack(int pkt_no,char*sendpacket){       int i,packsize;struct icmp *icmp;struct timeval *tval;icmp=(struct icmp*)sendpacket;icmp->icmp_type=ICMP_ECHO;   //设置类型为ICMP请求报文icmp->icmp_code=0;icmp->icmp_cksum=0;icmp->icmp_seq=pkt_no;icmp->icmp_id=pid;//设置当前进程ID为ICMP标示符packsize=ICMP_HEADSIZE+sizeof(struct timeval);tval= (struct timeval *)icmp->icmp_data;gettimeofday(tval,NULL);icmp->icmp_cksum=cal_chksum( (unsigned short *)icmp,packsize); return packsize;}unsigned short cal_chksum(unsigned short *addr,int len){       int nleft=len;int sum=0;unsigned short *w=addr;unsigned short answer=0;while(nleft>1)//把ICMP报头二进制数据以2字节为单位累加起来{       sum+=*w++;nleft-=2;}if( nleft==1)//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{*(unsigned char *)(&answer)=*(unsigned char *)w;sum+=answer;}sum=(sum>>16)+(sum&0xffff);sum+=(sum>>16);answer=~sum;return answer;}int recv_packet(int pkt_no,char *recvpacket){       int n,fromlen;fd_set rfds;FD_ZERO(&rfds);FD_SET(sockfd,&rfds);signal(SIGALRM,timeout);fromlen=sizeof(recv_addr);alarm(MAX_WAIT_TIME);while(1){select(sockfd+1, &rfds, NULL, NULL, NULL);if (FD_ISSET(sockfd,&rfds)){  if( (n=recvfrom(sockfd,recvpacket,PACKET_SIZE,0,(struct sockaddr *)&recv_addr,&fromlen)) <0)    {   if(errno==EINTR)return -1;perror("recvfrom error");return -2;      }}gettimeofday(&tvrecv,NULL); if(unpack(pkt_no,recvpacket,n)==-1)continue;return 1;}}int unpack(int cur_seq,char *buf,int len){    int iphdrlen;struct ip *ip;struct icmp *icmp;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4icmp=(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)return -1;       if( (icmp->icmp_type==ICMP_ECHOREPLY) && (icmp->icmp_id==pid) && (icmp->icmp_seq==cur_seq))return 0;else return -1;}void timeout(int signo){printf("Request Timed Out\n");}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;}void _CloseSocket(){close(sockfd);sockfd = 0;}