zerotier module: add option to join networks and open port

nixos/modules/services/networking/zerotierone.nix

···
 in
 {
   options.services.zerotierone.enable = mkEnableOption "ZeroTierOne";
+
+  options.services.zerotierone.joinNetworks = mkOption {
+    default = [];
+    example = [ "a8a2c3c10c1a68de" ];
+    type = types.listOf types.str;
+    description = ''
+      List of ZeroTier Network IDs to join on startup
+    '';
+  };
+
   options.services.zerotierone.package = mkOption {
     default = pkgs.zerotierone;
     defaultText = "pkgs.zerotierone";
···
       path = [ cfg.package ];
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
-      preStart =
-        ''
-        mkdir -p /var/lib/zerotier-one
+      preStart = ''
+        mkdir -p /var/lib/zerotier-one/networks.d
         chmod 700 /var/lib/zerotier-one
         chown -R root:root /var/lib/zerotier-one
-        '';
+      '' + (concatMapStrings (netId: ''
+        touch "/var/lib/zerotier-one/networks.d/${netId}.conf"
+      '') cfg.joinNetworks);
       serviceConfig = {
         ExecStart = "${cfg.package}/bin/zerotier-one";
         Restart = "always";
···
 
     # ZeroTier does not issue DHCP leases, but some strangers might...
     networking.dhcpcd.denyInterfaces = [ "zt0" ];
+
+    # ZeroTier receives UDP transmissions on port 9993 by default
+    networking.firewall.allowedUDPPorts = [ 9993 ];
 
     environment.systemPackages = [ cfg.package ];
   };