dns:linux中网址转IP的C编程

一 测试代码

本代码实现功能：获取www.baidu.com对应的IP地址。就是DNS干的事情。根据测试代码查看man手册获取详细说明。

#include <netdb.h>
#include <stdio.h>
#include <arpa/inet.h>
#include <string.h>

extern int h_errno;
int main(){
    char **ptemp;
    char str[100];
    struct hostent *p = gethostbyname("www.baidu.com");
    printf("official:%s\n",p->h_name);
    ptemp = p->h_aliases;
    for(;*ptemp != NULL;ptemp++){
        printf("aliases = %s\n",*ptemp);
    }
    switch(p->h_addrtype){
        case AF_INET:
            ptemp = p->h_addr_list;
            for(;*ptemp != NULL;ptemp++){
                printf("*ptemp = %s\n",*ptemp);
                memset(str,0,sizeof(str));
                printf("ip = %s\n",inet_ntop(p->h_addrtype,*ptemp,str,sizeof(str)/sizeof(str[0])));
                printf("*str = %s\n",str);
            }
    }
    return 0;
}

运行结果：

lkmao@ubuntu:~$ ./a.out
official:www.a.shifen.com
aliases = www.baidu.com
*ptemp = ױ'www.ױ&ifen.com
ip = 14.215.177.39
*str = 14.215.177.39
*ptemp = ױ&ifen.com
ip = 14.215.177.38
*str = 14.215.177.38
lkmao@ubuntu:~$ 

二 nslookup命令 

使用nslookup命令验证测试代码的直接结果。对比后，完全一致。

lkmao@ubuntu:~$ nslookup www.baidu.com
Server:         127.0.1.1
Address:        127.0.1.1#53

Non-authoritative answer:
www.baidu.com   canonical name = www.a.shifen.com.
Name:   www.a.shifen.com
Address: 14.215.177.38
Name:   www.a.shifen.com
Address: 14.215.177.39

lkmao@ubuntu:~$

三 涉及到的函数和头文件

详细信息完全可以查看man手册。

#include <netdb.h>
extern int h_errno;

struct hostent *gethostbyname(const char *name);

结构体：struct hostent

 The hostent structure is defined in <netdb.h> as follows:

           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 */
           }
           #define h_addr h_addr_list[0] /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name of the host.

       h_aliases
              An array of alternative names for the host, terminated by a null pointer.

       h_addrtype
              The type of address; always AF_INET or AF_INET6 at present.

       h_length
              The length of the address in bytes.

       h_addr_list
              An array of pointers to network addresses for the host (in network byte order), terminated by a null pointer.

       h_addr The first address in h_addr_list for backward compatibility.
 

 #include <arpa/inet.h>

const char *inet_ntop(int af, const void *src,
                    char *dst, socklen_t size);

 函数inet_ntop执行成功返回指向dst的指针，在测试结果中，就是IP地址的字符串形式。失败返回NULL;

 DESCRIPTION
       This  function  converts  the  network address structure src in the af address family into a character string.  The resulting string is copied to the buffer pointed to by dst, which must be a non-null
       pointer.  The caller specifies the number of bytes available in this buffer in the argument size.

       inet_ntop() extends the inet_ntoa(3) function to support multiple address families, inet_ntoa(3) is now considered to be deprecated in favor of inet_ntop().  The following address  families  are  cur‐
       rently supported:

       AF_INET
              src  points  to  a  struct  in_addr  (in  network  byte  order)  which  is converted to an IPv4 network address in the dotted-decimal format, "ddd.ddd.ddd.ddd".  The buffer dst must be at least
              INET_ADDRSTRLEN bytes long.

       AF_INET6
              src points to a struct in6_addr (in network byte order) which is converted to a representation of this address in the most appropriate IPv6 network address format for this address.  The  buffer
              dst must be at least INET6_ADDRSTRLEN bytes long.

RETURN VALUE
       On success, inet_ntop() returns a non-null pointer to dst.  NULL is returned if there was an error, with errno set to indicate the error. 

 小结