Monitor the latency of hostname resolution

dnsping.c

#include <arpa/inet.h>
#include <limits.h>
#include <math.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <unistd.h>

extern int h_errno;

void usage(const char *argv0) {
  fprintf(stderr, "Usage: %s [-c count] [-i secs] example.com\n", argv0);
}

void dsleep(double duration) {
  if(duration > 0) {
    double i, f = modf(duration, &i);
    struct timespec ts = { i, 1000000000.0 * f };
    while(nanosleep(&ts, &ts)) { }
  }
}

int main(int argc, char **argv) {
  int c;
  long count = 0;
  double interval = 1;
  char *domain = NULL;
  while(getopt(argc, argv, "c:i:") != -1) {
    switch(optopt) {
      case 'c': count = atoi(optarg); break;
      case 'i': interval = atof(optarg); break;
      default: usage(argv[0]); return 1;
    }
  }
  if(optind >= argc) {
    fprintf(stderr, "Expected a domain name to resolve.\n"); usage(argv[0]); return 1;
  }
  domain = argv[optind];
  if(count == 0) count = LONG_MAX;

  struct hostent *host = NULL;
  double elapsed = interval;
  for(long i = 0; i < count; i++) {
    dsleep(interval - elapsed);
    printf("%s -> ", domain);
    struct timeval t1, t2;
    gettimeofday(&t1, NULL);
    host = gethostbyname(domain);
    gettimeofday(&t2, NULL);
    if(host == NULL) {
      switch(h_errno) {
        case HOST_NOT_FOUND: printf("[HOST_NOT_FOUND]"); break;
        case NO_DATA:        printf("[NO_DATA]"); break;
        case NO_RECOVERY:    printf("[NO_RECOVERY]"); break;
        case TRY_AGAIN:      printf("[TRY_AGAIN]"); break;
        default:             printf("[%d]", h_errno);
      }
    } else {
      struct in_addr *addr = (struct in_addr*)(host->h_addr_list[0]);
      printf("%s(%s)", host->h_name, inet_ntoa(*addr));
    }
    elapsed = (t2.tv_sec - t1.tv_sec) + (t2.tv_usec - t1.tv_usec) / 1000000.0;
    printf(" %#.3fms\n", elapsed * 1000.0);
  }
  return host == NULL ? -1 : h_errno;
}

Example usage:

~ $ gcc dnsping.c -o dnsping
~ $ ./dnsping -c 3 -i 1 example.com
example.com -> example.com(93.184.216.34) 36.003ms
example.com -> example.com(93.184.216.34) 0.857ms
example.com -> example.com(93.184.216.34) 0.935ms
~ $