What tools and tests can detect DNS leaks when using Tor and how do you interpret their results?
Executive summary
Popular online DNS-leak checkers — including DNSLeakTest, BrowserLeaks, IPLeak, NordVPN’s test, and many VPN-provider checkers — attempt to discover which DNS servers your device uses by sending multiple crafted queries and reporting the responders (often exposing ISP-owned DNS as a “leak”) [1] [2] [3] [4]. Different sites use different query volumes and techniques (e.g., BrowserLeaks: 50 random names; NordVPN: standard 6 queries or extended 36) and so may return different visible DNS servers; interpreting results requires knowing each test’s method and your setup [2] [4].
1. What the main public tools do and how they differ
DNSLeakTest.com, DNSLeak.org and similar single-page tools present a quick check that reports which DNS resolvers answered the probe, flagging ISP resolvers as leaks [1] [5]. BrowserLeaks’ DNS test generates 50 randomly named domains (25 IPv4-only and 25 IPv6-only) to learn which resolver your browser uses and to catch IPv6-specific leaks [2]. IPLeak’s site combines DNS checks with WebRTC and torrent-IP checks and explains Windows/OS behaviors that can cause DNS traffic to bypass tunnels [3]. NordVPN and many VPN vendors offer two-level tests: a fast “standard” round (6 queries) and an extended multi-round test (36 queries) intended to discover intermittent or hard-to-find resolvers [4]. Comparative roundups (e.g., WhoerIP) highlight tools tailored to different users, from simple consumer checks to advanced anti-detect toolkits [6].
2. How these tests detect a “DNS leak” — the mechanics
These services make DNS queries for domain names they control or random names and observe which DNS servers return answers; if the answering server belongs to your ISP or an unexpected operator instead of your VPN/anonymous resolver, the test calls that a leak [2] [4]. Some tests also detect WebRTC or torrent-IP leaks that can expose your real IP even when DNS looks protected [3] [7]. Vendors’ marketing language frames any non-tunnel resolver as a privacy exposure because DNS queries reveal which hostnames you visit even if content is encrypted [4] [8].
3. Interpreting results — immediate rules of thumb
If a test lists your VPN provider’s DNS servers or an anonymized resolver, that indicates DNS is routed through the expected tunnel; if it lists your ISP or a geographically local resolver tied to your ISP, that indicates a leak [8] [3]. Discrepancies between tests are common: quick tests may miss intermittent leaks that an extended/multi-round test will catch, and IPv6 queries can reveal resolvers not used for IPv4 [2] [4]. Also note that seeing “foreign” IPs isn’t inherently hostile but does show who’s resolving names for you — different vendors emphasize different threat models [6].
4. Why test variability matters — trust the method, not just the label
Different services use different numbers/types of queries; e.g., BrowserLeaks’ large randomized set targets both IP versions to surface edge cases, while NordVPN’s standard test uses just six queries for speed and an extended option for thoroughness [2] [4]. VPN vendor pages often combine diagnostics with product claims; a vendor-run checker may flag issues differently and offer vendor-specific fixes, so cross-checking with independent services is prudent [9] [8].
5. Common causes of leaks and what the tests reveal about them
Misconfigured VPNs, OS-specific behavior (notably Windows’ per-interface DNS handling), enabled IPv6 when VPN lacks IPv6 support, or browser features like DoH/DoT that use separate resolvers can all cause DNS to exit the tunnel — tests surface which resolver answered so you can infer a cause [3] [7] [10]. Intermittent routing or particular applications may trigger leaks only under certain conditions; an extended test increases the chance of detection [4].
6. Practical steps after a positive leak result
Sources recommend cross-checking with multiple leak sites, running extended tests when available, disabling IPv6 if your VPN doesn’t support it, checking VPN “DNS leak protection” settings or using a provider that supplies its own resolvers, and verifying browser features like DNS-over-HTTPS aren’t sending queries outside the tunnel [4] [8] [7]. If tests still show your ISP resolver, the problem may be OS routing or a manual VPN configuration [3].
7. Caveats, vendor agendas, and limits of current reporting
Many test sites are run by or affiliated with VPN vendors and frame results to promote their products; claims that “only X VPN truly prevents DNS leaks” reflect vendor marketing as much as technical reality [8] [9]. Comparative roundups note modern complexities — DoH/DoT, IPv6, and anti-detect tooling — that make one-off tests less definitive and argue for periodic, multi-tool checks [6]. Available sources do not mention a single universally accepted “gold standard” lab test; rather, the field uses pragmatic multi-tool cross-checking (not found in current reporting).
Use multiple independent DNS-leak testers, run extended tests and IPv6-aware checks, and interpret the reported resolver IPs as the key signal: if the responder is your ISP or otherwise unexpected, you have a leak; if it’s your VPN or a privacy-focused resolver, DNS appears contained [2] [3] [4].