# Portions of this file are sourced from
# https://github.com/NickCao/flakes/blob/3b03efb676ea602575c916b2b8bc9d9cd13b0d85/nixos/mainframe/gravity.nix
{
  config,
  lib,
  pkgs,
  mylib,
  ...
}:
with lib;
let
  inherit (mylib.network) cidr;
  cfg = config.services.enthalpy;
in
{
  options.services.enthalpy.clat = {
    enable = mkEnableOption "464XLAT for IPv4 connectivity";
    address = mkOption {
      type = types.str;
      default = cidr.host 2 cfg.prefix;
      description = ''
        IPv6 address used for 464XLAT as the mapped source address.
      '';
    };
    segment = mkOption {
      type = types.listOf types.str;
      description = ''
        SRv6 segments used for NAT64.
      '';
    };
  };

  config = mkIf (cfg.enable && cfg.clat.enable) {
    systemd.services.enthalpy-clat = {
      path = with pkgs; [
        iproute2
        tayga
      ];
      preStart = ''
        ip -6 route replace 64:ff9b::/96 from ${cfg.clat.address} encap seg6 mode encap \
          segs ${concatStringsSep "," cfg.clat.segment} dev enthalpy mtu 1280
      '';
      script = ''
        exec tayga --config ${pkgs.writeText "tayga.conf" ''
          tun-device clat
          prefix 64:ff9b::/96
          ipv4-addr 192.0.0.1
          map 192.0.0.2 ${cfg.clat.address}
        ''}
      '';
      postStart = ''
        ip link set clat up
        ip -4 addr add 192.0.0.2/32 dev clat
        ip -6 route add ${cfg.clat.address} dev clat
        ip -4 route add 0.0.0.0/0 dev clat src 192.0.0.2
      '';
      preStop = ''
        ip -6 route del 64:ff9b::/96 from ${cfg.clat.address} encap seg6 mode encap \
          segs ${concatStringsSep "," cfg.clat.segment} dev enthalpy mtu 1280
      '';
      serviceConfig = {
        Type = "forking";
        Restart = "on-failure";
        RestartSec = 5;
        DynamicUser = true;
        RuntimeDirectory = "enthalpy";
        CapabilityBoundingSet = [ "CAP_NET_ADMIN" ];
        AmbientCapabilities = [ "CAP_NET_ADMIN" ];
        ProtectSystem = "full";
        ProtectHome = "yes";
        ProtectKernelTunables = true;
        ProtectControlGroups = true;
        PrivateTmp = true;
        SystemCallFilter = "~@cpu-emulation @debug @keyring @module @mount @obsolete @raw-io";
        MemoryDenyWriteExecute = "yes";
      };
      wants = [ "network.target" ];
      after = [ "network.target" ];
      wantedBy = [ "multi-user.target" ];
    };

    services.enthalpy.services.enthalpy-clat = { };
  };
}