MySQL Tuning

As of Security Onion MySQL (on the master server) should have a randomized root password set by default. You can still access MySQL using the following as an example of the syntax to run a command against securityonion_db (Sguil DB):

sudo mysql --defaults-file=/etc/mysql/debian.cnf -Dsecurityonion_db -e 'select * from event limit 10';


You can install and run mysqltuner to get some initial recommendations.

Install mysqltuner if you haven’t already:

sudo apt update && sudo apt install mysqltuner

Run mysqltuner with privileges:

sudo mysqltuner

You may also want to install mysqltuner via the following manner, given that Security Onion now uses defaults-file to handle MySQL database credentials:

wget -O && chmod +x
sudo ./

/etc/mysql/my.cnf vs /etc/mysql/conf.d/

Implement mysqltuner’s recommendations in /etc/mysql/my.cnf or create a new file in /etc/mysql/conf.d/ with the changes. We recommend /etc/mysql/conf.d/ so that your changes don’t get overwritten during MySQL package upgrades.

Restart MySQL

Changes don’t take effect until MySQL is restarted and you should ensure that Sguil and other services aren’t using MySQL before shutting it down.


Here are some common variables that may need to be tuned for your system:

  • open-files-limit
  • table_cache
  • key_buffer
  • max_connections

MySQL slow to start on boot

At boot time, MySQL checks all tables, which can take a long time. If you wish to disable this check, comment out check_for_crashed_tables in /etc/mysql/debian-start.


MySQL defaults table_definition_cache to 400. You may want to increase this value if one or more of the following conditions applies to you:

  • you have more than 400 MySQL .frm files
  • you’ve increased DAYSTOKEEP in /etc/nsm/securityonion.conf above its default value of 30 (each day requires 5 .frm files for OSSEC and 5 .frm files for each sniffing interface)
  • you’re running prepared statements

Check mysql table_definition cache (defaults to 400):

sudo mysql --defaults-file=/etc/mysql/debian.cnf -e "show global variables like 'table_definition_cache'"

Check current open_table_definitions (probably maxed out at table_definition_cache):

sudo mysql --defaults-file=/etc/mysql/debian.cnf -e "show global status like 'open_table_definitions'"

Check number of .frm files:

sudo find /var/lib/mysql/ -name "*.frm" |wc -l

Increase table_definition_cache above number of .frm files by creating a file called /etc/mysql/conf.d/securityonion-table_definition_cache.cnf (please note .cnf extension NOT .conf) and adding the following (replacing 4000 with your desired setting):

table_definition_cache = 4000

Reboot and then verify that open_table_definitions never gets limited by table_definition_cache.

For more information, please see: