Most "content filters" are really DNS filters: the device is told to use a resolver that returns nothing for blocked domains. It works — until the device decides to go somewhere else.

Three ways a DNS filter is walked around

  • Change the DNS server. A phone pointed at 8.8.8.8 simply stops asking the filtered resolver.
  • Turn on encrypted DNS. DoH and DoT hide the lookups, so the filter cannot see or answer them.
  • Open a VPN. The traffic and the DNS both leave the network inside an encrypted tunnel.

Each one is a setting or an app, not a hack. That is why a filter that only sits in the DNS layer is optional from the device's point of view.

What actually holds

The fix is to move enforcement to the network itself: the router redirects every DNS query regardless of what the device asks for, filters web traffic at the proxy layer, and closes the tunnel and encrypted-DNS channels that carry traffic past it. See how to stop VPNs bypassing your filter.