Checkmk DNS Cache

You have four options for configuring the IP address:

Option	Procedure	DNS happens
1.	You enter no IP address. The host name must be resolvable via DNS.	with activate changes
2.	You enter an IP address – in the standard dot notation.	never
3.	Instead of an IP address you can alternatively enter a DNS-resolvable host name.	during check execution
4.	Via rule set Hosts with dynamic DNS lookup during monitoring you determine hosts for a dynamic DNS. The result is similar to 3., except that the the Hostname field is used for DNS query.	during check execution

With the first option, Checkmk uses cached data in order to minimise repeated DNS requests during the activation of the changes — which is very important for accelerating the activation procedure. Furthermore, the cache ensures that a modified configuration can still be activated if the DNS stops working.

The catch is that Checkmk doesn’t automatically notice the change to an address in DNS. For this reason, in the Properties of host page there is the menu item Hosts > Update DNS cache which deletes the entire DNS cache and forces a new resolution the next time you activate the changes. This file is found under ~/var/check_mk/ipaddresses.cache in your site, by the way. Deleting this file has the same effect as running Update DNS cache.