tsunagi/README.md

# tsunagi

A proof-of-concept Rust library for **small private mesh networks** — a handful
of friends, home machines, a few servers. Units to dozens of participants, not
thousands.

The end user configures exactly two things:

```text
network_name
secret          # one shared secret; "password" and "secret" mean the same value
```

From those, every agent independently derives the same network space. There is
no central server, no network owner with special powers, no registration and no
majority vote. Anyone who knows the parameters can join; nobody has to trust
anybody else.

## What this proof of concept actually does

A working library with **real iroh connections** and integration tests:

- persistent device identity stored in SQLite, stable across restarts;
- several independent networks at once in one agent;
- deterministic network identity derived from name + secret;
- candidates supplied by a replaceable discovery component;
- real iroh connections plus an explicit mutual proof of network membership;
- a small versioned control protocol: handshake, hostname/capability
  announcement, ping/pong;
- automatic reconnect with bounded exponential backoff and jitter;
- status snapshots, an event stream and honest diagnostics;
- configuration restored after a restart;
- correct behaviour when the disposable cache is missing or corrupt;
- a **WireGuard data plane**, in userspace: its own key per network, an IPv6
  overlay with deterministically derived addresses and optional IPv4, real
  tunnels carried over iroh, and address ownership enforced rather than
  believed;
- a **command line agent**, `tsunagi`, with a local control socket.

### What it deliberately does **not** do

Not implemented, and not pretended to be: Mainline DHT, DNS, routing through
intermediate participants, a full CRDT, dynamically loaded plugins, a system
service, a complete CLI, or a local control socket. Snapshot synchronisation
and signed revocations are designed for but not implemented — see
[docs/sync-model.md](docs/sync-model.md). The WireGuard plugin's own limits,
including that its system backend is Linux-only, are in
[docs/wireguard.md](docs/wireguard.md#limits-and-future-work).

**Control and data are separated logically, not physically.** Both ride on
iroh, on different ALPNs and different connections, so the data plane inherits
iroh's hole punching and relay fallback instead of reimplementing them — while
the control protocol still knows nothing about packets and can keep a different
transport underneath it later. Filtering user traffic remains the operating
system's and the user's responsibility, not this library's.

## Requirements

- Rust 1.91 or newer (iroh 1.2 requires it), edition 2024. Pinned dependencies
  in `Cargo.lock`.
- No internet, no DHT, no public relay, no administrator rights and no changes
  to OS network settings are needed to build or test.
- WireGuard runs in userspace (boringtun): **no kernel module and no `wg`
  tool**. Only creating a real network interface needs `CAP_NET_ADMIN`, and
  `--no-tun` skips even that.

## Trying it on two machines

On the first machine:

```bash
cargo build --release
./target/release/tsunagi secret          # prints tsn1...; share it privately
./target/release/tsunagi doctor          # what this host can and cannot do

./target/release/tsunagi up --network lab --secret "$SECRET" --wireguard
```

It prints its endpoint id and then waits. On the second machine, pass that id:

```bash
./target/release/tsunagi up --network lab --secret "$SECRET" --wireguard \
  --peer <endpoint-id-from-the-first-machine>
```

Within a few seconds both print something like:

```text
  + peer b47c958462 connected over Direct rtt=Some(4.5ms)
  + data link to b47c958462 for wireguard: Direct via Ip(…), datagram 1382

--- status ---
control: 1 peer(s), 0 dial failure(s), 0 handshake failure(s)
wireguard: tsunkkcp43lmdje on fd15:1d9e:fa21:f201:…/64 mtu 1280, 1/1 tunnel(s) established
  4jO4kx9Z fd15:1d9e:fa21:f201:… handshake 3s ago tx=0 rx=0 dropped=0 path=Direct via Ip(…)
```

`1/1 tunnel(s) established` means a real WireGuard handshake completed.

## Checking that it works

From another shell on either machine:

```bash
tsunagi status
```

```text
endpoint  7d76ccbbc21bf30767e14422c0494740a2cecc02aa9f82d5b8d57bdae350e7fc
hostname  tsunagi-7d76ccbbc2
bound     0.0.0.0:41641

network lab (z2o4qwrvnj3zb6st2aoqg4abf342j662q2ujttqrsmz22argk2ba)  active
  peer b345d5271b  tsunagi-b345d5271b  Direct  rtt 24ms
  overlay tsunz2o4qwrvnj3 fd09:…:c1c6/64 and 100.110.49.177 mtu 1280  1/1 tunnel(s) up
    SDsEb/WF  fd09:…:c4b / 100.65.243.53  handshake 4s ago  tx 0 rx 0  Direct via Ip(…)
```

`1/1 tunnel(s) up` and a recent handshake mean the tunnel is live. Then send
real traffic to the peer's overlay address:

```bash
ping6 fd09:…:c4b        # or
ping  100.65.243.53
```

`tx` and `rx` in the status should start moving.

