PYTHON-5814 Configurable DNS domain validation for SRV records

.evergreen/resync-specs.sh

@@ -69,6 +69,19 @@ cpjson () {
 
 }
 
+# Copy the Public Suffix List bundled with the driver from the specs repo.
+# Unlike the spec tests, this is driver source, so it lives in pymongo/ rather
+# than test/.
+cp_psl () {
+    local src="$SPECS/source/public-suffix-list/public_suffix_list.dat"
+    if ! [ -f "$src" ]
+    then
+      echo "Could not find the public suffix list at $src" >&2
+      return 1
+    fi
+    cp "$src" "$PYMONGO"/pymongo/public_suffix_list.dat
+}
+
 for spec in "$@"
 do
   # Match the spec dir name, the python test dir name, and/or common abbreviations.
@@ -147,6 +160,11 @@ do
       ;;
     srv|SRV|initial-dns-seedlist-discovery|srv_seedlist)
       cpjson initial-dns-seedlist-discovery/tests/ srv_seedlist
+      # srvAllowedHostsSuffix validation uses the bundled Public Suffix List.
+      cp_psl
+      ;;
+    psl|public-suffix-list|public_suffix_list)
+      cp_psl
       ;;
     read-write-concern|read_write_concern)
       cpjson read-write-concern/tests/operation read_write_concern/operation

.pre-commit-config.yaml

@@ -116,6 +116,7 @@ repos:
     # - test/versioned-api/crud-api-version-1-strict.json:514: nin ==> inn, min, bin, nine
     # - test/test_client.py:188: te ==> the, be, we, to
     args: ["-L", "fle,fo,infinit,isnt,nin,te,aks"]
+    exclude: ^pymongo/public_suffix_list\.dat$
 
 - repo: local
   hooks:

doc/changelog.rst

@@ -6,6 +6,17 @@ Changes in Version 4.18.0 (2026/XX/XX)
 
 PyMongo 4.18 brings a number of changes including:
 
+- Added ``srvAllowedHostsSuffix`` as a URI option and keyword argument to
+  :class:`~pymongo.synchronous.mongo_client.MongoClient` and
+  :class:`~pymongo.asynchronous.mongo_client.AsyncMongoClient`. When connecting
+  via ``mongodb+srv://``, this option overrides the default requirement that
+  SRV-returned hosts share the same parent domain as the seed hostname,
+  allowing hosts under a different domain suffix to be accepted. The suffix must
+  not be a public suffix (per the `Public Suffix List
+  <https://publicsuffix.org/list/>`_). See the
+  :class:`~pymongo.synchronous.mongo_client.MongoClient` and
+  :class:`~pymongo.asynchronous.mongo_client.AsyncMongoClient` documentation for
+  security considerations.
 - Dropped support for MongoDB 4.2.
 - Added support for MongoDB 9.0.
 - Improved TLS connection performance by reusing TLS sessions across connections

pymongo/_psl.py

@@ -0,0 +1,61 @@
+# Copyright 2024-present MongoDB, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you
+# may not use this file except in compliance with the License.  You
+# may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.  See the License for the specific language governing
+# permissions and limitations under the License.
+
+"""Public Suffix List lookup for srvAllowedHostsSuffix validation."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Optional
+
+_PUBLIC_SUFFIXES: Optional[tuple[set[str], set[str], set[str]]] = None
+
+
+def _load_public_suffixes() -> tuple[set[str], set[str], set[str]]:
+    path = Path(__file__).parent / "public_suffix_list.dat"
+    suffixes: set[str] = set()
+    wildcards: set[str] = set()
+    exceptions: set[str] = set()
+    with open(path, encoding="utf-8") as f:
+        for line in f:
+            line = line.strip()  # noqa: PLW2901
+            if not line or line.startswith("//"):
+                continue
+            if line.startswith("!"):
+                exceptions.add(line[1:].lower())
+            elif line.startswith("*."):
+                wildcards.add(line[2:].lower())
+            else:
+                suffixes.add(line.lower())
+    return suffixes, wildcards, exceptions
+
+
+def is_public_suffix(domain: str) -> bool:
+    """Return True if domain is a public suffix per the bundled Public Suffix List."""
+    global _PUBLIC_SUFFIXES  # noqa: PLW0603
+    if _PUBLIC_SUFFIXES is None:
+        _PUBLIC_SUFFIXES = _load_public_suffixes()
+    suffixes, wildcards, exceptions = _PUBLIC_SUFFIXES
+
+    domain = domain.lower().strip(".")
+    if domain in exceptions:
+        return False
+    if domain in suffixes:
+        return True
+    parts = domain.split(".")
+    # this logic is to handle the wildcard rule, the domain could still be a public suffix if:
+    # - either `parts` is a single label, and thus it is a public suffix list (per the `*`) rule
+    # - or another wildcard rule such as *.xyz exists (stored as just xyz in `wildcards`), thus we check
+    #   if `parts[1:]` is in the list of wildcard rules.
+    return len(parts) == 1 or (len(parts) > 1 and ".".join(parts[1:]) in wildcards)

