{pkgs, ...}: let
  # Caddy's canonical certificate storage (XDG data dir). Renewals land here,
  # owned caddy:caddy 0600 — stalwart cannot read it directly, so we copy it
  # into a distribution dir stalwart's service (user "stalwart") can reach.
  caddyCertDir = "/var/lib/caddy/.local/share/caddy/certificates/acme-v02.api.letsencrypt.org-directory/mail.severijnse.eu";
  # Distribution dir read by stalwart (cert 0644, private key regranted to the
  # "stalwart" group by systemd.services.stalwart-cert-perm).
  distCertDir = "/var/lib/caddy/certificates/acme-v02.api.letsencrypt.org-directory/mail.severijnse.eu";
  zoneFile = "/var/lib/coredns/zones/severijnse.eu.db";
  # Records the SPKI hash applied at the last restart of stalwart, so cert
  # renewals trigger exactly one restart and unchanged certs never do.
  stateFile = "/var/lib/tlsa-update/.last-spki";

  syncScript = pkgs.writeShellScript "tlsa-update" ''
    set -euo pipefail
    OPENSSL="${pkgs.openssl}/bin/openssl"

    SRC_CERT="${caddyCertDir}/mail.severijnse.eu.crt"
    SRC_KEY="${caddyCertDir}/mail.severijnse.eu.key"
    DST_CERT="${distCertDir}/mail.severijnse.eu.crt"
    DST_KEY="${distCertDir}/mail.severijnse.eu.key"

    # Caddy has not obtained the certificate yet (e.g. first boot before HTTP-01).
    if [ ! -f "$SRC_CERT" ] || [ ! -f "$SRC_KEY" ]; then
      echo "tlsa-update: certificate not found at $caddyCertDir, skipping" >&2
      exit 0
    fi

    # 1) Propagate Caddy's renewed certificate into the distribution dir stalwart
    #    reads. Caddy stores certs 0600 caddy:caddy. The cert is world-readable;
    #    the private key is restricted to root (0640) and stalwart-cert-perm
    #    regrants it to the "stalwart" group so it is not exposed to other local
    #    users.
    install -D -m 0644 "$SRC_CERT" "$DST_CERT"
    install -D -m 0640 "$SRC_KEY"  "$DST_KEY"

    # 2) TLSA 3 1 1 = SHA-256 of the certificate's SubjectPublicKeyInfo (SPKI),
    #    NOT the whole certificate. Matching type 1 = SHA-256 of the SPKI DER.
    HEX=$("$OPENSSL" x509 -in "$DST_CERT" -noout -pubkey 2>/dev/null | "$OPENSSL" pkey -pubin -outform DER 2>/dev/null | "$OPENSSL" dgst -sha256 | cut -d' ' -f2)

    # 3) Update the CoreDNS zone. coredns.service regenerates this file from the
    #    Nix store on every start, so this unit (which is partOf coredns and runs
    #    after it) re-applies the correct TLSA after each rebuild/restart.
    if [ -f "${zoneFile}" ]; then
      sed -i -E "s/^(_25\._tcp\.mail.*TLSA 3 1 1).*/\1 $HEX/" "${zoneFile}"
      sed -i -E "s/^(_465\._tcp\.mail.*TLSA 3 1 1).*/\1 $HEX/" "${zoneFile}"
      sed -i -E "s/^(_993\._tcp\.mail.*TLSA 3 1 1).*/\1 $HEX/" "${zoneFile}"
      chown coredns:coredns "${zoneFile}"
      chmod 0640 "${zoneFile}"
    fi

    # 4) Reload services so the changes take effect immediately.
    systemctl reload coredns.service || true
    # Stalwart reads its TLS certs ($certDir) via %{file:...}% placeholders at
    # startup only; there is no signal-based reload (management API reload needs
    # admin credentials we must not store). Restart it, but only when the cert
    # actually changed (SPKI hash differs from the last applied run), so the
    # daily timer and inotify events for unchanged certs do not drop connections.
    # NOTE: stalwart.service `requires` this unit, so the restart must be issued
    # with --no-block (async): a synchronous restart waits for stalwart to come
    # back up, which in turn waits for this unit to finish — a deadlock. The state
    # file is updated BEFORE the restart so the tlsa-update run that stalwart's
    # required-activation re-triggers sees a matching hash and exits immediately.
    if [ "$(cat "${stateFile}" 2>/dev/null || true)" != "$HEX" ]; then
      printf '%s\n' "$HEX" > "${stateFile}"
      # Restarting stalwart re-runs its Requires=tlsa-update dependency; guard
      # with is-active so a boot-time run never races stalwart's initial start.
      if systemctl is-active --quiet stalwart.service 2>/dev/null; then
        systemctl --no-block restart stalwart.service || true
      fi
    fi

    echo "tlsa-update: TLSA set to $HEX"
  '';
in {
  systemd = {
    # Ensure the distribution dir exists (Caddy does not write here).
    tmpfiles.rules = [
      "d ${distCertDir} 0755 root root - -"
      "d /var/lib/tlsa-update 0755 root root - -"
    ];

    services.tlsa-update = {
      description = "Sync Caddy TLS certificate to mail server and update DANE/TLSA records";
      after = ["caddy.service" "coredns.service"];
      partOf = ["coredns.service"];
      wantedBy = ["multi-user.target"];
      path = with pkgs; [openssl coreutils gnused systemd];
      serviceConfig = {
        Type = "oneshot";
        ExecStart = "${syncScript}";
        User = "root";
        Group = "root";
      };
    };

    # Fire as soon as Caddy rewrites the certificate on renewal (the atomic rewrite
    # changes the directory mtime), eliminating the up-to-24h DANE drift window.
    paths.tlsa-update = {
      description = "Watch Caddy certificate directory for renewal";
      wantedBy = ["paths.target"];
      pathConfig = {
        PathModified = [caddyCertDir];
        Unit = "tlsa-update.service";
      };
    };

    # Fallback in case a renewal event is missed (e.g. inotify overflow).
    timers.tlsa-update = {
      wantedBy = ["timers.target"];
      timerConfig = {
        OnCalendar = "daily";
        Persistent = true;
      };
    };
  };
}