vpnhide/docs/diagnostics.md

Diagnostics — how VPN Hide self-tests hiding and attributes the result

This is the self-diagnosis model: how the app checks that hiding actually works for its own process and honestly reports who hid the VPN on each vector. For the hiding side (which backend covers which detection vector) see detection-vectors.md; for the app↔backend wire see protocol.md.

Devices this was validated on: Pixel 4a (sunfish, Magisk, 4.14, kmod/KPM/Zygisk), Pixel 8 Pro (husky, KernelSU-Next, GKI 6.1, KPM), and an Android 13 Zygisk device.

1. The problem it solves

A check result used to be a single tri-state boolean that conflated three unrelated meanings of "pass": (a) the backend hid the VPN, (b) SELinux denied the probe (EACCES), (c) there was nothing to leak on that surface. Counting all "passes" as backend wins made an installed-but-inactive backend read as "Partial" (its SELinux-blocked reads counted as passes) and a mostly-working Java layer read as "Not working" (any one failing probe painted the whole layer red).

2. Root-differential — the reliable classifier

A clean probe result is ambiguous on its own: hook-suppressed and nothing-to-leak look identical. Root (uid 0) is not a hook target, so a privileged read is the ground truth for "what is actually on this surface". The app runs each native probe twice — in-process (its own uid + SELinux domain + hooks) and as root — and diffs them:

root sees	app read	→ outcome
nothing	—	NothingToLeak (empty ground truth wins, even over EACCES)
VPN	EACCES	HiddenBySelinux
VPN	ok, clean	HiddenByBackend
—	app saw VPN	Leak

Priority: an empty ground truth is checked before EACCES — if root sees nothing, the SELinux block is moot, it is simply nothing-to-leak.

Ground truth is the same Rust probe binary run as root, not shell ip/cat. GroundTruthProbe extracts vhprobe from the APK, stages it to /data/local/tmp, and execs it via su; it emits the same JSON as the in-process JNI path (run_all_json), so the two views are directly comparable per check id. (This replaced an earlier gobley/UniFFI binding — the whole native surface is now one JSON-returning function built with plain cargo-ndk.)

3. Per-check outcome (CheckOutcome)

Leak · HiddenByBackend · HiddenBySelinux · NothingToLeak · NotMeasured(reason). Wire/log tokens: leak, hidden_backend, hidden_selinux, nothing_to_leak, not_measured_no_network, not_measured_no_ground_truth.

The Rust probe reports Pass / Fail / SelinuxBlocked (EACCES/EPERM, no longer folded into Pass) / NetworkBlocked (ECONNREFUSED from socket() — no network permission). Native checks classify via the root differential above. Java checks have no root differential (framework IPC), so they are binary — clean ⟹ HiddenByBackend, dirty ⟹ Leak — which is honest only because the self-in-tunnel gate (§5) guarantees a VPN artifact was present to hide.

4. Layer status & verdict (dashboard tiles)

Each dashboard tile is a LayerStatus: Absent (no module installed) · Inactive (installed, not loaded this boot) · Active(hidden, leaks). Presence is decided before the checks, so an unloaded backend can never render a verdict — it just reads "not active" (this is the type-level fix for the old "Partial"). An Active tile's verdict:

• leaks == 0 → Ok
• hidden > 0 && leaks > 0 → Partial (hides some, an owned vector still leaks)
• hidden == 0 && leaks > 0 → Broken (loaded but suppressed nothing)

hidden must be a measurement (the root differential), never inferred from a clean probe — otherwise Partial and Broken are indistinguishable. The native tile is judged only on vectors the active backend owns (has a hook for): a leak on a not-owned vector (e.g. /proc/net/dev under a kernel backend — no kernel hook exists) does not turn the tile red; it surfaces as a hero warning instead. So the tile answers "is this module doing its job" and the hero answers "is the VPN hidden at all". The Java tile uses the same rollup; LSPosed owns every Java check, so all its leaks count.

