network-interfaces service: add defaultGateway{,6}.interface

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

@@ -102,17 +102,21 @@ in
                 EOF
 
                 # Set the default gateway.
-                ${optionalString (cfg.defaultGateway != null && cfg.defaultGateway != "") ''
+                ${optionalString (cfg.defaultGateway != null && cfg.defaultGateway.address != "") ''
                   # FIXME: get rid of "|| true" (necessary to make it idempotent).
-                  ip route add default via "${cfg.defaultGateway}" ${
+                  ip route add default via "${cfg.defaultGateway.address}" ${
                     optionalString (cfg.defaultGatewayWindowSize != null)
-                      "window ${toString cfg.defaultGatewayWindowSize}"} || true
+                      "window ${toString cfg.defaultGatewayWindowSize}"} ${
+                    optionalString (cfg.defaultGateway.interface != null)
+                      "dev ${cfg.defaultGateway.interface}"}|| true
                 ''}
-                ${optionalString (cfg.defaultGateway6 != null && cfg.defaultGateway6 != "") ''
+                ${optionalString (cfg.defaultGateway6 != null && cfg.defaultGateway6.address != "") ''
                   # FIXME: get rid of "|| true" (necessary to make it idempotent).
-                  ip -6 route add ::/0 via "${cfg.defaultGateway6}" ${
+                  ip -6 route add ::/0 via "${cfg.defaultGateway6.address}" ${
                     optionalString (cfg.defaultGatewayWindowSize != null)
-                      "window ${toString cfg.defaultGatewayWindowSize}"} || true
+                      "window ${toString cfg.defaultGatewayWindowSize}"} ${
+                    optionalString (cfg.defaultGateway6.interface != null)
+                      "dev ${cfg.defaultGateway6.interface}"} || true
                 ''}
               '';
           };

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

@@ -38,6 +38,12 @@ in
     } {
       assertion = cfg.vswitches == {};
       message = "networking.vswichtes are not supported by networkd.";
+    } {
+      assertion = cfg.defaultGateway == null || cfg.defaultGateway.interface == null;
+      message = "networking.defaultGateway.interface is not supported by networkd.";
+    } {
+      assertion = cfg.defaultGateway6 == null || cfg.defaultGateway6.interface == null;
+      message = "networking.defaultGateway6.interface is not supported by networkd.";
     } ] ++ flip mapAttrsToList cfg.bridges (n: { rstp, ... }: {
       assertion = !rstp;
       message = "networking.bridges.${n}.rstp is not supported by networkd.";

nixos/modules/tasks/network-interfaces.nix

@@ -116,6 +116,28 @@ let
       };
     };
 
+  gatewayCoerce = address: { inherit address; };
+
+  gatewayOpts = { ... }: {
+
+    options = {
+
+      address = mkOption {
+        type = types.str;
+        description = "The default gateway address.";
+      };
+
+      interface = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = "enp0s3";
+        description = "The default gateway interface.";
+      };
+
+    };
+
+  };
+
   interfaceOpts = { name, ... }: {
 
     options = {
@@ -327,19 +349,27 @@ in
 
     networking.defaultGateway = mkOption {
       default = null;
-      example = "131.211.84.1";
-      type = types.nullOr types.str;
+      example = {
+        address = "131.211.84.1";
+        device = "enp3s0";
+      };
+      type = types.nullOr (types.coercedTo types.str gatewayCoerce (types.submodule gatewayOpts));
       description = ''
-        The default gateway.  It can be left empty if it is auto-detected through DHCP.
+        The default gateway. It can be left empty if it is auto-detected through DHCP.
+        It can be specified as a string or an option set along with a network interface.
       '';
     };
 
     networking.defaultGateway6 = mkOption {
       default = null;
-      example = "2001:4d0:1e04:895::1";
-      type = types.nullOr types.str;
+      example = {
+        address = "2001:4d0:1e04:895::1";
+        device = "enp3s0";
+      };
+      type = types.nullOr (types.coercedTo types.str gatewayCoerce (types.submodule gatewayOpts));
       description = ''
-        The default ipv6 gateway.  It can be left empty if it is auto-detected through DHCP.
+        The default ipv6 gateway. It can be left empty if it is auto-detected through DHCP.
+        It can be specified as a string or an option set along with a network interface.
       '';
     };