c-ares应用编程实例

本文参考：https://blog.csdn.net/lx_shudong/article/details/72638704
c-ares是一个异步DNS解析库，本文主要介绍c-ares在c/c++编程中的应用。
c-ares的官网是https://c-ares.haxx.se/
本文使用的版本是c-ares-1.12.0

前提

先编译好c-ares库。

cd /projects/test
mkdir test_c_ares
cd test_c_ares
tar zxvf c-ares-1.12.0.tar.gz
cd c-ares-1.12.0
./configure --prefix=/projects/test/test_c_ares/build --enable-shared
make clean
make
make install

实例

// compile cmd
// g++ test_c_ares.cpp -g -L/projects/test/test_c_ares/build/lib -I/projects/test/test_c_ares/build/include -lcares -lrt -o test
 
#include <cstdarg>
#include <time.h>
#include <iostream>
#include <netdb.h>
#include <arpa/inet.h>
#include <ares.h>
#include <sys/select.h>
#include <string.h>
 
#define IP_LEN 32
 
typedef struct {
	char host[64];
	char ip[10][IP_LEN]; 
	int count;   
}IpList;
 
void dns_callback (void* arg, int status, int timeouts, struct hostent* hptr)  //ares  处理完成，返回DNS解析的信息
{
    IpList *ips = (IpList*)arg;
	if( ips == NULL ) return;
    if(status == ARES_SUCCESS){
		strncpy(ips->host, hptr->h_name, sizeof(ips->host));
        char **pptr=hptr->h_addr_list;
		for(int i=0; *pptr!=NULL && i<10; pptr++,++i){
            inet_ntop(hptr->h_addrtype, *pptr, ips->ip[ips->count++], IP_LEN);
        } 
    }else{
        std::cout << "lookup failed: " << status << std::endl;
    }
}
 
void dns_resolve(const char* host, IpList &ips)
{
    ares_channel channel;    // 创建一个ares_channel
    
    int res;
    if((res = ares_init(&channel)) != ARES_SUCCESS) {     // ares 对channel 进行初始化
        std::cout << "ares feiled: " << res << std::endl;
        return;
    }
	//set dns server
    ares_set_servers_csv(channel, "114.114.114.114");
    //get host by name
    ares_gethostbyname(channel, host, AF_INET, dns_callback, (void*)(&ips));
 
    int nfds;  
    fd_set readers, writers;  
    timeval tv, *tvp;  
    while (true) 
    {  
        FD_ZERO(&readers);  
        FD_ZERO(&writers);  
        nfds = ares_fds(channel, &readers, &writers);   //获取ares channel使用的FD  
        if (nfds == 0) break;  
        tvp = ares_timeout(channel, NULL, &tv);      
        select(nfds, &readers, &writers, NULL, tvp);   //将ares的SOCKET FD 加入事件循环  
        ares_process(channel, &readers, &writers);  // 有事件发生 交由ares 处理
    }
    ares_destroy(channel);
}
 
int main(int argc, char **argv)
{
    int res;
    if(argc < 2 ) {
        std::cout << "usage: " << argv[0] << " ip.address"<< std::endl;
        return 1;
    }
 
	IpList ips;
	do{
		memset(&ips, 0, sizeof(ips));
        dns_resolve(argv[1],ips);
		std::cout <<"hostname:"<< ips.host << std::endl;
	    for(int i=0; i < ips.count; ++i){
			std::cout<< "address ip:" << ips.ip[i] << std::endl;
		}
		std::cout<< std::endl;
	}while(0);
	std::cout << "program end.." << std::endl;
    return 0;
}

注意：
可以设置DNS服务器，用ares_set_servers_csv(channel, “114.114.114.114”);
不同DNS服务器解析出的IP有可能不一样，看要解析的域名的实际情况。