A DNS leak occurs when DNS lookups bypass the secure path you intended, for example going to your ISP's resolver while the rest of your traffic uses a VPN. The traffic content is hidden, but the list of domains you look up is not.
Why it matters
Domain names alone reveal a lot about browsing habits, and they let a network operator log, filter or hijack lookups. Leaks defeat the privacy purpose of a VPN. They also cause confusing behavior in corporate setups, where an internal name fails because the query went to a public resolver instead of the company one.
Example
You connect to a VPN, then check which resolver answers your queries. If the resolver belongs to your home ISP rather than the VPN provider, you have a leak:
$ nslookup example.com
Server: 192.168.1.1Here the home router handles the lookup, not the VPN tunnel. Operating systems that send queries to all interfaces in parallel, or IPv6 queries that skip an IPv4-only tunnel, are common causes.
Common mistakes
- Trusting the VPN client indicator without testing.
- Disabling IPv4 handling but leaving IPv6 DNS untouched.
- Letting the browser use its own DoH provider while the system uses the VPN resolver.
Related
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