Website is not working due to high memory usage by MySQL: [Warning] 'user' entry ignored in --skip-name-resolve mode

Symptoms

• The following records appear in the MySQL log file /var/log/mysqld.log (the log file can be also /var/log/mysql/error.log or /var/log/mariadb/mariadb.log):

  CONFIG_TEXT: [Warning] 'user' entry 'user1@example.com' ignored in --skip-name-resolve mode.
  [Warning] 'user' entry 'user2@example.com' ignored in --skip-name-resolve mode.

• When connected to the server over SSH, the utility top shows that MySQL service consumes a lot of CPU time:

  CONFIG_TEXT: 8388604k total, 188204k used, 8200400k free, 10715096k cached
  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
  12353 mysql 20 0 15.6g 3.4g 5564 S 106.1 10.8 5953:36 mysqld

• The website is not working:

  CONFIG_TEXT: Service Temporarily Unavailable

Cause

MySQL server is configured to perform reverse DNS lookup of clients. In this case, the server uses only IP addresses and not hostnames to match connecting hosts to rows in the MySQL grant tables.

Resolution

1. Connect to the server using SSH.

2. Comment out the following parameter to /etc/my.cnf in the section [mysqld]:

   CONFIG_TEXT: skip-name-resolve

3. Restarted the service:

   # service mysqld restart

   Note: depending on the operating system or used MySQL drop-in replacement, the service can be also called mysql or mariadb.