From 12d3aa212f35b8cdd9a0ccd849b45beea46ed94d Mon Sep 17 00:00:00 2001
From: Antonio Quartulli <antonio@mandelbit.com>
Date: Mon, 28 Sep 2026 16:02:20 +0200
Subject: openvpn: T9364: default the MTU to 1420 when DCO is enabled

The ovpn Kernel module does no MSS clamping, so with a 1500 bytes MTU
full sized packets get fragmented on the underlay.

Add an "mtu" node, rendered as "tun-mtu". With "offload dco" it
defaults to 1420, unless a raw tun-mtu/link-mtu/udp-mtu option is
present. Reject "mtu" together with such an option, below 100, and
below 1280 when IPv6 runs inside the tunnel.
---
 interface-definitions/interfaces_openvpn.xml.in | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

(limited to 'interface-definitions')

diff --git a/interface-definitions/interfaces_openvpn.xml.in b/interface-definitions/interfaces_openvpn.xml.in
index 9ddd95518..26b004520 100644
--- a/interface-definitions/interfaces_openvpn.xml.in
+++ b/interface-definitions/interfaces_openvpn.xml.in
@@ -313,6 +313,20 @@
               </constraint>
             </properties>
           </leafNode>
+          <leafNode name="mtu">
+            <properties>
+              <help>Maximum Transmission Unit (MTU) - with DCO the default is 1420</help>
+              <valueHelp>
+                <format>u32:68-16000</format>
+                <description>Maximum Transmission Unit in byte</description>
+              </valueHelp>
+              <constraint>
+                <validator name="numeric" argument="--range 68-16000"/>
+              </constraint>
+              <constraintErrorMessage>MTU must be between 68 and 16000</constraintErrorMessage>
+            </properties>
+            <defaultValue>1500</defaultValue>
+          </leafNode>
           <node name="offload">
             <properties>
               <help>Configurable offload options</help>
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