浅谈Mysql连接数据库时host和user的匹配规则

--连接数据库时，host和user的匹配规则

官方文档：https://dev.mysql.com/doc/refman/5.7/en/connection-access.html

--host和user的匹配规则如下：

--是host为明确的最先匹配，host带%模糊的时候最后匹配，但host为''(空)位于%之后才匹配

--相同的host时候，比较user为明确的最先匹配，user为''(空)最后匹配

--相同的host和user时，排序是不确定的

When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows: 
Whenever the server reads the user table into memory, it sorts the rows. 
When a client attempts to connect, the server looks through the rows in sorted order. 
The server uses the first row that matches the client host name and user name. 
The server uses sorting rules that order rows with the most-specific Host values first. Literal host names and IP addresses are the most specific. (The specificity of a literal IP address is not affected by whether it has a netmask, so 198.51.100.13 and 198.51.100.0/255.255.255.0 are considered equally specific.) The pattern '%' means “any host” and is least specific. The empty string '' also means “any host” but sorts after '%'. Rows with the same Host value are ordered with the most-specific User values first (a blank User value means “any user” and is least specific). For rows with equally-specific Host and User values, the order is nondeterministic.

--查看当前的host及用户信息匹配顺序，先host顺序匹配、后user顺序匹配

mysql> SELECT authentication_string, host, user,account_locked FROM mysql.USER ORDER BY host desc ,user desc;
+-------------------------------------------+--------------+---------------+----------------+
| authentication_string      | host   | user   | account_locked |
+-------------------------------------------+--------------+---------------+----------------+
| *511C0A408C5065XXEC90D60YYA1AB9437281AF28 | localhost | root   | N    |
| *THISISNOTAVALIXXASSWORDYYATCANBEUSEDHERE | localhost | mysql.sys  | Y    |
| *THISISNOTAVALIXXASSWORDYYATCANBEUSEDHERE | localhost | mysql.session | Y    |
| *485CE31BA547A4XXC047659YY10DF200F361CD4E | localhost | bkpuser  | N    |
| *7B502777D8FF69XX4B56BC2YY2867F4B47321BA8 | 192.168.56.% | repl   | N    |
| *AECCE73463829AXX3968838YYF6F85E43C3F169C | %   | flyremote  | N    |
| *566AC8467DAAAEXXE247AE7YY0A770E9B97D9FB0 |    | flylocal  | N    |
+-------------------------------------------+--------------+---------------+----------------+
8 rows in set (0.00 sec)
 

--举个特殊例子

--建立两个特殊用户如下，一个用户名为''(空)、一个用户名和host都为''(空)

mysql> create user ''@'localhost' identified by "Kong123$";
Query OK, 0 rows affected (0.00 sec) 
mysql> create user ''@'' identified by "doubleKong123$";   
Query OK, 0 rows affected (0.00 sec)

--查看当前的host及用户信息匹配顺序，先host顺序匹配、后user顺序匹配

mysql> SELECT authentication_string, host, user,account_locked FROM mysql.USER ORDER BY host desc ,user desc;
+-------------------------------------------+--------------+---------------+----------------+
| authentication_string      | host   | user   | account_locked |
+-------------------------------------------+--------------+---------------+----------------+
| *511C0VVV8C5065CBEC90D6TTTT1AB9437281AF28 | localhost | root   | N    |
| *THISIVVVTAVALIDPASSWORTTTTTCANBEUSEDHERE | localhost | mysql.sys  | Y    |
| *THISIVVVTAVALIDPASSWORTTTTTCANBEUSEDHERE | localhost | mysql.session | Y    |
| *485CEVVVA547A48CC04765TTTT0DF200F361CD4E | localhost | bkpuser  | N    |
| *256D7VVV91F7363EBDADEFTTTTB74B2B318746FC | localhost |    | N    |
| *7B502VVVD8FF69164B56BCTTTT867F4B47321BA8 | 192.168.56.% | repl   | N    |
| *AECCEVVV63829A5F396883TTTT6F85E43C3F169C | %   | flyremote  | N    |
| *566ACVVV7DAAAE79E247AETTTTA770E9B97D9FB0 |    | flylocal  | N    |
| *AE162VVV68403D1D98A4C9TTTT50A508B8C56F3F |    |    | N    |
+-------------------------------------------+--------------+---------------+----------------+
9 rows in set (0.00 sec)

--这样本地登录flyremote用户时 会报错，因为按以上的顺序 优先匹配到了host为localhost、user为''(空)的用户，而不是flyremote用户 (因为user为''(空)的用户可以匹配任意用户名)

[root@hostmysql-m mysql]# mysql -uflyremote -pFlyremote123$
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1045 (28000): Access denied for user 'flyremote'@'localhost' (using password: YES)

--那就是说本地登录flyremote用户时， 用匹配到的host为localhost、user为''(空)的密码 Kong123$ ，就可以正常登陆了

[root@hostmysql-m mysql]# mysql -uflyremote -pKong123$
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 15
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, 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.

