The Domain Name Server (DNS) is responsible for resolving domains into the actual IP addresses to connect. When using a secure connection like a VPN tunnel , a DNS leak error occurs when DNS requests are sent over a normal (unencrypted) network instead of a secure tunnel. There are several ways to check for DNS leaks. While there are many VPN services that offer their own tools, one of the best ways is to use dnsleaktest.com . The tool is very simple and performs a series of tests that can help reveal leaks that don't appear all the time.