域名解析

域名解析相关api

#include <netdb.h>
extern int h_errno;

struct hostent *gethostent(void);
struct hostent *gethostbyname(const char *name);
void sethostent(int stayopen);
void endhostent(void);

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 */
           }

域名解析使用gethostbyname

/etc/hosts 文件

#include<netdb.h>
#include<unistd.h>
#include<stdlib.h>
#include<stdio.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[16];
    memset(ip, 0, sizeof(ip));
    inet_ntop(h->h_addrtype,
                h->h_addr_list[0], ip, sizeof(ip));
    printf("ip address:%s\n", ip);

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

// /etc/hosts 文件
int main(int argc, char* argv[]) {
    if (argc < 2){
        printf("usage:%s host\n",argv[0]);
        exit(1);
    }
    struct hostent *h;
    h = gethostbyname(argv[1]);
    if(h != NULL) {
        out_addr(h);
    } else {
        printf("no %s exist\n",argv[1]);
    }
    return 0;
}

image.png

gethostbyname在多线程中存在问题，另外gethostbyname只支持ipv4

使用gethostent做域名解析

#include<netdb.h>
#include<unistd.h>
#include<stdlib.h>
#include<stdio.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[16];
    memset(ip, 0, sizeof(ip));
    inet_ntop(h->h_addrtype,
                h->h_addr_list[0], ip, sizeof(ip));
    printf("ip address:%s\n", ip);

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

// /etc/hosts 文件
int main(int argc, char* argv[]) {
    if (argc < 2){
        printf("usage:%s host\n",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 \n",argv[1]);
    return 0;
}