五十七、linux 编程——UDP 编程 域名解析

57.1 介绍

57.1.1 域名解析

　　

57.1.2 域名解析函数

　　

　　gethostent 可以获取多组，gethostbyname 只可以获取一组

　　/etc/hosts 文件设置了域名和 IP 的绑定关系

　　

　　

57.2 例子

57.2.1 例子1

　　gethost.c

 #include <netdb.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <memory.h>
 #include <arpa/inet.h>

 void out_addr(struct hostent *h)
 {
     printf("hostname: %s\n", h->h_name);
     printf("addrtype: %s\n", h->h_addrtype == AF_INET ? "IPV4": "IPV6");
     char ip[];
     memset(ip, , sizeof(ip));
     inet_ntop(h->h_addrtype, h->h_addr_list[], ip, sizeof(ip));
     printf("ip address: %s\n", ip);

     int i = ;
     while(h->h_aliases[i] != NULL){
         printf("aliase: %s\n", h->h_aliases[i]);
         i++;
     }
 }

 int main(int argc, char *argv[])
 {
     if(argc < ){
         printf("usage: %s host\n", argv[]);
         exit();
     }

     struct hostent *h;
     h = gethostbyname(argv[]);
     if( NULL != h){
         out_addr(h);
     }
     else{
         perror("get hostbyname error");
     }

     return ;
 }

　　修改下 /etc/hosts 文件，给 localhost 增加一个别名

　　

　　编译运行：

　　

　　gethostbyname 有一些缺陷，支持 IPV4 但是不支持 IPV6，且不能用于多线程中。

57.2.2 例子2

　　gethost2.c

 #include <netdb.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <memory.h>
 #include <arpa/inet.h>

 void out_addr(struct hostent *h)
 {
     printf("hostname: %s\n", h->h_name);
     printf("addrtype: %s\n", h->h_addrtype == AF_INET ? "IPV4": "IPV6");
     char ip[];
     memset(ip, , sizeof(ip));
     inet_ntop(h->h_addrtype, h->h_addr_list[], ip, sizeof(ip));
     printf("ip address: %s\n", ip);

     int i = ;
     while(h->h_aliases[i] != NULL){
         printf("aliase: %s\n", h->h_aliases[i]);
         i++;
     }
 }

 int main(int argc, char *argv[])
 {
     if(argc < ){
         fprintf(stderr, "usage: %s host\n", argv[]);
         exit();
     }

     struct hostent *h;
     while((h = gethostent()) != NULL){
         if(!strcmp(argv[], h->h_name)){
             out_addr(h);
             exit();
         }
         else{
             int i = ;
             while(h->h_aliases[i] != NULL){
                 if(!strcmp(argv[], h->h_aliases[i])){
                     out_addr(h);
                     exit();
                 }
                 i++;
             }
         }
     }

     endhostent();
     printf("no %s exist\n", argv[]);

     return ;
 }

　　编译运行：

　　

57.2.3 例子3

　　修改前面 time_udp_client.c 文件

 #include <sys/types.h>
 #include <sys/socket.h>
 #include <arpa/inet.h>
 #include <unistd.h>
 #include <netdb.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <signal.h>
 #include <string.h>
 #include <time.h>

 int is_host(struct hostent *host, char *name)
 {
     if(!strcmp(host->h_name, name)) return ;
     int i = ;
     while(host->h_aliases[i] != NULL){
         if(!strcmp(host->h_aliases[i], name)) return ;
         i++;
     }

     return ;
 }

 unsigned int get_ip_by_name(char *name)
 {
     unsigned int ip = ;
     struct hostent *host;
     while((host = gethostent()) != NULL){
         if(is_host(host, name)){
             memcpy(&ip, host->h_addr_list[], );
             break;
         }
     }

     endhostent();
     return ip;
 }

 int main(int argc, char *argv[])
 {
     if(argc < ){
         printf("usage: %s ip port\n", argv[]);
         exit();
     }

     /** 步骤1: 创建 socket */
     int sockfd = socket(AF_INET, SOCK_DGRAM, );
     if(sockfd < ){
         perror("socket error");
         exit();
     }

     /** 步骤2: 调用 recvfrom 和 sendto 等函数和服务器端双向通信 */
     struct sockaddr_in serveraddr;
     memset(&serveraddr, , sizeof(serveraddr));
     serveraddr.sin_family = AF_INET; ///< ipv4
     serveraddr.sin_port = htons(atoi(argv[2])); ///< port

     unsigned int ip = get_ip_by_name(argv[]);
     if(ip != ){
         serveraddr.sin_addr.s_addr = ip;
     }
     else {
         inet_pton(AF_INET, argv[], &serveraddr.sin_addr.s_addr);
     }

     char buffer[] = "hello world";
     /** 向服务器端发送数据报文 */
     if(sendto(sockfd, buffer, sizeof(buffer), , (struct sockaddr*)&serveraddr, sizeof(serveraddr)) < ){
         perror("sendto error");
         exit();
     }
     else{
         /** 接受服务器端发送的数据报文 */
         memset(buffer, , sizeof(buffer));
         if(recv(sockfd, buffer, sizeof(buffer), ) < ){
             perror("recv error");
             exit();
         }
         else{
             printf("%s", buffer);
         }
     }

     close(sockfd);

     return ;
 }

　　编译，运行服务器和客户端测试：