This is something very simple but was not immediately obvious to me. I am setting up a Zabbix server as part of my consulting work and have many servers behind firewalls that need to use active checks. (Connect from server behind firewall/nat to the zabbix server, rather than have the zabbix server connect out to the monitored agents.) When you create the host set the name and dns name to the server’s name you put into the zabbix_agentd.conf file on the server you want to do active checks on. It will show up as unknown availability but that is erroneous.

Also, you need to create items with active checks. I recommend cloning a template and copying the items and triggers over. Then do a Mass Update and set all the items to “Zabbix Agent (Active)”