services.flannel: add an extraNetworkConfig configuration, add missing kernel module (#365317)

nixos/modules/services/networking/flannel.nix

@@ -7,13 +7,15 @@
 let
   cfg = config.services.flannel;
 
-  networkConfig = lib.filterAttrs (n: v: v != null) {
-    Network = cfg.network;
-    SubnetLen = cfg.subnetLen;
-    SubnetMin = cfg.subnetMin;
-    SubnetMax = cfg.subnetMax;
-    Backend = cfg.backend;
-  };
+  networkConfig =
+    (lib.filterAttrs (n: v: v != null) {
+      Network = cfg.network;
+      SubnetLen = cfg.subnetLen;
+      SubnetMin = cfg.subnetMin;
+      SubnetMax = cfg.subnetMax;
+      Backend = cfg.backend;
+    })
+    // cfg.extraNetworkConfig;
 in
 {
   options.services.flannel = {
@@ -139,6 +141,15 @@ in
         Type = "vxlan";
       };
     };
+
+    extraNetworkConfig = lib.mkOption {
+      description = "Extra configuration to be added to the net-conf.json/etcd-backed network configuration.";
+      type = (pkgs.formats.json { }).type;
+      default = { };
+      example = {
+        EnableIPv6 = true;
+      };
+    };
   };
 
   config = lib.mkIf cfg.enable {
@@ -184,6 +195,8 @@ in
       };
     };
 
+    boot.kernelModules = [ "br_netfilter" ];
+
     services.etcd.enable = lib.mkDefault (
       cfg.storageBackend == "etcd" && cfg.etcd.endpoints == [ "http://127.0.0.1:2379" ]
     );