IPv6 works out of the box: each member's address is derived from the network
id and collides with essentially nothing.

**IPv4 addresses are allocated and then remembered.** The default range is
`10.13.37.0/24`; the first member to join settles it and later members adopt
what they find, so `--ipv4-range` only matters for whoever starts the network:

```bash
tsunagi up --network lab --secret "$SECRET" --wireguard --ipv4-range 10.44.0.0/16
tsunagi up --network lab --secret "$SECRET" --wireguard --ipv4-range none   # IPv6 only
```

An address is claimed with a record signed by that member's persistent device
key, stored, and merged between every replica. A member that disappears for a
month comes back to the same address, because the claim outlived the session.
No vote is involved — see
[docs/wireguard.md](docs/wireguard.md#ipv4-allocated-signed-and-kept) and
[docs/sync-model.md](docs/sync-model.md).

Because the address is allocated at run time rather than derived, it is not
known until the agent has started and agreed with its peers. The agent then
assigns it to the interface itself.

## Privileges

On Linux the agent **manages its own overlay interface**. It creates the TUN
interface, sets the MTU, brings it up and assigns both overlay addresses, all
over netlink in process — no `ip` invocation, no shell, nothing that a
remote peer could influence.

That needs `CAP_NET_ADMIN`, granted once:

```bash
sudo setcap cap_net_admin+p /usr/local/bin/tsunagi
```

`+p` rather than `+ep`: the capability is then *permitted* but not
*effective*, and the agent raises it only around the handful of netlink calls
that need it — a few milliseconds at startup, and again if its address
allocation changes. Everything else, including every byte from the network,
is handled with it lowered. `+ep` works too; the agent lowers it on the way
in.

`tsunagi doctor` says which of these applies on the host it runs on.

### It cleans up after itself

The interface is tied to an open file descriptor and is deliberately **not**
made persistent, so the kernel removes it when the agent exits — on a clean
shutdown, on a panic, on `SIGKILL`, on power loss alike. Keeping it is what
would take an action; removing it is the default.

If something is left behind anyway — an interface made by an older version's
manual recipe, or one from a run killed in the instant between creating it and
recording it — the next start **replaces it**, along with any stale addresses
it carried. Two things are never touched:

* an interface that is not a TUN, because the name colliding with somebody's
  bridge is not a reason to destroy the bridge;
* a TUN that another process is holding open, because that is a working
  overlay belonging to somebody else — most likely a second agent on this
  host, which should be given a different `--wg-prefix`.

Both of those refuse with an explanation rather than guessing.

Two settings the manual recipe used to need are gone with it.
`keep_addr_on_down` existed only because an interface nobody held open lost
carrier and had its IPv6 addresses flushed, and `nodad` only because duplicate
address detection can never finish without carrier. An interface held open for
its whole life has carrier for its whole life.

### The MTU is 1280

That is the minimum IPv6 requires (RFC 8200), and Linux enforces it by
disabling IPv6 outright on an interface below it — the per-device
`/proc/sys/net/ipv6` entries vanish and adding an address fails with
`Invalid argument`. A smaller MTU cannot work at all, so the agent refuses one
rather than letting it fail later. See
[docs/wireguard.md](docs/wireguard.md#mtu) for the ceiling that pushes back
from the other side.

### Summary

| approach | agent runs as | notes |
|---|---|---|
| `setcap cap_net_admin+p` | ordinary user, one capability | recommended: nothing to prepare, nothing left behind. Lost on every rebuild or copy of the binary. |
| systemd service | `User=`, `AmbientCapabilities=CAP_NET_ADMIN` | the same, for an installed service |
| `sudo tsunagi up` | root | everything works, nothing is isolated |
| `--no-tun` | ordinary user, no capabilities | tunnels run and handshake, traffic never reaches the OS |

**Not implemented yet.** macOS and Windows have no provisioner: both need
real platform work — `utun` and `SystemConfiguration` on one, the IP Helper
API and a Wintun adapter on the other. There the agent says so and `--no-tun`
is the way to run it; the control plane and the tunnels are unaffected. The
decision logic that says *what* to change is shared and tested on every
platform, so only the execution is left to write.

## Checks

```bash
cargo fmt --all -- --check
cargo clippy --locked --workspace --all-targets -- -D warnings
cargo test --locked --workspace --all-targets
```

The whole suite runs offline on loopback. Set `TSUNAGI_TEST_LOG=tsunagi=debug`
to see agent logs while a test runs.

There are also two runnable demos, which are demos and not substitutes for the
tests:

```bash
cargo run --example two_agents       # control plane only
cargo run --example wireguard_mesh   # a WireGuard overlay carrying a real packet
```

Both run with no privileges and change nothing on the host.

