dnstest.c

/* gcc -D_GNU_SOURCE -o dnstest dnstest.c */
#include <stdio.h>
#include <unistd.h>
#include <netdb.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/time.h>
#include <arpa/nameser.h>
#include <resolv.h>

#define log(...) do {                                 \
    struct timeval tv;                                \
    gettimeofday(&tv, NULL);                          \
    fprintf(stderr, "%u:%u ", tv.tv_sec, tv.tv_usec); \
    fprintf(stderr, __VA_ARGS__);                     \
  } while(0)

void show_help(const char *name) {
  fprintf(stderr, "usage: %s [-w <tmout>] <host>\n",
          basename(name));
}

int main(int argc, char *argv[]) {
  int tmwait = 500000;
  const char *host = NULL;

  int c = 0;
  while ((c = getopt(argc, argv, "w:h")) != -1) {
    log("optind %d c %d\n", optind, c);
    switch (c) {
    case 'w':
      tmwait = atoi(optarg);
      if (tmwait == 0) {
        log("error: wait time conversion failed\n");
        exit(1);
      }
      break;
    case '?':
      show_help(argv[0]);
      exit(EXIT_FAILURE);
    }
  }

  if (optind >= argc) {
    log("error: host not provided\n");
    show_help(argv[0]);
    exit(EXIT_FAILURE);
  }

  host = argv[optind];

  log("using host: %s\n", host);

  while (1) {
    res_init();
    struct addrinfo *res = NULL;
    int err = getaddrinfo(host, NULL, NULL, &res);
    if (err != 0) {
      log("failed to resolve: %s\n", gai_strerror(err));
    } else {
      const char *name = inet_ntoa(((struct sockaddr_in *)res->ai_addr)->sin_addr);
      log("resolved to: %s\n", name);
      if (res)
        freeaddrinfo(res);
    }
    usleep(tmwait);
  }
}