Re: [RFC net-next] ipv6: Use destination address determined by IPVS

To: YOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@xxxxxxxxxxxxxx>, lvs-devel@xxxxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, Julian Anastasov <ja@xxxxxx>, Mark Brooks <mark@xxxxxxxxxxxxxxxx>
Subject: Re: [RFC net-next] ipv6: Use destination address determined by IPVS
From: Simon Horman <horms@xxxxxxxxxxxx>
Date: Wed, 16 Oct 2013 11:14:49 +0900
On Wed, Oct 16, 2013 at 02:53:22AM +0200, Hannes Frederic Sowa wrote:
> On Wed, Oct 16, 2013 at 09:02:31AM +0900, Simon Horman wrote:
> > In v3.9 6fd6ce2056de2709 ("ipv6: Do not depend on rt->n in
> > ip6_finish_output2()") changed the behaviour of ip6_finish_output2()
> > such that it creates and uses a neigh entry if none is found.
> > Subsequently the 'n' field was removed from struct rt6_info.
> > 
> > Unfortunately my analysis is that in the case of IPVS direct routing this
> > change leads to incorrect behaviour as in this case packets may be output
> > to a destination other than where they would be output according to the
> > route table. In particular, the destination address may actually be a local
> > address and empirically a neighbour lookup seems to result in it becoming
> > unreachable.
> > 
> > This patch resolves the problem by providing the destination address
> > determined by IPVS to ip6_finish_output2() in the skb callback.  Although
> > this seems to work I can see several problems with this approach:
> > 
> > * It is rather ugly, stuffing an IPVS exception right in
> >   the middle of IPv6 code. The overhead could be eliminated for many users
> >   by using a staic key. But none the less it is not attractive.
> > 
> > * The use of the skb callback is may not be valid
> >   as it crosses from IPVS to IPv6 code. A possible, though unpleasant,
> >   alternative is to add a new field to struct sk_buff.
> > 
> > * This covers all IPv6 packets output by IPVS but actually
> >   only those output using IPVS Direct-Routing need this.  One way to
> >   resolve this would be to add a more fine-grained ipvs_property to
> >   struct sk_buff.
> Hmm, that reminds me on the following bug report which would be nice we could
> solve in one go, too:

I think it should be possible to solve that using the
IP6CB() approach that Eric suggested. Hopefully we can make
that approach fly.
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