nixos/wireguard: add option to set mtu

Some networks can only transfer packets with a lower than normal maximum
transfer unit size. In these cases, it is necessary to set a MTU that
works for the given upstream network.

nixos/modules/services/networking/wireguard.nix

@@ -151,6 +151,19 @@ let
           differently.
         '';
       };
+
+      mtu = mkOption {
+        default = null;
+        type = with types; nullOr int;
+        example = 1280;
+        description = lib.mdDoc ''
+          Set the maximum transmission unit in bytes for the wireguard
+          interface. Beware that the wireguard packets have a header that may
+          add up to 80 bytes to the mtu. By default, the MTU is (1500 - 80) =
+          1420. However, if the MTU of the upstream network is lower, the MTU
+          of the wireguard network has to be adjusted as well.
+        '';
+      };
     };
 
   };
@@ -412,6 +425,7 @@ let
 
           ${ipPreMove} link add dev "${name}" type wireguard
           ${optionalString (values.interfaceNamespace != null && values.interfaceNamespace != values.socketNamespace) ''${ipPreMove} link set "${name}" netns "${ns}"''}
+          ${optionalString (values.mtu != null) ''${ipPreMove} link set "${name}" mtu ${toString values.mtu}''}
 
           ${concatMapStringsSep "\n" (ip:
             ''${ipPostMove} address add "${ip}" dev "${name}"''