Avoid Django's ALLOWED_HOSTS check

CHANGELOG.md

@@ -11,6 +11,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- `opentelemetry-instrumentation-django`: Fix `_DjangoMiddleware` to avoid hitting a host check ([#3651](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3651))
+
 ### Fixed
 
 - `opentelemetry-instrumentation`: Fix dependency conflict detection when instrumented packages are not installed by moving check back to before instrumentors are loaded. Add "instruments-any" feature for instrumentations that target multiple packages.

instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/middleware/otel_middleware.py

@@ -194,7 +194,7 @@ def process_request(self, request):
         # Read more about request.META here:
         # https://docs.djangoproject.com/en/3.0/ref/request-response/#django.http.HttpRequest.META
 
-        if self._excluded_urls.url_disabled(request.build_absolute_uri("?")):
+        if self._url_is_disabled(request):
             return
 
         is_asgi_request = _is_asgi_request(request)
@@ -305,7 +305,7 @@ def process_request(self, request):
     def process_view(self, request, view_func, *args, **kwargs):
         # Process view is executed before the view function, here we get the
         # route template from request.resolver_match.  It is not set yet in process_request
-        if self._excluded_urls.url_disabled(request.build_absolute_uri("?")):
+        if self._url_is_disabled(request):
             return
 
         if (
@@ -330,7 +330,7 @@ def process_view(self, request, view_func, *args, **kwargs):
                         duration_attrs[HTTP_ROUTE] = route
 
     def process_exception(self, request, exception):
-        if self._excluded_urls.url_disabled(request.build_absolute_uri("?")):
+        if self._url_is_disabled(request):
             return
 
         if self._environ_activation_key in request.META.keys():
@@ -340,7 +340,7 @@ def process_exception(self, request, exception):
     # pylint: disable=too-many-locals
     # pylint: disable=too-many-statements
     def process_response(self, request, response):
-        if self._excluded_urls.url_disabled(request.build_absolute_uri("?")):
+        if self._url_is_disabled(request):
             return response
 
         is_asgi_request = _is_asgi_request(request)
@@ -453,6 +453,15 @@ def process_response(self, request, response):
 
         return response
 
+    def _url_is_disabled(self, request):
+        """
+        Avoid `request.get_host` to bypass Django's `ALLOWED_HOSTS` check
+        """
+        url = "{}://{}{}".format(
+            request.scheme, request._get_raw_host(), request.path
+        )
+        return self._excluded_urls.url_disabled(url)
+
 
 def _parse_duration_attrs(
     req_attrs, sem_conv_opt_in_mode=_StabilityMode.DEFAULT

instrumentation/opentelemetry-instrumentation-django/tests/test_middleware.py

@@ -116,6 +116,10 @@ def setUpClass(cls):
     def setUp(self):
         super().setUp()
         setup_test_environment()
+        conf.settings.ALLOWED_HOSTS = [
+            # Django adds "testserver" within "setup_test_environment" so this check doesn't break during tests.
+            "unknown"
+        ]
         test_name = ""
         if hasattr(self, "_testMethodName"):
             test_name = self._testMethodName
@@ -230,6 +234,20 @@ def test_templated_route_get(self):
         self.assertEqual(span.attributes["http.scheme"], "http")
         self.assertEqual(span.attributes["http.status_code"], 200)
 
+    def test_when_middleware_triggers_host_check_then_returns_bad_request(self):
+        def return_host(request):
+            # "HttpRequest.get_host" checks for allowed hosts and raises
+            # `django.core.DisallowedHost` if it isn't allowed.
+            return request.get_host()
+
+        with patch.object(
+            _DjangoMiddleware, "process_request", side_effect=return_host
+        ) as process_mock:
+            response = Client().get("/route/2020/template/")
+
+        assert response.status_code == 400
+        process_mock.assert_called_once()
+
     def test_traced_get(self):
         Client().get("/traced/")