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
", h->h_name);
    printf("addrtype: %s
", h->h_addrtype == AF_INET ? "IPV4": "IPV6");
    char ip[16];
    memset(ip, 0, sizeof(ip));
    inet_ntop(h->h_addrtype, h->h_addr_list[0], ip, sizeof(ip));
    printf("ip address: %s
", ip);

    int i = 0;
    while(h->h_aliases[i] != NULL){
        printf("aliase: %s
", h->h_aliases[i]);
        i++;
    }
}

int main(int argc, char *argv[])
{
    if(argc < 2){
        printf("usage: %s host
", argv[0]);
        exit(1);
    }

    struct hostent *h;
    h = gethostbyname(argv[1]);
    if( NULL != h){
        out_addr(h);
    }
    else{
        perror("get hostbyname error");
    }

    return 0;
}

　　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
", h->h_name);
    printf("addrtype: %s
", h->h_addrtype == AF_INET ? "IPV4": "IPV6");
    char ip[16];
    memset(ip, 0, sizeof(ip));
    inet_ntop(h->h_addrtype, h->h_addr_list[0], ip, sizeof(ip));
    printf("ip address: %s
", ip);

    int i = 0;
    while(h->h_aliases[i] != NULL){
        printf("aliase: %s
", h->h_aliases[i]);
        i++;
    }
}

int main(int argc, char *argv[])
{
    if(argc < 2){
        fprintf(stderr, "usage: %s host
", argv[0]);
        exit(1);
    }

    struct hostent *h;
    while((h = gethostent()) != NULL){
        if(!strcmp(argv[1], h->h_name)){
            out_addr(h);
            exit(0);
        }
        else{
            int i = 0;
            while(h->h_aliases[i] != NULL){
                if(!strcmp(argv[1], h->h_aliases[i])){
                    out_addr(h);
                    exit(0);
                }
                i++;
            }
        }
    }

    endhostent();
    printf("no %s exist
", argv[1]);

    return 0;
}

　　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 1;
    int i = 0;
    while(host->h_aliases[i] != NULL){
        if(!strcmp(host->h_aliases[i], name)) return 1;
        i++;
    }

    return 0;
}

unsigned int get_ip_by_name(char *name)
{
    unsigned int ip = 0;
    struct hostent *host;
    while((host = gethostent()) != NULL){
        if(is_host(host, name)){
            memcpy(&ip, host->h_addr_list[0], 4);
            break;
        }
    }

    endhostent();
    return ip;
}

int main(int argc, char *argv[])
{
    if(argc < 3){
        printf("usage: %s ip port
", argv[0]);
        exit(1);
    }

    /** 步骤1: 创建 socket */
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if(sockfd < 0){
        perror("socket error");
        exit(1);
    }

    /** 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, 0, sizeof(serveraddr));
    serveraddr.sin_family = AF_INET; ///< ipv4
    serveraddr.sin_port = htons(atoi(argv[2])); ///< port
    
    unsigned int ip = get_ip_by_name(argv[1]);
    if(ip != 0){
        serveraddr.sin_addr.s_addr = ip;
    }
    else {
        inet_pton(AF_INET, argv[1], &serveraddr.sin_addr.s_addr);
    }
    
    char buffer[1024] = "hello world";
    /** 向服务器端发送数据报文 */
    if(sendto(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr*)&serveraddr, sizeof(serveraddr)) < 0){
        perror("sendto error");
        exit(1);
    }
    else{
        /** 接受服务器端发送的数据报文 */
        memset(buffer, 0, sizeof(buffer));
        if(recv(sockfd, buffer, sizeof(buffer), 0) < 0){
            perror("recv error");
            exit(1);
        }
        else{
            printf("%s", buffer);
        }
    }

    close(sockfd);

    return 0;
}

　　编译，运行服务器和客户端测试：