What is DNS-based ad blocking?

DNS-based ad blocking works by preventing requests to known ad, tracker, or telemetry domains before they reach the browser. Instead of blocking content in the page itself, the network layer stops those hostnames from resolving normally.

Because the browser never gets a usable address for a blocked domain, requests may fail, time out, or resolve differently depending on the DNS service and filtering rules in use. That makes it possible to infer whether a network is using ad blocking or privacy filtering.

Tool description

This tool checks whether ad blocking appears to be active by probing a control request and several known tracking URLs. It summarizes the result with a table that shows each probe host, its status, and how long the request took.

It is a practical diagnostic tool for understanding whether DNS filtering is affecting a network, browser, or device.

Features

  • Heuristic detection: Compares a control request with known tracker probes
  • Request timing: Shows how long each probe took to help distinguish blocked and reachable hosts
  • Simple status table: Makes it easy to scan the result without digging through raw network details

How it works

The tool sends no-cors fetch requests with a timeout and classifies each probe based on whether it appears reachable, blocked, or timed out. This is an inference method, so the result reflects network behavior rather than a direct ad-blocker API.

Limitations

  • It requires network access to run the probes.
  • The result is heuristic and can vary by DNS provider, browser, or firewall.
  • Some networks may return misleading responses instead of a clean block.