pymongo/asynchronous/mongo_client.py

@@ -446,6 +446,30 @@ def __init__(
             connect to. More specifically, when a "mongodb+srv://" connection string
             resolves to more than srvMaxHosts number of hosts, the client will randomly
             choose an srvMaxHosts sized subset of hosts.
+          - `srvAllowedHostsSuffix`: (string) Overrides the default requirement that
+            hosts returned by SRV DNS records share the same parent domain as the seed
+            hostname. When set, the driver accepts any returned host whose name ends
+            with this suffix (e.g. ``".atlas.mongodb.com"``). The value must not be a
+            public suffix (per the Public Suffix List). Only valid with
+            ``mongodb+srv://`` URIs.
+
+            .. warning::
+
+               This option relaxes a built-in DNS spoofing safeguard. Use the most
+               specific suffix possible for your deployment rather than a broad
+               company-wide domain. For example, instead of::
+
+                   AsyncMongoClient(
+                       "mongodb+srv://cluster.test.internal.example.com/",
+                       srvAllowedHostsSuffix=".example.com",
+                   )
+
+               which would accept any host across the entire domain, scope it further like so::
+
+                   AsyncMongoClient(
+                       "mongodb+srv://cluster.test.internal.example.com/",
+                       srvAllowedHostsSuffix=".internal.example.com",
+                   )
 
 
           | **Write Concern options:**
@@ -799,6 +823,7 @@ def __init__(
         fqdn = None
         srv_service_name = keyword_opts.get("srvservicename")
         srv_max_hosts = keyword_opts.get("srvmaxhosts")
+        srv_allowed_hosts_suffix = keyword_opts.get("srvallowedhostssuffix")
         if len([h for h in self._host if "/" in h]) > 1:
             raise ConfigurationError("host must not contain multiple MongoDB URIs")
         for entity in self._host:
@@ -849,6 +874,8 @@ def __init__(
             srv_service_name = opts.get("srvServiceName", common.SRV_SERVICE_NAME)
 
         srv_max_hosts = srv_max_hosts or opts.get("srvmaxhosts")
+        if srv_allowed_hosts_suffix is None:
+            srv_allowed_hosts_suffix = opts.get("srvallowedhostssuffix")
         opts = self._normalize_and_validate_options(opts, self._seeds)
 
         # Username and password passed as kwargs override user info in URI.
@@ -886,7 +913,9 @@ def __init__(
 
         self._retry_policy = _RetryPolicy(attempts=self._options.max_adaptive_retries)
 
-        self._init_based_on_options(self._seeds, srv_max_hosts, srv_service_name)
+        self._init_based_on_options(
+            self._seeds, srv_max_hosts, srv_service_name, srv_allowed_hosts_suffix
+        )
 
         self._opened = False
         self._closed = False
@@ -904,6 +933,7 @@ async def _resolve_srv(self) -> None:
         opts = common._CaseInsensitiveDictionary()
         srv_service_name = keyword_opts.get("srvservicename")
         srv_max_hosts = keyword_opts.get("srvmaxhosts")
+        srv_allowed_hosts_suffix = keyword_opts.get("srvallowedhostssuffix")
         for entity in self._host:
             # A hostname can only include a-z, 0-9, '-' and '.'. If we find a '/'
             # it must be a URI,
@@ -924,6 +954,7 @@ async def _resolve_srv(self) -> None:
                     connect_timeout=timeout,
                     srv_service_name=srv_service_name,
                     srv_max_hosts=srv_max_hosts,
+                    srv_allowed_hosts_suffix=srv_allowed_hosts_suffix,
                 )
                 seeds.update(res["nodelist"])
                 opts = res["options"]
@@ -956,6 +987,8 @@ async def _resolve_srv(self) -> None:
                 srv_service_name = opts.get("srvServiceName", common.SRV_SERVICE_NAME)
 
             srv_max_hosts = srv_max_hosts or opts.get("srvmaxhosts")
+            if srv_allowed_hosts_suffix is None:
+                srv_allowed_hosts_suffix = opts.get("srvAllowedHostsSuffix")
             opts = self._normalize_and_validate_options(opts, seeds)
 
