nixos/networking: add options for configuring a GRE tunnel

Add `networking.greTunnels` option that allows a GRE tunnel to be
configured in NixOS.

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

@@ -532,6 +532,33 @@ let
             '';
           });
 
+        createGreDevice = n: v: nameValuePair "${n}-netdev"
+          (let
+            deps = deviceDependency v.dev;
+          in
+          { description = "GRE Tunnel Interface ${n}";
+            wantedBy = [ "network-setup.service" (subsystemDevice n) ];
+            bindsTo = deps;
+            partOf = [ "network-setup.service" ];
+            after = [ "network-pre.target" ] ++ deps;
+            before = [ "network-setup.service" ];
+            serviceConfig.Type = "oneshot";
+            serviceConfig.RemainAfterExit = true;
+            path = [ pkgs.iproute2 ];
+            script = ''
+              # Remove Dead Interfaces
+              ip link show "${n}" >/dev/null 2>&1 && ip link delete "${n}"
+              ip link add name "${n}" type ${v.type} \
+                ${optionalString (v.remote != null) "remote \"${v.remote}\""} \
+                ${optionalString (v.local != null) "local \"${v.local}\""} \
+                ${optionalString (v.dev != null) "dev \"${v.dev}\""}
+              ip link set "${n}" up
+            '';
+            postStop = ''
+              ip link delete "${n}" || true
+            '';
+          });
+
         createVlanDevice = n: v: nameValuePair "${n}-netdev"
           (let
             deps = deviceDependency v.interface;
@@ -570,6 +597,7 @@ let
          // mapAttrs' createMacvlanDevice cfg.macvlans
          // mapAttrs' createFouEncapsulation cfg.fooOverUDP
          // mapAttrs' createSitDevice cfg.sits
+         // mapAttrs' createGreDevice cfg.greTunnels
          // mapAttrs' createVlanDevice cfg.vlans
          // {
            network-setup = networkSetup;

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

@@ -18,6 +18,7 @@ let
     concatLists (map (bond: bond.interfaces) (attrValues cfg.bonds))
     ++ concatLists (map (bridge: bridge.interfaces) (attrValues cfg.bridges))
     ++ map (sit: sit.dev) (attrValues cfg.sits)
+    ++ map (gre: gre.dev) (attrValues cfg.greTunnels)
     ++ map (vlan: vlan.interface) (attrValues cfg.vlans)
     # add dependency to physical or independently created vswitch member interface
     # TODO: warn the user that any address configured on those interfaces will be useless
@@ -245,6 +246,25 @@ in
           } ]);
         };
       })))
+      (mkMerge (flip mapAttrsToList cfg.greTunnels (name: gre: {
+        netdevs."40-${name}" = {
+          netdevConfig = {
+            Name = name;
+            Kind = gre.type;
+          };
+          tunnelConfig =
+            (optionalAttrs (gre.remote != null) {
+              Remote = gre.remote;
+            }) // (optionalAttrs (gre.local != null) {
+              Local = gre.local;
+            });
+        };
+        networks = mkIf (gre.dev != null) {
+          "40-${gre.dev}" = (mkMerge [ (genericNetwork (mkOverride 999)) {
+            tunnel = [ name ];
+          } ]);
+        };
+      })))
       (mkMerge (flip mapAttrsToList cfg.vlans (name: vlan: {
         netdevs."40-${name}" = {
           netdevConfig = {

nixos/modules/tasks/network-interfaces.nix

@@ -9,6 +9,7 @@ let
   interfaces = attrValues cfg.interfaces;
   hasVirtuals = any (i: i.virtual) interfaces;
   hasSits = cfg.sits != { };
+  hasGres = cfg.greTunnels != { };
   hasBonds = cfg.bonds != { };
   hasFous = cfg.fooOverUDP != { }
     || filterAttrs (_: s: s.encapsulation != null) cfg.sits != { };
@@ -996,6 +997,65 @@ in
       });
     };
 
