netdev - Re: problem forwarding IP fragments with DF bit set (caused by ipv4: fix path MTU discovery with connection tracking)

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Message-ID: <1398785841.29914.146.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 29 Apr 2014 08:37:21 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	mbizon@...ebox.fr
Cc:	Patrick McHardy <kaber@...sh.net>,
	Eric Dumazet <edumazet@...gle.com>, davem@...emloft.net,
	netdev <netdev@...r.kernel.org>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: problem forwarding IP fragments with DF bit set (caused by
 ipv4: fix path MTU discovery with connection tracking)

On Tue, 2014-04-29 at 17:23 +0200, Maxime Bizon wrote:
> On Tue, 2014-04-29 at 15:45 +0100, Patrick McHardy wrote:
> 
> > Right, that is not correct of course. We save the original packet size
> > and should either refragment to that size or send an ICMP frag required
> > if the original size exceeds the outgoing MTU.
> 
> yep indeed, if any fragment is bigger than outgoing MTU
> 
> > So your patch does look correct, however we should probably only set
> > local_df in conntrack defrag.
> 
> I thought of putting it in conntrack code first, but I had some doubts.
> There are a lot of possible paths that can be taken after reassembly
> (ipsec, tunnel, ...), and I did not audit all of them.
> 
> Since the patch that caused this regression modified the generic
> fragmentation code, it may have caused other silent breakage.
> 
> Eric/David, could I get your feeling about this ?

Hmm... maybe its finally time to rename local_df to ignore_df, as
Maze suggested some time ago ...


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