Verify that you have php-mysql installed:

dpkg -l | grep php-mysql

If needed, install it:

apt install -y php-cli php-mysql

MySQL LibreNMS script

Get the script and make it executable:

wget https://github.com/librenms/librenms-agent/raw/master/snmp/mysql -O /etc/snmp/mysql && chmod +x /etc/snmp/mysql

Create the /var/cache/librenms directory and make sure that it is owned by the user running the SNMP daemon:

mkdir -p /var/cache/librenms/ && chown -R Debian-snmp:Debian-snmp /var/cache/librenms

Create /etc/snmp/mysql.cnf:

<?php
$mysql_user = 'root';
$mysql_pass = 'toor';
$mysql_host = 'localhost';
$mysql_port = 3306;

SNMP Configuration

Adapt your /etc/snmp/snmpd.conf with:

extend mysql /etc/snmp/mysql

Restart the service.

Adapt the user and path to your configuration:

sudo -u Debian-snmp /etc/snmp/mysql