--查看当前用户连接方式 和 当前用户认证方式

mysql> select user(),CURRENT_USER();
+---------------------+----------------+
| user()    | CURRENT_USER() |
+---------------------+----------------+
| flyremote@localhost | @localhost  |
+---------------------+----------------+
1 row in set (0.06 sec)

--用带入ip的方式登录flyremote用户时 无问题， ip匹配到了% ，user匹配到了flyremote

[root@hostmysql-m mysql]# mysql -uflyremote -pFlyremote123$ -h127.11.22.33 
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 12
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, 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>

--查看当前用户连接方式 和 当前用户认证方式

mysql> select user(),CURRENT_USER();
+------------------------+----------------+
| user()     | CURRENT_USER() |
+------------------------+----------------+
| flyremote@127.11.22.33 | flyremote@% |
+------------------------+----------------+
1 row in set (0.00 sec)

--任意用户、任意host，只要密码和建立的第二个空用户空host的密码"doubleKong123$"匹配了， 就可以进入mysql

--测试一个不存在的用户hahaha

[root@hostmysql-m ~]# mysql -uhahaha -pdoubleKong123$ -h127.11.22.33
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6
Server version: 5.7.23-log MySQL Community Server (GPL) 
Copyright (c) 2000, 2018, 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>

--查看当前用户连接方式 和 当前用户认证方式

mysql> select user(),CURRENT_USER();
+---------------------+----------------+
| user()    | CURRENT_USER() |
+---------------------+----------------+
| hahaha@127.11.22.33 | @    |
+---------------------+----------------+
1 row in set (0.01 sec)

--解决方案：

1、手工删除空用户和空host用户确保安全

或者

2、使用 mysql_secure_installation 来进行安全配置

--安全配置如下，其中有删除匿名用户的操作

This program enables you to improve the security of your MySQL installation in the following ways:
 You can set a password for root accounts.
 You can remove root accounts that are accessible from outside the local host.
 You can remove anonymous-user accounts.
 You can remove the test database (which by default can be accessed by all users, even anonymous users), and privileges that permit anyone to access databases with names that start with test_.

--删除匿名用户的源码 mysql_secure_installation.cc 如下：

 //Remove anonymous users
 remove_anonymous_users(); 
/**
 Removes all the anonymous users for better security.
*/
void remove_anonymous_users()
{
 int reply;
 reply= get_response((const char *) "By default, a MySQL installation has an "
      "anonymous user,\nallowing anyone to log "
      "into MySQL without having to have\na user "
      "account created for them. This is intended "
      "only for\ntesting, and to make the "
      "installation go a bit smoother.\nYou should "
      "remove them before moving into a production\n"
      "environment.\n\nRemove anonymous users? "
      "(Press y|Y for Yes, any other key for No) : ", 'y');
 
 if (reply == (int) 'y' || reply == (int) 'Y')
 {
 const char *query;
 query= "SELECT USER, HOST FROM mysql.user WHERE USER=''";
 if (!execute_query(&query, strlen(query)))
  DBUG_PRINT("info", ("query success!"));
 MYSQL_RES *result= mysql_store_result(&mysql);
 if (result)
  drop_users(result);
 mysql_free_result(result);
 fprintf(stdout, "Success.\n\n");
 }
 else
 fprintf(stdout, "\n ... skipping.\n\n");
}

补充：mysql 用户表中多个host时的匹配规则

mysql数据库中user表的host字段，是用来控制用户访问数据库“权限”的。

可以使用“%”，表示所有的网段；

也可以使用具体的ip地址，表示只有该ip的客户端才可以登录到mysql服务器；

也可以使用“_”进行模糊匹配，表示某个网段的客户端可以登录到mysql服务器。

如果在user表中存在一个用户两条不同host值的记录，那么mysql服务器该如何匹配该用户的权限呢？

mysql采用的策略是：当服务器读取user表时，它首先以最具体的Host值排序（主机名和IP号是最具体的） 。有相同Host值的条目首先以最具体的User匹配。

举例：

如下，有两条root用户，那么只有localhost的root客户端可以登录到mysql服务器。

| root | localhost | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |
| root | %   | *81F5E21E35407D884A6CD4A731AEBFB6AF209E1B |

以上为个人经验，希望能给大家一个参考，也希望大家多多支持潘少俊衡。如有错误或未考虑完全的地方，望不吝赐教。