netdev - Re: PMTUD broken inside network namespace with multipath routing

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Date:   Tue, 1 Sep 2020 13:40:14 +0300
From:   mastertheknife <mastertheknife@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: PMTUD broken inside network namespace with multipath routing

Hello David.

I was able to solve it while troubleshooting some fragmentation issue.
The VTI interfaces had MTU of 1480 by default. I reduced to them to
the real PMTUD (1366) and now its all working just fine.
I am not sure how its related and why, but seems like it solved the issue.

P.S: while reading the relevant code in the kernel, i think i spotted
some mistake in net/ipv4/route.c, in function "update_or_create_fnhe".
It looks like it loops over all the exceptions for the nexthop entry,
but always overwriting the first (and only) entry, so effectively only
1 exception can exist per nexthop entry.
Line 678:
"if (fnhe) {"
Should probably be:
"if (fnhe && fnhe->fnhe_daddr == daddr) {"


Thank you for your efforts,
Kfir Itzhak

On Fri, Aug 14, 2020 at 10:08 AM mastertheknife
<mastertheknife@...il.com> wrote:
>
> 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 192.168.252.5 in that case
> 192.168.252.250 and 192.168.252.252 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,
> Kfir
>
>
> On Wed, Aug 12, 2020 at 10:21 PM David Ahern <dsahern@...il.com> 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 192.168.252.209/24 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 192.168.252.100
> > > root@...t:~# ip netns exec testns ip route add 192.168.249.0/24
> > > nexthop via 192.168.252.250 nexthop via 192.168.252.252
> > > # 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 192.168.252.250 and
> > 192.168.252.252