From a68d6f1a9a9e9b471085c5d0231d1e3a0fad92c8 Mon Sep 17 00:00:00 2001
From: Dirk Ziegelmeier <dziegelmeier@de.pepperl-fuchs.com>
Date: Mon, 6 May 2019 12:37:19 +0200
Subject: [PATCH] Fix bug #56136: The netif->mtu6 was updated by Router
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Using patch from Gao Quingahui plus improvement
---
 src/core/ipv6/nd6.c | 2 +-
 src/core/netif.c    | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/core/ipv6/nd6.c b/src/core/ipv6/nd6.c
index 1e7c94ce..d65ed4e1 100644
--- a/src/core/ipv6/nd6.c
+++ b/src/core/ipv6/nd6.c
@@ -697,7 +697,7 @@ nd6_input(struct pbuf *p, struct netif *inp)
 #if LWIP_ND6_ALLOW_RA_UPDATES
           if (inp->mtu) {
             /* don't set the mtu for IPv6 higher than the netif driver supports */
-            inp->mtu6 = LWIP_MIN(inp->mtu, (u16_t)mtu32);
+            inp->mtu6 = LWIP_MIN(LWIP_MIN(inp->mtu, inp->mtu6), (u16_t)mtu32);
           } else {
             inp->mtu6 = (u16_t)mtu32;
           }
diff --git a/src/core/netif.c b/src/core/netif.c
index 15200a27..e95cb672 100644
--- a/src/core/netif.c
+++ b/src/core/netif.c
@@ -1031,6 +1031,10 @@ netif_set_link_down(struct netif *netif)
 
   if (netif->flags & NETIF_FLAG_LINK_UP) {
     netif_clear_flags(netif, NETIF_FLAG_LINK_UP);
+#if LWIP_IPV6 && LWIP_ND6_ALLOW_RA_UPDATES
+    netif->mtu6 = netif->mtu;
+#endif
+
     NETIF_LINK_CALLBACK(netif);
 #if LWIP_NETIF_EXT_STATUS_CALLBACK
     {