Re: [PATCH 05/19] netfilter: nf_conntrack_ipv6: improve fragmentation handling

Patrick McHardy
2012-08-27T10:41:15Z

On Mon, 27 Aug 2012, Jesper Dangaard Brouer wrote:

>>> When loading "nf_conntrack_ipv6" I run into issues.
>>>
>>> When sending a fragmented UDP packet. With these patches, the IPVS
>>> stack will no longer see the fragmented packets, but instead see one
>>> large SKB. This will trigger a MTU path check in e.g.
>>> ip_vs_dr_xmit_v6() and an ICMPv6 too big packet is send back.
>>>
>>> IPVS: ip_vs_dr_xmit_v6(): frag needed
>>>
>>> Perhaps we could change/fix the MTU check in IPVS?
>>> (This would also solve issues I've seen with TSO/GSO frames, hitting
>>> this code path).
>>
>> I guess this should use the same check as in IPv6 output, check
>> whether IP6CB(skb)->max_frag_size is != 0 and > MTU and only send
>> an ICMPv6 error in that case.
>
> Hans have (already) proposed this solution:
>
> if ((!skb->local_df && skb->len > mtu && !skb_is_gso(skb)) ||
> (IP6CB(skb)->frag_max_size && IP6CB(skb)->frag_max_size > mtu)) {
>
> And I have tested it works.
> But I'm not sure about, if we really need the "!skb->local_df" check ?

Not necessarily, alternatively you could use:

if ((!IP6CB(skb)->frag_max_size &&
(skb->len > mtu && !skb_is_gso(skb)) ||
IP6CB(skb)->frag_max_size > mtu)

We just need to make sure that if frag_max_size is set that skb->len
is ignored. Either way is fine, but I'd suggest to keep it similar to
the variant used in IPv6.

> Thus, we should extend you patchset with a patch, that also address the
> MTU checks in IPVS.

Can you send over a patch for inclusion in the patchset? Besides this, its
ready for submission.