gistfile1.txt

 % curl localhost:9200/_cat/nodes ##### 2 node cluster
127.0.0.1 25 100 20 2.44   mdi - qKD-e03
127.0.0.1 12 100 29 2.44   mdi * qxG7nuF
 % curl localhost:9200/_foo ##### 5.6 node, a 400
{"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"No endpoint or operation is available at [_foo]"}],"type":"illegal_argument_exception","reason":"No endpoint or operation is available at [_foo]"},"status":400}
% curl localhost:9201/_foo ##### 6.0 node, a 404
{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","resource.type":"index_or_alias","resource.id":"_foo","index":"_foo"}],"type":"index_not_found_exception","reason":"no such index","index_uuid":"_na_","resource.type":"index_or_alias","resource.id":"_foo","index":"_foo"},"status":404}

** Kill the 5.6 node **

% curl localhost:9201/_cat/nodes ##### 6.0 node
127.0.0.1 27 90 12 2.86   mdi * qKD-e03
% curl localhost:9201/_foo ##### 6.0 node, a 400 now! 
{"error":{"root_cause":[{"type":"invalid_index_name_exception","reason":"Invalid index name [_foo], must not start with '_'.","index_uuid":"_na_","index":"_foo"}],"type":"invalid_index_name_exception","reason":"Invalid index name [_foo], must not start with '_'.","index_uuid":"_na_","index":"_foo"},"status":400}