前面說過， 利用ping命令探測網路是否可通， 但很多時候， 服務端或者防火牆禁止了ping命令， 也就是說， ping不通， 不表示網路不通， 所以仍有可能能建立tcp連線。怎麼檢測tcp連線是否可通呢？ 用telnet命令就可以搞起， 但問題是， 很多時候（尤其是批量探測的時候）， 我們需要給telnet命令設定一個超時時間， 很遺憾， 這是telnet命令所不支援的。 那要怎麼搞？ 還是自己寫程式吧， 如下：

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <stdarg.h>
#include <fcntl.h>
#include <time.h>


int telnetCheckTcp(const char *ip, int port, int tSecond)
{
	int sockClient = socket(AF_INET, SOCK_STREAM, 0);
	int iFinal = 0;

	struct sockaddr_in addrSrv;
	addrSrv.sin_addr.s_addr = inet_addr(ip);
	addrSrv.sin_family = AF_INET;
	addrSrv.sin_port = htons(port);

	fcntl(sockClient, F_SETFL, fcntl(sockClient, F_GETFL, 0)|O_NONBLOCK);  
	
	int iRet = connect(sockClient, ( const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));  // 返回-1不一定是異常
	if (iRet != 0)	
	{  
		if(errno != EINPROGRESS)
		{
			iFinal = -1;	
		}
		else  
		{
			struct timeval tm = {tSecond, 0}; 
			fd_set wset, rset;	
			FD_ZERO(&wset); 
			FD_ZERO(&rset); 
			FD_SET(sockClient, &wset);	
			FD_SET(sockClient, &rset); 
			int time1 = time(NULL);
			int n = select(sockClient + 1, &rset, &wset, NULL, &tm);  
			int time2 = time(NULL);

			if(n < 0)	
			{  
			    iFinal = -2;	  
			}  
			else if(n == 0)  
			{  
			    iFinal = -3;	  
			}  
			else if (n == 1)  
			{
			   if(FD_ISSET(sockClient, &wset))	
			   {  
				   iFinal = 0;	 
				   fcntl(sockClient, F_SETFL, fcntl(sockClient, F_GETFL, 0) & ~O_NONBLOCK);  
			   }  
			   else  
			   {  
				   iFinal = -4;	
			   }  
			}
			else
			{
				iFinal = -5;	
			}
		}  
	}  

	close(sockClient);
	return iFinal;
}


int main(int argc, char *argv[])
{
	if(argc != 4)
	{
		printf("error\n");
		return -1;
	}

	int iFinal = telnetCheckTcp(argv[1], atoi(argv[2]), atoi(argv[3]));
	printf("iFinal is %d\n", iFinal);

	return 0;
}
 

[email protected]:~$ ./a.out 220.181.112.244 80 2
iFinal is 0
[email protected]:~$ ./a.out 220.181.112.244 81 2
iFinal is -3
[email protected]:~$ ./a.out 220.181.112.244 443 2
iFinal is 0
[email protected]:~$ ./a.out 220.181.112.244 445 2
iFinal is -3
[email protected]:~$ ./a.out 1.1.1.1 80 2
iFinal is -3
 
[email protected]:~$ 

        批量探測怎麼搞呢？ 來看看：

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <malloc.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/ioctl.h>
#include <stdarg.h>
#include <fcntl.h>
#include <time.h>

#include <iostream>
#include <fstream>
using namespace std;


int telnetCheckTcp(const char *ip, int port, int tSecond)
{
	int sockClient = socket(AF_INET, SOCK_STREAM, 0);
	int iFinal = 0;

	struct sockaddr_in addrSrv;
	addrSrv.sin_addr.s_addr = inet_addr(ip);
	addrSrv.sin_family = AF_INET;
	addrSrv.sin_port = htons(port);

	fcntl(sockClient, F_SETFL, fcntl(sockClient, F_GETFL, 0)|O_NONBLOCK);  
	
	int iRet = connect(sockClient, ( const struct sockaddr *)&addrSrv, sizeof(struct sockaddr_in));  // 返回-1不一定是異常
	if (iRet != 0)	
	{  
		if(errno != EINPROGRESS)
		{
			iFinal = -1;	
		}
		else  
		{
			struct timeval tm = {tSecond, 0}; 
			fd_set wset, rset;	
			FD_ZERO(&wset); 
			FD_ZERO(&rset); 
			FD_SET(sockClient, &wset);	
			FD_SET(sockClient, &rset); 
			int time1 = time(NULL);
			int n = select(sockClient + 1, &rset, &wset, NULL, &tm);  
			int time2 = time(NULL);

			if(n < 0)	
			{  
			    iFinal = -2;	  
			}  
			else if(n == 0)  
			{  
			    iFinal = -3;	  
			}  
			else if (n == 1)  
			{
			   if(FD_ISSET(sockClient, &wset))	
			   {  
				   iFinal = 0;	 
				   fcntl(sockClient, F_SETFL, fcntl(sockClient, F_GETFL, 0) & ~O_NONBLOCK);  
			   }  
			   else  
			   {  
				   iFinal = -4;	
			   }  
			}
			else
			{
				iFinal = -5;	
			}
		}  
	}  

	close(sockClient);
	return iFinal;
}


int main(int argc, char *argv[])  // ./a.out ipfilename port timeout
{
	if(argc != 4)
	{
		cout << "error" << endl;
		return -1;
	}

	ifstream in(argv[1]);
	string filename;
	string line;

	unsigned int i = 0;
	if(in) // 有該檔案
	{
		while (getline (in, line)) // line中不包括每行的換行符
		{
			// 這裡最好做ip格式判斷
			i++;
			int iFinal = telnetCheckTcp(line.c_str(), atoi(argv[2]), atoi(argv[3]));
			if(iFinal == 0)
			{
				printf("iFinal is %d, ip is %s,  index is %d\n", iFinal, line.c_str(), i);
			}
			else
			{
				printf("iFinal is %d, cannot connect to %s, index is %d\n", iFinal, line.c_str(), i);
			}
		}
	}
	else // 沒有該檔案
	{
		cout <<"no such file" << endl;
	}

	return 0;
}
 

[email protected]:~$ ./a.out a.txt 443 2
iFinal is 0, ip is 220.181.112.244,  index is 1
iFinal is -3, cannot connect to 1.1.1.1, index is 2
[email protected]:~$ 
[email protected]:~$ 
[email protected]:~$ ./a.out a.txt 80 2
iFinal is 0, ip is 220.181.112.244,  index is 1
iFinal is -3, cannot connect to 1.1.1.1, index is 2
[email protected]:~$ 
[email protected]:~$ 
[email protected]:~$ ./a.out a.txt 100 2
iFinal is -3, cannot connect to 220.181.112.244, index is 1
iFinal is -3, cannot connect to 1.1.1.1, index is 2
[email protected]:~$