Linux系统提供了系统调用，可以查询到设备的域名信息

在头文件netdb.h中，提供了两个系统调用gethostbyaddr和gethostbyname

#include <netdb.h>
struct hostent *gethostbyname(const char*name);
struct hostent *gethostbyaddr(const char *addr, int len, 0);
// 调用结果成功，则返回非空指针；错误则返回空指针，且同时设置h_errno。

其中返回的结构struct hostent用于记录主机信息条目具体为

/* Description of data base entry for a single host.  */
struct hostent
{
  char *h_name;			/* Official name of host.  */
  char **h_aliases;		/* Alias list.  */
  int h_addrtype;		/* Host address type.  */
  int h_length;			/* Length of address.  */
  char **h_addr_list;		/* List of addresses from name server.  */
#ifdef __USE_MISC
# define	h_addr	h_addr_list[0] /* Address, for backward compatibility.*/
#endif
};

下面是一个简单的示例，用于说明这两个系统调用的使用方法

#include <iostream>
#include <netdb.h>
#include <arpa/inet.h>
#include <string.h>

void process_print_res(const struct hostent *res)
{
    std::cout << "official name: " <<res->h_name <<std::endl;
    for(char **p = res->h_aliases; *p!=nullptr; ++p){
        std::cout <<"alias names: " <<*p <<std::endl;
    }
    std::cout << "addr net type: ";

    switch (res->h_addrtype)
    {
    case AF_INET:
        std::cout << "ipv4";
        break;
    case AF_INET6:
        std::cout << "ipv6";
        break;
    default:
        std::cout << "unknown";
        break;
    }
    std::cout <<std::endl;

    for(char **p = res->h_addr_list; *p!=nullptr; ++p){
        struct in_addr tmp;
        tmp.s_addr = ((struct in_addr *)*p)->s_addr;
        char *addr = inet_ntoa(tmp);
        std::cout <<"address: " <<addr <<std::endl;
    }
}

int main(int argc, char *argv[]) {
    struct in_addr addr;
    struct hostent *p_host;

    if(argc != 2){
        std::cout <<"usage: " <<argv[0] << " <domain_name or ip>" <<std::endl; 
        return 0;
    }

    if(0 != inet_aton(argv[1], &addr)){
        // input is a address
        p_host = gethostbyaddr((const void*)&addr, sizeof(addr), AF_INET);
        if(!p_host){
            std::cout<<"get host by addr err" <<std::endl;
            return 0;
        }
    } else {
        // input is a hostname
        p_host = gethostbyname(argv[1]);
        if(!p_host){
            std::cout<<"get host by name err" <<std::endl;
        }
    }

    process_print_res(p_host);

    return 0;
}