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

with lib;

let

  cfg = config.services.stunnel;
  yesNo = val: if val then "yes" else "no";

  verifyChainPathAssert = n: c: {
    assertion = c.verifyHostname == null || (c.verifyChain || c.verifyPeer);
    message =  "stunnel: \"${n}\" client configuration - hostname verification " +
      "is not possible without either verifyChain or verifyPeer enabled";
  };

  serverConfig = {
    options = {
      accept = mkOption {
        type = types.either types.str types.int;
        description = ''
          On which [host:]port stunnel should listen for incoming TLS connections.
          Note that unlike other softwares stunnel ipv6 address need no brackets,
          so to listen on all IPv6 addresses on port 1234 one would use ':::1234'.
        '';
      };

      connect = mkOption {
        type = types.int;
        description = "To which port the decrypted connection should be forwarded.";
      };

      cert = mkOption {
        type = types.path;
        description = "File containing both the private and public keys.";
      };
    };
  };

  clientConfig = {
    options = {
      accept = mkOption {
        type = types.str;
        description = "IP:Port on which connections should be accepted.";
      };

      connect = mkOption {
        type = types.str;
        description = "IP:Port destination to connect to.";
      };

      verifyChain = mkOption {
        type = types.bool;
        default = true;
        description = "Check if the provided certificate has a valid certificate chain (against CAPath).";
      };

      verifyPeer = mkOption {
        type = types.bool;
        default = false;
        description = "Check if the provided certificate is contained in CAPath.";
      };

      CAPath = mkOption {
        type = types.nullOr types.path;
        default = null;
        description = "Path to a directory containing certificates to validate against.";
      };

      CAFile = mkOption {
        type = types.nullOr types.path;
        default = "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt";
        description = "Path to a file containing certificates to validate against.";
      };

      verifyHostname = mkOption {
        type = with types; nullOr str;
        default = null;
        description = "If set, stunnel checks if the provided certificate is valid for the given hostname.";
      };
    };
  };


in

{

  ###### interface

  options = {

    services.stunnel = {

      enable = mkOption {
        type = types.bool;
        default = false;
        description = "Whether to enable the stunnel TLS tunneling service.";
      };

      user = mkOption {
        type = with types; nullOr str;
        default = "nobody";
        description = "The user under which stunnel runs.";
      };

      group = mkOption {
        type = with types; nullOr str;
        default = "nogroup";
        description = "The group under which stunnel runs.";
      };

      logLevel = mkOption {
        type = types.enum [ "emerg" "alert" "crit" "err" "warning" "notice" "info" "debug" ];
        default = "info";
        description = "Verbosity of stunnel output.";
      };

      fipsMode = mkOption {
        type = types.bool;
        default = false;
        description = "Enable FIPS 140-2 mode required for compliance.";
      };

      enableInsecureSSLv3 = mkOption {
        type = types.bool;
        default = false;
        description = "Enable support for the insecure SSLv3 protocol.";
      };


      servers = mkOption {
        description = "Define the server configuations.";
        type = with types; attrsOf (submodule serverConfig);
        example = {
          fancyWebserver = {
            accept = 443;
            connect = 8080;
            cert = "/path/to/pem/file";
          };
        };
        default = { };
      };

      clients = mkOption {
        description = "Define the client configurations.";
        type = with types; attrsOf (submodule clientConfig);
        example = {
          foobar = {
            accept = "0.0.0.0:8080";
            connect = "nixos.org:443";
            verifyChain = false;
          };
        };
        default = { };
      };
    };
  };


  ###### implementation

  config = mkIf cfg.enable {

    assertions = concatLists [
      (singleton {
        assertion = (length (attrValues cfg.servers) != 0) || ((length (attrValues cfg.clients)) != 0);
        message = "stunnel: At least one server- or client-configuration has to be present.";
      })

      (mapAttrsToList verifyChainPathAssert cfg.clients)
    ];

    environment.systemPackages = [ pkgs.stunnel ];

    environment.etc."stunnel.cfg".text = ''
      ${ if cfg.user != null then "setuid = ${cfg.user}" else "" }
      ${ if cfg.group != null then "setgid = ${cfg.group}" else "" }

      debug = ${cfg.logLevel}

      ${ optionalString cfg.fipsMode "fips = yes" }
      ${ optionalString cfg.enableInsecureSSLv3 "options = -NO_SSLv3" }

      ; ----- SERVER CONFIGURATIONS -----
      ${ lib.concatStringsSep "\n"
           (lib.mapAttrsToList
             (n: v: ''
               [${n}]
               accept = ${toString v.accept}
               connect = ${toString v.connect}
               cert = ${v.cert}

             '')
           cfg.servers)
      }

      ; ----- CLIENT CONFIGURATIONS -----
      ${ lib.concatStringsSep "\n"
           (lib.mapAttrsToList
             (n: v: ''
               [${n}]
               client = yes
               accept = ${v.accept}
               connect = ${v.connect}
               verifyChain = ${yesNo v.verifyChain}
               verifyPeer = ${yesNo v.verifyPeer}
               ${optionalString (v.CAPath != null) "CApath = ${v.CAPath}"}
               ${optionalString (v.CAFile != null) "CAFile = ${v.CAFile}"}
               ${optionalString (v.verifyHostname != null) "checkHost = ${v.verifyHostname}"}
               OCSPaia = yes

             '')
           cfg.clients)
      }
    '';

    systemd.services.stunnel = {
      description = "stunnel TLS tunneling service";
      after = [ "network.target" ];
      wants = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
      restartTriggers = [ config.environment.etc."stunnel.cfg".source ];
      serviceConfig = {
        ExecStart = "${pkgs.stunnel}/bin/stunnel ${config.environment.etc."stunnel.cfg".source}";
        Type = "forking";
      };
    };

    meta.maintainers = with maintainers; [
      # Server side
      lschuermann
      # Client side
      das_j
    ];
  };

}