1/* Implement suppression of AAAA queries.
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 <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. */
27static bool
28qtype_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. */
40bool
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}
144