错误内容：
Could not execute ruok towards ZK host xxx:2181. Add this line to the 'zoo.cfg' configuration file on each zookeeper node: '4lw.commands.whitelist=mntr,conf,ruok'. See also chapter 'Setting Up an External ZooKeeper Ensemble' in the Solr Reference Guide.
其中xxx是主机名或IP地址
解决方法：
就是按提示里面说的，在zoo.cfg里面添加一行4lw.commands.whitelist=mntr,conf,ruok
vim zookeeper/conf/zoo.cfg
在末尾加入一行：
4lw.commands.whitelist=mntr,conf,ruok
保存后退出，然后重启ZooKeeper：
zkServer.sh restart