/*
     This file is part of GNUnet.
     Copyright (C) 2011 GNUnet e.V.

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

     GNUnet 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
     Affero General Public License for more details.

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

     SPDX-License-Identifier: AGPL3.0-or-later
 */

/**
 * @file src/dns/gnunet-dns-redirector.c
 * @brief Tool to change DNS replies (for testing)
 * @author Christian Grothoff
 */

#include "platform.h"
#include "gnunet_util_lib.h"
#include "gnunet_dns_service.h"
#include "gnunet_dnsparser_lib.h"

/**
 * Handle to DNS service.
 */
static struct GNUNET_DNS_Handle *handle;

/**
 * New target for A records.
 */
static char *n4;

/**
 * New target for AAAA records.
 */
static char *n6;

/**
 * Global return value (0 success).
 */
static int ret;

/**
 * Selected level of verbosity.
 */
static unsigned int verbosity;


/**
 * Modify the given DNS record.
 *
 * @param record record to modify
 */
static void
modify_record (const struct GNUNET_DNSPARSER_Record *record)
{
  char buf[INET6_ADDRSTRLEN];

  switch (record->type)
  {
  case GNUNET_DNSPARSER_TYPE_A:
    if (record->data.raw.data_len != sizeof(struct in_addr))
      return;
    if (NULL != n4)
    {
      if (verbosity > 1)
        fprintf (stderr,
                 "Changing A record from `%s' to `%s'\n",
                 inet_ntop (AF_INET, record->data.raw.data, buf, sizeof(buf)),
                 n4);
      GNUNET_assert (1 == inet_pton (AF_INET, n4, record->data.raw.data));
    }
    break;

  case GNUNET_DNSPARSER_TYPE_AAAA:
    if (record->data.raw.data_len != sizeof(struct in6_addr))
      return;
    if (NULL != n6)
    {
      if (verbosity > 1)
        fprintf (stderr,
                 "Changing AAAA record from `%s' to `%s'\n",
                 inet_ntop (AF_INET6, record->data.raw.data, buf, sizeof(buf)),
                 n6);
      GNUNET_assert (1 == inet_pton (AF_INET6, n6, record->data.raw.data));
    }
    break;

  case GNUNET_DNSPARSER_TYPE_NS:
  case GNUNET_DNSPARSER_TYPE_CNAME:
  case GNUNET_DNSPARSER_TYPE_PTR:
  case GNUNET_DNSPARSER_TYPE_SOA:
  case GNUNET_DNSPARSER_TYPE_MX:
  case GNUNET_DNSPARSER_TYPE_TXT:
    break;

  default:
    break;
  }
}


/**
 * Signature of a function that is called whenever the DNS service
 * encounters a DNS request and needs to do something with it.  The
 * function has then the chance to generate or modify the response by
 * calling one of the three "GNUNET_DNS_request_*" continuations.
 *
 * When a request is intercepted, this function is called first to
 * give the client a chance to do the complete address resolution;
 * "rdata" will be NULL for this first call for a DNS request, unless
 * some other client has already filled in a response.
 *
 * If multiple clients exist, all of them are called before the global
 * DNS.  The global DNS is only called if all of the clients'
 * functions call GNUNET_DNS_request_forward.  Functions that call
 * GNUNET_DNS_request_forward will be called again before a final
 * response is returned to the application.  If any of the clients'
 * functions call GNUNET_DNS_request_drop, the response is dropped.
 *
 * @param cls closure
 * @param rh request handle to user for reply
 * @param request_length number of bytes in request
 * @param request udp payload of the DNS request
 */
static void
modify_request (void *cls,
                struct GNUNET_DNS_RequestHandle *rh,
                size_t request_length,
                const char *request)
{
  struct GNUNET_DNSPARSER_Packet *p;
  unsigned int i;
  char *buf;
  size_t len;
  int ret;

  p = GNUNET_DNSPARSER_parse (request, request_length);
  if (NULL == p)
  {
    fprintf (stderr, "Received malformed DNS packet, leaving it untouched\n");
    GNUNET_DNS_request_forward (rh);
    return;
  }
  for (i = 0; i < p->num_answers; i++)
    modify_record (&p->answers[i]);
  buf = NULL;
  ret = GNUNET_DNSPARSER_pack (p, 1024, &buf, &len);
  GNUNET_DNSPARSER_free_packet (p);
  if (GNUNET_OK != ret)
  {
    if (GNUNET_NO == ret)
      fprintf (stderr,
               "Modified DNS response did not fit, keeping old response\n");
    else
      GNUNET_break (0);  /* our modifications should have been sane! */
    GNUNET_DNS_request_forward (rh);
  }
  else
  {
    if (verbosity > 0)
      fprintf (stdout,
               "Injecting modified DNS response\n");
    GNUNET_DNS_request_answer (rh, len, buf);
  }
  GNUNET_free (buf);
}


/**
 * Shutdown.
 */
static void
do_disconnect (void *cls)
{
  if (NULL != handle)
  {
    GNUNET_DNS_disconnect (handle);
    handle = NULL;
  }
}


/**
 * Main function that will be run by the scheduler.
 *
 * @param cls closure
 * @param args remaining command-line arguments
 * @param cfgfile name of the configuration file used (for saving, can be NULL!)
 * @param cfg configuration
 */
static void
run (void *cls, char *const *args, const char *cfgfile,
     const struct GNUNET_CONFIGURATION_Handle *cfg)
{
  struct in_addr i4;
  struct in6_addr i6;

  if ((n4 != NULL) &&
      (1 != inet_pton (AF_INET, n4, &i4)))
  {
    fprintf (stderr,
             "`%s' is nto a valid IPv4 address!\n",
             n4);
    return;
  }
  if ((n6 != NULL) &&
      (1 != inet_pton (AF_INET6, n6, &i6)))
  {
    fprintf (stderr,
             "`%s' is nto a valid IPv6 address!\n",
             n6);
    return;
  }

  handle =
    GNUNET_DNS_connect (cfg,
                        GNUNET_DNS_FLAG_POST_RESOLUTION,
                        &modify_request,
                        NULL);
  GNUNET_SCHEDULER_add_shutdown (&do_disconnect, NULL);
}


int
main (int argc, char *const *argv)
{
  struct GNUNET_GETOPT_CommandLineOption options[] = {
    GNUNET_GETOPT_option_string ('4',
                                 "ipv4",
                                 "IPV4",
                                 gettext_noop ("set A records"),
                                 &n4),

    GNUNET_GETOPT_option_string ('6',
                                 "ipv4",
                                 "IPV6",
                                 gettext_noop ("set AAAA records"),
                                 &n6),

    GNUNET_GETOPT_option_verbose (&verbosity),
    GNUNET_GETOPT_OPTION_END
  };

  if (GNUNET_OK != GNUNET_STRINGS_get_utf8_args (argc, argv, &argc, &argv))
    return 2;

  ret = (GNUNET_OK ==
         GNUNET_PROGRAM_run (argc, argv, "gnunet-dns-redirector",
                             gettext_noop
                               ("Change DNS replies to point elsewhere."),
                             options,
                             &run, NULL)) ? ret : 1;
  GNUNET_free_nz ((void *) argv);
  return ret;
}


/* end of gnunet-dns-redirector.c */
