Validate underscores in host names successfully

For eg, `minio_server` should be treated as valid hostname.

Fixes #760

Author: Praveenrajmani

minio/helpers.py

@@ -53,7 +53,7 @@
 
 _VALID_BUCKETNAME_REGEX = re.compile('^[a-z0-9][a-z0-9\\.\\-]+[a-z0-9]$')
 _ALLOWED_HOSTNAME_REGEX = re.compile(
-    '^((?!-)[A-Z\\d-]{1,63}(?<!-)\\.)*((?!-)[A-Z\\d-]{1,63}(?<!-))$',
+    '^((?!-)(?!_)[A-Z\_\\d-]{1,63}(?<!-)(?<!_)\\.)*((?!_)(?!-)[A-Z\_\\d-]{1,63}(?<!-)(?<!_))$',
     re.IGNORECASE)
 
 _EXTRACT_REGION_REGEX = re.compile('s3[.-]?(.+?).amazonaws.com')

tests/unit/get_s3_endpoint_test.py

@@ -27,3 +27,6 @@ def test_get_s3_endpoint(self):
     def test_is_valid_endpoint(self):
         eq_(True, is_valid_endpoint('s3.amazonaws.com'))
         eq_(True, is_valid_endpoint('s3.cn-north-1.amazonaws.com.cn'))
+        eq_(True, is_valid_endpoint('minio_server:9000'))
+        eq_(True, is_valid_endpoint('s3.server_1.amazonaws.com'))
+