Fix s3bucketpath handling for IP based requests (#765)

* check HTTP header for IPv4 or IPv6 addresses and default to path based S3

* improved IPv4 and IPv6 checking with optional ports

* typo

moto/s3/responses.py

@@ -4,6 +4,8 @@ import re
 
 import six
 from six.moves.urllib.parse import parse_qs, urlparse
+
+import socket
 import xmltodict
 
 from moto.core.responses import _TemplateEnvironmentMixin
@@ -44,10 +46,29 @@ class ResponseObject(_TemplateEnvironmentMixin):
 
     def subdomain_based_buckets(self, request):
         host = request.headers.get('host', request.headers.get('Host'))
-        if host.startswith("localhost"):
+
+        if not host or host.startswith("localhost"):
             # For localhost, default to path-based buckets
             return False
 
+        match = re.match(r'^([^\[\]:]+)(:\d+)?$', host)
+        if match:
+            try:
+                socket.inet_pton(socket.AF_INET, match.groups()[0])
+                 # For IPv4, default to path-based buckets
+                return False
+            except socket.error:
+                pass
+
+        match = re.match(r'^\[(.+)\](:\d+)?$', host)
+        if match:
+            try:
+                socket.inet_pton(socket.AF_INET6, match.groups()[0])
+                # For IPv6, default to path-based buckets
+                return False
+            except socket.error:
+                pass
+
         path_based = (host == 's3.amazonaws.com' or re.match(r"s3[\.\-]([^.]*)\.amazonaws\.com", host))
         return not path_based