A DNS leak happens when your DNS lookups bypass your proxy or VPN and go out through your regular internet connection, exposing your real IP address or ISP even while your traffic appears to use the proxy. It's a common but often overlooked privacy gap in proxy and VPN setups.

DNS leaks typically happen due to misconfiguration, an operating system or application defaulting to its own DNS resolver instead of routing lookups through the proxy tunnel. Testing for DNS leaks (using a dedicated leak-test tool) is a standard step when setting up any privacy-sensitive proxy configuration, since a leak can undermine the entire point of using a proxy in the first place.

DNS leaks can also happen with application-level proxy setups, for example when an application sends traffic through a SOCKS5 proxy but still resolves hostnames locally instead of having the proxy resolve them.