Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets

From: Florian Westphal <[email-masked]>
Date: Mon, 13 Jul 2020 10:04:13 +0200

Stefano Brivio <[email-masked]> wrote:
> Hi,
> 
> On Sun, 12 Jul 2020 22:07:03 +0200
> Florian Westphal <[email-masked]> wrote:
> 
> > vxlan and geneve take the to-be-transmitted skb, prepend the
> > encapsulation header and send the result.
> > 
> > Neither vxlan nor geneve can do anything about a lowered path mtu
> > except notifying the peer/upper dst entry.
> 
> It could, and I think it should, update its MTU, though. I didn't
> include this in the original implementation of PMTU discovery for UDP
> tunnels as it worked just fine for locally generated and routed
> traffic, but here we go.

I don't think its a good idea to muck with network config in response
to untrusted entity.

> As PMTU discovery happens, we have a route exception on the lower
> layer for the given path, and we know that VXLAN will use that path,
> so we also know there's no point in having a higher MTU on the VXLAN
> device, it's really the maximum packet size we can use.

No, in the setup that prompted this series the route exception is wrong.
The current "fix" is a shell script that flushes the exception as soon
as its added to keep the tunnel working...

> > Some setups, however, will use vxlan as a bridge port (or openvs vport).
> 
> And, on top of that, I think what we're missing on the bridge is to
> update the MTU when a port lowers its MTU. The MTU is changed only as
> interfaces are added, which feels like a bug. We could use the lower
> layer notifier to fix this.

I will defer to someone who knows bridges better but I think that
in bridge case we 100% depend on a human to set everything.

bridge might be forwarding frames of non-ip protocol and I worry that
this is a self-induced DoS when we start to alter configuration behind
sysadmins back.

> I tried to represent the issue you're hitting with a new test case in
> the pmtu.sh selftest, also included in the diff. Would that work for
> Open vSwitch?

No idea, I don't understand how it can work at all, we can't 'chop
up'/mangle l2 frame in arbitrary fashion to somehow make them pass to
the output port.  We also can't influence MTU config of the links peer.

> If OVS queries the MTU of VXLAN devices, I guess that should be enough.

What should it be doing...?