2. 端口绑定和域名解析
2.1 端口绑定：SO_REUSEADDR选项
int opt = ;//1表示启用该选项
//设置为可重新使用端口，每次启动该端口时，会重新绑定端口。相当于端口被复位并被重新。
//绑定。因此，以后以最后这次绑定为准
if((ret = setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt))) < ){
perror("setsockopt error");
exit();
}
2.2 域名解析
（1）域名解析过程
（2）域名解析函数
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头文件 |
#include <netdb.h> |
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函数 |
struct hostent* gethostent(void); struct hostent* gethostbyname(const char* hostname); void sethostent(int stayopen); void endhostent(void); //注意get和endhostent应成对使用！ |
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参数 |
struct hostent{
|
|
功能 |
域名解析 |
|
备注 |
查看域名和IP映射关系：#more /etc/hosts |
（3）hostent结构体
（4）域名解析案例
//域名解析，并将结果保存在hostent结构体中
if((hptr = gethostbyname("www.google.com")) == NULL){
fprintf(stderr, "gethostbyname call failed.%s\n",
hstrerror(h_errno));
return -;
} printf("official name: %s\n", hptr->h_name); //输出正式名
for(pptr = hptr->h_aliases; *pptr != NULL; pptr++){ //输出对应各个别名
printf("\t alias: %s\n", *pptr);
} if(hptr->h_addrtype != AF_INET){ //判断是否为IPv4地址
fprintf(stderr, "Invalid address type %d\n", hptr->addrtype);
return -;
} pptr = hptr->h_addr_list;
for(; *pptr != NULL; pptr++){ //输出点分十进制的IP地址（字符串）
printf("\t address: %s\n",
inet_ntop(hptr->h_addrtype, *pptr, str, sizeof(str)));
}
【编程实验】根据域名解析出IP地址
//gethost.c
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <string.h> int is_host(struct hostent* host, char* name)
{
//如果正式名为name，则直接返回
if(!strcmp(host->h_name, name)) return ; //查找别名为name
int i = ;
while(host->h_aliases[i] != NULL){
if(!strcmp(host->h_aliases[i], name)) return ;
i++;
}
} //根据主机名获取IP
unsigned int get_ip_by_name(char* name)
{
unsigned int ip = ;
struct hostent* host;
//遍历/etc/hosts文件，找出所有的主机名。
//每调用一次gethostent将读取文件的一行（对应一个主机）
while((host = gethostent()) != NULL){
if(is_host(host, name)){
//h_addr_list[0]中所存放在IP是网络字节序，共4个字节。
memcpy(&ip, host->h_addr_list[], );
break;
}
}
endhostent();
return ip;
} 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[];
memset(ip, , sizeof(ip));
inet_ntop(h->h_addrtype, h->h_addr_list[], ip, sizeof(ip));//绑定的第1个IP地址
printf("ip address: %s\n", ip); //别名
int i = ;
while(h->h_aliases[i] != NULL){
printf("aliases: %s\n", h->h_aliases[i]);
i++;
}
} int main(int argc, char* argv[])
{
if(argc < ){
printf("usage: %s host\n", argv[]);
exit();
} struct hostent* h;
h = gethostbyname(argv[]); if(h != NULL){
out_addr(h);
}else{
printf("no %s exist\n", argv[]);
} //根据主机名，输出IP
unsigned int ipNet = get_ip_by_name(argv[]); //网络字节序IP
char ip[];
memset(ip, , sizeof(ip));
inet_ntop(AF_INET, &ipNet, ip, sizeof(ip));//绑定的第1个IP地址
printf("ip address: %s\n", ip); return ;
}
/*host文件
* [root@localhost 14.udp]# more /etc/hosts
* 127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
* 192.168.32.100 santaclaus www.5iedu.com
* ::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
*
*输出结果
[root@localhost 14.udp]# bin/gethost santaclaus
hostname: santaclaus
addrtype: IPv4
ip address: 192.168.32.100
aliases: www.5iedu.com
ip address: 192.168.32.100
[root@localhost 14.udp]# bin/gethost www.5iedu.com
hostname: santaclaus
addrtype: IPv4
ip address: 192.168.32.100
aliases: www.5iedu.com
ip address: 192.168.32.100
[root@localhost 14.udp]# bin/gethost localhost
hostname: localhost
addrtype: IPv4
ip address: 127.0.0.1
aliases: localhost.localdomain
aliases: localhost4
aliases: localhost4.localdomain4
aliases: localhost.localdomain
aliases: localhost6
aliases: localhost6.localdomain6
ip address: 127.0.0.1
*/