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: <20140429143512.GC12781@macbook.localnet>
Date:	Tue, 29 Apr 2014 15:35:14 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Maxime Bizon <mbizon@...ebox.fr>
Cc:	Eric Dumazet <edumazet@...gle.com>, davem@...emloft.net,
	netdev <netdev@...r.kernel.org>
Subject: Re: problem forwarding IP fragments with DF bit set (caused by ipv4:
 fix path MTU discovery with connection tracking)

On Mon, Apr 28, 2014 at 07:59:40PM +0200, Maxime Bizon wrote:
> 
> On Mon, 2014-04-28 at 18:37 +0200, Maxime Bizon wrote:
> > 
> > conntrack causes the packets to be re-assembled, but since the
> > resulting skb now has IP_DF set, it fails the (DF + MTU) test in
> > ip_forward.c and causes ICMP frag_needed to be sent. 
> 
> hum isn't it just a matter of doing this ?

But why should we do this? The entire point of the patch was to fix PMTUD.

> --- a/net/ipv4/ip_fragment.c
> +++ b/net/ipv4/ip_fragment.c
> @@ -493,6 +493,8 @@ found:
>             skb->len + ihl > qp->q.max_size)
>                 qp->q.max_size = skb->len + ihl;
>  
> +       skb->local_df = 1;
> +
>         if (qp->q.last_in == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
>             qp->q.meat == qp->q.len) {
>                 unsigned long orefdst = skb->_skb_refdst;
> 
> 
> -- 
> Maxime
> 
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