diff --git a/src/command_modules/azure-cli-acs/HISTORY.rst b/src/command_modules/azure-cli-acs/HISTORY.rst
index e8054f1ee4b..a20d5573e02 100644
--- a/src/command_modules/azure-cli-acs/HISTORY.rst
+++ b/src/command_modules/azure-cli-acs/HISTORY.rst
@@ -5,6 +5,7 @@ Release History
 unreleased
 +++++++++++++++++++
 * correct preview regions
+* format default dns_name_prefix properly
 
 2.0.13 (2017-08-15)
 +++++++++++++++++++
diff --git a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/_params.py b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/_params.py
index e7909cc3eea..2b2d80ceeea 100644
--- a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/_params.py
+++ b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/_params.py
@@ -81,7 +81,7 @@ def _get_feature_in_preview_message():
 # some admin names are prohibited in acs, such as root, admin, etc. Because we have no control on the orchestrators, so default to a safe name.
 register_cli_argument('acs', 'admin_username', options_list=('--admin-username',), default='azureuser', required=False)
 register_cli_argument('acs', 'api_version', options_list=('--api-version',), required=False, help=_get_feature_in_preview_message() + 'Use API version of ACS to perform az acs operations. Available options: 2017-01-31, 2017-07-01. Default to use the latest version for the location')
-register_cli_argument('acs', 'dns_name_prefix', options_list=('--dns-prefix', '-d'))
+register_cli_argument('acs', 'dns_name_prefix', options_list=('--dns-prefix', '-d'), help='default use the format of <clustername>-<resourcegroupname>-<subid>, will trim the length and replace sensitive characters if needed')
 register_cli_argument('acs', 'container_service_name', options_list=('--name', '-n'), help='The name of the container service', completer=get_resource_name_completion_list('Microsoft.ContainerService/ContainerServices'))
 
 register_cli_argument('acs', 'ssh_key_value', required=False, help='SSH key file value or key file path.', type=file_type, default=os.path.join('~', '.ssh', 'id_rsa.pub'), completer=FilesCompleter())
diff --git a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/custom.py b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/custom.py
index c557a385b89..413f7be3582 100644
--- a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/custom.py
+++ b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/custom.py
@@ -21,6 +21,7 @@
 import webbrowser
 import stat
 import ssl
+import re
 import yaml
 import dateutil.parser
 from dateutil.relativedelta import relativedelta
@@ -370,6 +371,15 @@ def _get_subscription_id():
     return sub_id
 
 
+def _get_default_dns_prefix(name, resource_group_name, subscription_id):
+    # Use subscription id to provide uniqueness and prevent DNS name clashes
+    name_part = re.sub('[^A-Za-z0-9-]', '', name)[0:10]
+    if not name_part[0].isalpha():
+        name_part = (str('a') + name_part)[0:10]
+    resource_group_part = re.sub('[^A-Za-z0-9-]', '', resource_group_name)[0:16]
+    return '{}-{}-{}'.format(name_part, resource_group_part, subscription_id[0:6])
+
+
 # pylint: disable=too-many-locals
 # pylint: disable-msg=too-many-arguments
 def acs_create(resource_group_name, deployment_name, name, ssh_key_value, dns_name_prefix=None,
@@ -482,8 +492,7 @@ def acs_create(resource_group_name, deployment_name, name, ssh_key_value, dns_na
 
     subscription_id = _get_subscription_id()
     if not dns_name_prefix:
-        # Use subscription id to provide uniqueness and prevent DNS name clashes
-        dns_name_prefix = '{}-{}-{}'.format(name, resource_group_name, subscription_id[0:6])
+        dns_name_prefix = _get_default_dns_prefix(name, resource_group_name, subscription_id)
 
     register_providers()
     groups = _resource_client_factory().resource_groups
diff --git a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/tests/test_custom.py b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/tests/test_custom.py
index 6296054d004..5a2c67bda5b 100644
--- a/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/tests/test_custom.py
+++ b/src/command_modules/azure-cli-acs/azure/cli/command_modules/acs/tests/test_custom.py
@@ -14,7 +14,7 @@
 from msrestazure.azure_exceptions import CloudError
 
 from azure.cli.command_modules.acs.custom import (merge_kubernetes_configurations,
-                                                  _acs_browse_internal, _add_role_assignment)
+                                                  _acs_browse_internal, _add_role_assignment, _get_default_dns_prefix)
 from azure.mgmt.containerservice.models import (ContainerServiceOrchestratorTypes,
                                                 ContainerService,
                                                 ContainerServiceOrchestratorProfile)
@@ -22,6 +22,18 @@
 
 
 class AcsCustomCommandTest(unittest.TestCase):
+    def test_get_default_dns_prefix(self):
+        name = 'test5678910'
+        resource_group_name = 'resource_group_with_underscore'
+        sub_id = '123456789'
+
+        dns_name_prefix = _get_default_dns_prefix(name, resource_group_name, sub_id)
+        self.assertEqual(dns_name_prefix, "test567891-resourcegroupwit-123456")
+
+        name = '1test5678910'
+        dns_name_prefix = _get_default_dns_prefix(name, resource_group_name, sub_id)
+        self.assertEqual(dns_name_prefix, "a1test5678-resourcegroupwit-123456")
+
     def test_add_role_assignment_basic(self):
         role = 'Owner'
         sp = '1234567'