用户管理模块之mysql.user

不使用-h参数来指定登录host，默认会连接localhost，仅当mysql.user表中有一条对应的localhost访问授权(username@%不对任何主机做限制也不行)时登录才成功，否则登录会被拒绝。
虚拟机VMUest上安装两个MySQL实例，两个实例搭建了Master(端口3306)-Slave(端口3307)，主从数据完全一致。

mysql> select Host,User,Password from mysql.user;
+-----------------------+--------+-------------------------------------------+
| Host                  | User   | Password                                  |
+-----------------------+--------+-------------------------------------------+
| localhost             | root   | *6B4F89A54E2D27ECD7E8DA05B4AB8FD9D1D8B119 |
| localhost.localdomain | root   |                                           |
| 127.0.0.1             | root   |                                           |
| ::1                   | root   |                                           |
| localhost             |        |                                           |
| localhost.localdomain |        |                                           |
| %                     | mydba  | *80BF8C1F4008F25267DB194E29D2E8BC20C836ED |
| %                     | backup | *975B2CD4FF9AE554FE8AD33168FBFC326D2021DD |
| 192.168.85.%          | repl   | *A424E797037BF97C19A2E88CF7891C5C2038C039 |
+-----------------------+--------+-------------------------------------------+
9 rows in set

使用root用户登录

#不指定-h参数，默认就会走localhost
[uest@VMUest ~]$ mysql -uroot -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> exit
Bye
[uest@VMUest ~]$ mysql -uroot -p -P3307
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> exit
Bye
#指定-h+ip参考，同时指定-P端口
[uest@VMUest ~]$ mysql -uroot -p -h127.0.0. -P3306
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> exit
Bye
[uest@VMUest ~]$ mysql -uroot -p -h127.0.0. -P3307
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> exit
Bye
#指定-h+localhost参考，同时指定-P端口
[uest@VMUest ~]$ mysql -uroot -p -hlocalhost -P3307
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> exit
Bye
[uest@VMUest ~]$ 

从上面的结果可以看出只有在指定-h+ip，同时指定-P端口的情况下，才能连接到指定的端口中。localhost实际就是指定通过socket连接！！！
注意：我们的mysql.user表中root@'127.0.0.1'对应的Password为空(即不需要输入密码)，但实际登录过程需要输入密码。这是受skip_name_resolve参数影响

mysql> show variables like '%skip_name_resolve%';
+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| skip_name_resolve | OFF   |
+-------------------+-------+
1 row in set

该参数的值为OFF，root@'127.0.0.1'会转化为root@'localhost'登录，它使用的是root@'localhost'定义的密码。可尝试开启skip_name_resolve参数(配置文件my.cnf中添加skip_name_resolve=1)，然后使用空密码登录。
mysql.user表中第5条记录Host='localhost'，User和Password字段为空，也就是localhost可以不指定用户、密码直接登录

[uest@VMUest ~]$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log Source distribution

Copyright (c) , , Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@port;
+--------+
| @@port |
+--------+
|    |
+--------+
 row in set (0.00 sec)

mysql> 

还是有一些小细节需要注意。引用iVictor一段话：'t1'@'%'中包含't1'@'127.0.0.1'，如果开启skip_name_resolve参数，则't1'@'%'中定义的密码可用于't1'@'127.0.0.1'的登录；如果没有开启该参数，则't1'@'127.0.0.1'会转化为't1'@'localhost'登录，如果用户表存在't1'@'localhost'条目，根据最精确匹配原则(Host->User)，需要使用't1'@'localhost'对应的密码登录；如果用户表不存在't1'@'localhost'条目，'t1'@'%'定义的密码依旧适用。
从库开启skip_name_resolve参数，从库可以使用 mysql -h127.0.0.1 -P3307 -umydba -p 登录，对应user表中条目mydba@'%'；主库没有开启skip_name_resolve参数，它如果尝试使用 mysql -h127.0.0.1 -P3306 -umydba -p 登录，会转化为mydba@'localhost'，user表中没有此条目，登录拒绝！

MySQL存放于内存结构中的权限信息何时被更新：
FLUSH PRIVILEGES会强行让MySQL更新Load到内存中的权限信息；GRANT、REVOKE或者CREATE USER和DROP USER操作会直接更新内存中的权限信息；重启MySQL会让MySQL完全从grant tables中读取权限信息。
内存结构中的权限信息更新之后对已经连接上的用户何时生效：
对于Global Level的权限信息的修改，仅仅只有更改之后新建连接才会用到，对于已经连接上的session并不会受到影响。
对于Database Level的权限信息的修改，只有当客户端请求执行了“use database_name”命令之后，才会在重新校验中使用到新的权限信息。
对于Table Level和Column Level的权限，在下一次需要使用到该权限的Query被请求的时候生效。