域名系统 DNS

#include  /* See NOTES */
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
 
int sock;
struct sockaddr_in sa;
 
/*
generate_question:解析域名数据
*/
void generate_question(char*dns_name,char*buf,int*len)
{
    char*pos = dns_name;//指向域名当前位置
    char*ptr = buf;
    int n = 0;
    *len = 0;
    while(1)
    {
            n = strlen(pos) - (strstr(pos , ".") ? strlen(strstr(pos , ".")) : 0);//在 pos 中查找"."子串
        //返回"."在 pos 中第一次出现的地址，若没有找到返回 NULL
        printf("%d\n", n);
        *ptr++ = (unsigned char)n;
        memcpy(ptr,pos,n);
        printf("%lu\n",strlen(ptr));
        printf("%s\n", ptr);
        *len += n + 1;
        ptr += n;
        if(!strstr(pos , "."))
        {
            *ptr = (unsigned char)0;
            ptr ++;
            *len += 1;
            break;
        }
        pos += n + 1;
        printf("%s\n", pos);
    }
}
 
/*
send_dns_request:解析域名数据，并发送数据包
*/
void send_dns_request(char * dns_name)
{
    unsigned char request[256] = {0}; //保存整个请求报文
    unsigned char *ptr = request;
    unsigned char question[128];//存储域名解析数据
    int question_len;
    //产生请求(把字符串表示的域名转换成 DNS 要求的格式)
    generate_question(dns_name , question ,&question_len);
    printf("%s\n", question);
    printf("%d\n", question_len);
    *((unsigned short*)ptr) = 1; //会话标识 ID
    ptr += 2;
    *((unsigned short*)ptr) = htons(0x0100); //flags
    ptr += 2;
    *((unsigned short*)ptr) = htons(1); //Quetions 问题数，通常为 1
    ptr += 2;
    *((unsigned short*)ptr) = 0;
    ptr += 2;
    *((unsigned short*)ptr) = 0;
    ptr += 2;
    *((unsigned short*)ptr) = 0;
    ptr += 2;
    //把域名(www.baidu.com)装换为相应的问题格式保存在此处,以\0 结尾
    memcpy(ptr , question , question_len);
    ptr += question_len;
    *((unsigned short*)ptr) = htons(1); //获取 IPV4 地址，查询类型
    ptr += 2;
    *((unsigned short*)ptr) = htons(1); //指互联网地址，查询类，一般为 1，表明是 Internet 数据
    int re = sendto(sock, request,question_len+16 ,0, (structsockaddr*)&sa, sizeof(sa));
    printf("re = %d\n",re);
    int i;
    for(i=0;i//以 16 进制格式打印出来
    {
        printf("%02X ",(unsigned char)request[i]);
    } 
    printf("===================\n");
}
 
/*
    recv_dns_response:获取域名所对应的 IP 地址，并打印
*/
void recv_dns_response()
{
    struct sockaddr_in src_addr;
    socklen_t addrlen = sizeof(src_addr);
    char buf[255] = {0};
    int r = recvfrom(sock, buf, 255, 0, (struct sockaddr*)&src_addr,&addrlen);
    printf("r == %d\n", r);
    if(r > 0)//收到大于 0 的数据
    {
        int i;
        for(i=r-4;i//以 16 进制格式打印出来
        {
            if(i != r-1)
                printf("%d.",(unsigned char)buf[i]);
            else
                printf("%d\n",(unsigned char)buf[i]);
        } 
    }
}
 
//例./main www.baidu.com
int main(int argc, char *argv[])
{
    if(argc != 2)
    {
        printf("Usage : %s \n",argv[0]);
        return -1;
    }
    //step 1: 创建一个套接字
    sock = socket(AF_INET, SOCK_DGRAM, 0);
    if (sock == -1)
    {
        perror("socket error:");
        return -1;
    }
 
    //step 2: 绑定一个地址(ip+端口号)
    memset(&sa, 0,sizeof(sa));
    sa.sin_family = AF_INET;
    sa.sin_port = htons(53); //按"网络字节序"来保存一个整数
    sa.sin_addr.s_addr = inet_addr("114.114.114.114");
    //发送解析请求
    send_dns_request(argv[1]);
    //接收分析结果
    recv_dns_response();
    close(sock);
    return 0;
}

域名解析函数(gethostbyname)

#include 
#include 
extern int h_errno;

struct hostent *gethostbyname(const char *name);

struct hostent
{
    char *h_name; /* 主机正式名称 */
    char **h_aliases; /* 别名列表。 */
    int h_addrtype; /* 主机地址类型。*/
    int h_length; /* 地址的长度。 */
    char **h_addr_list; /*来自名称服务器的地址列表。 */
    #ifdef __USE_MISC
    # define h_addr h_addr_list[0] /* 地址，用于向后兼容。*/
    #endif
};

#include 
#include 
#include 
#include 
extern int h_errno;
 
int main(int argc, char **argv)
{
    if (argc != 2) {
        printf("Use example: %s www.google.com\n", *argv);
        return -1;
    }
    char *name = argv[1];
    struct hostent *hptr;
    hptr = gethostbyname(name);
    if (hptr == NULL) {
        printf("gethostbyname error for host: %s: %s\n", name, 
        hstrerror(h_errno));
        return -1;
    }
    //输出主机的规范名
    printf("\tofficial: %s\n", hptr->h_name);
    //输出主机的别名
    char **pptr;
    char str[INET_ADDRSTRLEN];
    for (pptr=hptr->h_aliases; *pptr!=NULL; pptr++) {
        printf("\ttalias: %s\n", *pptr);
    }
    //输出 ip 地址
    switch (hptr->h_addrtype) {
        case AF_INET:
            pptr = hptr->h_addr_list;
            for (; *pptr!=NULL; pptr++) {
                printf("\taddress: %s\n",inet_ntop(hptr->h_addrtype, hptr->h_addr, str, sizeof(str)));
        }
            break;
        default:
            printf("unknown address type\n");
            break;
    }
    return 0;
}

#include 

int gethostname(char *name, size_t len);

char buf[256] = {0};
int t = gethostname(buf,256);
if(t == 0)
{
    printf("%s\n",buf);
}

#include 

int sethostname(const char *name, size_t len);