Command-line DNS stub resolver

dnsstubq.c

/*
   Command-line DNS stub resolver
   Copyright (C) 2018-2021 SATOH Fumiyasu @ OSS Technology Corp., Japan

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <string.h>
#include <ctype.h>
#include <netdb.h>
#ifdef _AIX
#include <sys/socket.h>
#endif
#include <arpa/inet.h>
#include <errno.h>

const char *progname;

void dnsstubq_error(const char *fmt, ...)
{
  va_list ap;

  fprintf(stderr, "%s: ERROR: ", progname);
  va_start(ap, fmt);
  vfprintf(stderr, fmt, ap);
  va_end(ap);
}

int dnsstubq_name2addr(const char *name)
{
  struct addrinfo ai_hint, *ai_list, *ai_p;
  char addr[NI_MAXHOST];
  int err;

  memset(&ai_hint, 0, sizeof(ai_hint));
  ai_hint.ai_family = AF_UNSPEC;
  ai_hint.ai_socktype = SOCK_STREAM;
  err = getaddrinfo(name, NULL, &ai_hint, &ai_list);
  if (err) {
    dnsstubq_error("%s\n", gai_strerror(err));
    return 1;
  }

  err = 0;
  for (ai_p = ai_list; ai_p; ai_p = ai_p->ai_next) {
    void *sa_p;

    switch (ai_p->ai_family) {
    case AF_INET:
      sa_p = &((struct sockaddr_in *)(ai_p->ai_addr))->sin_addr.s_addr;
      break;
    case AF_INET6:
      sa_p = &((struct sockaddr_in6 *)(ai_p->ai_addr))->sin6_addr;
      break;
    default:
      dnsstubq_error("Unknown address family: %d\n", ai_p->ai_family);
      err = 1;
      continue;
    }

    if (inet_ntop(ai_p->ai_family, sa_p, addr, sizeof(addr)) == NULL) {
      dnsstubq_error("inet_ntop() failed: %s\n", strerror(errno));
      err = 1;
      continue;
    }

    printf("%s\n", addr);
  }

  freeaddrinfo(ai_list);

  return err;
}

int dnsstubq_addr2name(const char *addr)
{
  const struct sockaddr *sa_p;
  socklen_t sa_len;
  struct sockaddr_in sa4;
  struct sockaddr_in6 sa6;
  char name[NI_MAXHOST];
  int rc, err;

  if (strchr(addr, ':')) {
    memset(&sa6, 0, sizeof(sa6));
    sa6.sin6_family = AF_INET6;
    sa_p = (struct sockaddr *)&sa6;
    sa_len = sizeof(sa6);
    rc = inet_pton(AF_INET6, addr, &(sa6.sin6_addr));
  } else {
    memset(&sa4, 0, sizeof(sa4));
    sa4.sin_family = AF_INET;
    sa_p = (struct sockaddr *)&sa4;
    sa_len = sizeof(sa4);
    rc = inet_pton(AF_INET, addr, &(sa4.sin_addr));
  }

  switch (rc) {
  case 0:
    dnsstubq_error("Invalid address: %s\n", addr);
    return 1;
  case -1:
    dnsstubq_error("%s\n", strerror(errno));
    return 1;
  }

  err = getnameinfo(sa_p, sa_len, name, sizeof(name), NULL, 0, NI_NAMEREQD);
  if (err) {
    dnsstubq_error("%s\n", gai_strerror(err));
    return 1;
  }

  printf("%s\n", name);

  return 0;
}

int main(int argc, char **argv)
{
  const char *q;
  int q_hostname_p;
  int err;

  progname = argv[0];

  if (argc != 2) {
    printf("Usage: %s <HOSTNAME|IP-ADDRESS>\n", progname);
    return 1;
  }

  q = argv[1];

  q_hostname_p = 1;
  if (strchr(q, ':')) {
    // IPv6?
    q_hostname_p = 0;
  } else {
    const char *p;
    for (p = q; *p != '\0'; p++) {
      if (isdigit(*p) || *p == '.') {
	continue;
      }
      break;
    }
    if (*p == '\0') {
      // IPv4?
      q_hostname_p = 0;
    }
  }

  if (q_hostname_p) {
    err = dnsstubq_name2addr(q);
  } else {
    err = dnsstubq_addr2name(q);
  }

  return err;
}