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What is DNS leak?
Avoid leaking IP address of my home DNS server - Information Security Stack Exchange
Firefox [Windows 64bits] leaking DNS (to Google!) when set to use DNS over HTTPS
DNS leak is mainly for when you are using VPN and wanna make sure that there are no leaks.
My personal opinion on this, you wanna a network solution which is easy to have instead of a browser add-on. You have AD-Guard or Pihole, seriously, a kid can install Pi-Hole.
Otherwise, it's like trying to carry water in a sieve.
More on reddit.comOpenVPN DNS leaking
I recall posts a few days ago that other VPN providers were susceptible to this. Report via Feedback Hub, please, and post a link here. It could be an issue with Windows 11 network stack.
More on reddit.comsecurity flaw revealing DNS requests
So I thought as long as I use VPN no one knows what I am doing. Now I hear of DNS leak.
can someone ELI5 it to me? Why does it happen? What can they know if it does?
I did a doileak test and I can only see the server I am connecting to via VPN.
I do use my ISP DNS and I think its built into the router's firmware so I can't change it.
A DNS leak occurs when a DNS request is sent to the ISP DNS server, so the ISP knows which domains the client is accessing. In your case, it's irrelevant.
If you are not using a VPN or proxy, your connection to all internet sites goes thru your ISP anyway, because they are the ones routing internet packets to your computer, they have to know your IP address and they have to know the sites you want to access.
You are probably using Adguard because your goals are to reduce the amount of ads on your devices instead of concealing your navigation habits, so a DNS leak is not an issue.
To avoid your ISP knowing the DNS requests you make, use DNS over TLS or DNS over HTTPS on your internal DNS server. It will encrypt the queries to the upstream DNS server.
It will not change much because if your internet connection does not go thru a VPN server, your ISP will know the sites you accessed. And if your connection goes thru a VPN server, your VPN provider knows what you accessed. And even if you have a self-hosted VPN server running on a VM somewhere, the VM provider knows what you accessed.
So the answer was given by Steffen Ullrich in the second comment : I need to use a VPN for my DNS server so that all its requests are done with another IP than my public one !