Added proftpd mysql authentication from phpbb3
In /etc/proftpd/sql.conf:
SQLEngine       on
SQLBackend      mysql
SQLAuthTypes    PHPBBHASH
SQLAuthenticate users
SQLConnectInfo  dbuser@dbhost dbtable dbpassword

SQLNamedQuery   auth SELECT "username_clean, user_password, webserver_uid, webserver_gid, concat\('/path/to/phpbb/files/', username_clean\), NULL from phpbb3_users where username_clean='%U' and user_ftp=1"