resolv: Implement trust-ad option for /etc/resolv.conf [BZ #20358]

This introduces a concept of trusted name servers, for which the
AD bit is passed through to applications.  For untrusted name
servers (the default), the AD bit in responses are cleared, to
provide a safe default.

This approach is very similar to the one suggested by Pavel Šimerda
in <https://bugzilla.redhat.com/show_bug.cgi?id=1164339#c15>.

The DNS test framework in support/ is enhanced with support for
setting the AD bit in responses.

Tested on x86_64-linux-gnu.

Change-Id: Ibfe0f7c73ea221c35979842c5c3b6ed486495ccc

1 files changed, 200 insertions, 0 deletions

diff --git a/resolv/tst-resolv-trustad.c b/resolv/tst-resolv-trustad.c
new file mode 100644
index 0000000..fc9ae54
--- /dev/null
+++ b/resolv/tst-resolv-trustad.c
@@ -0,0 +1,200 @@
+/* Test the behavior of the trust-ad option.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <resolv.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/check_nss.h>
+#include <support/resolv_test.h>
+#include <support/support.h>
+
+/* This controls properties of the response.  volatile because
+   __res_send is incorrectly declared as __THROW.  */
+static volatile unsigned char response_number;
+static volatile bool response_ad_bit;
+static volatile bool query_ad_bit;
+
+static void
+response (const struct resolv_response_context *ctx,
+          struct resolv_response_builder *b,
+          const char *qname, uint16_t qclass, uint16_t qtype)
+{
+  TEST_COMPARE (qclass, C_IN);
+  TEST_COMPARE (qtype, T_A);
+  TEST_COMPARE_STRING (qname, "www.example");
+
+  HEADER header;
+  memcpy (&header, ctx->query_buffer, sizeof (header));
+  TEST_COMPARE (header.ad, query_ad_bit);
+
+  struct resolv_response_flags flags = { .ad = response_ad_bit, };
+  resolv_response_init (b, flags);
+  resolv_response_add_question (b, qname, qclass, qtype);
+  resolv_response_section (b, ns_s_an);
+  resolv_response_open_record (b, qname, qclass, T_A, 0x12345678);
+  char addr[4] = { 192, 0, 2, response_number };
+  resolv_response_add_data (b, addr, sizeof (addr));
+  resolv_response_close_record (b);
+}
+
+static void
+check_answer (const unsigned char *buffer, size_t buffer_length,
+              bool expected_ad)
+{
+  HEADER header;
+  TEST_VERIFY (buffer_length > sizeof (header));
+  memcpy (&header, buffer, sizeof (header));
+  TEST_COMPARE (0, header.aa);
+  TEST_COMPARE (expected_ad, header.ad);
+  TEST_COMPARE (0, header.opcode);
+  TEST_COMPARE (1, header.qr);
+  TEST_COMPARE (0, header.rcode);
+  TEST_COMPARE (1, header.rd);
+  TEST_COMPARE (0, header.tc);
+  TEST_COMPARE (1, ntohs (header.qdcount));
+  TEST_COMPARE (1, ntohs (header.ancount));
+  TEST_COMPARE (0, ntohs (header.nscount));
+  TEST_COMPARE (0, ntohs (header.arcount));
+
+  char *description = xasprintf ("response=%d ad=%d",
+                                 response_number, expected_ad);
+  char *expected = xasprintf ("name: www.example\n"
+                              "address: 192.0.2.%d\n", response_number);
+  check_dns_packet (description, buffer, buffer_length, expected);
+  free (expected);
+  free (description);
+}
+
+static int
+do_test (void)
+{
+  struct resolv_test *aux = resolv_test_start
+    ((struct resolv_redirect_config)
+     {
+       .response_callback = response,
+     });
+
+  /* By default, the resolver is not trusted, and the AD bit is
+     cleared.  */
+
+  static const unsigned char hand_crafted_query[] =
+    {
+     10, 11,                    /* Transaction ID.  */
+     1, 0x20,                   /* Query with RD, AD flags.  */
+     0, 1,                      /* One question.  */
+     0, 0, 0, 0, 0, 0,          /* The other sections are empty.  */
+     3, 'w', 'w', 'w', 7, 'e', 'x', 'a', 'm', 'p', 'l', 'e', 0,
+     0, T_A,                    /* A query.  */
+     0, 1,                      /* Class IN.  */
+    };
+
+  ++response_number;
+  response_ad_bit = false;
+
+  unsigned char buffer[512];
+  memset (buffer, 255, sizeof (buffer));
+  query_ad_bit = true;
+  int ret = res_send (hand_crafted_query, sizeof (hand_crafted_query),
+                      buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+
+  ++response_number;
+  memset (buffer, 255, sizeof (buffer));
+  query_ad_bit = false;
+  ret = res_query ("www.example", C_IN, T_A, buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+  response_ad_bit = true;
+
+  response_ad_bit = true;
+
+  ++response_number;
+  query_ad_bit = true;
+  ret = res_send (hand_crafted_query, sizeof (hand_crafted_query),
+                  buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+
+  ++response_number;
+  memset (buffer, 255, sizeof (buffer));
+  query_ad_bit = false;
+  ret = res_query ("www.example", C_IN, T_A, buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+
+  /* No AD bit set in generated queries.  */
+  memset (buffer, 255, sizeof (buffer));
+  ret = res_mkquery (QUERY, "www.example", C_IN, T_A,
+                     (const unsigned char *) "", 0, NULL,
+                     buffer, sizeof (buffer));
+  HEADER header;
+  memcpy (&header, buffer, sizeof (header));
+  TEST_VERIFY (!header.ad);
+
+  /* With RES_TRUSTAD, the AD bit is passed through if it set in the
+     response.  It is also included in queries.  */
+
+  _res.options |= RES_TRUSTAD;
+  query_ad_bit = true;
+
+  response_ad_bit = false;
+
+  ++response_number;
+  memset (buffer, 255, sizeof (buffer));
+  ret = res_send (hand_crafted_query, sizeof (hand_crafted_query),
+                  buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+
+  ++response_number;
+  memset (buffer, 255, sizeof (buffer));
+  ret = res_query ("www.example", C_IN, T_A, buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, false);
+
+  response_ad_bit = true;
+
+  ++response_number;
+  memset (buffer, 0, sizeof (buffer));
+  ret = res_send (hand_crafted_query, sizeof (hand_crafted_query),
+                  buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, true);
+
+  ++response_number;
+  memset (buffer, 0, sizeof (buffer));
+  ret = res_query ("www.example", C_IN, T_A, buffer, sizeof (buffer));
+  TEST_VERIFY (ret > 0);
+  check_answer (buffer, ret, true);
+
+  /* AD bit set in generated queries.  */
+  memset (buffer, 0, sizeof (buffer));
+  ret = res_mkquery (QUERY, "www.example", C_IN, T_A,
+                     (const unsigned char *) "", 0, NULL,
+                     buffer, sizeof (buffer));
+  memcpy (&header, buffer, sizeof (header));
+  TEST_VERIFY (header.ad);
+
+  resolv_test_end (aux);
+
+  return 0;
+}
+
+#include <support/test-driver.c>