nixpkgs/nixos/modules/system/boot/initrd-ssh.nix

{ config, lib, pkgs, ... }:

with lib;

let

  cfg = config.boot.initrd.network.ssh;

in

{

  options = {

    boot.initrd.network.ssh.enable = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Start SSH service during initrd boot. It can be used to debug failing
        boot on a remote server, enter pasphrase for an encrypted partition etc.
        Service is killed when stage-1 boot is finished.
      '';
    };

    boot.initrd.network.ssh.port = mkOption {
      type = types.int;
      default = 22;
      description = ''
        Port on which SSH initrd service should listen.
      '';
    };

    boot.initrd.network.ssh.shell = mkOption {
      type = types.str;
      default = "/bin/ash";
      description = ''
        Login shell of the remote user. Can be used to limit actions user can do.
      '';
    };

    boot.initrd.network.ssh.hostRSAKey = mkOption {
      type = types.nullOr types.path;
      default = null;
      description = ''
        RSA SSH private key file in the Dropbear format.

        WARNING: Unless your bootloader supports initrd secrets, this key is
        contained insecurely in the global Nix store. Do NOT use your regular
        SSH host private keys for this purpose or you'll expose them to
        regular users!
      '';
    };

    boot.initrd.network.ssh.hostDSSKey = mkOption {
      type = types.nullOr types.path;
      default = null;
      description = ''
        DSS SSH private key file in the Dropbear format.

        WARNING: Unless your bootloader supports initrd secrets, this key is
        contained insecurely in the global Nix store. Do NOT use your regular
        SSH host private keys for this purpose or you'll expose them to
        regular users!
      '';
    };

    boot.initrd.network.ssh.hostECDSAKey = mkOption {
      type = types.nullOr types.path;
      default = null;
      description = ''
        ECDSA SSH private key file in the Dropbear format.

        WARNING: Unless your bootloader supports initrd secrets, this key is
        contained insecurely in the global Nix store. Do NOT use your regular
        SSH host private keys for this purpose or you'll expose them to
        regular users!
      '';
    };

    boot.initrd.network.ssh.authorizedKeys = mkOption {
      type = types.listOf types.str;
      default = config.users.users.root.openssh.authorizedKeys.keys;
      description = ''
        Authorized keys for the root user on initrd.
        Note that Dropbear doesn't support OpenSSH's Ed25519 key type.
      '';
    };

  };

  config = mkIf (config.boot.initrd.network.enable && cfg.enable) {
    assertions = [
      { assertion = cfg.authorizedKeys != [];
        message = "You should specify at least one authorized key for initrd SSH";
      }
    ];

    boot.initrd.extraUtilsCommands = ''
      copy_bin_and_libs ${pkgs.dropbear}/bin/dropbear
      cp -pv ${pkgs.glibc.out}/lib/libnss_files.so.* $out/lib
    '';

    boot.initrd.extraUtilsCommandsTest = ''
      $out/bin/dropbear -V
    '';

    boot.initrd.network.postCommands = ''
      echo '${cfg.shell}' > /etc/shells
      echo 'root:x:0:0:root:/root:${cfg.shell}' > /etc/passwd
      echo 'passwd: files' > /etc/nsswitch.conf

      mkdir -p /var/log
      touch /var/log/lastlog

      mkdir -p /etc/dropbear

      mkdir -p /root/.ssh
      ${concatStrings (map (key: ''
        echo ${escapeShellArg key} >> /root/.ssh/authorized_keys
      '') cfg.authorizedKeys)}

      dropbear -s -j -k -E -p ${toString cfg.port} ${optionalString (cfg.hostRSAKey == null && cfg.hostDSSKey == null && cfg.hostECDSAKey == null) "-R"}
    '';

    boot.initrd.secrets =
     (optionalAttrs (cfg.hostRSAKey != null) { "/etc/dropbear/dropbear_rsa_host_key" = cfg.hostRSAKey; }) //
     (optionalAttrs (cfg.hostDSSKey != null) { "/etc/dropbear/dropbear_dss_host_key" = cfg.hostDSSKey; }) //
     (optionalAttrs (cfg.hostECDSAKey != null) { "/etc/dropbear/dropbear_ecdsa_host_key" = cfg.hostECDSAKey; });

  };

}