From 7d284c5fa7e4c6e80be3b2b8546be2e27c8efc5f Mon Sep 17 00:00:00 2001
From: Beniamino Galvani <bgalvani@redhat.com>
Date: Wed, 14 Jan 2026 08:40:44 +0100
Subject: [PATCH] l3cfg: fix CLAT MTU handling

The current code takes the IPv6 MTU value from the IPv6 default
route. However, that value is always zero because NM doesn't set it
usually. Instead, it should use the IPv6 MTU sysctl value. The problem
is that at this point NM hasn't written the sysctl yet, and we need
some logic to find the actual value.
---
 src/core/nm-l3cfg.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/src/core/nm-l3cfg.c b/src/core/nm-l3cfg.c
index 0c4434742c..5be8369bbf 100644
--- a/src/core/nm-l3cfg.c
+++ b/src/core/nm-l3cfg.c
@@ -4361,7 +4361,6 @@ _l3cfg_update_combined_config(NML3Cfg               *self,
                         nm_l3_config_data_get_best_default_route(l3cd, AF_INET6));
                 }
                 if (best_v6_route) {
-                    int                  mtu;
                     NMPlatformIP4Address addr = {
                         .ifindex      = self->priv.ifindex,
                         .address      = self->priv.p->clat_address_4->addr,
@@ -4369,18 +4368,39 @@ _l3cfg_update_combined_config(NML3Cfg               *self,
                         .addr_source  = NM_IP_CONFIG_SOURCE_CLAT,
                         .plen         = 32,
                     };
+                    const NMPlatformLink *pllink;
+                    guint                 mtu = 0;
+                    guint                 val = 0;
 
                     best_v6_gateway.addr_family = AF_INET6;
                     best_v6_gateway.addr.addr6  = best_v6_route->gateway;
 
-                    mtu = best_v6_route->mtu;
+                    /* Determine the IPv6 MTU of the interface. Unfortunately,
+                     * the logic to set the MTU is in NMDevice and here we need
+                     * some duplication to find the actual value.
+                     * TODO: move the MTU handling into l3cfg. */
 
-                    if (!mtu || mtu > 1500) {
-                        /* v4 Internet MTU */
+                    /* Get the link MTU */
+                    pllink = nm_l3cfg_get_pllink(self, TRUE);
+                    if (pllink)
+                        mtu = pllink->mtu;
+                    if (mtu == 0)
                         mtu = 1500;
+
+                    /* Update it with the IPv6 MTU value from the connection
+                     * or from RA */
+                    val = nm_l3_config_data_get_ip6_mtu_static(l3cd);
+                    if (val == 0) {
+                        val = nm_l3_config_data_get_ip6_mtu_ra(l3cd);
                     }
-                    /* 20 for the ipv6 vs ipv4 header plus 8 for a potential
-                       fragmentation extension header */
+                    if (val != 0 && val < mtu) {
+                        mtu = val;
+                    }
+                    if (mtu < 1280)
+                        mtu = 1280;
+
+                    /* Leave 20 additional bytes for the ipv4 -> ipv6 header translation,
+                     * plus 8 for a potential fragmentation extension header */
                     mtu -= 28;
 
                     rx.r4 = (NMPlatformIP4Route) {