DNS lookup program simply gcc *.c

lookup.c

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

#pragma pack(1)

typedef unsigned int u32;
typedef unsigned short u16;
typedef unsigned char u8;

char *get_type(u16 type) {
  switch(type) {
    case 0x0005:
      return "cname";
    case 0x0001:
      return "a";
    default:
      return "unknown";
  }
}

struct dns_header {
  u16 trans_id;
  u16 flags;
  u16 questions;
  u16 ans_rrs;
  u16 auth_rrs;
  u16 other_rrs;
};

struct dns_ans {
  u16 name;
  u16 type;
  u16 class;
  u32 ttl;
  u16 len;
  u8 data[];
};

int next_dot(char *buff) {
  int i=0;
  while(*buff) {
    if(*buff == '.') {
      return i;
    }
    i++;
    buff++;
  }
  return i;
}

int prep(char *buff,char *dest) {
  char *start = buff;
  int sz = 2;
  struct dns_header *hdr = (struct dns_header*)buff;
  hdr->trans_id = ((u16)dest)^0x5555;
  hdr->flags = htons(0x0100);
  hdr->questions = htons(1);
  hdr->ans_rrs = 0;
  hdr->auth_rrs = 0;
  hdr->other_rrs = 0;
  buff+=12;
  while(sz) {
    sz=next_dot(dest);
    *buff++=(u8)sz;
    memcpy(buff,dest,sz);
    dest+=sz;
    buff+=sz;
    if(!*dest) {
      break;
    }
    dest++;
  }
  *buff++=0;
  *buff++=0;
  *buff++=1;
  *buff++=0;
  *buff++=1;
  return buff-start;
}

void print_info(char *pkt) {
  int i;
  u8 size;
  int count;
  struct dns_header *hdr = (struct dns_header *)pkt;
  struct dns_ans *ans;
  char *val;
  printf("Status: %s Found\n",(ntohs(hdr->flags)&0x0007)?"Not":"");
  count = ntohs(hdr->ans_rrs);
  printf("Answers: %d\n",count);
  pkt+=12;
  while(*pkt) {
    pkt+=*pkt+1;
  }
  pkt+=5;
  ans = (struct dns_ans *)pkt;
  for(i=1;i<=count;i++) {
    printf(" %d:\n",i);
    ans->type = ntohs(ans->type);
    printf("  Type: %s 0x%04x\n",get_type(ans->type),ans->type);
    ans->class = ntohs(ans->class);
    printf("  Class: %s 0x%04x\n","unknown", ans->class);
    ans->ttl = ntohl(ans->ttl);
    printf("  TTL: %us (%uh%um%us)\n",ans->ttl,ans->ttl/3600,(ans->ttl/60)%60,ans->ttl%60);
    ans->len = ntohs(ans->len);
    printf("  Size: %u bytes\n",ans->len);
    if(ans->type == 0x0005) {
      val = (char *)ans->data;
      size = *val;
      do {
        *val = '.';
        val+=size+1;
        size = *val;
        if(size == 0xc0) {
          size = 0;
        }
      } while(size);
      *val=0;
      printf("  Value: %s\n",(char *)ans->data+1);
    }
    if(ans->type == 0x0001) {
      printf("  Value: %d.%d.%d.%d\n",ans->data[0],ans->data[1],ans->data[2],ans->data[3]);
    }
    pkt+=ans->len+12;
    ans=(struct dns_ans *)pkt;
  }
}

void lookup(char *server,char *dest) {
  char pktbuf[512];
  int len = prep(pktbuf,dest);
  int sockfd;
  struct sockaddr_in servaddr,recvaddr;
  socklen_t size;
  printf("looking up %s on %s\n",dest,server);
  sockfd = socket(AF_INET,SOCK_DGRAM,0);
  if(sockfd < 0) {
    fprintf(stderr,"Failed to get socket!\n");
    return;
  }
  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = inet_addr(server);
  servaddr.sin_port = htons(53);
  if(servaddr.sin_addr.s_addr == INADDR_NONE) {
    fprintf(stderr,"Bad server address!\n");
    return;
  }
  sendto(sockfd,pktbuf,len,0,(struct sockaddr *)&servaddr,sizeof(servaddr));
  recvfrom(sockfd,pktbuf,512,0,(struct sockaddr *)&recvaddr,&size);
  print_info(pktbuf);
  return;
}

main.c

#include <stdio.h>
#include <unistd.h>
#include <getopt.h>

extern void lookup(char *server,char *dest);

int main(int argc, char **argv) {
  char c=0;
  char *server=0;
  char *dest=0;
  struct option long_options[] = {
    {"server",required_argument,0,'s'},
    {"target",required_argument,0,'t'},
    {0,0,0,0}
  };
  int option_index = 0;
  while(c!=-1) {
    c = getopt_long(argc,argv,"s:t:",long_options,&option_index);
    if(c=='s') {
      server = optarg;
    }
    if(c=='t') {
      dest = optarg;
    }
  }
  if(server == 0) {
    fprintf(stderr,"Server not set.\n");
    return -1;
  }
  if(dest == 0) {
    if(optind>=argc) {
      fprintf(stderr,"Destination not set.\n");
      return -1;
    }
    dest = argv[optind];
  }
  printf("%s\n%s\n",server,dest);
  lookup(server,dest);
  printf("\n");
  return 0;
}