On this page
- Every connection starts with a DNS lookup; a filter can answer "no such server" for ad names.
- A local VPN lets an app see only the lookups, while other traffic goes straight out.
- Only small lookup packets are processed, so speed and battery barely change.
- Ads from the same server as content, and apps with their own DNS, are not covered.
Most explanations of ad blocking jump to technical words. This one starts from what actually happens when an app shows an ad, and ends with why some ads survive.
What a DNS lookup is
Servers on the internet are reached by numeric addresses, but apps use names such as ads.example.com. Before an app can connect, it asks a DNS server "what is the number for this name?", like looking up a phone number from a contact. Nothing else can happen until that answer arrives, which is why it is such a good place to stop an ad.
What a DNS ad blocker does
A DNS ad blocker keeps a list of names that belong to advertising and tracking companies. When an app asks for one of those names, the blocker replies "no such name" (the reply is called NXDOMAIN). The app thinks the ad server does not exist, gives up, and the ad is never downloaded. For all other names the blocker passes the question to a real DNS server and returns the answer.
How a local VPN catches the lookups
An app cannot normally see other apps' traffic. Android has a VPN feature that lets an app route traffic through itself, and ad blockers use it with a trick: the app tells Android to send only DNS traffic to a fake address that lives inside the phone. Every other kind of traffic, such as your video and downloads, goes straight to the internet and never touches the app.
- You tap the shield. Android asks permission for a VPN connection once.
- The app creates a tunnel and tells Android "send DNS to this address".
- An app on your phone asks for a name; the question arrives at the blocker.
- The blocker checks the name and its parent domains against the list.
- Blocked: it answers "no such name". Allowed: it asks a real DNS server and returns the answer.
Why it is fast and light
A lookup is a few hundred bytes. A web page or video is millions. Because only the lookups go through the blocker, the extra work is tiny compared with loading content, so battery and speed barely change. A VPN that carried all your traffic would be a different story.
Why it cannot block every ad
- Same name for content and ads. If a video and its ad come from one server, blocking the name breaks the video. See why YouTube app ads stay.
- Apps with their own DNS. Some apps and browsers use encrypted DNS of their own and skip the phone's settings.
- Empty spaces. The app still reserves room for the ad even if it never arrives.
- Ads inside a web page. A DNS filter cannot see page content, so it cannot hide a banner served from the site's own server. A blocking browser can.
Privacy: who sees what
The filter sees the names apps ask for. A well-built one checks them in memory, keeps only counts, and does not send them anywhere. The DNS server the blocker asks for allowed names sees those names, so the choice of upstream matters. ClearAd uses Cloudflare's 1.1.1.1 by default. Private DNS has the same shape, with the provider you chose seeing the names.
The block list is the product
The list decides how good a blocker is. Too short and ads slip through; too aggressive and apps break. Good lists are curated, updated often and have a way to allow a wrongly blocked name. Licences matter too: some popular lists carry non-commercial or share-alike terms. You can see the sources ClearAd uses in the app under More, Block list sources.
Frequently asked questions
How does DNS ad blocking work?
The blocker answers "no such server" when an app looks up a known ad or tracker name, so the ad never downloads.
Does a DNS ad blocker slow down my phone?
Barely. Only small lookup packets pass through it; your videos and downloads go straight to the internet.
Can DNS blocking hide ads inside web pages?
No. It cannot see page content. Use a browser with ad blocking for that.
Block ads in your apps with ClearAd
No root. 15 free minutes every day. Blocks most ads, not all. Installed from this site as an APK.