# NetCage

> A per-app firewall for Android. Pick any installed app and cut its internet access — foreground
> and background, Wi-Fi and mobile — with one switch.

NetCage is an Android application, package `com.aoneahsan.netcage`, prepared for release on Google Play, not yet published.
This site is its documentation. It is prerendered, so every page's HTML already contains its full
text — no JavaScript is needed to read it. The Markdown source is public at
https://github.com/aoneahsan/netcage-docs under `docs/`, where the page at `/<name>` is
`docs/<name>.md`.

## How it works, in one paragraph

Android does not let an ordinary app revoke another app's `INTERNET` permission or write firewall
rules. It does allow a `VpnService` with app-based routing. NetCage builds a VPN tunnel that goes
nowhere: a private address, a default route, and `addAllowedApplication()` for caged apps only.
Packets that arrive are refused with a TCP reset or an ICMP port-unreachable, built from the
packet's own header, so a caged app reports no internet immediately instead of timing out. Only that
header is read — never the packet contents, and never a hostname. Non-caged apps are untouched:
their traffic never enters the process, so there is no latency and no battery cost for them.

## What NetCage is not

- **Not an ad blocker.** It does no domain filtering, no hosts file, no DNS interception, and no
  content blocking of any kind. It blocks an app, entirely or not at all.
- **Not a packet inspector.** Refusing a caged app's connection needs the destination address and
  port at the front of the packet, and nothing else. The contents are never read, no hostname is
  ever resolved, and nothing is logged, forwarded, or transmitted.
- **Not a proxy or a real VPN.** The tunnel has no server on the other end. That is the mechanism.

## Docs

- [Overview](https://netcage-docs.aoneahsan.com/)
- [How it works](https://netcage-docs.aoneahsan.com/how-it-works)
- [Install and setup](https://netcage-docs.aoneahsan.com/install-and-setup)
- [Caging apps](https://netcage-docs.aoneahsan.com/caging-apps)
- [Profiles](https://netcage-docs.aoneahsan.com/profiles)
- [Schedules and day passes](https://netcage-docs.aoneahsan.com/schedules-and-day-passes)
- [Widgets and the Quick Settings tile](https://netcage-docs.aoneahsan.com/widgets-and-tile)
- [Account and sync](https://netcage-docs.aoneahsan.com/account-and-sync)
- [Settings and your data](https://netcage-docs.aoneahsan.com/settings-and-data)
- [Privacy](https://netcage-docs.aoneahsan.com/privacy)
- [Limitations](https://netcage-docs.aoneahsan.com/limitations)
- [FAQ](https://netcage-docs.aoneahsan.com/faq)
- [Changelog](https://netcage-docs.aoneahsan.com/changelog)
- [For agents](https://netcage-docs.aoneahsan.com/for-agents)

## Links

- Product site: https://netcage.aoneahsan.com
- Developer: https://aoneahsan.com