[dns-client] feature to auto set the default server address (#6787)

This commit adds a new feature in DNS client to automatically set and
update the server's IPv6 address in the default config. This is done
only when user does not explicitly set or specify the server address.
This behavior requires SRP client and its auto-start feature to be
enabled. SRP client will then monitor the Thread Network Data for
DNS/SRP Service entries to select an SRP server. The selected SRP
server address is also set as the DNS server address in the default
config.

This commit also adds `test_dns_client_config_auto_start.py` which
verifies the behavior of the new feature. In particular, that the DNS
client's default config gets changed and mirrors the SRP client
selected server address (when not set by user) and that it remains
unchanged if user explicitly sets the address in the default config.

tests/scripts/thread-cert/Makefile.am

@@ -160,6 +160,7 @@ EXTRA_DIST                                                         = \
     test_crypto.py                                                   \
     test_dataset_updater.py                                          \
     test_diag.py                                                     \
+    test_dns_client_config_auto_start.py                             \
     test_dnssd.py                                                    \
     test_ipv6.py                                                     \
     test_ipv6_fragmentation.py                                       \
@@ -219,6 +220,7 @@ check_SCRIPTS                                                      = \
     test_crypto.py                                                   \
     test_dataset_updater.py                                          \
     test_diag.py                                                     \
+    test_dns_client_config_auto_start.py                             \
     test_dnssd.py                                                    \
     test_ipv6.py                                                     \
     test_ipv6_fragmentation.py                                       \

tests/scripts/thread-cert/node.py

@@ -2487,6 +2487,32 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect_done()
 
+    def dns_get_config(self):
+        """
+        Returns the DNS config as a list of property dictionary (string key and string value).
+
+        Example output:
+        {
+            'Server': '[fd00:0:0:0:0:0:0:1]:1234'
+            'ResponseTimeout': '5000 ms'
+            'MaxTxAttempts': '2'
+            'RecursionDesired': 'no'
+        }
+        """
+        cmd = f'dns config'
+        self.send_command(cmd)
+        output = self._expect_command_output(cmd)
+        config = {}
+        for line in output:
+            k, v = line.split(': ')
+            config[k] = v
+        return config
+
+    def dns_set_config(self, config):
+        cmd = f'dns config {config}'
+        self.send_command(cmd)
+        self._expect_done()
+
     def dns_resolve(self, hostname, server=None, port=53):
         cmd = f'dns resolve {hostname}'
         if server is not None:

tests/scripts/thread-cert/test_dns_client_config_auto_start.py

@@ -0,0 +1,149 @@
+#!/usr/bin/env python3
+#
+#  Copyright (c) 2021, The OpenThread Authors.
+#  All rights reserved.
+#
+#  Redistribution and use in source and binary forms, with or without
+#  modification, are permitted provided that the following conditions are met:
+#  1. Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+#  2. Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#  3. Neither the name of the copyright holder nor the
+#     names of its contributors may be used to endorse or promote products
+#     derived from this software without specific prior written permission.
+#
+#  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+#  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+#  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+#  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+#  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+#  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+#  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+#  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+#  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+#  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+#  POSSIBILITY OF SUCH DAMAGE.
+#
+
+import unittest
+
+import command
+import thread_cert
+
+# Test description:
+#
+#   This test verifies the behavior of DNS client feature where the SRP
+#   server address (from SRP client auto-start feature) is used as the
+#   default IPv6 address for DNS resolver config when user does not
+#   explicitly set a default address
+#
+# Topology: two nodes
+#
+#   leader (SRP server & client & DNS client) --- router (SRP client & DNS client)
+#
+
+LEADER = 1
+ROUTER = 2
+
+DEFAULT_ADDRESS = 'fd00:1:2:3:4:5:6:7'
+
+
+class DnsClientConfigAutoStart(thread_cert.TestCase):
+    USE_MESSAGE_FACTORY = False
+    SUPPORT_NCP = False
+
+    TOPOLOGY = {
+        LEADER: {
+            'name': 'LEADER',
+            'mode': 'rdn',
+        },
+        ROUTER: {
+            'name': 'ROUTER',
+            'mode': 'rdn',
+        },
+    }
+
+    def test(self):
+        leader = self.nodes[LEADER]
+        router = self.nodes[ROUTER]
+
+        # Form network.
+
+        leader.start()
+        self.simulator.go(5)
+        self.assertEqual(leader.get_state(), 'leader')
+
+        router.start()
+        self.simulator.go(5)
+        self.assertEqual(router.get_state(), 'router')
+
+        # On leader set DNS config explicitly.
+
+        leader.dns_set_config(DEFAULT_ADDRESS)
+        config = leader.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(DEFAULT_ADDRESS))
+
+        # Start leader to act as SRP server.
+
+        leader.srp_server_set_enabled(True)
+
+        # Enable SRP client auto start on both nodes.
+
+        leader.srp_client_enable_auto_start_mode()
+        router.srp_client_enable_auto_start_mode()
+        self.simulator.go(5)
+
+        # Verify that on router the default DNS config is changed
+        # to the same address being used by SRP client.
+
+        srp_server_address = router.srp_client_get_server_address()
+        self.assertEqual(leader.srp_client_get_server_address(), srp_server_address)
+
+        config = router.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(srp_server_address))
+
+        # Verify that on leader the default DNS config remains
+        # as before (the address explicitly set earlier).
+
+        config = leader.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(DEFAULT_ADDRESS))
+
+        # On leader clear DNS config (the explicitly set address)
+        # and verify that it adopts the SRP server address.
+
+        leader.dns_set_config("::")
+        config = leader.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(srp_server_address))
+
+        # On leader set DNS config explicitly again.
+
+        leader.dns_set_config(DEFAULT_ADDRESS)
+        config = leader.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(DEFAULT_ADDRESS))
+
+        # Stop SRP server on leader and start it on router.
+
+        leader.srp_server_set_enabled(False)
+        router.srp_server_set_enabled(True)
+        self.simulator.go(5)
+
+        # Verify that SRP client on router switched to new SRP server.
+
+        self.assertNotEqual(srp_server_address, router.srp_client_get_server_address())
+        self.assertEqual(router.srp_client_get_server_address(), leader.srp_client_get_server_address())
+
+        # Verify that config on router gets changed while on leader
+        # it remains unchanged.
+
+        config = router.dns_get_config()
+        srp_server_address = router.srp_client_get_server_address()
+        self.assertEqual(config['Server'], '[{}]:53'.format(srp_server_address))
+
+        config = leader.dns_get_config()
+        self.assertEqual(config['Server'], '[{}]:53'.format(DEFAULT_ADDRESS))
+
+
+if __name__ == '__main__':
+    unittest.main()