Hello @Mutterz ,

The default location is /etc/my.cnf or /etc/mysql/my.cnf

Please note that /etc/mysql/my.cnf file is not really a file but a symlink. If you want to make some changes to your MySQL variables, you need to make the change in the MySQL daemon config file located at /etc/mysql/mysql.conf.d/mysqld.cnf.

https://www.digitalocean.com/community/questions/assistance-with-mysql-my-cnf-file-preventing-mysql-from-starting?answer=63542

Hope that helps!

Cheers,
Lalitha