There is two modes for Zabbix checks:
– Passive check : a simple data request. Zabbix server or proxy asks for some data (for example, CPU load) and Zabbix agent sends back the result to the server.
– Active check : the agent must first retrieve from the server(s) a list of items for independent processing. Then the agent then periodically sends the new values to the server(s).
Monitoring a Zabbix agent behind a firewall or a NAT without any port redirection requires to use the active mode.
It’s not recommended to monitor over internet without using a VPN.