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

let
  apparmorEnabled = config.security.apparmor.enable;
  dnscrypt-proxy = pkgs.dnscrypt-proxy;
  cfg = config.services.dnscrypt-proxy;
  stateDirectory = "/var/lib/dnscrypt-proxy";

  localAddress = "${cfg.localAddress}:${toString cfg.localPort}";

  # The minisign public key used to sign the upstream resolver list.
  # This is somewhat more flexible than preloading the key as an
  # embedded string.
  upstreamResolverListPubKey = pkgs.fetchurl {
    url = https://raw.githubusercontent.com/jedisct1/dnscrypt-proxy/master/minisign.pub;
    sha256 = "18lnp8qr6ghfc2sd46nn1rhcpr324fqlvgsp4zaigw396cd7vnnh";
  };

  # Internal flag indicating whether the upstream resolver list is used
  useUpstreamResolverList = cfg.resolverList == null && cfg.customResolver == null;

  resolverList =
    if (cfg.resolverList != null)
      then cfg.resolverList
      else "${stateDirectory}/dnscrypt-resolvers.csv";

  resolverArgs = if (cfg.customResolver != null)
    then
      [ "--resolver-address=${cfg.customResolver.address}:${toString cfg.customResolver.port}"
        "--provider-name=${cfg.customResolver.name}"
        "--provider-key=${cfg.customResolver.key}"
      ]
    else
      [ "--resolvers-list=${resolverList}"
        "--resolver-name=${cfg.resolverName}"
      ];

  # The final command line arguments passed to the daemon
  daemonArgs =
    [ "--local-address=${localAddress}" ]
    ++ optional cfg.tcpOnly "--tcp-only"
    ++ optional cfg.ephemeralKeys "-E"
    ++ resolverArgs;
in

{
  meta = {
    maintainers = with maintainers; [ joachifm ];
    doc = ./dnscrypt-proxy.xml;
  };

  options = {
    services.dnscrypt-proxy = {
      enable = mkOption {
        default = false;
        type = types.bool;
        description = "Whether to enable the DNSCrypt client proxy";
      };

      localAddress = mkOption {
        default = "127.0.0.1";
        type = types.str;
        description = ''
          Listen for DNS queries to relay on this address. The only reason to
          change this from its default value is to proxy queries on behalf
          of other machines (typically on the local network).
        '';
      };

      localPort = mkOption {
        default = 53;
        type = types.int;
        description = ''
          Listen for DNS queries to relay on this port. The default value
          assumes that the DNSCrypt proxy should relay DNS queries directly.
          When running as a forwarder for another DNS client, set this option
          to a different value; otherwise leave the default.
        '';
      };

      resolverName = mkOption {
        default = "dnscrypt.eu-nl";
        type = types.nullOr types.str;
        description = ''
          The name of the upstream DNSCrypt resolver to use, taken from
          <filename>${resolverList}</filename>.  The default resolver is
          located in Holland, supports DNS security extensions, and
          <emphasis>claims</emphasis> to not keep logs.
        '';
      };

      resolverList = mkOption {
        default = null;
        type = types.nullOr types.path;
        description = ''
          List of DNSCrypt resolvers.  The default is to use the list of
          public resolvers provided by upstream.
        '';
      };

      customResolver = mkOption {
        default = null;
        description = ''
          Use an unlisted resolver (e.g., a private DNSCrypt provider). For
          advanced users only. If specified, this option takes precedence.
        '';
        type = types.nullOr (types.submodule ({ ... }: { options = {
          address = mkOption {
            type = types.str;
            description = "IP address";
            example = "208.67.220.220";
          };

          port = mkOption {
            type = types.int;
            description = "Port";
            default = 443;
          };

          name = mkOption {
            type = types.str;
            description = "Fully qualified domain name";
            example = "2.dnscrypt-cert.opendns.com";
          };

          key = mkOption {
            type = types.str;
            description = "Public key";
            example = "B735:1140:206F:225D:3E2B:D822:D7FD:691E:A1C3:3CC8:D666:8D0C:BE04:BFAB:CA43:FB79";
          };
        }; }));
      };

      tcpOnly = mkOption {
        default = false;
        type = types.bool;
        description = ''
          Force sending encrypted DNS queries to the upstream resolver over
          TCP instead of UDP (on port 443). Use only if the UDP port is blocked.
        '';
      };

      ephemeralKeys = mkOption {
        default = false;
        type = types.bool;
        description = ''
          Compute a new key pair for every query.  Enabling this option
          increases CPU usage, but makes it more difficult for the upstream
          resolver to track your usage of their service across IP addresses.
          The default is to re-use the public key pair for all queries, making
          tracking trivial.
        '';
      };
    };
  };

  config = mkIf cfg.enable {

    assertions = [
      { assertion = (cfg.customResolver != null) || (cfg.resolverName != null);
        message   = "please configure upstream DNSCrypt resolver";
      }
    ];

    security.apparmor.profiles = optional apparmorEnabled (pkgs.writeText "apparmor-dnscrypt-proxy" ''
      ${dnscrypt-proxy}/bin/dnscrypt-proxy {
        /dev/null rw,
        /dev/urandom r,

