resolv/res-noaaaa.c

   1 /* Implement suppression of AAAA queries.
   2    Copyright (C) 2022-2024 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 <resolv.h>
  20 #include <string.h>
  21 #include <resolv-internal.h>
  22 #include <resolv_context.h>
  23 #include <arpa/nameser.h>
  24
  25 /* Returns true if the question type at P matches EXPECTED, and the
  26    class is IN.  */
  27 static bool
  28 qtype_matches (const unsigned char *p, int expected)
  29 {
  30   /* This assumes that T_A/C_IN constants are less than 256, which
  31      they are.  */
  32   return p[0] == 0 && p[1] == expected && p[2] == 0 && p[3] == C_IN;
  33 }
  34
  35 /* Handle RES_NOAAAA translation of AAAA queries.  To produce a Name
  36    Error (NXDOMAIN) response for domain names that do not exist, it is
  37    still necessary to send a query.  Using question type A is a
  38    conservative choice.  In the returned answer, it is necessary to
  39    switch back the question type to AAAA.  */
  40 bool
  41 __res_handle_no_aaaa (struct resolv_context *ctx,
  42                       const unsigned char *buf, int buflen,
  43                       unsigned char *ans, int anssiz, int *result)
  44 {
  45   /* AAAA mode is not active, or the query looks invalid (will not be
  46      able to be parsed).  */
  47   if ((ctx->resp->options & RES_NOAAAA) == 0
  48       || buflen <= sizeof (HEADER))
  49     return false;
  50
  51   /* The replacement A query is produced here.  */
  52   struct
  53   {
  54     HEADER header;
  55     unsigned char question[NS_MAXCDNAME + 4];
  56   } replacement;
  57   memcpy (&replacement.header, buf, sizeof (replacement.header));
  58
  59   if (replacement.header.qr
  60       || replacement.header.opcode != 0
  61       || replacement.header.rcode != 0
  62       || ntohs (replacement.header.qdcount) != 1
  63       || ntohs (replacement.header.ancount) != 0
  64       || ntohs (replacement.header.nscount) != 0)
  65     /* Not a well-formed question.  Let the core resolver code produce
  66        the proper error.  */
  67     return false;
  68
  69   /* Disable EDNS0.  */
  70   replacement.header.arcount = htons (0);
  71
  72   /* Extract the QNAME.  */
  73   int ret = __ns_name_unpack (buf, buf + buflen, buf + sizeof (HEADER),
  74                               replacement.question, NS_MAXCDNAME);
  75   if (ret < 0)
  76     /* Format error.  */
  77     return false;
  78
  79   /* Compute the end of the question name.  */
  80   const unsigned char *after_question = buf + sizeof (HEADER) + ret;
  81
  82   /* Check that we are dealing with an AAAA query.  */
  83   if (buf + buflen - after_question < 4
  84       || !qtype_matches (after_question, T_AAAA))
  85     return false;
  86
  87   /* Find the place to store the type/class data in the replacement
  88      query.  */
  89   after_question = replacement.question;
  90   /* This cannot fail because __ns_name_unpack above produced a valid
  91      domain name.  */
  92   (void) __ns_name_skip (&after_question, &replacement.question[NS_MAXCDNAME]);
  93   unsigned char *start_of_query = (unsigned char *) &replacement;
  94   const unsigned char *end_of_query = after_question + 4;
  95
  96   /* Produce an A/IN query.  */
  97   {
  98     unsigned char *p = (unsigned char *) after_question;
  99     p[0] = 0;
 100     p[1] = T_A;
 101     p[2] = 0;
 102     p[3] = C_IN;
 103   }
 104
 105   /* Clear the output buffer, to avoid reading undefined data when
 106      rewriting the result from A to AAAA.  */
 107   memset (ans, 0, anssiz);
 108
 109   /* Always perform the message translation, independent of the error
 110      code.  */
 111   ret = __res_context_send (ctx,
 112                             start_of_query, end_of_query - start_of_query,
 113                             NULL, 0, ans, anssiz,
 114                             NULL, NULL, NULL, NULL, NULL);
 115
 116   /* Patch in the AAAA question type if there is room and the A query
 117      type was received.  */
 118   after_question = ans + sizeof (HEADER);
 119   if (__ns_name_skip (&after_question, ans + anssiz) == 0
 120       && ans + anssiz - after_question >= 4
 121       && qtype_matches (after_question, T_A))
 122     {
 123       ((unsigned char *) after_question)[1] = T_AAAA;
 124
 125       /* Create an aligned copy of the header.  Hide all data except
 126          the question from the response.  Put back the header.  There is
 127          no need to change the response code.  The zero answer count turns
 128          a positive response with data into a no-data response.  */
 129       memcpy (&replacement.header, ans, sizeof (replacement.header));
 130       replacement.header.ancount = htons (0);
 131       replacement.header.nscount = htons (0);
 132       replacement.header.arcount = htons (0);
 133       memcpy (ans, &replacement.header, sizeof (replacement.header));
 134
 135       /* Truncate the reply.  */
 136       if (ret <= 0)
 137         *result = ret;
 138       else
 139         *result = after_question - ans + 4;
 140     }
 141
 142   return true;
 143 }