fix default mtu handling

internal/server/usermanager.go

@@ -390,6 +390,9 @@ func (u *UserManager) validateOrCreateDevice(dev wgtypes.Device, ipAddresses []s
 		device.Mtu = 0
 		device.PersistentKeepalive = 16 // Default
 		device.IPsStr = strings.Join(ipAddresses, ", ")
+		if mtu == wireguard.WireGuardDefaultMTU {
+			mtu = 0
+		}
 		device.Mtu = mtu
 
 		res := u.db.Create(&device)

internal/wireguard/net.go

@@ -7,6 +7,8 @@ import (
 	"github.com/milosgajdos/tenus"
 )
 
+const WireGuardDefaultMTU = 1420
+
 func (m *Manager) GetIPAddress() ([]string, error) {
 	wgInterface, err := tenus.NewLinkFrom(m.Cfg.DeviceName)
 	if err != nil {
@@ -107,7 +109,7 @@ func (m *Manager) SetMTU(mtu int) error {
 	}
 
 	if mtu == 0 {
-		mtu = 1420 // WireGuard default MTU
+		mtu = WireGuardDefaultMTU
 	}
 
 	if err := wgInterface.SetLinkMTU(mtu); err != nil {