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Date:   Fri, 14 Aug 2020 10:08:30 +0300
From:   mastertheknife <>
To:     David Ahern <>
Subject: Re: PMTUD broken inside network namespace with multipath routing

Hello David,

It's on a production system, vmbr2 is a bridge with eth.X VLAN
interface inside for the connectivity on that 252.0/24 network. vmbr2
has address in that case and are CentOS8 LXCs on another host,
with libreswan inside for any/any IPSECs with VTi interfaces.

Everything is kernel 5.4.44 LTS

I wish i could fully reproduce all of it in a script, but i am not
sure how to create such hops that return this ICMP

Thank you,

On Wed, Aug 12, 2020 at 10:21 PM David Ahern <> wrote:
> On 8/12/20 6:37 AM, mastertheknife wrote:
> > Hello David,
> >
> > I tried and it seems i can reproduce it:
> >
> > # Create test NS
> > root@...t:~# ip netns add testns
> > # Create veth pair, veth0 in host, veth1 in NS
> > root@...t:~# ip link add veth0 type veth peer name veth1
> > root@...t:~# ip link set veth1 netns testns
> > # Configure veth1 (NS)
> > root@...t:~# ip netns exec testns ip addr add dev veth1
> > root@...t:~# ip netns exec testns ip link set dev veth1 up
> > root@...t:~# ip netns exec testns ip route add default via
> > root@...t:~# ip netns exec testns ip route add
> > nexthop via nexthop via
> > # Configure veth0 (host)
> > root@...t:~# brctl addif vmbr2 veth0
> vmbr2's config is not defined.
> ip li add vmbr2 type bridge
> ip li set veth0 master vmbr2
> ip link set veth0 up
> anything else? e.g., address for vmbr2? What holds and

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