             # Username and password passed as kwargs override user info in URI.
@@ -965,10 +998,16 @@ async def _resolve_srv(self) -> None:
                 username, password, self._resolve_srv_info["dbase"], opts, _IS_SYNC
             )
 
-            self._init_based_on_options(seeds, srv_max_hosts, srv_service_name)
+            self._init_based_on_options(
+                seeds, srv_max_hosts, srv_service_name, srv_allowed_hosts_suffix
+            )
 
     def _init_based_on_options(
-        self, seeds: Collection[tuple[str, int]], srv_max_hosts: Any, srv_service_name: Any
+        self,
+        seeds: Collection[tuple[str, int]],
+        srv_max_hosts: Any,
+        srv_service_name: Any,
+        srv_allowed_hosts_suffix: Any,
     ) -> None:
         self._event_listeners = self._options.pool_options._event_listeners
         self._topology_settings = TopologySettings(
@@ -987,6 +1026,7 @@ def _init_based_on_options(
             load_balanced=self._options.load_balanced,
             srv_service_name=srv_service_name,
             srv_max_hosts=srv_max_hosts,
+            srv_allowed_hosts_suffix=srv_allowed_hosts_suffix,
             server_monitoring_mode=self._options.server_monitoring_mode,
             topology_id=self._topology_settings._topology_id if self._topology_settings else None,
         )

pymongo/asynchronous/monitor.py

@@ -366,6 +366,7 @@ async def _get_seedlist(self) -> Optional[list[tuple[str, Any]]]:
                 self._fqdn,
                 self._settings.pool_options.connect_timeout,
                 self._settings.srv_service_name,
+                srv_allowed_hosts_suffix=self._settings.srv_allowed_hosts_suffix,
             )
             seedlist, ttl = await resolver.get_hosts_and_min_ttl()
             if len(seedlist) == 0:

pymongo/asynchronous/settings.py

@@ -52,6 +52,7 @@ def __init__(
         load_balanced: Optional[bool] = None,
         srv_service_name: str = common.SRV_SERVICE_NAME,
         srv_max_hosts: int = 0,
+        srv_allowed_hosts_suffix: Optional[str] = None,
         server_monitoring_mode: str = common.SERVER_MONITORING_MODE,
         topology_id: Optional[ObjectId] = None,
     ):
@@ -79,6 +80,7 @@ def __init__(
         self._load_balanced = load_balanced
         self._srv_service_name = srv_service_name
         self._srv_max_hosts = srv_max_hosts or 0
+        self._srv_allowed_hosts_suffix = srv_allowed_hosts_suffix
         self._server_monitoring_mode = server_monitoring_mode
         if topology_id is not None:
             self._topology_id = topology_id
@@ -156,6 +158,11 @@ def srv_max_hosts(self) -> int:
         """The srvMaxHosts."""
         return self._srv_max_hosts
 
+    @property
+    def srv_allowed_hosts_suffix(self) -> Optional[str]:
+        """The srvAllowedHostsSuffix."""
+        return self._srv_allowed_hosts_suffix
+
     @property
     def server_monitoring_mode(self) -> str:
         """The serverMonitoringMode."""

pymongo/asynchronous/srv_resolver.py

@@ -20,6 +20,7 @@
 import random
 from typing import TYPE_CHECKING, Any, Optional, Union
 
+from pymongo._psl import is_public_suffix
 from pymongo.common import CONNECT_TIMEOUT
 from pymongo.errors import ConfigurationError
 
@@ -71,11 +72,21 @@ def __init__(
         connect_timeout: Optional[float],
         srv_service_name: str,
         srv_max_hosts: int = 0,
+        srv_allowed_hosts_suffix: Optional[str] = None,
     ):
-        self.__fqdn = fqdn
+        self.__fqdn = fqdn.lower()
         self.__srv = srv_service_name
         self.__connect_timeout = connect_timeout or CONNECT_TIMEOUT
         self.__srv_max_hosts = srv_max_hosts or 0
+        self.__srv_allowed_hosts_suffix = (
+            "." + srv_allowed_hosts_suffix.lower().strip(".") if srv_allowed_hosts_suffix else None
+        )  # ensure there's a . at the beginning of the domain
+        if self.__srv_allowed_hosts_suffix is not None and is_public_suffix(
+            self.__srv_allowed_hosts_suffix
+        ):
+            raise ConfigurationError(
+                f"srvAllowedHostsSuffix must not be a public suffix, got: {srv_allowed_hosts_suffix}"
+            )
         # Validate the fully qualified domain name.
         try:
             ipaddress.ip_address(fqdn)
@@ -124,7 +135,7 @@ async def _get_srv_response_and_hosts(
 
