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

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Message-ID: <20131016005322.GA18135@order.stressinduktion.org>
Date:	Wed, 16 Oct 2013 02:53:22 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Simon Horman <horms@...ge.net.au>
Cc:	YOSHIFUJI Hideaki / 吉藤英明 
	<yoshfuji@...ux-ipv6.org>, lvs-devel@...r.kernel.org,
	netdev@...r.kernel.org, Julian Anastasov <ja@....bg>,
	Mark Brooks <mark@...dbalancer.org>
Subject: Re: [RFC net-next] ipv6: Use destination address determined by IPVS

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: http://www.spinics.net/lists/netdev/msg250785.html

Greetings,

  Hannes

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