5. Self-in-tunnel gate

Diagnostics are meaningless if VPN Hide itself is not routed through the VPN: split- tunnelled out, there is no VPN artifact for its own probes to be hidden from, so every check would read misleadingly clean. Before running any checks the app asks vhprobe --uid <selfUid> (root, hook-inert) whether its own uid is routed through the VPN. Two passes over the policy rules (both address families): learn the VPN egress table id(s) from rules that egress via a VPN interface (oif tun*), then check whether a uidrange rule steers this uid into exactly that table. This is stricter than the broad netlink_getrule diagnostic predicate — every uid sits in some per-network table (wlan/rmnet, also non-standard), so it must pin the VPN table specifically. If not routed, the UI shows an "add VPN Hide to your tunnel" prompt instead of clean results. A null answer (no root) does not block.

6. Empirical facts that shape the checks

• SELinux carries part of "protection", and it is invisible without the differential. On both test kernels, 8 of 13 native probes "pass" under enforcing only because SELinux denies the read. Three of those — /proc/net/if_inet6, /proc/net/dev, /sys/class/net — have no backend coverage at all under a kernel backend (no kernel hook exists for those procfs/sysfs paths, by design); on a permissive device they leak tun0. This is why the permissive-SELinux warning exists and why SELinux attribution is dev-facing, not a user alarm.
• The VPN lives in protected sockets + per-UID policy tables, not the main route table. A split-tunnel VPN app marks its sockets and installs ip rule … uidrange <uid> lookup tun0; it does not put a default route in the main table. So /proc/net/route (main table only) shows no VPN for the target and resolves to NothingToLeak — while RTM_GETRULE (policy rules) is the real routing detection vector, added as a probe mirroring the fib_nl_fill_rule kernel filter.
• Some checks never fire on any config: both /proc/net/route reads (native + Java) and the removed system-proxy check. They only added false confidence; the route reads are kept because the differential now labels them NothingToLeak honestly, the proxy check was dropped.
• Suppression counters can distinguish "hook not loaded" from "hook not working" (a per-hook Δ>0 during a probe is proof the hook did real work). They are not used by diagnostics — the root differential already gives the full 4-way + hidden without them — and stay only in the Statistics tab.

7. Native check → owning hook (KPM/kmod), verified on Pixel 4a

The kernel backend's 10 hooks map to the diagnostic checks below; the "gaps" rows are SELinux/zygisk territory by design, not bugs. Full hiding matrix in detection-vectors.md.

check id	probes	kernel hook	notes
ioctl_flags, ioctl_mtu	SIOCGIF* by name	dev_ioctl	ENODEV for tun0
ioctl_conf	SIOCGIFCONF	sock_ioctl	tun0 absent from ifconf
getifaddrs, netlink_getlink	RTM_GETLINK / getifaddrs	rtnl_fill_ifinfo, inet*_fill_ifaddr
netlink_getroute	RTM_GETROUTE v4/v6	fib_dump_info, rt6_fill_node
netlink_getrule	RTM_GETRULE policy rules	fib_nl_fill_rule	v4+v6; kernel-only vector
proc_route	/proc/net/route	fib_route_seq_show	main table — empty for split-tunnel VPN
proc_ipv6_route	/proc/net/ipv6_route	ipv6_route_seq_show
proc_if_inet6	/proc/net/if_inet6	(none)	no kernel seq_show hook — zygisk openat or SELinux only
proc_dev	/proc/net/dev	(none)	zygisk openat or SELinux only
sys_class_net	/sys/class/net	(none)	SELinux only

Java-level checks (LSPosed) cover the framework side — hasTransport(VPN), NET_CAPABILITY_NOT_VPN, VpnTransportInfo, getAllNetworks, LinkProperties, getNetworkForType(TYPE_VPN), the push NetworkCallback (issue #70), and the legacy getActiveNetworkInfo / getNetworkInfo(TYPE_VPN) APIs.