[netdata] allow zero-len external route prefix (#6784)

This commit allows a zero-len prefix `::/0` to be added as an external
route in Network Data. Such a prefix can act as a default route. This
commit also adds `test_zero_len_external_route.py` testcase to verify
the network's behavior using a zero-len external route.

tests/scripts/thread-cert/test_zero_len_external_route.py

@@ -0,0 +1,145 @@
+#!/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 thread_cert
+
+# Test description:
+#
+#   This test verifies the behavior of the network when a zero length external route prefix "::/0" is added.
+#
+# Topology:
+#
+#    leader --- router1
+#       \         /
+#        \       /
+#         router2
+#
+
+LEADER = 1
+ROUTER1 = 2
+ROUTER2 = 3
+
+
+class ZeroLengthExternalRoute(thread_cert.TestCase):
+    USE_MESSAGE_FACTORY = False
+    SUPPORT_NCP = False
+
+    TOPOLOGY = {
+        LEADER: {
+            'name': 'LEADER',
+            'mode': 'rdn',
+        },
+        ROUTER1: {
+            'name': 'ROUTER2',
+            'mode': 'rdn',
+        },
+        ROUTER2: {
+            'name': 'ROUTER2',
+            'mode': 'rdn',
+        },
+    }
+
+    def test(self):
+        leader = self.nodes[LEADER]
+        router1 = self.nodes[ROUTER1]
+        router2 = self.nodes[ROUTER2]
+
+        # Start the nodes and form the network.
+
+        leader.start()
+        self.simulator.go(5)
+        self.assertEqual(leader.get_state(), 'leader')
+
+        router1.start()
+        router2.start()
+        self.simulator.go(5)
+        self.assertEqual(router1.get_state(), 'router')
+        self.assertEqual(router2.get_state(), 'router')
+
+        # Add an on-mesh prefix with SLAAC on router2 (without default
+        # route flag).
+
+        router2.add_prefix('fd00:1234::/64', 'paos')
+        router2.register_netdata()
+
+        # Add a zero length external route on router1. Also add an IPv6
+        # address on router1.
+
+        router1.add_route('::/0', stable=True)
+        router1.add_ipaddr('fd00:abcd::1')
+        router1.register_netdata()
+        self.simulator.go(5)
+
+        # Ping from leader the address added on router1. The zero
+        # length external route should ensure the message is routed
+        # to router1.
+
+        self.assertTrue(leader.ping('fd00:abcd::1'))
+
+        # Change the on-mesh prefix on router2 to now also have
+        # the default route flag.
+
+        router2.remove_prefix('fd00:1234::/64')
+        router2.add_prefix('fd00:5678::/64', 'paros')
+        router2.register_netdata()
+        self.simulator.go(5)
+
+        # Again ping from leader the same address. The explicit
+        # external route (even with zero length) on router1 should
+        # still be preferred over the default route flag of the
+        # on-mesh prefix from router2.
+
+        self.assertTrue(leader.ping('fd00:abcd::1'))
+
+        # Remove the external route on router1.
+
+        router1.remove_route("::/0")
+        router1.register_netdata()
+        self.simulator.go(5)
+
+        # Now the ping should fail since the message would be routed
+        # to router2 (due to its on-mesh prefix with default route
+        # flag).
+
+        self.assertFalse(leader.ping('fd00:abcd::1'))
+
+        # Remove the address from router1 and add it on router2 and
+        # ping it again from leader to verify that the message is
+        # being routed to router2.
+
+        router1.del_ipaddr('fd00:abcd::1')
+        router2.add_ipaddr('fd00:abcd::1')
+        self.simulator.go(5)
+        self.assertTrue(leader.ping('fd00:abcd::1'))
+
+
+if __name__ == '__main__':
+    unittest.main()