hyperhive/nix/host-modules/swarm-ca.nix

# The swarm root CA: the anchor a whole swarm shares, and the issuer of
# each hive's own CA (which is where it gets used — see ./hive-tls.nix).
#
# Why a hierarchy at all: cross-hive trust is hand-pinned today
# (`swarm.peers.<d>.caCert`), so every hive must name every peer — O(n²)
# configuration that a new hive can only join by editing all the others.
# One root makes it O(1): trust the root once and every present *and
# future* peer validates.
#
# Two provisioning modes, ONE structure — what differs is who puts the
# artifacts on disk, never what the artifacts are:
#
#   - autoconfigured (the default while the hive declares no peers): the
#     unit below generates the root here on first boot.
#   - operator-provided (multi-host): the operator installs the root cert
#     — and, on a host that does not hold the root key, the hive CA too —
#     into the state dirs, and this unit does nothing. Splitting a
#     single-host swarm across hosts is then a matter of moving the
#     service dirs and turning `autoConfigure` off, not of a second code
#     path that has to be kept agreeing with the first.
#
# The root KEY is the reason this is a runtime file and not a nix option:
# the store is world-readable and content-addressed, so a key committed
# to a flake is a key *published* to every consumer of that flake. The
# root CERT has the opposite property, and is already distributed at
# build time via `swarm.peers.<d>.caCert`. That asymmetry is what makes
# the layering work at all.
{
  lib,
  config,
  pkgs,
  ...
}:
let
  cfg = config.services.hyperhive.swarm.ca;
  hyperhiveCfg = config.services.hyperhive;

  # The subject CN is a label for a human reading a chain, not an
  # identity anything authenticates against. Fall through swarm name →
  # hive domain → a constant so a hive that has set neither still
  # evaluates; a missing `domain` is reported by its own assertion in
  # hive-network.nix, and shouldn't also surface here as a null.
  swarmLabel =
    if hyperhiveCfg.swarm.name != null then
      hyperhiveCfg.swarm.name
    else if hyperhiveCfg.domain != null then
      hyperhiveCfg.domain
    else
      "hyperhive";
in
{
  options.services.hyperhive.swarm.ca = {
    autoConfigure = lib.mkOption {
      type = lib.types.bool;
      default = hyperhiveCfg.swarm.peers == { };
      defaultText = lib.literalExpression "services.hyperhive.swarm.peers == { }";
      example = false;
      description = ''
        Generate the swarm root CA on this host when it is missing.

        Defaults to true exactly while this hive declares no peers —
        an all-on-one-host swarm has no cross-hive trust to establish,
        so it should cost no configuration. Declaring a peer turns it
        off, because a hive that has peers is by definition not the
        only place a root could come from, and a second independently
        generated root is not a swarm root at all. Set it explicitly
        to `true` on the host that does hold the root, or to `false`
        to require the operator to provide it.

        Turning this off does not disable anything else: the root is
        read from the same `stateDir` either way.
      '';
    };

    stateDir = lib.mkOption {
      type = lib.types.str;
      default = "/var/lib/swarm-ca";
      description = ''
        Host directory holding the swarm root CA: `root.pem` (the
        anchor, safe to distribute — this is what a peer hive is
        pointed at via `swarm.peers.<d>.caCert`) and `root-key.pem`
        (0600, the one file that must never reach the nix store or
        another host). The directory itself is 0700: nothing reads
        out of it but the hive CA issuance in `hive-tls.nix`.

        Moving the swarm CA to its own host is a matter of moving this
        directory and setting `autoConfigure = false` here.
      '';
    };

    validityDays = lib.mkOption {
      type = lib.types.int;
      default = 10950;
      description = ''
        Validity window of the swarm root CA in days (default ~30y).
        Deliberately longer than `services.hyperhive.tls.caValidityDays`:
        the root must outlive the hive CAs it issues, or those chains
        expire out from under hives that are still perfectly happy with
        their own intermediate. Rotating a root is the one operation in
        this system with no partial-failure mode — it invalidates every
        peer at once, paced by the slowest peer's rebuild — so it is
        never automatic and this window is meant to be uneventful.
      '';
    };
  };

  config = lib.mkIf (hyperhiveCfg.enable && cfg.autoConfigure) {
    systemd.services.swarm-ca = {
      description = "Generate the swarm root CA when absent";
      wantedBy = [ "multi-user.target" ];
      path = [ pkgs.openssl ];
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
        UMask = "0077";
        # Pin the journal identity (else it's the `script` store-path wrapper).
        SyslogIdentifier = "swarm-ca";
      };
      script = ''
        set -euo pipefail
        d=${lib.escapeShellArg cfg.stateDir}
        install -d -m 0700 "$d"

        root="$d/root.pem"
        rootk="$d/root-key.pem"

        # Note the asymmetry with the hive CA in hive-tls.nix, which
        # regenerates itself once expired: a root is never replaced
        # automatically, not even an expired one. Consumers hold this
        # cert, so replacing it is a swarm-wide flag day that wants an
        # operator running it deliberately, with both roots trusted
        # across the overlap.
        if [ -s "$root" ] && [ -s "$rootk" ]; then
          echo "swarm root CA already present at $root — leaving it alone"
          exit 0
        fi

        # Half a root is not a root. Generating a fresh key beside an
        # already-distributed cert (or the reverse) leaves every
        # consumer trusting an anchor that no longer signs anything —
        # and it would look like it worked.
        if [ -e "$root" ] || [ -e "$rootk" ]; then
          echo "swarm root CA half-provisioned ($root / $rootk) — refusing to generate over it" >&2
          exit 1
        fi

        echo "generating swarm root CA at $root"
        # pathlen:1 — the root signs hive CAs, which sign leaves. One
        # intermediate below the root and no deeper.
        openssl req -x509 -newkey rsa:4096 -nodes -sha256 \
          -days ${toString cfg.validityDays} \
          -keyout "$rootk" -out "$root" \
          -subj "/CN=swarm-ca ${swarmLabel}" \
          -addext "basicConstraints=critical,CA:TRUE,pathlen:1" \
          -addext "keyUsage=critical,keyCertSign,cRLSign"
        chmod 0600 "$rootk"
        chmod 0644 "$root"
      '';
    };
  };
}