linux 域名轉ip的方法
使用gethostbyname函式,然後通過inet_ntoa得到ip地址,inet_ntoa函式功能將一個IP轉換成一個網際網路標準點分格式的字串;
或者使用gethostbyname_r函式;
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <netdb.h>
#include <sys/types.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct hostent *h;
char hostname[40];
printf("請輸入域名/n");
scanf("%s",hostname);
getchar();
if((h=gethostbyname(hostname))==NULL)
{
fprintf(stderr,"不能得到IP/n");
exit(1);
}
printf("HostName :%s/n",h->h_name);
printf("IP Address :%s/n",inet_ntoa(*((struct in_addr *)h->h_addr)));
return EXIT_SUCCESS;
}
//
使用gethostbyname_r 注意的地方,如果name是xxx.xxx.xxx.xxx,這樣的格式,返回值為0,不一定代表轉換的result正確;當name本身不是一個合格的ip地址後,根本沒有呼叫dns,也就是說我們用的時候,如果name是ip地址,那麼必須保證它是正確的,在判斷返回為0而且result!=NULL才表示函式呼叫成功!
關於gethostbyname_r的返回值
man手冊是這麼說的
/* GNU extensions */
struct hostent *gethostbyname2(const char *name, int af);
int gethostbyname_r (const char *name,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
int gethostbyname2_r (const char *name, int af,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
......
The functions gethostbyname() and gethostbyaddr() may return pointers
to static data, which may be overwritten by later calls. Copying the
struct hostent does not suffice, since it contains pointers - a deep
copy is required.
Glibc2 also has a gethostbyname2() that works like gethostbyname(),
but permits to specify the address family to which the address must
belong.
Glibc2 also has reentrant versions gethostbyname_r() and gethostby-
name2_r(). These return 0 on success and nonzero on error. The result
of the call is now stored in the struct with address ret. After the
call, *result will be NULL on error or point to the result on success.
Auxiliary data is stored in the buffer buf of length buflen. (If the
buffer is too small, these functions will return ERANGE.) No global
variable h_errno is modified, but the address of a variable in which
to store error numbers is passed in h_errnop.
按照man的說法
gethostbyname_r()呼叫成功時返回0
*result指向解析成功的資料結構, *result如果為NULL則表示解析出錯
man手冊是這麼說的
/* GNU extensions */
struct hostent *gethostbyname2(const char *name, int af);
int gethostbyname_r (const char *name,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
int gethostbyname2_r (const char *name, int af,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);
......
The functions gethostbyname() and gethostbyaddr() may return pointers
to static data, which may be overwritten by later calls. Copying the
struct hostent does not suffice, since it contains pointers - a deep
copy is required.
Glibc2 also has a gethostbyname2() that works like gethostbyname(),
but permits to specify the address family to which the address must
belong.
Glibc2 also has reentrant versions gethostbyname_r() and gethostby-
name2_r(). These return 0 on success and nonzero on error. The result
of the call is now stored in the struct with address ret. After the
call, *result will be NULL on error or point to the result on success.
Auxiliary data is stored in the buffer buf of length buflen. (If the
buffer is too small, these functions will return ERANGE.) No global
variable h_errno is modified, but the address of a variable in which
to store error numbers is passed in h_errnop.
按照man的說法
gethostbyname_r()呼叫成功時返回0
*result指向解析成功的資料結構, *result如果為NULL則表示解析出錯
如果被解析的字串是類似"xxx.xxx.xxx.xxx"的 數字+點 字串
那麼gethostbyname_r並沒有發出DNS請求
而是直接去計算了
192.1468.80.38顯然不是合法的IP地址, 但gethostbyname_r()返回0
此時*result是NULL
那麼gethostbyname_r並沒有發出DNS請求
而是直接去計算了
192.1468.80.38顯然不是合法的IP地址, 但gethostbyname_r()返回0
此時*result是NULL
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