diff --git a/src/azure-cli/azure/cli/command_modules/acs/_validators.py b/src/azure-cli/azure/cli/command_modules/acs/_validators.py
index 126cdd0433b..77a93943379 100644
--- a/src/azure-cli/azure/cli/command_modules/acs/_validators.py
+++ b/src/azure-cli/azure/cli/command_modules/acs/_validators.py
@@ -147,11 +147,11 @@ def validate_linux_host_name(namespace):
     in the CLI pre-flight.
     """
     # https://stackoverflow.com/questions/106179/regular-expression-to-match-dns-hostname-or-ip-address
-    rfc1123_regex = re.compile(r'^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))*$')  # pylint:disable=line-too-long
+    rfc1123_regex = re.compile(r'^[a-zA-Z0-9]$|^[a-zA-Z0-9][-_a-zA-Z0-9]{0,61}[a-zA-Z0-9]$')  # pylint:disable=line-too-long
     found = rfc1123_regex.findall(namespace.name)
     if not found:
-        raise CLIError('--name cannot exceed 63 characters and can only contain '
-                       'letters, numbers, or dashes (-).')
+        raise InvalidArgumentValueError('--name cannot exceed 63 characters and can only contain '
+                                        'letters, numbers, underscores (_) or dashes (-).')
 
 
 def validate_snapshot_name(namespace):