diff --git a/django/http/request.py b/django/http/request.py
index e222081450..9f9f32b1b4 100644
--- a/django/http/request.py
+++ b/django/http/request.py
@@ -68,14 +68,19 @@ class HttpRequest(object):
             if server_port != ('443' if self.is_secure() else '80'):
                 host = '%s:%s' % (host, server_port)
 
-        allowed_hosts = ['*'] if settings.DEBUG else settings.ALLOWED_HOSTS
+        # There is no hostname validation when DEBUG=True
+        if settings.DEBUG:
+            return host
+
         domain, port = split_domain_port(host)
-        if domain and validate_host(domain, allowed_hosts):
+        if domain and validate_host(domain, settings.ALLOWED_HOSTS):
             return host
         else:
             msg = "Invalid HTTP_HOST header: %r." % host
             if domain:
                 msg += "You may need to add %r to ALLOWED_HOSTS." % domain
+            else:
+                msg += "The domain name provided is not valid according to RFC 1034/1035"
             raise DisallowedHost(msg)
 
     def get_full_path(self):
diff --git a/tests/requests/tests.py b/tests/requests/tests.py
index 8c56e48f58..5bd3e5141f 100644
--- a/tests/requests/tests.py
+++ b/tests/requests/tests.py
@@ -620,12 +620,20 @@ class HostValidationTests(SimpleTestCase):
         }
         self.assertEqual(request.get_host(), 'example.com')
 
+        # Invalid hostnames would normally raise a SuspiciousOperation,
+        # but we have DEBUG=True, so this check is disabled.
+        request = HttpRequest()
+        request.META = {
+            'HTTP_HOST': "invalid_hostname.com",
+        }
+        self.assertEqual(request.get_host(), "invalid_hostname.com")
 
     @override_settings(ALLOWED_HOSTS=[])
     def test_get_host_suggestion_of_allowed_host(self):
         """get_host() makes helpful suggestions if a valid-looking host is not in ALLOWED_HOSTS."""
         msg_invalid_host = "Invalid HTTP_HOST header: %r."
         msg_suggestion = msg_invalid_host + "You may need to add %r to ALLOWED_HOSTS."
+        msg_suggestion2 = msg_invalid_host + "The domain name provided is not valid according to RFC 1034/1035"
 
         for host in [ # Valid-looking hosts
             'example.com',
@@ -664,6 +672,14 @@ class HostValidationTests(SimpleTestCase):
                 request.get_host
             )
 
+        request = HttpRequest()
+        request.META = {'HTTP_HOST': "invalid_hostname.com"}
+        self.assertRaisesMessage(
+            SuspiciousOperation,
+            msg_suggestion2 % "invalid_hostname.com",
+            request.get_host
+        )
+
 
 @skipIf(connection.vendor == 'sqlite'
         and connection.settings_dict['TEST_NAME'] in (None, '', ':memory:'),