bunshin/README.md

分身 bunshin

A Rust implementation of the Smol Mail server: a minimalist, end-to-end encrypted mail protocol over a Noise-secured TCP connection, with an optional Reticulum (RNS) mesh carrier behind the rns build feature. bunshin implements the server side only — receiving, storing and serving sealed mail — not the client.

The server never sees plaintext, sender identities or any private key. It learns only which mailbox an envelope is for, its size, and when it arrived.

The flake's main purpose is turnkey deployment on a NixOS host: import nixosModules.default, point it at a key, and nixos-rebuild switch.

Deploying on NixOS

Add bunshin as a flake input and import the module:

{
  inputs.bunshin.url = "https://code.randogoth.com/randogoth/bunshin";

  outputs = { self, nixpkgs, bunshin, ... }: {
    nixosConfigurations.myhost = nixpkgs.lib.nixosSystem {
      modules = [
        bunshin.nixosModules.default
        {
          services.bunshin = {
            enable = true;
            keyFile = "/var/lib/bunshin/server.key"; # provisioned out of band, see below
            openFirewall = true;
            inviteTokenFile = "/run/secrets/bunshin-invite"; # or inviteToken directly
          };
        }
      ];
    };
  };
}

services.bunshin also takes host, port, dataDir, maxEnvelope, quota, requestsQuota, retentionDays, requestsRetentionDays, maxTokens, rateConnections, rateSends, rateTokens and domains; see flake.nix for defaults. The module renders a systemd unit that runs bunshin serve under DynamicUser; it does not generate a key.

For the RNS carrier, build packages.rns, set services.bunshin.package to it, and enable services.bunshin.rns with its keyFile; see RNS.md for the protocol and the remaining options.

Multiple domains

One process can serve several logical servers, each with its own static key, port and mailbox database. bunshin routes by port — the wire protocol has no domain field, and each domain's clients simply pin user@host → host:port, public key. How a user@domain address resolves to that host:port (published addresses, DNS SRV records, a proxy) is outside the server.

The agreed discovery convention is a DNS SRV record: _smolmail._tcp.<domain>. SRV 0 1 <port> <host>, lowest priority wins, and no record falls back to the default port 1961. SRV is discovery only — a lying record lands the client on a server with the wrong static key and the handshake aborts, so the pin stays out of band. Client-side only: fumi implements the lookup (rev 1468f3c), pinning by host:port with the default port inheriting the bare-host pin and other ports trust-on-first-use; bunshin serves ports and pins keys, and needs nothing for it.

serve --config <file> takes a TOML file: one [defaults] table inherited by every [domains.<name>] table (names are 1-63 bytes of [a-z0-9._-]). Per domain, key and port are required; db defaults to mail-<name>.db, and any of the other settings can be overridden per domain:

[defaults]
host = "127.0.0.1"
quota = 67108864

[domains.example_org]
key = "/var/lib/bunshin/example_org.key"
port = 11961

[domains.example_net]
key = "/var/lib/bunshin/example_net.key"
port = 11962
invite_token_file = "/run/secrets/example_net-invite"

Every knob settable on the command line is settable in either table; unknown settings are rejected, not silently defaulted. --config cannot be combined with the single-domain flags or the --rns-* flags — the RNS carrier is single-domain, so it stays on the legacy flags. All keys are read and all ports bound before any domain serves, so a missing key or a taken port fails the process at startup.

On NixOS, set services.bunshin.domains instead; the module renders the TOML, gives each domain a database under dataDir, opens each domain's port with openFirewall, and loads per-domain invite token files via LoadCredential. The existing flat options keep serving the single-domain case unchanged.

services.bunshin = {
  enable = true;
  domains = {
    example_org.keyFile = "/var/lib/bunshin/example_org.key";
    example_org.port = 11961;
    example_net.keyFile = "/var/lib/bunshin/example_net.key";
    example_net.port = 11962;
    example_net.inviteTokenFile = "/run/keys/example_net-invite";
  };
};

Before the first deploy, generate the server's static key once (from a dev shell or nix run) and place it at the configured keyFile:

nix run "https://code.randogoth.com/randogoth/bunshin" -- keygen --key server.key

keygen writes the server's static X25519 key (used for the Noise handshake, distinct from any user's Ed25519 identity) and prints its public key in base32. Publish that public key through a trusted channel — clients pin it, and a mismatch aborts the handshake.

RNS carrier

The rns feature (built by packages.rns, wired to the module through services.bunshin.rns) serves the same mailbox over the Reticulum Network Stack alongside TCP: bunshin serve --rns runs both carriers in one process against one database. The mesh address is the smolmail.server destination hash — there is no key to pin, the address is the pin.

bunshin rns-keygen --key server.rns.key
bunshin serve --rns --key server.key --rns-key server.rns.key --db mail.db --rns-udp 0.0.0.0:4242

rns-keygen writes the 64-byte Reticulum identity and prints the destination hash; publish smol+rns://<user>@<hash> as the server's address. RNS-specific limits (--rns-max-envelope, --rns-fetch-budget, --rns-max-links, --rns-rate-link-requests, --rns-rate-link-bytes) default to mesh-friendly 32 KiB caps; --rns-link-idle (default 300 s) reaps links whose client went silent, since nothing else times a link out; the UDP interface is the one supported transport interface so far. See RNS.md.

Building

Requires a Rust toolchain (stable, edition 2021) and a C compiler — rusqlite's bundled feature compiles SQLite from source rather than linking a system copy, so no separate SQLite install is needed. The rns feature additionally needs cmake, ninja and a microReticulum checkout pointed at by MICRORETICULUM_SOURCE_DIR — the flake's dev shell provides all of it.

cargo build --release
cargo test

The binary lands at target/release/bunshin. With Nix, skip the toolchain setup entirely: nix build produces the same binary at ./result/bin/bunshin.

Running directly, outside the NixOS module:

bunshin keygen --key server.key
bunshin serve --key server.key --db mail.db --host 0.0.0.0 --port 1961

serve accepts --max-envelope, --quota, --requests-quota, --retention-days, --requests-retention-days, --max-tokens, --invite-token, --rate-connections, --rate-sends and --rate-tokens to control size limits, the mailbox's two quota tiers, their retention, the accept-token cap, registration gating and abuse control. Run bunshin serve --help for defaults.

--verbose (or services.bunshin.verbose in the NixOS module) raises logging to debug level, which adds server-side metrics on both carriers: per-operation timing, request and response sizes, handshake duration, per-session summaries, and RNS link events. The default info level stays quiet on success.

Status

Implements SPEC.md version 1.1 in full: AUTH, RESOLVE, SEND, FETCH, DELETE and REGISTER, including accept tokens and the main/requests tier split, fetch cursors, 32-byte message ids, REGISTER proof of possession, and dual-signed key rotation chains.

With the rns feature, implements the version 1.2 RNS transport: the same six operations over Reticulum Links to an announced smolmail.server destination, with link-bound AUTH/REGISTER signatures, per-link request and byte limits, and a concurrent-link cap. One server process can serve both carriers over one store.

License

Licensed under the Apache License, Version 2.0 (LICENSE). Third-party dependency licenses are listed in THIRD_PARTY_NOTICES.md.