This app uses the VpnService. Usage of the VpnService is required to change the DNS servers for all types of networks (otherwise it would work for Wifi only), as well as offer advanced security features. No actual VPN connection is established and no data leaves the device via VPN.
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When navigating to a website known by it's name, say example.com, your device ask specific servers - DNS servers - how to address the website. DNS is an old protocol which, except for smaller changes, hasn't been touched since its creation in 1987. Naturally the Internet changed a lot in this time, rendering the protocol outdated in some of its core aspects.

This app tackles one of the bigger problems with DNS: Encryption.
Whilst nearly all traffic on the Internet is encrypted now, DNS requests (i.e. questions for a names address) and the response aren't. This enables attackers to intercept, read and modify your requests.

Nebulo is a DNS changer which implements DNS-over-HTTPs and DNS-over-TLS and DoH3 to safely send your DNS requests to the target server. This way only you and the DNS server are able to read the requests you are sending. 

Core features:
 - Configure the app once and then forget about it. After initial configuration it works fully autonomous
 - No ads and no tracking
 - Custom servers can be used
 - Low battery consumption

This app is open source. The source code can be accessed from within the app.