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Date:   Mon, 13 May 2019 21:22:49 +0200
From:   Mikael Magnusson <>
Subject: IPv6 PMTU discovery fails with source-specific routing

Hello list,

I think I have found a regression in 4.15+ kernels. IPv6 PMTU discovery 
doesn't seem to work with source-specific routing (AKA source-address 
dependent routing, SADR).

I made a test script (see attachment). It sets up a test environment 
with three network namespaces (a, b and c) using SADR. The link between 
b and c is configured with MTU 1280. It then runs a ping test with large 

I have tested a couple of kernels on Ubuntu 19.04 with the following 

mainline 4.14.117-0414117-generic SUCCESS
ubuntu   4.15.0-1036-oem          FAIL
mainline 5.1.0-050100-generic     FAIL

The mainline kernels are from, and the other from the 
Ubuntu 19.04 repository.

I have attached a patch against 5.1 which seems to fix the problem in 
the test case.

It's bug #1788623 in Ubuntu:


View attachment "0001-net-ipv6-route-Fix-PMTU-for-source-specific-routes.patch" of type "text/x-patch" (950 bytes)

Download attachment "" of type "application/x-shellscript" (1433 bytes)

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