一個簡單的DNS客戶端demo

// 一個簡單的DNS客戶端demo
// DNS協議參考：http://www.cnblogs.com/topdog/archive/2011/11/15/2250185.html
// 代碼參考：http://www.isayme.org/socket-udp-dns-ping-ip.html

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

//#define USE_TCP // DNS協議默認使用UDP，但其實使用TCP也是可以的，試試就知道。

#define SRV_PORT 53
#define BUF_SIZE 1024

const char srv_ip[] = "208.67.222.222"; // 可用的DNS服務器地址

typedef unsigned short U16;

typedef struct _DNS_HDR
{
    U16 id;
    U16 tag;
    U16 numq;
    U16 numa;
    U16 numa1;
    U16 numa2;
} DNS_HDR;

typedef struct _DNS_QER
{
    U16 type;
    U16 classes;
} DNS_QER;

int main(int argc, char** argv)
{
    int clifd,len = 0,i;
    struct sockaddr_in servaddr;
#ifdef USE_TCP
    int socklen = sizeof(servaddr);
#endif
    char buf[BUF_SIZE];
    char *p;
    DNS_HDR *dnshdr = (DNS_HDR *)buf;
    DNS_QER *dnsqer = (DNS_QER *)(buf + sizeof(DNS_HDR));

    if (argc != 2)
    {
        printf("todo:usage\n");
        return -1;
    }

    if ((clifd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        printf("create socket error!\n");
        return -1;
    }

    bzero(&servaddr, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    inet_aton(srv_ip, &servaddr.sin_addr);
    servaddr.sin_port = htons(SRV_PORT);

#ifdef USE_TCP
    if (connect(clifd, (struct sockaddr *)&servaddr, socklen) < 0)
    {
        printf("can't connect to %s!\n ", argv[1]);
        return -1;
    }
#endif

    memset(buf, 0, BUF_SIZE);
    dnshdr->id   = htons(0x0001); // 一個用戶發送查詢的時候定義的隨機數，當服務器返回結果的時候，返回包的ID與用戶發送的一致
    dnshdr->tag  = htons(0x0100); // 一個期望遞歸的標準查詢請求
    dnshdr->numq = htons(0x0001); // 報文請求段中的問題記錄數
    dnshdr->numa = 0;             // 報文回答段中的回答記錄數，由服務器返回時設置

    // 以下按照DNS協議的規則填充要查詢的域名
    strcpy(buf + sizeof(DNS_HDR) + 1, argv[1]);
    p = buf + sizeof(DNS_HDR) + 1;
    i = 0;
    while (p < (buf + sizeof(DNS_HDR) + 1 + strlen(argv[1])))
    {
        if (*p == '.')
        {
            *(p - i - 1) = i;
            i = 0;
        }
        else
        {
            i++;
        }
        p++;
    }
    *(p - i - 1) = i;

    dnsqer = (DNS_QER *)(buf + sizeof(DNS_HDR) + 2 + strlen(argv[1]));
    dnsqer->type    = htons(0x0001); // 查詢的資源記錄類型
    dnsqer->classes = htons(0x0001); // 指定信息的協議組

#ifdef USE_TCP
    len = send(clifd, buf, sizeof(DNS_HDR) + sizeof(DNS_QER) + strlen(argv[1]) + 2, 0);
    len = recv(clifd, buf, BUF_SIZE, 0);
#else
    len = sendto(clifd, buf, sizeof(DNS_HDR) + sizeof(DNS_QER) + strlen(argv[1]) + 2, 0, (struct sockaddr *)&servaddr, sizeof(servaddr));
    i = sizeof(struct sockaddr_in);
    len = recvfrom(clifd, buf, BUF_SIZE, 0, (struct sockaddr *)&servaddr, (socklen_t *)&i);
#endif

    if (len < 0)
    {
        printf("send or recv error\n");
        return -1;
    }

    if (dnshdr->numa == 0)
    {
        printf("ack error\n");
        return -1;
    }

    p = buf + len - 4;
    printf("%s ==> %u.%u.%u.%u\n", argv[1],
           (unsigned char)(*p),
           (unsigned char)(*(p + 1)),
           (unsigned char)(*(p + 2)),
           (unsigned char)(*(p + 3))
          );

    close(clifd);
    return 0;
}