UDP编程-DNS解析器的分析与实现（C语言）

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <arpa/inet.h>
#define DNS_SERVER_PORT  53
#define DNS_SERVER_IP  "114.114.114.114"
#define DNS_HOST    0x01
#define DNS_CNAME   0x05
struct dns_header {
  unsigned short id;
  unsigned short flags;
  unsigned short questions; // 1
  unsigned short answer;
  unsigned short authority;
  unsigned short additional;
};
struct dns_question {
  int length;//name length
  unsigned short qtype;
  unsigned short qclass;
  unsigned char *name; // 
};
//响应dns项
struct dns_item {
  char *domain;
  char *ip;
};
//client sendto dns server
int dns_create_header(struct dns_header *header) {
  if (header == NULL) return -1;
  memset(header, 0, sizeof(struct dns_header));
  //random，2个一起生成随机数，不是线程安全的
  srandom(time(NULL));
  header->id = random();
  header->flags = htons(0x0100);
  header->questions = htons(1); 
    //其他字段由响应生成，不用请求构造
  return 0;
}
// hostname: www.0voice.com
// name: 3www60voice3com0
//根据hostname构造问题域
int dns_create_question(struct dns_question *question, const char *hostname) {
  if (question == NULL || hostname == NULL) return -1;
  memset(question, 0, sizeof(struct dns_question));
  question->name = (char*)malloc(strlen(hostname) + 2);
  if (question->name == NULL) {
  return -2;
  }
  question->length = strlen(hostname) + 2;
  question->qtype = htons(1); //
  question->qclass = htons(1);
  // name 
  const char delim[2] = ".";//分隔符
  char *qname = question->name;
  char *hostname_dup = strdup(hostname); // strdup --> malloc
  char *token = strtok(hostname_dup, delim); // www.0voice.com ,token的结果是www
  while (token != NULL) {
  size_t len = strlen(token);
  *qname = len;
  qname ++;
  strncpy(qname, token, len+1);//len+1是由于qname的最后必须以0结尾
  qname += len;
  //strtok不是线程安全的，要由前面的截取后再次截取
  token = strtok(NULL, delim); //0voice.com ,  com
  }
  free(hostname_dup);
}
// struct dns_header *header [in]
// struct dns_question *question [in]
// char *request [out]
// rlen: request buffer length [in]
int dns_build_request(struct dns_header *header, struct dns_question *question, char *request, int rlen) {
  if (header == NULL || question == NULL || request == NULL) return -1;
  memset(request, 0, rlen);
  // header --> request
  memcpy(request, header, sizeof(struct dns_header));
  int offset = sizeof(struct dns_header);
  // question --> request
  memcpy(request+offset, question->name, question->length);
  offset += question->length;
  memcpy(request+offset, &question->qtype, sizeof(question->qtype));
  offset += sizeof(question->qtype);
  memcpy(request+offset, &question->qclass, sizeof(question->qclass));
  offset += sizeof(question->qclass);
  return offset;
}
static int is_pointer(int in) {
  return ((in & 0xC0) == 0xC0);
}
//chunk[in]:response buffer
//ptr[in]:start search ptr
//out[out]:
//len[out]:
static void dns_parse_name(unsigned char *chunk, unsigned char *ptr, char *out, int *len) {
  int flag = 0, n = 0, alen = 0;
  char *pos = out + (*len);
  while (1) {
  flag = (int)ptr[0];
  if (flag == 0) break;
  if (is_pointer(flag)) {
    n = (int)ptr[1];
    ptr = chunk + n;
    dns_parse_name(chunk, ptr, out, len);
    break;
  } else {
    ptr ++;
    memcpy(pos, ptr, flag);
    pos += flag;
    ptr += flag;
    *len += flag;
    if ((int)ptr[0] != 0) {
    memcpy(pos, ".", 1);
