c-ares/ares_create_query.c

/* Copyright 1998 by the Massachusetts Institute of Technology.
 *
 * Permission to use, copy, modify, and distribute this
 * software and its documentation for any purpose and without
 * fee is hereby granted, provided that the above copyright
 * notice appear in all copies and that both that copyright
 * notice and this permission notice appear in supporting
 * documentation, and that the name of M.I.T. not be used in
 * advertising or publicity pertaining to distribution of the
 * software without specific, written prior permission.
 * M.I.T. makes no representations about the suitability of
 * this software for any purpose.  It is provided "as is"
 * without express or implied warranty.
 */

#include "ares_setup.h"

#ifdef HAVE_NETINET_IN_H
#  include <netinet/in.h>
#endif
#ifdef HAVE_ARPA_NAMESER_H
#  include <arpa/nameser.h>
#else
#  include "nameser.h"
#endif
#ifdef HAVE_ARPA_NAMESER_COMPAT_H
#  include <arpa/nameser_compat.h>
#endif

#include "ares.h"
#include "ares_dns.h"
#include "ares_private.h"

#ifndef T_OPT
#  define T_OPT  41 /* EDNS0 option (meta-RR) */
#endif

/* Header format, from RFC 1035:
 *                                  1  1  1  1  1  1
 *    0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                      ID                       |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |QR|   Opcode  |AA|TC|RD|RA|   Z    |   RCODE   |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                    QDCOUNT                    |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                    ANCOUNT                    |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                    NSCOUNT                    |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                    ARCOUNT                    |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *
 * AA, TC, RA, and RCODE are only set in responses.  Brief description
 * of the remaining fields:
 *      ID      Identifier to match responses with queries
 *      QR      Query (0) or response (1)
 *      Opcode  For our purposes, always QUERY
 *      RD      Recursion desired
 *      Z       Reserved (zero)
 *      QDCOUNT Number of queries
 *      ANCOUNT Number of answers
 *      NSCOUNT Number of name server records
 *      ARCOUNT Number of additional records
 *
 * Question format, from RFC 1035:
 *                                  1  1  1  1  1  1
 *    0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                                               |
 *  /                     QNAME                     /
 *  /                                               /
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                     QTYPE                     |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *  |                     QCLASS                    |
 *  +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
 *
 * The query name is encoded as a series of labels, each represented
 * as a one-byte length (maximum 63) followed by the text of the
 * label.  The list is terminated by a label of length zero (which can
 * be thought of as the root domain).
 */

int ares_create_query(const char *name, int dnsclass, int type,
                      unsigned short id, int rd, unsigned char **buf,
                      int *buflen, int max_udp_size)
{
  int len;
  unsigned char *q;
  const char *p;

  /* Set our results early, in case we bail out early with an error. */
  *buflen = 0;
  *buf = NULL;

  /* Compute the length of the encoded name so we can check buflen.
   * Start counting at 1 for the zero-length label at the end. */
  len = 1;
  for (p = name; *p; p++)
    {
      if (*p == '\\' && *(p + 1) != 0)
        p++;
      len++;
    }
  /* If there are n periods in the name, there are n + 1 labels, and
   * thus n + 1 length fields, unless the name is empty or ends with a
   * period.  So add 1 unless name is empty or ends with a period.
   */
  if (*name && *(p - 1) != '.')
    len++;

  /* Immediately reject names that are longer than the maximum of 255
   * bytes that's specified in RFC 1035 ("To simplify implementations,
   * the total length of a domain name (i.e., label octets and label
   * length octets) is restricted to 255 octets or less."). We aren't
   * doing this just to be a stickler about RFCs. For names that are
   * too long, 'dnscache' closes its TCP connection to us immediately
   * (when using TCP) and ignores the request when using UDP, and
   * BIND's named returns ServFail (TCP or UDP). Sending a request
   * that we know will cause 'dnscache' to close the TCP connection is
   * painful, since that makes any other outstanding requests on that
   * connection fail. And sending a UDP request that we know
   * 'dnscache' will ignore is bad because resources will be tied up
   * until we time-out the request.
   */
  if (len > MAXCDNAME)
    return ARES_EBADNAME;

  *buflen = len + HFIXEDSZ + QFIXEDSZ + (max_udp_size ? EDNSFIXEDSZ : 0);
  *buf = malloc(*buflen);
  if (!*buf)
      return ARES_ENOMEM;

  /* Set up the header. */
  q = *buf;
  memset(q, 0, HFIXEDSZ);
  DNS_HEADER_SET_QID(q, id);
  DNS_HEADER_SET_OPCODE(q, QUERY);
  if (rd) {
    DNS_HEADER_SET_RD(q, 1);
  }
  else {
    DNS_HEADER_SET_RD(q, 0);
  }
  DNS_HEADER_SET_QDCOUNT(q, 1);

  if (max_udp_size) {
      DNS_HEADER_SET_ARCOUNT(q, 1);
  }

  /* A name of "." is a screw case for the loop below, so adjust it. */
  if (strcmp(name, ".") == 0)
    name++;

  /* Start writing out the name after the header. */
  q += HFIXEDSZ;
  while (*name)
    {
      if (*name == '.')
        return ARES_EBADNAME;

      /* Count the number of bytes in this label. */
      len = 0;
      for (p = name; *p && *p != '.'; p++)
        {
          if (*p == '\\' && *(p + 1) != 0)
            p++;
          len++;
        }
      if (len > MAXLABEL)
        return ARES_EBADNAME;

      /* Encode the length and copy the data. */
      *q++ = (unsigned char)len;
      for (p = name; *p && *p != '.'; p++)
        {
          if (*p == '\\' && *(p + 1) != 0)
            p++;
          *q++ = *p;
        }

      /* Go to the next label and repeat, unless we hit the end. */
      if (!*p)
        break;
      name = p + 1;
    }

  /* Add the zero-length label at the end. */
  *q++ = 0;

  /* Finish off the question with the type and class. */
  DNS_QUESTION_SET_TYPE(q, type);
  DNS_QUESTION_SET_CLASS(q, dnsclass);

  if (max_udp_size)
  {
      q += QFIXEDSZ;
      memset(q, 0, EDNSFIXEDSZ);
      q++;
      DNS_RR_SET_TYPE(q, T_OPT);
      DNS_RR_SET_CLASS(q, max_udp_size);
  }

  return ARES_SUCCESS;
}