Mozilla: What is a DNS Leak?

DNS leaks usually happen when a VPN is programmed incorrectly or if the VPN service you’re using is simply unreliable. Sometimes a VPN you’re using, like a free VPN provider for example, may not have its own DNS servers at all and use untrustworthy ones that you might not be aware of. Faulty VPNs can also leak your DNS queries by routing them outside of the protection of the VPN.