环境：

mysql> select version();
+------------+
| version() |
+------------+
| 5.7.17-log |
+------------+
1 row in set (0.00 sec)

报错：

mysql> create user repl@'192.168.100.51' identified by '123456';
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

出现这个问题是因为安装了插件，如下：

mysql> select name,dl from mysql.plugin;
+-------------------+----------------------+
| name | dl |
+-------------------+----------------------+
| validate_password | validate_password.so |
+-------------------+----------------------+
1 row in set (0.00 sec)

具体规则是：

mysql> SHOW VARIABLES LIKE 'validate_password%';
+--------------------------------------+--------+
| Variable_name | Value |
+--------------------------------------+--------+
| validate_password_check_user_name | OFF |
| validate_password_dictionary_file | |
| validate_password_length | 8 |
| validate_password_mixed_case_count | 1 |
| validate_password_number_count | 1 |
| validate_password_policy | MEDIUM |
| validate_password_special_char_count | 1 |
+--------------------------------------+--------+

插件的安装启用：

1.插件对应的库对象文件需在配置选项plugin_dir指定的目录中。

2.可使用--plugin-load=validate_password.so在server启动时载入插件，或者将plugin-load=validate_password.so写入配置文件。

3.也可以通过如下语句在server运行时载入插件（会注册进mysql.plugins表）mysql> INSTALL PLUGIN validate_password SONAME 'validate_password.so';

为阻止该插件在运行时被删除可在配置文件中添加：
[mysqld]
plugin-load=validate_password.so
validate-password=FORCE_PLUS_PERMANENT

更多信息：

https://dev.mysql.com/doc/refman/5.7/en/validate-password-plugin-installation.html