dendritic/modules/services/step-ca/mtls.nix

{ inputs, lib, ... }:
let
  opts = {
    enable = lib.mkEnableOption "Enable mTLS";
    caURL = lib.mkOption {
      description = "URL to the certificate authority";
      type = lib.types.str;
    };
    subject = lib.mkOption {
      description = "The Common Name, DNS Name, or IP address that will be set as the Subject Common Name for the certificate. If no Subject Alternative Names (SANs) are configured (via the --san flag) then the subject will be set as the only SAN.";
      type = lib.types.str;
    };
    keyFilename = lib.mkOption {
      description = "String filename for the private key";
      type = lib.types.str;
      default = "key.pem";
    };
    certFilename = lib.mkOption {
      description = "String filename for the public certificate";
      type = lib.types.str;
      default = "cert.pem";
    };
    bundleFilename = lib.mkOption {
      description = "String filename for the mTLS key bundle";
      type = lib.types.str;
      default = "mtls.pem";
    };
    san = lib.mkOption {
      description = "List of SAN to give the mTLS cert";
      type = lib.types.listOf lib.types.str;
      default = [ ];
    };
    provisioner = lib.mkOption {
      type = lib.types.str;
      default = "admin";
    };
    renew = {
      enable = lib.mkOption {
        description = "Enable automatic mTLS certificate renewal using a systemd timer.";
        type = lib.types.bool;
        default = true;
      };
      onCalendar = lib.mkOption {
        description = "systemd OnCalendar schedule for mTLS certificate renewal checks.";
        type = lib.types.str;
        default = "*:1/15";
      };
      randomizedDelaySec = lib.mkOption {
        description = "Randomized delay added to renewal timer runs to avoid synchronized renewals.";
        type = lib.types.str;
        default = "5m";
      };
      reloadUnits = lib.mkOption {
        description = "systemd units to try-reload-or-restart after a successful certificate renewal.";
        type = lib.types.listOf lib.types.str;
        default = [ ];
      };
      postCommands = lib.mkOption {
        description = "Shell commands to run after a successful certificate renewal.";
        type = lib.types.listOf lib.types.lines;
        default = [ ];
      };
    };
  };
in
{
  flake.modules.nixos.mtls = { config, lib, pkgs, ... }:
  let
    cfg = config.mtls;
    certDir = "/etc/step/certs";
    tlsKey = "${certDir}/${cfg.keyFilename}";
    tlsCert = "${certDir}/${cfg.certFilename}";
    mtlsBundle = "${certDir}/${cfg.bundleFilename}";
    rootCA = "${certDir}/root_ca.crt";
    sanArgs = lib.concatMapStringsSep " " (san: "--san \"${san}\"") cfg.san;
    renewReloadScript = lib.concatMapStringsSep "\n" (unit: ''
      if ${lib.getExe' pkgs.systemd "systemctl"} --quiet is-active "${unit}"; then
        ${lib.getExe' pkgs.systemd "systemctl"} try-reload-or-restart "${unit}"
      fi
    '') cfg.renew.reloadUnits;
    renewPostCommands = lib.concatStringsSep "\n" cfg.renew.postCommands;
  in
  {
    options.mtls = opts;
    config = lib.mkIf cfg.enable {
      environment.systemPackages = with pkgs; lib.optionals cfg.enable [
        (writeShellScriptBin "mtls-generate" ''
          set -euo pipefail
          ${lib.getExe pkgs.step-cli} ca certificate \
            ${cfg.subject} ${tlsCert} ${tlsKey} \
            --ca-url ${cfg.caURL} \
            --root ${rootCA} \
            --provisioner ${cfg.provisioner} \
            ${sanArgs}
          cat ${tlsCert} ${tlsKey} > ${mtlsBundle}
        '')
        (writeShellScriptBin "mtls-check" ''
          ${lib.getExe pkgs.openssl} x509 \
          -noout -subject -issuer \
          -ext subjectAltName,extendedKeyUsage \
          -enddate -in ${mtlsBundle}
        '')
      ];

      systemd.services.mtls-renew = lib.mkIf cfg.renew.enable {
        description = "Renew the mTLS certificate when Smallstep marks it ready";
        wantedBy = [ ];
        after = [ "network-online.target" ];
        wants = [ "network-online.target" ];
        path = [ pkgs.coreutils pkgs.step-cli pkgs.systemd ];
        serviceConfig = {
          Type = "oneshot";
          User = "root";
          Group = "root";
        };
        script = ''
          set -euo pipefail

          if ${lib.getExe pkgs.step-cli} certificate needs-renewal "${tlsCert}"; then
            echo "Renewing mTLS certificate"
          else
            rc=$?
            if [ "$rc" -eq 1 ]; then
              echo "mTLS certificate does not need renewal"
              exit 0
            fi

            if [ "$rc" -eq 2 ]; then
              echo "mTLS certificate missing: ${tlsCert}" >&2
              exit 1
            fi

            echo "step certificate needs-renewal failed with rc=$rc" >&2
            exit "$rc"
          fi

          ${lib.getExe pkgs.step-cli} ca renew --force "${tlsCert}" "${tlsKey}"

          umask 077
          cat "${tlsCert}" "${tlsKey}" > "${mtlsBundle}"

          ${renewReloadScript}
          ${renewPostCommands}
        '';
      };

      systemd.timers.mtls-renew = lib.mkIf cfg.renew.enable {
        description = "Periodic Smallstep renewal for the mTLS certificate";
        wantedBy = [ "timers.target" ];
        timerConfig = {
          Persistent = true;
          OnCalendar = cfg.renew.onCalendar;
          AccuracySec = "1us";
          RandomizedDelaySec = cfg.renew.randomizedDelaySec;
          Unit = "mtls-renew.service";
        };
      };
    };
  };

  flake.modules.homeManager.mtls = { config, lib, pkgs, ... }:
  let
    cfg = config.mtls;
    certDir = cfg.certDir;
    tlsKey = "${certDir}/${cfg.keyFilename}";
    tlsCert = "${certDir}/${cfg.certFilename}";
    mtlsBundle = "${certDir}/${cfg.bundleFilename}";
    rootCA = "${certDir}/root_ca.crt";
    sanArgs = lib.concatMapStringsSep " " (san: "--san \"${san}\"") cfg.san;
  in
  {
    options.mtls = opts // {
      certDir = lib.mkOption {
        description = "String path to where the mtls certs will be stored.";
        type = lib.types.str;
        default ="${config.home.homeDirectory}/.step/certs";
      };
    };

    config = {
      home.packages = with pkgs; [
        step-cli
        (writeShellScriptBin "mtls-generate" ''
          set -euo pipefail
          ${lib.getExe pkgs.step-cli} ca certificate \
            john-pc-ubuntu ${tlsCert} ${tlsKey} \
            --provisioner ${cfg.provisioner} \
            ${sanArgs}
          cat ${tlsCert} ${tlsKey} > ${mtlsBundle}
        '')
        (writeShellScriptBin "mtls-check" ''
          ${lib.getExe pkgs.openssl} x509 \
          -noout -subject -issuer \
          -ext subjectAltName,extendedKeyUsage \
          -enddate -in ${mtlsBundle}
        '')
      ];
    };
  };
}