nixos/network-interfaces: Add ipv6 gateway support

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -85,6 +85,12 @@ in
                     optionalString (cfg.defaultGatewayWindowSize != null)
                       "window ${cfg.defaultGatewayWindowSize}"} || true
                 ''}
+                ${optionalString (cfg.defaultGateway6 != null && cfg.defaultGateway6 != "") ''
+                  # FIXME: get rid of "|| true" (necessary to make it idempotent).
+                  ip -6 route add ::/0 via "${cfg.defaultGateway6}" ${
+                    optionalString (cfg.defaultGatewayWindowSize != null)
+                      "window ${cfg.defaultGatewayWindowSize}"} || true
+                ''}
               '';
           };
 

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -51,6 +51,8 @@ in
           DHCP = override (dhcpStr cfg.useDHCP);
         } // optionalAttrs (cfg.defaultGateway != null) {
           gateway = override [ cfg.defaultGateway ];
+        } // optionalAttrs (cfg.defaultGateway6 != null) {
+          gateway = override [ cfg.defaultGateway6 ];
         } // optionalAttrs (domains != [ ]) {
           domains = override domains;
         };

nixos/modules/tasks/network-interfaces.nix

@@ -256,6 +256,15 @@ in
       '';
     };
 
+    networking.defaultGateway6 = mkOption {
+      default = null;
+      example = "2001:4d0:1e04:895::1";
+      type = types.nullOr types.str;
+      description = ''
+        The default ipv6 gateway.  It can be left empty if it is auto-detected through DHCP.
+      '';
+    };
+
     networking.defaultGatewayWindowSize = mkOption {
       default = null;
       example = 524288;