From e1e47628d7a34bcd94f21c93cff60f0a210dd28a Mon Sep 17 00:00:00 2001
From: Callan Bryant <callan.bryant@gmail.com>
Date: Sun, 11 May 2025 09:47:11 +0100
Subject: [PATCH] wiring to set MTU

Yet to figure out:

1. the right place to set it
2. How to set it to change it on sync

https://github.com/WireGuard/wireguard-go/blob/master/main.go reference
---
 cmd/cli/config.go | 2 ++
 cmd/root.go       | 1 +
 lib/link.go       | 2 ++
 3 files changed, 5 insertions(+)

diff --git a/cmd/cli/config.go b/cmd/cli/config.go
index 17cfe25..f04ea31 100644
--- a/cmd/cli/config.go
+++ b/cmd/cli/config.go
@@ -67,6 +67,7 @@ type DsnetConfig struct {
 	Peers      []PeerConfig `validate:"dive"`
 	// used for server and client
 	PersistentKeepalive int `validate:"gte=0,lte=255"`
+	MTU			 int `validate:"gte=0,lte=65535"`
 }
 
 // LoadConfigFile parses the json config file, validates and stuffs
@@ -88,6 +89,7 @@ func LoadConfigFile() (*DsnetConfig, error) {
 		// used _even if value is zero!_
 		// Effectively, this is a migration
 		PersistentKeepalive: 25,
+		MTU: 1420,
 	}
 
 	err = json.Unmarshal(raw, &conf)
diff --git a/cmd/root.go b/cmd/root.go
index 3ae3508..4cb60b2 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -206,6 +206,7 @@ func init() {
 	viper.SetDefault("config_file", "/etc/dsnetconfig.json")
 	viper.SetDefault("fallback_wg_bing", "wireguard-go")
 	viper.SetDefault("listen_port", 51820)
+	viper.SetDefault("MTU", 1420)
 	viper.SetDefault("interface_name", "dsnet")
 
 	// if last handshake (different from keepalive, see https://www.wireguard.com/protocol/)
diff --git a/lib/link.go b/lib/link.go
index 1e2c257..d77648c 100644
--- a/lib/link.go
+++ b/lib/link.go
@@ -65,6 +65,8 @@ func (s *Server) CreateLink() error {
 			if err != nil {
 				return fmt.Errorf("could not add ipv4 addr %s to interface %s: %v", addr.IP, s.InterfaceName, err)
 			}
+			// set MTU
+			err = netlink.LinkSetMTU(link, s.MTU)
 		}
 	}