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Date:   Mon, 3 Aug 2020 17:24:50 +0300
From:   mastertheknife <>
To:     David Ahern <>
Subject: Re: PMTUD broken inside network namespace with multipath routing

Hi David,

In this case, both paths are in the same layer2 network, there is no
symmetric multi-path routing.
If original message takes path 1, ICMP response will come from path 1
If original message takes path 2, ICMP response will come from path 2
Also, It works fine outside of LXC.

Thank you,

On Mon, Aug 3, 2020 at 4:32 PM David Ahern <> wrote:
> On 8/3/20 5:14 AM, mastertheknife wrote:
> > What seems to be happening, is that when multipath routing is used
> > inside LXC (or any network namespace), the kernel doesn't generate a
> > routing exception to force the lower MTU.
> > I believe this is a bug inside the kernel.
> >
> Known problem. Original message can take path 1 and ICMP message can
> path 2. The exception is then created on the wrong path.

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