netdev - Re: GRO/GSO hiding PMTU?

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Message-ID: <20110210235022.GA25293@gondor.apana.org.au>
Date:	Fri, 11 Feb 2011 10:50:22 +1100
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: GRO/GSO hiding PMTU?

On Thu, Feb 10, 2011 at 02:55:55PM -0800, David Miller wrote:
>
> I suspect that the packet arrives on eth1, accumulates into GRO, and
> thus marked as GSO as well, then GSO/TSO on output to eth0 is
> re-segmenting things transparently, and we're not getting the ICMP
> frag-needed message and the packet drop because of the skb_is_gso()
> check in ip_forward().
> 
> 	if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) &&
> 		     (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
> 		IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
> 		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
> 			  htonl(dst_mtu(&rt->dst)));
> 		goto drop;
> 	}
> 
> So if that's what is happening, that's cute, but I think we need to
> fix this :-)

Yes this is a known problem and we do need to fix this, even if
it doesn't appear to be the cause of your immediate issue :)

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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