Ubuntu安装MySQL成功后，普通用户无权访问：ERROR 1045 (28000): Access denied

Ubuntu安装Mysql很简单，但是也会遇到一些问题。

我的Ubuntu版本是18.04.1，安装mysql版本是5.7。

Ubuntu安装MySQL的步骤如下：

1. sudo apt-get install mysql-server

2. apt-get install mysql-client

3.  sudo apt-get install libmysqlclient-dev

安装以后，通过mysql --version命令查看版本信息：

mysql  Ver 14.14 Distrib 5.7.26, for Linux (x86_64) using  EditLine wrapper

mysql是安装成功了，但是直接输入mysql，却显示没有权限：

ERROR 1045 (28000): Access denied for user 'qingyuan'@'localhost' (using password: NO)

根据网上的建议，查看mysql到底有没有启动：

sudo netstat -tap |grep mysql

输出显示是成功的：

tcp        0      0 localhost:mysql         0.0.0.0:*               LISTEN      4892/mysqld   

继续跟随网上建议，关闭mysql，然后重启

systemctl restart mysqld

输出如下：

Failed to start mysqld.service: Unit mysqld.service not found.

输出显示：没有mysqld.service这个单元。

输入mysqld命令，输出如下：

mysqld: Can't change dir to '/var/lib/mysql/' (Errcode: 13 - Permission denied)
2019-07-01T13:44:44.228936Z 0 [Warning] Changed limits: max_open_files: 1024 (requested 5000)
2019-07-01T13:44:44.229209Z 0 [Warning] Changed limits: table_open_cache: 431 (requested 2000)
2019-07-01T13:44:44.479228Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2019-07-01T13:44:44.596275Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.597278Z 0 [Note] mysqld (mysqld 5.7.26-0ubuntu0.18.04.1) starting as process 3561 ...
2019-07-01T13:44:44.905374Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.905433Z 0 [Warning] Can't create test file /var/lib/mysql/ubuntu.lower-test
2019-07-01T13:44:44.936710Z 0 [Warning] One can only use the --user switch if running as root

2019-07-01T13:44:44.937074Z 0 [ERROR] failed to set datadir to /var/lib/mysql/
2019-07-01T13:44:44.937156Z 0 [ERROR] Aborting

2019-07-01T13:44:44.939133Z 0 [Note] Binlog end
2019-07-01T13:44:44.958845Z 0 [Note] mysqld: Shutdown complete

但是，切换到root后，却可以直接访问mysql了

这到底是为什么？

这得从Ubuntu下安装高版本的MySql有关。在Ubuntu上安装MySQL时，对于root用户，可以设置空密码。

输入如下指令查看mysql账户信息：

SELECT User, Host, HEX(authentication_string) FROM mysql.user;

输出为： 

+------------------+-----------+------------------------------------------------------------------------------------+
| User             | Host      | HEX(authentication_string)                                                         |
+------------------+-----------+------------------------------------------------------------------------------------+
| root             | localhost |                                                                                    |
| mysql.session    | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 |
| mysql.sys        | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 |
| debian-sys-maint | localhost | 2A33383636414146424141463045393833383341423435353142373433363739333531383934423043 |
+------------------+-----------+------------------------------------------------------------------------------------+

我们看到，root的密码确实为空。

试着改变root的密码，输入如下指令：

ALTER USER 'root'@'localhost' IDENTIFIED BY 'test';

输出为：

Query OK, 0 rows affected (0.00 sec)

然后发现，root的密码仍然为空，继续尝试：

SET PASSWORD FOR 'root'@'localhost' = PASSWORD('test');

输出：

Query OK, 0 rows affected, 2 warnings (0.02 sec)

仍然没有成功，并且有两个警告。我们看看警告是什么：

show warnings;

输出：

+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                 |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1287 | 'SET PASSWORD FOR  = PASSWORD('')' is deprecated and will be removed in a future release. Please use SET PASSWORD FOR  = '' instead |
| Note    | 1699 | SET PASSWORD has no significance for user 'root'@'localhost' as authentication plugin does not support it.                                                                              |
+---------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.04 sec)

输出显示当前使用的授权插件不支持修改密码，那我们看一下当前的授权插件是什么：

SELECT user, plugin FROM mysql.user;

输出：

+------------------+-----------------------+
| user             | plugin                |
+------------------+-----------------------+
| root             | auth_socket           |
| mysql.session    | mysql_native_password |
| mysql.sys        | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+

我们看到，root采用的是授权插件是auth_socket，也即通过套接字进行鉴权。如果你安装的版本是5.7，并且没有为root用户提供密码（安装时并不需要），那么就会使用auth_socket插件，这个插件不关心也不需要密码，它只是通过UNIX套接字的用户名进行验证，如果验证通过，那么就鉴权成功。这就是root用户可以正常使用mysql的原因，因为root用户在mysql的user表中。

如果我们想配置密码，那么就需要同时改变插件并且设置密码。如果是先改变插件，然后设置密码就会不起作用。插件也会回滚到auth_socket。

输入如下命令：

update mysql.user set authentication_string=PASSWORD('123456'), plugin='mysql_native_password' where user='root';

输出如下：

Query OK, 1 row affected, 1 warning (0.09 sec)
Rows matched: 1  Changed: 1  Warnings: 1

 查看操作后的用户信息

mysql> SELECT User, Host, HEX(authentication_string) ,plugin FROM mysql.user;
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+
| User             | Host      | HEX(authentication_string)                                                         | plugin                |
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+
| root             | localhost | 2A36424234383337454237343332393130354545343536384444413744433637454432434132414439 | mysql_native_password |
| mysql.session    | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 | mysql_native_password |
| mysql.sys        | localhost | 2A5448495349534E4F544156414C494450415353574F52445448415443414E42455553454448455245 | mysql_native_password |
| debian-sys-maint | localhost | 2A33383636414146424141463045393833383341423435353142373433363739333531383934423043 | mysql_native_password |
+------------------+-----------+------------------------------------------------------------------------------------+-----------------------+

此时root用户确实有了密码 。我们查看一下warning是什么：

mysql> show warnings;
+---------+------+-------------------------------------------------------------------+
| Level   | Code | Message                                                           |
+---------+------+-------------------------------------------------------------------+
| Warning | 1681 | 'PASSWORD' is deprecated and will be removed in a future release. |
+---------+------+-------------------------------------------------------------------+
1 row in set (0.00 sec)

警告显示，PASSWORD被废弃并且在未来版本中要被移除。可以通过如下命令

ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY '123456';

退出mysql，重启mysql，退出root用户，切换到普通用户，用设置的密码123456，root用户登录mysql。

mysql -u root -p

然后输入密码：123456，登录成功：

qingyuan@ubuntu:~$ mysql -u root -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.26-0ubuntu0.18.04.1 (Ubuntu)

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参考资料

1. Ubuntu18.04安装MySQL后普通用户无法登录

2. Change user password in MySQL 5.7 with “plugin: auth_socket”