nix-config/modules/nixos/namespaced-vpn.nix

{
  config,
  lib,
  pkgs,
  ...
}:
with lib;
let
  cfg = config.services.namespaced-vpn;
in
{
  options.services.namespaced-vpn = {
    enable = mkEnableOption "namespaced vpn";

    ips = mkOption {
      type = types.listOf types.str;
    };

    publicKey = mkOption {
      type = types.str;
    };

    endpoint = mkOption {
      type = types.str;
    };

    privateKeyFile = mkOption {
      type = types.path;
    };

    namespace = mkOption {
      type = types.str;
      default = "vpn";
    };

    wgInterface = mkOption {
      type = types.str;
      default = "wg0";
    };

    hostVethIP = mkOption {
      type = types.str;
      default = "10.10.10.1/30";
    };

    namespaceVethIP = mkOption {
      type = types.str;
      default = "10.10.10.2/30";
    };
  };

  config = mkIf cfg.enable {
    networking.iproute2.enable = true;

    systemd.services."netns@" = {
      description = "Network namespace %i";
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
        ExecStart = "${pkgs.iproute2}/bin/ip netns add %i";
        ExecStop = "${pkgs.iproute2}/bin/ip netns del %i";
      };
    };

    systemd.services."veth-setup@" = {
      description = "Setup veth pair for %i namespace";
      after = [ "netns@%i.service" ];
      requires = [ "netns@%i.service" ];
      serviceConfig = {
        Type = "oneshot";
        RemainAfterExit = true;
        ExecStart =
          let
            script = pkgs.writers.writeBash "veth-up" ''
              ns="$1"
              ${pkgs.iproute2}/bin/ip link add veth-init-$ns type veth peer name veth-ns-$ns
              ${pkgs.iproute2}/bin/ip link set veth-ns-$ns netns $ns
              ${pkgs.iproute2}/bin/ip link set veth-init-$ns up
              ${pkgs.iproute2}/bin/ip netns exec $ns ${pkgs.iproute2}/bin/ip link set veth-ns-$ns up
              ${pkgs.iproute2}/bin/ip netns exec $ns ${pkgs.iproute2}/bin/ip addr add $NS_IP dev veth-ns-$ns
              ${pkgs.iproute2}/bin/ip netns exec $ns ${pkgs.iproute2}/bin/ip route add $HOST_IP via $(echo $NS_IP |  sed -E 's/\/[0-9]+$//') dev veth-ns-$ns
            '';
          in
          "${script} %i";
        ExecStop = "${pkgs.iproute2}/bin/ip link del veth-%i";
      };
    };

    systemd.services."veth-setup@${cfg.namespace}" = {
      overrideStrategy = "asDropin";
      environment = {
        HOST_IP = cfg.hostVethIP;
        NS_IP = cfg.namespaceVethIP;
      };
    };

    systemd.network.networks."50-init-${cfg.namespace}" = {
      matchConfig.Name = "veth-init-${cfg.namespace}";
      networkConfig = {
        Address = "${cfg.hostVethIP}";
      };
    };

    systemd.services."wireguard-${cfg.wgInterface}".requires = [
      "netns@${cfg.namespace}.service"
      "veth-setup@${cfg.namespace}.service"
    ];

    systemd.network.wait-online.ignoredInterfaces = [
      cfg.wgInterface
      "veth-init-${cfg.namespace}"
      "veth-ns-${cfg.namespace}"
    ];

    networking.wireguard = {
      enable = true;
      useNetworkd = false;

      interfaces."${cfg.wgInterface}" = {
        ips = cfg.ips;

        peers = [
          {
            publicKey = cfg.publicKey;
            allowedIPs = [
              "0.0.0.0/0"
              "::0/0"
            ];
            endpoint = cfg.endpoint;
            persistentKeepalive = 25;
          }
        ];

        listenPort = 51820;
        privateKeyFile = cfg.privateKeyFile;
        socketNamespace = "init";
        interfaceNamespace = cfg.namespace;
      };
    };

  };
}