Lifecycle

vpnhotspotd is started lazily by DaemonController when the app sends the first daemon command. The daemon stays alive only while the controller has active calls. When the last call is closed, Kotlin closes the control connection; the Rust control loop then stops all daemon-owned runtime state and exits.

Process Startup

Kotlin locates the native vpnhotspotd library in the APK and runs it through Android's linker from a root command. It creates:

• an abstract Unix-domain server socket name for the control channel;
• stdout and stderr pipes that are drained into Timber;
• one root command invocation that starts the daemon with the socket name.

The Rust entry point accepts exactly one argument: that socket name. It connects back to the abstract Unix socket, splits the stream, starts a writer task for outbound frames, initializes the nonfatal reporter, and builds process-wide bookkeeping:

• one lazy rtnetlink runtime slot;
• one NAT66 ICMP dispatcher shared by NAT66 sessions and bound to the app-owned NFQUEUE number 30000;
• a session map keyed by the start-session call ID;
• one process-wide upstream-interface aggregate for optional IPsec probes;
• one optional neighbour monitor;
• one process-wide flag for the NAT66 firewall base chains.

The rtnetlink runtime is created on the first command that needs netlink or routing state: session start, neighbour monitoring, static-address replacement, or Clean. Commands that do not need netlink, such as traffic-counter reads, do not open the rtnetlink connection. Once created, the runtime remains process-wide until daemon exit.

The daemon does not listen for arbitrary clients. The app-side controller owns the listening socket and accepts only a peer whose Unix socket credentials have uid=0; non-root peers are closed and the controller keeps waiting within the startup timeout. The daemon connects to that single controller.

Calls

The control loop decodes one client envelope at a time and dispatches each non-cancel command into a task tracked by call ID. CancelCommand is handled before dispatch and cancels the active call's cancellation token.

There are two call shapes:

• one-shot calls return one reply or one terminal error;
• event-style calls keep the call ID active, deliver one or more event frames, and finish only when the controller cancels them, the daemon reports an error, the control connection closes, or the daemon sends a completion.

StartSessionCommand and StartNeighbourMonitorCommand are event calls. ReplaceSessionCommand, ReadTrafficCountersCommand, ReplaceStaticAddressesCommand, and CleanRoutingCommand are one-shot calls.

StartSessionCommand sends an event ACK after the session is established, then keeps the call active as the session owner. The session event stream may later carry optional daemon-to-routing requests, such as an IPsec forwarding-policy update request. StartNeighbourMonitorCommand sends an initial neighbour/topology snapshot and then streams updates. The protobuf schema still describes the frames; "event-style call" only describes the controller lifetime shape.

Call IDs are part of lifecycle ownership. A session is stored under the call ID that started it. Closing that event call is the normal request to stop that session.

In these docs, a session means the daemon-owned runtime for one downstream interface named by SessionConfig.downstream. It bundles that interface's DNS proxy listeners, optional NAT66 proxy state, and routing mutations. It is not a client connection and it is not an upstream network.

Session Startup

StartSessionCommand reserves a session slot before doing setup. The daemon rejects a second active session for the same downstream interface. If IPv6 NAT is requested, the process-wide IPv6 NAT firewall base chains are attempted before the session runtime starts. Failure there is reported as a structured nonfatal tied to the start call and IPv6 NAT is disabled for that session start.

Session::start constructs the session in this order:

1. Ensure the process-wide rtnetlink runtime exists, then wait for a downstream IPv4 address through netlink notifications and the caller's cancellation token.
2. Start the DNS runtime bound to that downstream IPv4 address. TCP and UDP listener setup is staged per MAC and protocol as independent best-effort capabilities.
3. Start NAT66 if requested. NAT66 TCP and UDP listener setup is staged per MAC and protocol; RA and ICMP setup remain session-level capabilities. Failures are reported as structured nonfatals tied to the start call. If the initial client set is empty, NAT66 is deferred with no interception. If clients are present and NAT66 produces no commit-ready TCP or UDP listener, the session continues with IPv6 NAT disabled.
4. Start routing using the staged DNS and optional NAT66 capabilities produced by the runtimes. Routing applies each mutation best effort and reports setup failures without rolling back unrelated successful mutations.
5. Publish only capabilities whose daemon resource and routing rule both committed. Staged per-MAC resources whose routing rules failed are cancelled before the ACK. If clients are present and routing commits no NAT66 TCP or UDP capability, NAT66 is stopped and the session is published with IPv6 NAT disabled.