+    networking.greTunnels = mkOption {
+      default = { };
+      example = literalExpression ''
+        {
+          greBridge = {
+            remote = "10.0.0.1";
+            local = "10.0.0.22";
+            dev = "enp4s0f0";
+            type = "tap";
+          };
+        }
+      '';
+      description = ''
+        This option allows you to define Generic Routing Encapsulation (GRE) tunnels.
+      '';
+      type = with types; attrsOf (submodule {
+        options = {
+
+          remote = mkOption {
+            type = types.nullOr types.str;
+            default = null;
+            example = "10.0.0.1";
+            description = ''
+              The address of the remote endpoint to forward traffic over.
+            '';
+          };
+
+          local = mkOption {
+            type = types.nullOr types.str;
+            default = null;
+            example = "10.0.0.22";
+            description = ''
+              The address of the local endpoint which the remote
+              side should send packets to.
+            '';
+          };
+
+          dev = mkOption {
+            type = types.nullOr types.str;
+            default = null;
+            example = "enp4s0f0";
+            description = ''
+              The underlying network device on which the tunnel resides.
+            '';
+          };
+
+          type = mkOption {
+            type = with types; enum [ "tun" "tap" ];
+            default = "tap";
+            example = "tap";
+            apply = v: if v == "tun" then "gre" else "gretap";
+            description = ''
+              Whether the tunnel routes layer 2 (tap) or layer 3 (tun) traffic.
+            '';
+          };
+        };
+      });
+    };
+
     networking.vlans = mkOption {
       default = { };
       example = literalExpression ''
@@ -1225,6 +1285,7 @@ in
     boot.kernelModules = [ ]
       ++ optional hasVirtuals "tun"
       ++ optional hasSits "sit"
+      ++ optional hasGres "gre"
       ++ optional hasBonds "bonding"
       ++ optional hasFous "fou";
 

nixos/tests/networking.nix

@@ -489,6 +489,77 @@ let
               client2.wait_until_succeeds("ping -c 1 fc00::2")
         '';
     };
+    gre = let
+      node = { pkgs, ... }: with pkgs.lib; {
+        networking = {
+          useNetworkd = networkd;
+          useDHCP = false;
+        };
+      };
+    in {
+      name = "GRE";
+      nodes.client1 = args@{ pkgs, ... }:
+        mkMerge [
+          (node args)
+          {
+            virtualisation.vlans = [ 1 2 ];
+            networking = {
+              greTunnels = {
+                greTunnel = {
+                  local = "192.168.2.1";
+                  remote = "192.168.2.2";
+                  dev = "eth2";
+                  type = "tap";
+                };
+              };
+              bridges.bridge.interfaces = [ "greTunnel" "eth1" ];
+              interfaces.eth1.ipv4.addresses = mkOverride 0 [];
+              interfaces.bridge.ipv4.addresses = mkOverride 0 [
+                { address = "192.168.1.1"; prefixLength = 24; }
+              ];
+            };
+          }
+        ];
+      nodes.client2 = args@{ pkgs, ... }:
+        mkMerge [
+          (node args)
+          {
+            virtualisation.vlans = [ 2 3 ];
+            networking = {
+              greTunnels = {
+                greTunnel = {
+                  local = "192.168.2.2";
+                  remote = "192.168.2.1";
+                  dev = "eth1";
+                  type = "tap";
+                };
+              };
+              bridges.bridge.interfaces = [ "greTunnel" "eth2" ];
+              interfaces.eth2.ipv4.addresses = mkOverride 0 [];
+              interfaces.bridge.ipv4.addresses = mkOverride 0 [
+                { address = "192.168.1.2"; prefixLength = 24; }
+              ];
+            };
+          }
+        ];
+      testScript = { ... }:
+        ''
+          start_all()
+
+          with subtest("Wait for networking to be configured"):
+              client1.wait_for_unit("network.target")
+              client2.wait_for_unit("network.target")
+
+              # Print diagnostic information
+              client1.succeed("ip addr >&2")
+              client2.succeed("ip addr >&2")
+
+          with subtest("Test GRE tunnel bridge over VLAN"):
+              client1.wait_until_succeeds("ping -c 1 192.168.1.2")
+
+              client2.wait_until_succeeds("ping -c 1 192.168.1.1")
+        '';
+    };
     vlan = let
       node = address: { pkgs, ... }: with pkgs.lib; {
         #virtualisation.vlans = [ 1 ];