In order for Pingdom's probe servers to make requests to a specific server, an IP address is needed. Most servers are identified by a hostname, for example example.com, this address is unfortunately not routable on the Internet - an IP address needs to be looked up first.
This is done by making a DNS lookup to the name server example.com.
Our probes used to store these lookups internally, or used a central cache we previously had. This made for some really fast lookup times, but it could take a while before the probe servers detected changes to these DNS records.
We eventually changed the way these lookups are made: each request our probes make will include a regular DNS lookup without caching from now on. This will increase the response times somewhat for some checks, but it's now reflecting more truly what a new request to your URL would look like. Changes to DNS records will be detected by the probes faster and issues with DNS lookups will be alerted on quicker as well.
We also provide a specific DNS check which make lookups for you URL to a specific name server to see if the correct IP address is returned.