#include <netdb.h>
struct hostent *gethostbyname(const char *hostname);//成功时返回hostent结构体指针，失败时返回NULL指针

struct hostent
{char *h_name;         //正式主机名char **h_aliases;     //主机别名int h_addrtype;       //主机IP地址类型：IPV4-AF_INETint h_length;       //主机IP地址字节长度，对于IPv4是四字节，即32位char **h_addr_list;   //主机的IP地址列表
};

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netdb.h>
void error_handling(char *message);int main(int argc, char *argv[])
{int i;struct hostent *host;if (argc != 2){printf("Usage:%s<addr>\n", argv[0]);exit(1);}host = gethostbyname(argv[1]);if (!host)error_handling("gethost... error");printf("Official name:%s\n", host->h_name);for (i = 0; host->h_aliases[i]; i++)printf("Aliases %d:%s\n", i + 1, host->h_aliases[i]);printf("Address type:%s\n", (host->h_addrtype == AF_INET) ? "AF_INET" : "AF_INET6");for (i = 0; host->h_addr_list[i]; i++)printf("IP addr %d:%s\n", i + 1, inet_ntoa(*(struct in_addr *)host->h_addr_list[i]));return 0;
}
void error_handling(char *message)
{fputs(message, stderr);fputc('\n', stderr);exit(1);
}

# gcc gethostbyname.c -o gethostbyname
# ./gethostbyname www.taobao.com
Official name:www.taobao.com.danuoyi.tbcache.com
Aliases 1:www.taobao.com
Address type:AF_INET
IP addr 1:61.154.126.109
IP addr 2:218.67.61.254

#include <sys/socket.h>
struct hostent *gethostbyaddr(const char *addr, socklen_t len, int family);//成功时返回hostent结构体变量地址值，失败时返回NULL指针

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netdb.h>
void error_handling(char *message);int main(int argc, char *argv[])
{int i;struct hostent *host;struct sockaddr_in addr;if (argc != 2){printf("Usage : %s <IP>\n", argv[0]);exit(1);}memset(&addr, 0, sizeof(addr));addr.sin_addr.s_addr = inet_addr(argv[1]);host = gethostbyaddr((char *)&addr.sin_addr, 4, AF_INET);if (!host)error_handling("gethost... error");printf("Official name: %s \n", host->h_name);for (i = 0; host->h_aliases[i]; i++)printf("Aliases %d: %s \n", i + 1, host->h_aliases[i]);printf("Address type: %s \n",(host->h_addrtype == AF_INET) ? "AF_INET" : "AF_INET6");for (i = 0; host->h_addr_list[i]; i++)printf("IP addr %d: %s \n", i + 1,inet_ntoa(*(struct in_addr *)host->h_addr_list[i]));return 0;
}void error_handling(char *message)
{fputs(message, stderr);fputc('\n', stderr);exit(1);
}

# gcc gethostbyaddr.c -o gethostbyaddr
# ./gethostbyaddr 218.67.61.254
Official name: 254.61.67.218.broad.sm.fj.dynamic.163data.com.cn
Address type: AF_INET
IP addr 1: 218.67.61.254