src/loop.c

/* dnsmasq is Copyright (c) 2000-2021 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()
{
   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 (!(serv->flags & 
	   (SERV_LITERAL_ADDRESS | SERV_NO_ADDR | SERV_USE_RESOLV | SERV_NO_REBIND | SERV_HAS_DOMAIN | SERV_FOR_NODOTS | SERV_LOOP)))
       {
	 ssize_t len = loop_make_probe(serv->uid);
	 int fd;
	 
	 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(query[i]))
      return 0;

  uid = strtol(query, NULL, 16);

  for (serv = daemon->servers; serv; serv = serv->next)
     if (!(serv->flags & 
	   (SERV_LITERAL_ADDRESS | SERV_NO_ADDR | SERV_USE_RESOLV | SERV_NO_REBIND | SERV_HAS_DOMAIN | SERV_FOR_NODOTS | SERV_LOOP)) &&
	 uid == serv->uid)
       {
	 serv->flags |= SERV_LOOP;
	 check_servers(); /* log new state */
	 return 1;
       }

  return 0;
}

#endif