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

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Date:   Tue, 1 Sep 2020 13:44:12 +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,

A quick correction; The issue is not solved, it was a mistake in my
testing. The issue is still there.


Kfir

On Tue, Sep 1, 2020 at 1:40 PM mastertheknife <mastertheknife@...il.com> wrote:
>
> 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