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  }