        /etc/passwd r,
        /etc/group r,
        ${config.environment.etc."nsswitch.conf".source} r,

        ${getLib pkgs.glibc}/lib/*.so mr,
        ${pkgs.tzdata}/share/zoneinfo/** r,

        network inet stream,
        network inet6 stream,
        network inet dgram,
        network inet6 dgram,

        ${getLib pkgs.gcc.cc}/lib/libssp.so.* mr,
        ${getLib pkgs.libsodium}/lib/libsodium.so.* mr,
        ${getLib pkgs.systemd}/lib/libsystemd.so.* mr,
        ${getLib pkgs.xz}/lib/liblzma.so.* mr,
        ${getLib pkgs.libgcrypt}/lib/libgcrypt.so.* mr,
        ${getLib pkgs.libgpgerror}/lib/libgpg-error.so.* mr,
        ${getLib pkgs.libcap}/lib/libcap.so.* mr,
        ${getLib pkgs.lz4}/lib/liblz4.so.* mr,
        ${getLib pkgs.attr}/lib/libattr.so.* mr,

        ${resolverList} r,
      }
    '');

    users.users.dnscrypt-proxy = {
      description = "dnscrypt-proxy daemon user";
      isSystemUser = true;
      group = "dnscrypt-proxy";
    };
    users.groups.dnscrypt-proxy = {};

    systemd.services.init-dnscrypt-proxy-statedir = optionalAttrs useUpstreamResolverList {
      description = "Initialize dnscrypt-proxy state directory";
      script = ''
        mkdir -pv ${stateDirectory}
        chown -c dnscrypt-proxy:dnscrypt-proxy ${stateDirectory}
        cp --preserve=timestamps -uv \
          ${pkgs.dnscrypt-proxy}/share/dnscrypt-proxy/dnscrypt-resolvers.csv \
          ${stateDirectory}
      '';
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
      };
    };

    systemd.services.update-dnscrypt-resolvers = optionalAttrs useUpstreamResolverList {
      description = "Update list of DNSCrypt resolvers";

      requires = [ "init-dnscrypt-proxy-statedir.service" ];
      after = [ "init-dnscrypt-proxy-statedir.service" ];

      path = with pkgs; [ curl minisign ];
      script = ''
        cd ${stateDirectory}
        curl -fSsL -o dnscrypt-resolvers.csv.tmp \
          https://download.dnscrypt.org/dnscrypt-proxy/dnscrypt-resolvers.csv
        curl -fSsL -o dnscrypt-resolvers.csv.minisig.tmp \
          https://download.dnscrypt.org/dnscrypt-proxy/dnscrypt-resolvers.csv.minisig
        mv dnscrypt-resolvers.csv.minisig{.tmp,}
        minisign -q -V -p ${upstreamResolverListPubKey} \
          -m dnscrypt-resolvers.csv.tmp -x dnscrypt-resolvers.csv.minisig
        mv dnscrypt-resolvers.csv{.tmp,}
      '';

      serviceConfig = {
        PrivateTmp = true;
        PrivateDevices = true;
        ProtectHome = true;
        ProtectSystem = true;
      };
    };

    systemd.timers.update-dnscrypt-resolvers = optionalAttrs useUpstreamResolverList {
      timerConfig = {
        OnBootSec = "5min";
        OnUnitActiveSec = "6h";
      };
      wantedBy = [ "timers.target" ];
    };

    systemd.sockets.dnscrypt-proxy = {
      description = "dnscrypt-proxy listening socket";
      socketConfig = {
        ListenStream = localAddress;
        ListenDatagram = localAddress;
      };
      wantedBy = [ "sockets.target" ];
    };

    systemd.services.dnscrypt-proxy = {
      description = "dnscrypt-proxy daemon";

      after = [ "network.target" ]
        ++ optional apparmorEnabled "apparmor.service"
        ++ optional useUpstreamResolverList "init-dnscrypt-proxy-statedir.service";

      requires = [ "dnscrypt-proxy.socket "]
        ++ optional apparmorEnabled "apparmor.service"
        ++ optional useUpstreamResolverList "init-dnscrypt-proxy-statedir.service";

      serviceConfig = {
        Type = "simple";
        NonBlocking = "true";
        ExecStart = "${dnscrypt-proxy}/bin/dnscrypt-proxy ${toString daemonArgs}";

        User = "dnscrypt-proxy";

        PrivateTmp = true;
        PrivateDevices = true;
        ProtectHome = true;
      };
    };
  };
}