netdev - Re: [PATCH -next, V3 0/2] net: force refragmentation for DF reassembed skbs

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Date:	Sat, 23 May 2015 00:52:04 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>, Florian Westphal <fw@...len.de>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH -next, V3 0/2] net: force refragmentation for DF reassembed
 skbs

On Fri, May 22, 2015, at 21:03, David Miller wrote:
> From: Florian Westphal <fw@...len.de>
> Date: Fri, 22 May 2015 16:32:49 +0200
> 
> > IP (ttl 64, id 12345, offset 0, flags [+, DF], proto UDP (17), length 1204)
> >     192.168.7.1.42 > 10.23.42.2.42: UDP, length 1400
> > IP (ttl 64, id 12345, offset 1184, flags [DF], proto UDP (17), length 244)
> >     192.168.7.1 > 10.23.42.2: ip-proto-17
> 
> I almost consider a fragment with DF set an oxymoron.
> 
> How does this happen?
> 
> DF is a mechanism for PMTU discovery.  Therefore it is the end nodes
> which set DF on packets before fragmentation can even take place.
> Since DF is set initially, the logical consequence is that the packet
> cannot be legally fragmented along the way.
> 
> Are tunnels doing something to create this situation?

Hmm, interesting...

I think openvswitch still has this behavior that it always sets
ignore_df but DF bit control is up to user space.
Another situation where this can arise if a local socket with
ip_sk_ignore_df(sk) = 1 (we keep socket reference as long as possible ;)
) sends packets into a tunnel, like gre, where pmtudisc is enabled. It
should also look like those packets. This is because we don't clear
ignore_df during passing down the skb.

Bye,
Hannes
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