Refs #26666 -- Added ALLOWED_HOSTS validation when running tests.

Also used ALLOWED_HOSTS to check for external hosts in assertRedirects().

docs/ref/settings.txt

@@ -90,14 +90,18 @@ If the ``Host`` header (or ``X-Forwarded-Host`` if
 list, the :meth:`django.http.HttpRequest.get_host()` method will raise
 :exc:`~django.core.exceptions.SuspiciousOperation`.
 
-When :setting:`DEBUG` is ``True`` or when running tests, host validation is
-disabled; any host will be accepted. Thus it's usually only necessary to set it
-in production.
+When :setting:`DEBUG` is ``True``, host validation is disabled; any host will
+be accepted. ``ALLOWED_HOSTS`` is :ref:`checked when running tests
+<topics-testing-advanced-multiple-hosts>`.
 
 This validation only applies via :meth:`~django.http.HttpRequest.get_host()`;
 if your code accesses the ``Host`` header directly from ``request.META`` you
 are bypassing this security protection.
 
+.. versionchanged:: 1.11
+
+    In older versions, ``ALLOWED_HOSTS`` wasn't checked when running tests.
+
 .. setting:: APPEND_SLASH
 
 ``APPEND_SLASH``

docs/releases/1.11.txt

@@ -262,6 +262,11 @@ Miscellaneous
 * CSRF failures are logged to the ``django.security.csrf ``` logger instead of
   ``django.request``.
 
+* :setting:`ALLOWED_HOSTS` validation is no longer disabled when running tests.
+  If your application includes tests with custom host names, you must include
+  those host names in :setting:`ALLOWED_HOSTS`. See
+  :ref:`topics-testing-advanced-multiple-hosts`.
+
 * Using a foreign key's id (e.g. ``'field_id'``) in ``ModelAdmin.list_display``
   displays the related object's ID. Remove the ``_id`` suffix if you want the
   old behavior of the string representation of the object.

docs/spelling_wordlist

@@ -498,6 +498,7 @@ multiline
 multilinestring
 multipart
 multipolygon
+multitenancy
 multithreaded
 multithreading
 Mumbai

docs/topics/testing/advanced.txt

@@ -67,6 +67,61 @@ The following is a simple unit test using the request factory::
             response = MyView.as_view()(request)
             self.assertEqual(response.status_code, 200)
 
+.. _topics-testing-advanced-multiple-hosts:
+
+Tests and multiple host names
+=============================
+
+The :setting:`ALLOWED_HOSTS` setting is validated when running tests. This
+allows the test client to differentiate between internal and external URLs.
+
+Projects that support multitenancy or otherwise alter business logic based on
+the request's host and use custom host names in tests must include those hosts
+in :setting:`ALLOWED_HOSTS`.
+
+The first and simplest option to do so is to add the hosts to your settings
+file. For example, the test suite for docs.djangoproject.com includes the
+following::
+
+    from django.test import TestCase
+
+    class SearchFormTestCase(TestCase):
+        def test_empty_get(self):
+            response = self.client.get('/en/dev/search/', HTTP_HOST='docs.djangoproject.dev:8000')
+            self.assertEqual(response.status_code, 200)
+
+and the settings file includes a list of the domains supported by the project::
+
+    ALLOWED_HOSTS = [
+        'www.djangoproject.dev',
+        'docs.djangoproject.dev',
+        ...
+    ]
+
+Another option is to add the required hosts to :setting:`ALLOWED_HOSTS` using
+:meth:`~django.test.override_settings()` or
+:attr:`~django.test.SimpleTestCase.modify_settings()`. This option may be
+preferable in standalone apps that can't package their own settings file or
+for projects where the list of domains is not static (e.g., subdomains for
+multitenancy). For example, you could write a test for the domain
+``http://otherserver/`` as follows::
+
+    from django.test import TestCase, override_settings
+
+    class MultiDomainTestCase(TestCase):
+        @override_settings(ALLOWED_HOSTS=['otherserver'])
+        def test_other_domain(self):
+            response = self.client.get('http://otherserver/foo/bar/')
+
+Disabling :setting:`ALLOWED_HOSTS` checking (``ALLOWED_HOSTS = ['*']``) when
+running tests prevents the test client from raising a helpful error message if
+you follow a redirect to an external URL.
+
+.. versionchanged:: 1.11
+
+    Older versions didn't validate ``ALLOWED_HOSTS`` while testing so these
+    techniques weren't necessary.
+
 .. _topics-testing-advanced-multidb:
 
 Tests and multiple databases