dotfiles/nixos/modules/router/netns-if.nix

{
  config,
  lib,
  pkgs,
  ...
}: let
  inherit
    (lib)
    mkOption
    mkIf
    mkMerge
    types
    getExe'
    flip
    mapAttrs'
    flatten
    mapAttrsToList
    nameValuePair
    concatMapStringsSep
    mkEnableOption
    ;

  cfg = config.networking.netnsIf;
in {
  options.networking.netnsIf = {
    assignments = mkOption {
      description = ''
        Assign network interfaces to a network namespaces by MAC address.
      '';
      type = types.attrsOf (types.listOf types.str);
      default = {};
    };
  };

  config =
    mkIf (cfg.assignments != {})
    {
      systemd.services = mkMerge [
        {
          "netns@" = {
            description = "Ensure the %I network namespace exists";

            serviceConfig = {
              Type = "oneshot";
            };

            scriptArgs = "%I";
            script = ''
              set -eo pipefail
              _network_namespace="$1"

              if ! [ -n "$(${getExe' pkgs.iproute2 "ip"} netns list | grep "$_network_namespace")" ] ; then
                ${getExe' pkgs.iproute2 "ip"} netns add "$_network_namespace"
              fi
            '';
          };
        }

        (flip mapAttrs' cfg.assignments (n: _:
          nameValuePair "netns-${n}-if@" {
            description = "Move %I interface to namespace";
            requires = ["netns@${n}.service"];
            after = ["netns@${n}.service"];

            scriptArgs = "%I";
            script = ''
              set -eo pipefail
              _kernel_name="$(basename "$1")"

              ${getExe' pkgs.iproute2 "ip"} link set "$_kernel_name" netns "${n}"
            '';
          }))
      ];

      services.udev.extraRules =
        flip (concatMapStringsSep "\n") (flatten (mapAttrsToList (n: v:
          map (x: {
            interface = x;
            namespace = n;
          })
          v)
        cfg.assignments)) ({
          interface,
          namespace,
        }: ''
          SUBSYSTEM=="net", ACTION=="add", DEVPATH!="/devices/virtual/*", ATTR{address}=="${interface}", TAG+="systemd", ENV{SYSTEMD_WANTS}="netns-${namespace}-if@.service"
        '');
    };
}