diff --git a/moto/s3/responses.py b/moto/s3/responses.py
index 18698c275..cec2de6d9 100644
--- a/moto/s3/responses.py
+++ b/moto/s3/responses.py
@@ -40,11 +40,13 @@ class ResponseObject(_TemplateEnvironmentMixin):
         return template.render(buckets=all_buckets)
 
     def subdomain_based_buckets(self, request):
-        host = request.headers['host']
+        host = request.headers.get('host', request.headers.get('Host'))
         if host.startswith("localhost"):
             # For localhost, default to path-based buckets
             return False
-        return host != 's3.amazonaws.com' and not re.match("s3.(.*).amazonaws.com", host)
+
+        path_based = (host == 's3.amazonaws.com' or re.match("s3.(.*).amazonaws.com", host))
+        return not path_based
 
     def is_delete_keys(self, request, path, bucket_name):
         if self.subdomain_based_buckets(request):
diff --git a/moto/s3/urls.py b/moto/s3/urls.py
index dda4b273a..6d9ff514e 100644
--- a/moto/s3/urls.py
+++ b/moto/s3/urls.py
@@ -1,4 +1,6 @@
 from __future__ import unicode_literals
+
+from moto.compat import OrderedDict
 from .responses import S3ResponseInstance
 
 url_bases = [
@@ -6,13 +8,13 @@ url_bases = [
     "https?://(?P<bucket_name>[a-zA-Z0-9\-_.]*)\.?s3(.*).amazonaws.com"
 ]
 
-url_paths = {
+url_paths = OrderedDict([
     # subdomain bucket
-    '{0}/$': S3ResponseInstance.bucket_response,
+    ('{0}/$', S3ResponseInstance.bucket_response),
 
     # subdomain key of path-based bucket
-    '{0}/(?P<key_or_bucket_name>.+)': S3ResponseInstance.ambiguous_response,
+    ('{0}/(?P<key_or_bucket_name>.+)', S3ResponseInstance.ambiguous_response),
 
     # path-based bucket + key
-    '{0}/(?P<bucket_name_path>[a-zA-Z0-9\-_./]+)/(?P<key_name>.+)': S3ResponseInstance.key_response,
-}
+    ('{0}/(?P<bucket_name_path>[a-zA-Z0-9\-_./]+)/(?P<key_name>.+)', S3ResponseInstance.key_response),
+])