The focus system only ever filtered this phone's own traffic: /system/etc/hosts
via the system resolver, dns_enforcer and the curfew net layer via OUTPUT. A
tethered second phone's packets are FORWARDed + NAT'd through us on a path none
of that covers, so a second phone on the hotspot browses freely on our mobile
data and defeats focus mode.
Add tether_enforcer.sh: an always-on daemon that acts while focus mode is ON
(at home) or a force-test file is present, converging three levers each tick:
1. disable Android tether offload (settings global tether_offload_disabled=1,
snapshot/restore) so forwarded traffic goes through netfilter;
2. blanket REJECT of the FORWARD chain (iptables + ip6tables, FOCUS_TETHER_BLOCK
pinned at position 1), rebuilt only when tampered via a chain_intact gate so
it does not fork an `iptables -L` every second (that overheated the phone
during curfew-net tuning). Covers WiFi, USB and BT tethering;
3. best-effort `cmd wifi stop-softap` (Android 11+) so the hotspot visibly
flips off.
Reverts offload + tears down FORWARD on transition away from home / on SIGTERM.
Wiring: config.sh flags, focus_ctl.sh (tether-status/start/stop/log/test-on/off),
magisk_service.sh boot autostart, deploy.sh push/cp/restart. README section and
a working subprocess regression test (lib/tests/test_tether_enforcer.sh, 17/17).
Verified on-device (BL9000, Android 13): our REJECT jump sits ABOVE Android's
tetherctrl_FORWARD chain, tether_offload_disabled=1 took effect, and
`cmd wifi stop-softap` reports "Soft AP stopped successfully".
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q6i3cdhKBoUrKxy4rmx6hH
