从任何 IP 均可链接

首先,服务器防火墙层面:
The problem was in the firewall configuration. Eventually, the solution came when doing:

sudo firewall-cmd --zone=public --permanent --add-service=mysql
And then, restart the firewall:

sudo systemctl restart firewalld
https://stackoverflow.com/a/33332726

其次: 核心配置流程参考下面答案
https://stackoverflow.com/a/32716003
另,注意,centOS 的 mysql 配置位置为:(http://www.inmotionhosting.com/support/website/general-server-setup/edit-mysql-my-cnf)
sudo nano /etc/my.cnf
直接添加:bind-address = 0.0.0.0
(https://stackoverflow.com/a/16162146)