netdev - Re: [RFC PATCH net-next 0/7 v2]IPv6:netfilter: defragment

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Date:	Thu, 25 Mar 2010 10:23:38 +0100 (CET)
From:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
cc:	Patrick McHardy <kaber@...sh.net>,
	Shan Wei <shanwei@...fujitsu.com>,
	YOSHIFUJI Hideaki <hideaki.yoshifuji@...il.com>,
	David Miller <davem@...emloft.net>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Yasuyuki KOZAKAI <yasuyuki.kozakai@...hiba.co.jp>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	netfilter-devel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 0/7 v2]IPv6:netfilter: defragment

On Thu, 25 Mar 2010, YOSHIFUJI Hideaki wrote:

> (2010/03/24 5:10), Jozsef Kadlecsik wrote:
> 
> > On Wed, 24 Mar 2010, YOSHIFUJI Hideaki wrote:
> > 
> > > > In this case without conntrack, IPv6 would send an ICMPv6 message,
> > > > so in my opinion the transparent thing to do would be to still send
> > > > them. Of course only if reassembly is done on an end host.
> > > 
> > > Well, no.  conntrack should just forward even uncompleted fragments
> > > to next process (e.g. core ipv6 code), and then the core would send
> > > ICMP error back.  ICMP should be sent by the core ipv6 code according
> > > to decision of itself, not according to netfilter.
> > 
> > But what state could be associated by conntrack to the uncompleted
> > fragments but the INVALID state? In consequence, in any sane setup, the
> > uncompleted fragments will be dropped silently by a filter table rule
> > and no ICMP error message will be sent back.
> > 
> > Therefore I think iff the destination of the fragments is the host
> > itself, then conntrack should generate an ICMP error message. But that
> > error message must be processed by conntrack to set its state and then
> > the fate of the generated packet can be decided by a rule.
> 
> Well.... no.
> 
> First of all. in "sane" setup, people should configure according
> to their own requirements.  They may or may not want send back
> icmp packet.  And, even if the core is to send icmp back, the
> state would be correctly assigned.

I meant the state of the fragmented packets. If we let the uncompleted 
fragments to enter conntrack, as far as I see their state will be INVALID. 
Or should we add an exception and set their state to UNTRACKED in 
conntrack?
 
> We cannot (and should not) do something "cleaver" (excluding
> packet drops) in conntrack in PRE_ROUTING chain.

Actually, I have to agree with you.
 
> One reason is that in PRE_ROUTING context, we can NOT determine
> if the address we see in the IP header is really the final
> destination.  The overall situation is the same even if the
> routing entry corresponding the "current" destination points
> the node itself, or even if the node is configured as host.
> 
> It might seems that we could do something in the "filter"
> table in LOCAL_IN, FORWARD or LOCAL_OUT (after routing header
> process).
> 
> But well, we unfortunately cannot do this (at least
> automatically) because even in LOCAL_IN, the final
> destination has not been decided until we process all
> of extension headers.
> 
> Sending ICMP in netfilter (especially in conntrack) is too
> patchy, and is not right.  If we do the right thing (and
> I believe we should do so),  I'd propose to have hooks
> around handlers inside ip6_input_finish().
> 
> ...I remember that I was thinking about this before.
> 
> For my conclusion, first option is just to drop
> uncompleted fragments as we do today.  Second option
> would be  to forward them to the next process so that
> core code could send ICMPv6 etc. or, we could have
> new code to send ICMPV6_TIME_EXCEED in REJECT target.
> In longer term, I think it is better to introduce
> per-exthdr hooks.

I agree with your conclusion too, except a few question.

It is unclear for me how can you forward the packets to the next process: 
above you pointed out that in defrag/reassembly before conntrack we do not 
know yet whether the packets are destined to the host or not. So again, 
how would you let through the fragments on conntrack then?

I don't know how could the REJECT target help in any way.

This is not a simple case at all and I have to think that the "best" way 
just to drop the packets as we currently do.

Best regards,
Jozsef
-
E-mail  : kadlec@...ckhole.kfki.hu, kadlec@...l.kfki.hu
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Address : KFKI Research Institute for Particle and Nuclear Physics
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