Fix Host ‘IP’ is blocked because of many connection errors on MySQL/MariaDB

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I am trying to login to my mysql/mysqld server using the following Unix command:
$ mysql -u foo -h 172.16.5.100 -p dbnmame
But greeted with the following error on screen:
ERROR 1129 (HY000): Host ‘172.16.5.100’ is blocked because of many connection errors; unblock with ‘mysqladmin flush-hosts’
How do I fix this problem on a Linux or Unix-like system?

The number of connections permitted is controlled by the MySQL/MariaDB max_connections system variable. The default value is 151 to improve performance when MySQL is used with the Web server. You might run into a problem if you are running a high trafficked web site or MariaDB server in clustered mode or using a Galera master to master DB cluster. If you need to support more connections, you should set a more substantial value for this variable.

How to find out current value of max_connections

Type the following mysql command:
$ mysql -u root -p
Now issue the following sql command:
mysql> show variables like "max_connections";
Sample outputs:

+-----------------+--------+
| Variable_name   | Value  |
+-----------------+--------+
| max_connections | 151    |
+-----------------+--------+
1 row in set (0.00 sec)

How do I increase max_connections value?

Edit my.cnf or mariadb.conf.d/50-server.cnf in /etc/ directory using a text editor such as vi command or nano command:
$ sudo vim /etc/mariadb.conf.d/50-server.cnf
Add/append/edit the following line under [mysqld] section (say set value to 1000. Max limit is 100000):
max_connections = 1000
Save and close the file. Next, restart the mysqld service, run:
$ sudo systemctl restart mysql
If you are using a CentOS/RHEL/Fedora/Oracle/Scientific Linux, run:
$ sudo systemctl restart mysqld
If you are using a FreeBSD unix, run the following to restart the system:
$ sudo /usr/local/etc/rc.d/mysql-server restart
Verify new limits with the following command:
$ mysql -u root -p -e 'show variables like "max_connections";'

Fig.01: How to view mac_connection limits when using MySQL/MariaDB
Fig.01: How to view mac_connection limits when using MySQL/MariaDB

A note about mysqladmin command

To flush all cached hosts and remove this limit run the following bash command
$ mysqladmin flush-hosts
OR pass username and prompt for a password:
$ mysqladmin -u root -p flush-hosts

A note about “Too many open files” error

You need to changes the number of file descriptors available to mysqld if you get the error. To see current FD limits, run:
$ mysql -u root -p -e 'show variables like "open_files_limit";'
Sample outputs:

Enter password: 
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| open_files_limit | 500005 |
+------------------+--------+

Again edit my.cnf if limit is too small and getting an error in your log file:
$ sudo vi my.cnf
OR
$ sudo vim /etc/mariadb.conf.d/50-server.cnf
Set the value as per requirements in in [mysqld] section
open_files_limit = 1024000
Save and close the file. Make sure you restart the mysqld as described above. If you are using GNU/Linux systemd based distro such as RHEL/CentOS 7, create a file named:
$ sudo vi /etc/systemd/system/mysqld.service
Append the following:
[Service]
User=mysql
Group=mysql
LimitNOFILE=1024000

Restart needed services:
$ sudo systemctl daemon-reload
$ sudo systemctl restart mysqld

Older Linux distro must edit the /etc/security/limits.conf file.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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