zilencer: Disambiguate no MX records from the domain not existing.

This switches to dnspython, since it offers the higher-level
`resolve_name` method to look up both A and AAAA records, and set
timeouts.

zerver/tests/test_push_notifications.py

@@ -8,7 +8,6 @@ from typing import Any, Dict, Iterator, List, Mapping, Optional, Tuple, Union
 from unittest import mock, skipUnless
 
 import aioapns
-import DNS
 import orjson
 import responses
 import time_machine
@@ -19,6 +18,7 @@ from django.http.response import ResponseHeaders
 from django.test import override_settings
 from django.utils.crypto import get_random_string
 from django.utils.timezone import now
+from dns.resolver import NoAnswer as DNSNoAnswer
 from requests.exceptions import ConnectionError
 from requests.models import PreparedRequest
 from typing_extensions import override
@@ -5247,17 +5247,24 @@ class PushBouncerSignupTest(ZulipTestCase):
         self.assert_json_error(result, "Please use your real email address.")
 
         request["contact_email"] = "admin@zulip.com"
-        with mock.patch("DNS.mxlookup", side_effect=DNS.Base.ServerError("test", 1)):
+        with mock.patch("zilencer.views.dns_resolver.Resolver") as resolver:
+            resolver.return_value.resolve.side_effect = DNSNoAnswer
+            resolver.return_value.resolve_name.return_value = ["whee"]
             result = self.client_post("/api/v1/remotes/server/register", request)
             self.assert_json_error(
-                result, "zulip.com does not exist or is not configured to accept email."
+                result, "zulip.com is invalid because it does not have any MX records"
             )
 
-        with mock.patch("DNS.mxlookup", return_value=[]):
+        with mock.patch("zilencer.views.dns_resolver.Resolver") as resolver:
+            resolver.return_value.resolve.side_effect = DNSNoAnswer
+            resolver.return_value.resolve_name.side_effect = DNSNoAnswer
             result = self.client_post("/api/v1/remotes/server/register", request)
-            self.assert_json_error(
-                result, "zulip.com does not exist or is not configured to accept email."
-            )
+            self.assert_json_error(result, "zulip.com does not exist")
+
+        with mock.patch("zilencer.views.dns_resolver.Resolver") as resolver:
+            resolver.return_value.resolve.return_value = ["whee"]
+            result = self.client_post("/api/v1/remotes/server/register", request)
+            self.assert_json_success(result)
 
 
 class TestUserPushIdentityCompat(ZulipTestCase):

zilencer/views.py

@@ -5,7 +5,6 @@ from email.headerregistry import Address
 from typing import Any, Dict, List, Optional, Type, TypedDict, TypeVar, Union
 from uuid import UUID
 
-import DNS
 import orjson
 from django.conf import settings
 from django.core.exceptions import ValidationError
@@ -18,6 +17,8 @@ from django.utils.timezone import now as timezone_now
 from django.utils.translation import gettext as _
 from django.utils.translation import gettext as err_
 from django.views.decorators.csrf import csrf_exempt
+from dns import resolver as dns_resolver
+from dns.exception import DNSException
 from pydantic import BaseModel, ConfigDict, Json, StringConstraints
 from pydantic.functional_validators import AfterValidator
 from typing_extensions import Annotated
@@ -170,19 +171,25 @@ def register_remote_server(
         raise JsonableError(_("Invalid email address."))
 
     # Check if the domain has an MX record
+    resolver = dns_resolver.Resolver()
+    resolver.timeout = 3
+    dns_mx_check_successful = False
     try:
-        records = DNS.mxlookup(contact_email_domain)
-        dns_mx_check_successful = True
-        if not records:
-            dns_mx_check_successful = False
-    except DNS.Base.ServerError:
-        dns_mx_check_successful = False
+        if resolver.resolve(contact_email_domain, "MX"):
+            dns_mx_check_successful = True
+    except DNSException:
+        pass
     if not dns_mx_check_successful:
-        raise JsonableError(
-            _("{domain} does not exist or is not configured to accept email.").format(
-                domain=contact_email_domain
+        # Check if the A/AAAA exist, for better error reporting
+        try:
+            resolver.resolve_name(contact_email_domain)
+            raise JsonableError(
+                _("{domain} is invalid because it does not have any MX records").format(
+                    domain=contact_email_domain
+                )
             )
-        )
+        except DNSException:
+            raise JsonableError(_("{domain} does not exist").format(domain=contact_email_domain))
 
     try:
         validate_uuid(zulip_org_id)