使用c-ares進行DNS域名解析

1、c-ares是一個C語言實現的DNS非阻塞異步解析，libcurl, libevent, gevent, nodejs wireshark都使用了c-ares，因此在一般開發一些服務的客戶端Client，在連接服務器Server時，需要做的工作的就是域名解析。

2、源碼下載路徑： https://c-ares.haxx.se/download/

3、編譯使用

    （1）Win32環境

      源碼下載解壓後，進入vs文件夾，使用VS2013等工具打開工程，即可以編譯Win32的caresd.lib和caresd.dll

static int CloudServerHostIsIP(const char * serverhost)
{
	struct in_addr addr;
	int lsuccess;
	lsuccess= inet_pton(AF_INET, serverhost, &addr);
	return lsuccess> 0 ? 0 : -1;
}

static void DNSCallBack(void* arg, int status, int timeouts, struct hostent* host)
{
	char **lpSrc;
	char  * lpHost = (char *)arg;

	if (status == ARES_SUCCESS)
	{
		for (lpSrc = host->h_addr_list; *lpSrc; lpSrc++)
		{
			char addr_buf[32] = "";
			ares_inet_ntop(host->h_addrtype, *lpSrc, addr_buf, sizeof(addr_buf));
			if (strlen(addr_buf) != 0)
			{
				strcpy(lpHost, addr_buf);
				break;
			}
		}
	}
}

static int DomainNameReSolution(const char * lpDomainName, char * lpHost)
{

	int lsuccess= 0;

	ares_channel channel;
	lsuccess= ares_library_init(ARES_LIB_INIT_ALL);
	if ((lsuccess= ares_init(&channel)) != ARES_SUCCESS) return -1;

	int trytime = 3;
	do{
		fd_set readers, writers;
		timeval tv;
		tv.tv_sec = 5;
		tv.tv_usec = 0;
		FD_ZERO(&readers);
		FD_ZERO(&writers);
		ares_gethostbyname(channel, lpDomainName, AF_INET, DNSCallBack, (char *)lpHost);
		int nfds = ares_fds(channel, &readers, &writers);
		if (nfds == 0){
			continue;
		}
		int count = select(nfds, &readers, NULL, NULL, &tv);
		if (count > 0){
			ares_process(channel, &readers, &writers);
			lsuccess= 0;
			break;
		}
		else{
			lsuccess= -1;
		}
	} while (trytime-- > 0);

	ares_destroy(channel);
	ares_library_cleanup();
	return lsuccess;
}