## Usage

```rust,no_run
use std::sync::Arc;
use tsunagi::config::{AgentConfig, StoragePaths, TransportPolicy};
use tsunagi::discovery::SharedMemoryDiscovery;
use tsunagi::identity::{NetworkName, NetworkSecret};
use tsunagi::proto::ControlMessage;
use tsunagi::{Agent, Result};

// The library never starts a runtime, installs a logger, handles signals,
// forks, or calls process::exit. The binary owns all of that.
#[tokio::main]
async fn main() -> Result<()> {
    let config = AgentConfig::new(StoragePaths::user_default()?)
        .with_transport(TransportPolicy::N0Defaults)
        .with_discovery(Arc::new(SharedMemoryDiscovery::new()));

    let agent = Agent::spawn(config).await?;

    let name = NetworkName::new("kitchen-table")?;
    let secret = NetworkSecret::generate();       // 32 random bytes
    println!("share this: {}", secret.encode().as_str());

    let network = agent.join_network(&name, &secret).await?;

    let mut events = agent.subscribe();
    tokio::spawn(async move {
        while let Ok(event) = events.recv().await {
            println!("{event:?}");
        }
    });

    for peer in agent.network_status(network).await?.connected_peers() {
        agent
            .send(network, peer, ControlMessage::Ping { seq: 1, payload: vec![] })
            .await?;
    }

    agent.shutdown().await;
    Ok(())
}
```

`TransportPolicy::LocalOnly` is the default, so a plain `AgentConfig::new` never
reaches the internet by accident. Opt into `DirectOnly` or `N0Defaults`
explicitly.

## How peers find each other

Two different lookups are involved, and only one of them is this project's:

**1. Resolving one endpoint's address — iroh's, and it works today.**
With `--transport relay` or `--transport direct`, iroh publishes a signed
record of this endpoint's addresses, keyed by its endpoint id, to the public
service run by Number 0 — "n0", the company behind iroh — at `dns.iroh.link`,
over pkarr and DNS, and resolves other endpoints the same way. That is why `--peer <endpoint-id>` works with no address attached:
iroh looks it up. None of that code is ours.

**2. Finding who is in a network — ours, and it is still manual.**
`NetworkDiscovery` maps a secret-derived `DiscoveryKey` to a set of *candidate*
members. Two backends exist: `StaticBootstrap` (what `--peer` feeds) and an
in-memory one for tests. The planned Mainline DHT backend, which would let
members find each other from the network secret alone, is **not implemented**.
So today you bootstrap by passing one peer's id; after that the mesh is
whatever those agents reach.

What this means in practice:

- With `relay` or `direct`, **your endpoint id and IP addresses are published
  to a public third-party service** (Number 0's, unless you change it). They are not secret, and the network secret is
  never published, but an observer of that service learns that your endpoint
  exists and where it is. `--transport local` publishes nothing.
- A relay, when one is needed, sees the volume and timing of your traffic — not
  its contents. The default relays are Number 0's, in the US, EU and
  Asia-Pacific.

## Storage

Two physically separate SQLite files, placed wherever the library's
configuration says (`StoragePaths`). A future system service supplies its own
paths; tests always use temporary directories.

| file           | holds                                            | when damaged            |
|----------------|--------------------------------------------------|-------------------------|
| `state.sqlite` | device identity, network configuration, hostname | clear error, never reset |
| `cache.sqlite` | address hints and other recoverable data          | discarded and recreated  |

The WireGuard plugin keeps its own keys in its own `wireguard.sqlite`, wherever
its configuration points, because plugin keys are neither the iroh identity nor
the network secret.

The command line agent puts everything under the platform's per-user
directories by default; `--state-dir` and `--cache-dir` override them.

One state directory belongs to one live agent, enforced with a real OS file
lock rather than an existence check.

## Documentation

- [docs/architecture.md](docs/architecture.md) — module boundaries and runtime.
- [docs/wireguard.md](docs/wireguard.md) — the WireGuard plugin: overlay
  addressing, announcements, backends, reconciliation.
- [docs/protocol.md](docs/protocol.md) — identity derivation, framing, handshake.
- [docs/sync-model.md](docs/sync-model.md) — the planned signed-state model and
  what is deliberately not built yet.
- [docs/threat-model.md](docs/threat-model.md) — threat model and known limits.
- [docs/testing.md](docs/testing.md) — what the suite covers and what it does not.
- [AGENTS.md](AGENTS.md) — rules for anyone (human or agent) changing this repo.

## Security in one paragraph

Membership is proved by an HMAC over a transcript keyed by a value derived from
the shared secret, bound to the specific iroh connection through the TLS
exporter, to the network id, to both endpoint identities and to distinct role
labels. This targets high-entropy secrets: there is no PAKE here, so a short
human passphrase is guessable offline by anyone who can reach the handshake.
Anyone who knows the secret is a full participant and can create many
identities. Read [docs/threat-model.md](docs/threat-model.md) before relying on
any of this.

## Licence

MIT OR Apache-2.0.