If you have a host which is only capable of active agent checks (i.e. the host is not reachable from the zabbix server, but the host can reach the zabbix server) you can configure it as described below.

Configure the agentd with:

 Hostname=activehost
 DisableActive=0
 DisablePassive=1

Configured the host within zabbix server with IP 0.0.0.0, no DNS, no Zabbix agent port and with the same name as used above on the agent in Hostname.

I created a Full clone of the template to be assigned to this host and named it Template_Linux-Active.

At the Items screen for this new template I selected all items and did a massupdate to change the Type of these items to Zabbix Agent (active)

Then I linked this 'active' template to my host.

NOTE: The error message “Cannot connect to” beside the host did not disappear. Apparently the active agent checks do not update this error. I dropped the host an recreated it. Now I have a full working active agent.