域名解析

#include<netinet/in.h>

struct in_addr {
in_addr_t s_addr;	//32-bit IPv4 address,BE
};
struct sockaddr_in {
uint8_t			sin_len;	//该结构长度 16-bit
sa_family_t		sin_family;	//AF_INET
in_port_t		sin_port;	//端口号
struct in_addr	sin_addr;	//32-bit IP地址
char			sin_zero[8];　　//保留
};

#include<netdb.h>
#include<sys/socket.h>

//通过域名获取IP地址
struct hostent *gethostbyname(const char *name);
/*	name:域名，eg:"www.baidu.com"
*/
//通过二进制的IP地址找到对应的域名
struct hostent *gethostbyaddr(const void *addr, socklen_t len, int type);
/*	addr:二进制ip
len:ip长度，对于IPv4是4
type:AF_INET
*/
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 */
}
/*	char* 字符串都是以 '\0' 结束的
char** 字符串指针都是以 NULL 结束的
*/

//ip地址格式转换
#include <arpa/inet.h>
//点分十进制转换为数值形式
int inet_pton(int af, const char *src, void *dst);
/*	af: AF_INET/AF_INET6，对应IPv4/IPv6
src: 点分十进制字符串，eg: "255.255.0.0"
dst: 存放转换结果的，对应解析为 0x0000FFFF,大端
返回：成功，1；src无效，0；出错，-1；
*/
//数值形式转换为点分十进制形式
const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);
/*	src:网络字节序，二进制形式，eg: 0x0000ffff
dst:存放转换结果，点分十进制字符串
size:指定字符串长度
返回：成功，dst;出错，NULL
*/

//主机字节序和网络字节序转换
#include <arpa/inet.h>

uint32_t htonl(uint32_t hostlong);

uint16_t htons(uint16_t hostshort);

uint32_t ntohl(uint32_t netlong);

uint16_t ntohs(uint16_t netshort);

#include<stdio.h>
#include<stdlib.h>
#include<netdb.h>
#include<arpa/inet.h>
void printHostent(struct hostent *p)
{
printf("%s\n",p->h_name);
char **q=p->h_aliases;
while(*q!=NULL){
printf("%s\n",*q);
++q;
}
printf("%d %d\n",p->h_addrtype,p->h_length);
q=p->h_addr_list;
char str[16];
while(*q!=NULL){
printf("%s\n",inet_ntop(AF_INET,*q,str,16));
//printf("%s\n",str);
++q;
}
}
int main()
{
printf("struct hostent of www.baidu.com:\n");
char *host="www.baidu.com";
struct hostent *p=gethostbyname(host);
if(p==NULL)
printf("gethostbyname failed\n");
else
printHostent(p);

printf("\npresentation ip---numeric ip\n");
unsigned int a;
inet_pton(AF_INET,"255.255.0.0",&a);
printf("numeric ip network byte order: %x\n",a);
printf("host byte order: %x\n",ntohl(a));
char str[16];
inet_ntop(AF_INET,&a,str,16);
printf("presentation ip: %s\n",str);
}