    pos += 1;
    (*len) += 1;
    }
  }
  }
}
//buffer[in]: dns response buffer
//dimains[out]:domain-ip item
//请参考wireshark响应的字段解析，
//大家可以输入www.taobao.com，然后再wireshark看响应中cname等字段表示的意义
//注意：只能解析出ipv4，不支持ipv6
static int dns_parse_response(char *buffer, struct dns_item **domains) {
  int i = 0;
  unsigned char *ptr = buffer;
  ptr += 4;
  int querys = ntohs(*(unsigned short*)ptr);//Questions
  ptr += 2;
  int answers = ntohs(*(unsigned short*)ptr); //Answer RRs
  ptr += 6;
  for (i = 0;i < querys;i ++) {//遍历Queries的个数（长度不定）
  while (1) {
    int flag = (int)ptr[0];
    ptr += (flag + 1);
    if (flag == 0) break;
  }
  ptr += 4;//
  }
  //cnanme:别名
  char cname[128], aname[128], ip[20], netip[4];
  int len, type, ttl, datalen;
  int cnt = 0;
  struct dns_item *list = (struct dns_item*)calloc(answers, sizeof(struct dns_item));//need free 
  if (list == NULL) {
  return -1;
  }
  for (i = 0;i < answers;i ++) {
  bzero(aname, sizeof(aname));
  len = 0;
  dns_parse_name(buffer, ptr, aname, &len);
  ptr += 2;
  type = htons(*(unsigned short*)ptr);
  ptr += 4;
  ttl = htons(*(unsigned short*)ptr);
  ptr += 4;
  datalen = ntohs(*(unsigned short*)ptr);
  ptr += 2;
  if (type == DNS_CNAME) {//如果是CNAME需要再次解析，然后根据CNAME才能解析出ip
    bzero(cname, sizeof(cname));
    len = 0;
    dns_parse_name(buffer, ptr, cname, &len);//根据canme解析出ip
    ptr += datalen;
  } else if (type == DNS_HOST) {
    bzero(ip, sizeof(ip));
    if (datalen == 4) {
    memcpy(netip, ptr, datalen);
    inet_ntop(AF_INET , netip , ip , sizeof(struct sockaddr));
    printf("%s has address %s\n" , aname, ip);
    printf("\tTime to live: %d minutes , %d seconds\n", ttl / 60, ttl % 60);
    list[cnt].domain = (char *)calloc(strlen(aname) + 1, 1);
    memcpy(list[cnt].domain, aname, strlen(aname));
    list[cnt].ip = (char *)calloc(strlen(ip) + 1, 1);
    memcpy(list[cnt].ip, ip, strlen(ip)); 
    cnt ++;
    }
    ptr += datalen;
  }
  }
  *domains = list;
  ptr += 2;
  return cnt; 
}
int dns_client_commit(const char *domain) {
  int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
  if (sockfd < 0) {
  return  -1;
  }
  struct sockaddr_in servaddr = {0};
  servaddr.sin_family = AF_INET;
  servaddr.sin_port = htons(DNS_SERVER_PORT);
  servaddr.sin_addr.s_addr = inet_addr(DNS_SERVER_IP);
  //DNS是UDP编程，connect不是必须的，这里的好处是为了增加发送请求成功的机率
  int ret = connect(sockfd, (struct sockaddr*)&servaddr, sizeof(servaddr));
  printf("connect : %d\n", ret);
  struct dns_header header = {0};
  dns_create_header(&header);
  struct dns_question question = {0};
  dns_create_question(&question, domain);
  char request[1024] = {0};
  int length = dns_build_request(&header, &question, request, 1024);
  // request
  int slen = sendto(sockfd, request, length, 0, (struct sockaddr*)&servaddr, sizeof(struct sockaddr));
  //recvfrom -block :until recv response data
  char response[1024] = {0};
  struct sockaddr_in addr;
  size_t addr_len = sizeof(struct sockaddr_in);
  int n = recvfrom(sockfd, response, sizeof(response), 0, (struct sockaddr*)&addr, (socklen_t*)&addr_len);
  struct dns_item *dns_domain = NULL;
  dns_parse_response(response, &dns_domain);
  free(dns_domain);
  return n;
}
int main(int argc, char *argv[]) {
  if (argc < 2)
  {
  printf("please input domain!\n");
  return -1;
  } 
  dns_client_commit(argv[1]);
}