/* dnsmasq is Copyright (c) 2000-2025 Simon Kelley

   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; version 2 dated June, 1991, or

   (at your option) version 3 dated 29 June, 2007.

   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 "dnsmasq.h"

#ifdef HAVE_LOOP

static ssize_t loop_make_probe(u32 uid);

void loop_send_probes(void)

{

   struct server *serv;

   struct randfd_list *rfds = NULL;

   if (!option_bool(OPT_LOOP_DETECT))

     return;

   /* Loop through all upstream servers not for particular domains, and send a query to that server which is

      identifiable, via the uid. If we see that query back again, then the server is looping, and we should not use it. */

   for (serv = daemon->servers; serv; serv = serv->next)

     if (strlen(serv->domain) == 0 &&

   !(serv->flags & (SERV_FOR_NODOTS)))

       {

   ssize_t len = loop_make_probe(serv->uid);

   int fd;

   serv->flags &= ~SERV_LOOP;

   if ((fd = allocate_rfd(&rfds, serv)) == -1)

     continue;

   while (retry_send(sendto(fd, daemon->packet, len, 0, 

          &serv->addr.sa, sa_len(&serv->addr))));

       }

   free_rfds(&rfds);

}

static ssize_t loop_make_probe(u32 uid)

{

  struct dns_header *header = (struct dns_header *)daemon->packet;

  unsigned char *p = (unsigned char *)(header+1);

  /* packet buffer overwritten */

  daemon->srv_save = NULL;

  header->id = rand16();

  header->ancount = header->nscount = header->arcount = htons(0);

  header->qdcount = htons(1);

  header->hb3 = HB3_RD;

  header->hb4 = 0;

  SET_OPCODE(header, QUERY);

  *p++ = 8;

  sprintf((char *)p, "%.8x", uid);

  p += 8;

  *p++ = strlen(LOOP_TEST_DOMAIN);

  strcpy((char *)p, LOOP_TEST_DOMAIN); /* Add terminating zero */

  p += strlen(LOOP_TEST_DOMAIN) + 1;

  PUTSHORT(LOOP_TEST_TYPE, p);

  PUTSHORT(C_IN, p);

  return p - (unsigned char *)header;

}

int detect_loop(char *query, int type)

{

  int i;

  u32 uid;

  struct server *serv;

  if (!option_bool(OPT_LOOP_DETECT))

    return 0;

  if (type != LOOP_TEST_TYPE ||

      strlen(LOOP_TEST_DOMAIN) + 9 != strlen(query) ||

      strstr(query, LOOP_TEST_DOMAIN) != query + 9)

    return 0;

  for (i = 0; i < 8; i++)

    if (!isxdigit((unsigned char)query[i]))

      return 0;

  uid = strtol(query, NULL, 16);

  for (serv = daemon->servers; serv; serv = serv->next)

    if (strlen(serv->domain) == 0 &&

  !(serv->flags & SERV_LOOP) &&

  uid == serv->uid)

      {

  serv->flags |= SERV_LOOP;

  check_servers(1); /* log new state - don't send more probes. */

  return 1;

      }

  return 0;

}

#endif