Downstream IPv4 discovery is still required before a session can be established. After that point, DNS, NAT66, and routing setup failures remove only the affected MAC/protocol capability or mutation from the best-effort setup result.

After the session is installed, Rust publishes a session-control handle and sends an event ACK. Read, replace, and stop operations enqueue commands through that handle; the start-session task owns the session runtime and processes those commands in order. When cancelled normally, it removes the control handle from the slot, drains already queued commands, removes the session from daemon state, and stops the session runtimes.

After the ACK, the daemon updates a process-wide aggregate of upstream interface names across all active sessions. On Android 12+, if an interface name enters that aggregate set, the daemon spawns a best-effort probe that runs /system/bin/dumpsys ipsec. When the probe completes, each matching IPv4 tunnel forwarding-policy request is emitted to one currently active session call that still references the target's upstream interface. No-match is quiet; dumpsys or parser failures are structured nonfatals tied to one currently active session call for the probed interfaces, if any. The daemon does not separately supervise a stuck dumpsys process, and it does not track or clean up IPsec policy state; tunnel and policy teardown remain platform-owned.

Session Replacement

ReplaceSessionCommand updates the config for an existing session. The downstream interface is immutable; replacing it is rejected because routing and session ownership are keyed to that interface. Replacement is ordered through the session-control command loop, so it cannot interleave with traffic-counter reads or session stop.

Inside that ordered replacement, the session holds its config mutex as a commit gate for DNS and NAT66 readers:

• routing reconciles from the previous committed config/capability set to the next desired config/capability set;
• NAT66 records replacement state that matters for later cleanup;
• the shared config snapshot is replaced;
• NAT66 is notified after the mutex is released.

This means active DNS/NAT66 work that needs a config snapshot can pause behind replacement, but it cannot observe the next config before routing has committed the matching interception state.

Client changes are MAC-scoped. Replacement stages DNS and NAT66 resources for new MAC/protocol capabilities, reconciles routing, publishes only committed capabilities, and cancels removed or uncommitted per-MAC resources. Before a MAC or counter source is removed, the session exposes its final daemon-owned counters through the next traffic-counter read.

When the next client set is empty, replacement publishes no NAT66 routing capabilities. Existing NAT66 runtime state may stay alive only to preserve session-owned counters and deferred NAT66 eligibility for a later non-empty client set.

If process-wide firewall-base setup failed, NAT66 produced no runtime for a non-empty client set, or routing committed no NAT66 TCP/UDP capability for a non-empty client set, later replacements keep ipv6_nat disabled for that session. An empty client set is not failure; replacement may start NAT66 when a later neighbour snapshot adds a MAC.

After a successful replacement, the same process-wide IPsec upstream aggregate is updated from the session's new upstream interface set. A replacement only triggers an IPsec probe for interface names that enter the aggregate set; client changes, downstream changes, and primary/fallback role changes do not trigger a probe by themselves.

Shutdown And Clean

Normal session stop cancels the session stop token first so DNS and NAT66 listeners normally choose shutdown over reporting teardown-time socket errors. Shutdown does not wait for listener or per-packet tasks to drain before removing routing state. It then stops NAT66, which may withdraw router-advertised prefixes during stop.

When the control connection closes, the daemon cancels active calls, waits for call tasks, stops the neighbour monitor, stops all sessions without extra withdraw-cleanup, clears the IPsec aggregate, removes process-wide IPv6 NAT firewall base state, drops the writer, and exits.

CleanRoutingCommand is stronger than normal shutdown. It:

• drains all session slots from daemon state;
• marks those sessions as cleaning so their start-session tasks do not try to remove the same state again;
• detaches and completes the corresponding event calls;
• stops sessions with withdraw_cleanup = true;
• clears the process-wide IPsec upstream aggregate;
• removes the process-wide NAT66 firewall base chains;
• runs deterministic routing cleanup that reconstructs app-owned state from current kernel/interface state and the prefix seed.

Clean must not depend on private app databases, preferences, or daemon memory. Anything that can outlive the process needs a deterministic cleanup path. Traffic history is not a cleanup input. Per-MAC listeners, redirect rules, TPROXY rules, and the single NAT66 ICMPv6 NFQUEUE rule are removed through normal routing cleanup or deterministic Clean reconstruction.

Neighbour Monitor

The daemon allows one neighbour monitor at a time. Starting a monitor registers single-consumer netlink neighbour and link event slots, sends an initial dump with bridge topology, then streams deltas until the event call is cancelled.

Stopping the monitor drops both netlink registrations and waits for the monitor task. Link events trigger bridge topology snapshots only when the topology actually changes.