From eb49174f7b3e4222e69a21ee9a1cb59f84b73c8d Mon Sep 17 00:00:00 2001
From: digital <git-voopaipa@dinid.net>
Date: Wed, 31 Aug 2022 22:22:40 +0200
Subject: [PATCH] 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 | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/nixos/modules/services/networking/wireguard.nix b/nixos/modules/services/networking/wireguard.nix
index 8cc4fa9faf3612f..6a5d7c6b0410328 100644
--- a/nixos/modules/services/networking/wireguard.nix
+++ b/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}"''