resolv/ns_rr_cursor_init.c

   1 /* Initialize a simple DNS packet parser.
   2    Copyright (C) 2022-2023 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <arpa/nameser.h>
  20 #include <errno.h>
  21 #include <stdbool.h>
  22 #include <string.h>
  23
  24 bool
  25 __ns_rr_cursor_init (struct ns_rr_cursor *c,
  26                      const unsigned char *buf, size_t len)
  27 {
  28   c->begin = buf;
  29   c->end = buf + len;
  30
  31   /* Check for header size and 16-bit question count value (it must be 1).  */
  32   if (len < 12 || buf[4] != 0 || buf[5] != 1)
  33     {
  34       __set_errno (EMSGSIZE);
  35       c->current = c->end;
  36       return false;
  37     }
  38   c->current = buf + 12;
  39
  40   int consumed = __ns_name_length_uncompressed (c->current, c->end);
  41   if (consumed < 0)
  42     {
  43       __set_errno (EMSGSIZE);
  44       c->current = c->end;
  45       c->first_rr = NULL;
  46       return false;
  47     }
  48   c->current += consumed;
  49
  50   /* Ensure there is room for question type and class.  */
  51   if (c->end - c->current < 4)
  52     {
  53       __set_errno (EMSGSIZE);
  54       c->current = c->end;
  55       c->first_rr = NULL;
  56       return false;
  57     }
  58   c->current += 4;
  59   c->first_rr = c->current;
  60
  61   return true;
  62 }