PYTHON-5288: SRV hostname validation fails when resolver and resolved hostnames are identical with three domain levels

pymongo/asynchronous/srv_resolver.py

@@ -137,7 +137,8 @@ async def _get_srv_response_and_hosts(
 
         # Validate hosts
         for node in nodes:
-            if self.__fqdn == node[0].lower():
+            srv_host = node[0].lower()
+            if self.__fqdn == srv_host and len(srv_host.split(".")) < 3:
                 raise ConfigurationError(
                     "Invalid SRV host: return address is identical to SRV hostname"
                 )

pymongo/synchronous/srv_resolver.py

@@ -137,7 +137,8 @@ def _get_srv_response_and_hosts(
 
         # Validate hosts
         for node in nodes:
-            if self.__fqdn == node[0].lower():
+            srv_host = node[0].lower()
+            if self.__fqdn == srv_host and len(srv_host.split(".")) < 3:
                 raise ConfigurationError(
                     "Invalid SRV host: return address is identical to SRV hostname"
                 )

test/asynchronous/test_dns.py

@@ -220,12 +220,15 @@ async def mock_resolve(query, record_type, *args, **kwargs):
                 mock_resolver.side_effect = mock_resolve
                 domain = case["query"].split("._tcp.")[1]
                 connection_string = f"mongodb+srv://{domain}"
-                try:
+                if "expected_error" not in case:
                     await parse_uri(connection_string)
-                except ConfigurationError as e:
-                    self.assertIn(case["expected_error"], str(e))
                 else:
-                    self.fail(f"ConfigurationError was not raised for query: {case['query']}")
+                    try:
+                        await parse_uri(connection_string)
+                    except ConfigurationError as e:
+                        self.assertIn(case["expected_error"], str(e))
+                    else:
+                        self.fail(f"ConfigurationError was not raised for query: {case['query']}")
 
     async def test_1_allow_srv_hosts_with_fewer_than_three_dot_separated_parts(self):
         with patch("dns.asyncresolver.resolve"):
@@ -264,6 +267,12 @@ async def test_3_throw_when_return_address_is_identical_to_srv_hostname(self):
                 "mock_target": "mongo.local",
                 "expected_error": "Invalid SRV host",
             },
+            # When the SRV hostname has three or more dot-separated parts
+            # it is valid for the returned hostnames to be identical.
+            {
+                "query": "_mongodb._tcp.blogs.mongodb.com",
+                "mock_target": "blogs.mongodb.com",
+            },
         ]
         await self.run_initial_dns_seedlist_discovery_prose_tests(test_cases)
 

test/test_dns.py

@@ -218,12 +218,15 @@ def mock_resolve(query, record_type, *args, **kwargs):
                 mock_resolver.side_effect = mock_resolve
                 domain = case["query"].split("._tcp.")[1]
                 connection_string = f"mongodb+srv://{domain}"
-                try:
+                if "expected_error" not in case:
                     parse_uri(connection_string)
-                except ConfigurationError as e:
-                    self.assertIn(case["expected_error"], str(e))
                 else:
-                    self.fail(f"ConfigurationError was not raised for query: {case['query']}")
+                    try:
+                        parse_uri(connection_string)
+                    except ConfigurationError as e:
+                        self.assertIn(case["expected_error"], str(e))
+                    else:
+                        self.fail(f"ConfigurationError was not raised for query: {case['query']}")
 
     def test_1_allow_srv_hosts_with_fewer_than_three_dot_separated_parts(self):
         with patch("dns.resolver.resolve"):
@@ -262,6 +265,12 @@ def test_3_throw_when_return_address_is_identical_to_srv_hostname(self):
                 "mock_target": "mongo.local",
                 "expected_error": "Invalid SRV host",
             },
+            # When the SRV hostname has three or more dot-separated parts
+            # it is valid for the returned hostnames to be identical.
+            {
+                "query": "_mongodb._tcp.blogs.mongodb.com",
+                "mock_target": "blogs.mongodb.com",
+            },
         ]
         self.run_initial_dns_seedlist_discovery_prose_tests(test_cases)