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

with lib;

let

  inherit (pkgs) lsh;

  cfg = config.services.lshd;

in

{

  ###### interface

  options = {

    services.lshd = {

      enable = mkOption {
        default = false;
        description = ''
          Whether to enable the GNU lshd SSH2 daemon, which allows
          secure remote login.
        '';
      };

      portNumber = mkOption {
        default = 22;
        description = ''
          The port on which to listen for connections.
        '';
      };

      interfaces = mkOption {
        default = [];
        description = ''
          List of network interfaces where listening for connections.
          When providing the empty list, `[]', lshd listens on all
          network interfaces.
        '';
        example = [ "localhost" "1.2.3.4:443" ];
      };

      hostKey = mkOption {
        default = "/etc/lsh/host-key";
        description = ''
          Path to the server's private key.  Note that this key must
          have been created, e.g., using "lsh-keygen --server |
          lsh-writekey --server", so that you can run lshd.
        '';
      };

      syslog = mkOption {
        default = true;
        description = ''Whether to enable syslog output.'';
      };

      passwordAuthentication = mkOption {
        default = true;
        description = ''Whether to enable password authentication.'';
      };

      publicKeyAuthentication = mkOption {
        default = true;
        description = ''Whether to enable public key authentication.'';
      };

      rootLogin = mkOption {
        default = false;
        description = ''Whether to enable remote root login.'';
      };

      loginShell = mkOption {
        default = null;
        description = ''
          If non-null, override the default login shell with the
          specified value.
        '';
        example = "/nix/store/xyz-bash-10.0/bin/bash10";
      };

      srpKeyExchange = mkOption {
        default = false;
        description = ''
          Whether to enable SRP key exchange and user authentication.
        '';
      };

      tcpForwarding = mkOption {
        default = true;
        description = ''Whether to enable TCP/IP forwarding.'';
      };

      x11Forwarding = mkOption {
        default = true;
        description = ''Whether to enable X11 forwarding.'';
      };

      subsystems = mkOption {
        description = ''
          List of subsystem-path pairs, where the head of the pair
          denotes the subsystem name, and the tail denotes the path to
          an executable implementing it.
        '';
      };

    };

  };


  ###### implementation

  config = mkIf cfg.enable {

    services.lshd.subsystems = [ ["sftp" "${pkgs.lsh}/sbin/sftp-server"] ];

    jobs.lshd =
      { description = "GNU lshd SSH2 daemon";

        startOn = "started network-interfaces";
        stopOn = "stopping network-interfaces";

        environment =
          { LD_LIBRARY_PATH = config.system.nssModules.path; };

        preStart =
          ''
            test -d /etc/lsh || mkdir -m 0755 -p /etc/lsh
            test -d /var/spool/lsh || mkdir -m 0755 -p /var/spool/lsh

            if ! test -f /var/spool/lsh/yarrow-seed-file
            then
                # XXX: It would be nice to provide feedback to the
                # user when this fails, so that they can retry it
                # manually.
                ${lsh}/bin/lsh-make-seed --sloppy \
                   -o /var/spool/lsh/yarrow-seed-file
            fi

            if ! test -f "${cfg.hostKey}"
            then
                ${lsh}/bin/lsh-keygen --server | \
                ${lsh}/bin/lsh-writekey --server -o "${cfg.hostKey}"
            fi
          '';

        exec = with cfg;
          ''
            ${lsh}/sbin/lshd --daemonic \
              --password-helper="${lsh}/sbin/lsh-pam-checkpw" \
              -p ${toString portNumber} \
              ${if interfaces == [] then ""
                else (concatStrings (map (i: "--interface=\"${i}\"")
                                         interfaces))} \
              -h "${hostKey}" \
              ${if !syslog then "--no-syslog" else ""} \
              ${if passwordAuthentication then "--password" else "--no-password" } \
              ${if publicKeyAuthentication then "--publickey" else "--no-publickey" } \
              ${if rootLogin then "--root-login" else "--no-root-login" } \
              ${if loginShell != null then "--login-shell=\"${loginShell}\"" else "" } \
              ${if srpKeyExchange then "--srp-keyexchange" else "--no-srp-keyexchange" } \
              ${if !tcpForwarding then "--no-tcpip-forward" else "--tcpip-forward"} \
              ${if x11Forwarding then "--x11-forward" else "--no-x11-forward" } \
              --subsystems=${concatStringsSep ","
                                              (map (pair: (head pair) + "=" +
                                                          (head (tail pair)))
                                                   subsystems)}
          '';
      };

    security.pam.services.lshd = {};

  };

}