         # Construct address tuples
         nodes = [
-            (maybe_decode(res.target.to_text(omit_final_dot=True)), res.port)  # type: ignore[attr-defined]
+            (maybe_decode(res.target.to_text(omit_final_dot=True)).lower(), res.port)  # type: ignore[attr-defined]
             for res in results
         ]
 
@@ -135,12 +146,16 @@ async def _get_srv_response_and_hosts(
                 raise ConfigurationError(
                     "Invalid SRV host: return address is identical to SRV hostname"
                 )
-            try:
-                nlist = srv_host.split(".")[1:][-self.__slen :]
-            except Exception as exc:
-                raise ConfigurationError(f"Invalid SRV host: {node[0]}") from exc
-            if self.__plist != nlist:
-                raise ConfigurationError(f"Invalid SRV host: {node[0]}")
+            if self.__srv_allowed_hosts_suffix is not None:
+                if not srv_host.endswith(self.__srv_allowed_hosts_suffix):
+                    raise ConfigurationError(f"Invalid SRV host: {node[0]}")
+            else:
+                try:
+                    nlist = srv_host.split(".")[1:][-self.__slen :]
+                except Exception as exc:
+                    raise ConfigurationError(f"Invalid SRV host: {node[0]}") from exc
+                if self.__plist != nlist:
+                    raise ConfigurationError(f"Invalid SRV host: {node[0]}")
         if self.__srv_max_hosts:
             nodes = random.sample(nodes, min(self.__srv_max_hosts, len(nodes)))
         return results, nodes

pymongo/asynchronous/uri_parser.py

@@ -48,6 +48,7 @@ async def parse_uri(
     connect_timeout: Optional[float] = None,
     srv_service_name: Optional[str] = None,
     srv_max_hosts: Optional[int] = None,
+    srv_allowed_hosts_suffix: Optional[str] = None,
 ) -> dict[str, Any]:
     """Parse and validate a MongoDB URI.
 
@@ -116,6 +117,7 @@ async def parse_uri(
             connect_timeout,
             srv_service_name,
             srv_max_hosts,
+            srv_allowed_hosts_suffix,
         )
     )
     result["options"] = _make_options_case_sensitive(result["options"])
@@ -131,6 +133,7 @@ async def _parse_srv(
     connect_timeout: Optional[float] = None,
     srv_service_name: Optional[str] = None,
     srv_max_hosts: Optional[int] = None,
+    srv_allowed_hosts_suffix: Optional[str] = None,
 ) -> dict[str, Any]:
     if uri.startswith(SCHEME):
         is_srv = False
@@ -158,14 +161,17 @@ async def _parse_srv(
 
     hosts = unquote_plus(hosts)
     srv_max_hosts = srv_max_hosts or options.get("srvMaxHosts")
+    srv_allowed_hosts_suffix = srv_allowed_hosts_suffix or options.get("srvAllowedHostsSuffix")
     if is_srv:
         nodes = split_hosts(hosts, default_port=None)
         fqdn, _port = nodes[0]
 
         # Use the connection timeout. connectTimeoutMS passed as a keyword
         # argument overrides the same option passed in the connection string.
         connect_timeout = connect_timeout or options.get("connectTimeoutMS")
-        dns_resolver = _SrvResolver(fqdn, connect_timeout, srv_service_name, srv_max_hosts)
+        dns_resolver = _SrvResolver(
+            fqdn, connect_timeout, srv_service_name, srv_max_hosts, srv_allowed_hosts_suffix
+        )
         nodes = await dns_resolver.get_hosts()
         dns_options = await dns_resolver.get_options()
         if dns_options:

pymongo/common.py

@@ -748,6 +748,7 @@ def validate_server_monitoring_mode(option: str, value: str) -> str:
     "zlibcompressionlevel": validate_zlib_compression_level,
     "srvservicename": validate_string,
     "srvmaxhosts": validate_non_negative_integer,
+    "srvallowedhostssuffix": validate_string,
     "timeoutms": validate_timeoutms,
     "servermonitoringmode": validate_server_monitoring_mode,
     "maxadaptiveretries": validate_non_negative_integer,