netdev - Re: [PATCH 1/1] netfilter: Add possibility to turn off netfilters defrag per netns

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Date:	Wed, 4 Jan 2012 10:49:09 +0100 (CET)
From:	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
To:	Jan Engelhardt <jengelh@...ozas.de>
cc:	Hans Schillstrom <hans.schillstrom@...csson.com>,
	Patrick McHardy <kaber@...sh.net>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"hans@...illstrom.com" <hans@...illstrom.com>
Subject: Re: [PATCH 1/1] netfilter: Add possibility to turn off netfilters
 defrag per netns

On Wed, 4 Jan 2012, Jan Engelhardt wrote:

> On Wednesday 2012-01-04 10:03, Jozsef Kadlecsik wrote:
> 
> >On Wed, 4 Jan 2012, Hans Schillstrom wrote:
> >
> >> On Wednesday 04 January 2012 09:28:05 Jozsef Kadlecsik wrote:
> >> > 
> >> > On Wed, 4 Jan 2012, Hans Schillstrom wrote:
> >> > 
> >> > > In some cases it not desirable to have auto defrag.
> >> > > Ex. in a cluster where packets can arrive on different blades.
> >> > > In that case it is possible to use containers (LXC) and send
> >> > > all fragments to one place where defrag is enabled.
> >> > > 
> >> > > This patch makes it possible to turn off the defrag per network name space,
> >> > > by setting net.netfilter.nf_conntrack_nodefrag to 1.
> >> > > Both IPv4 and IPv6 is effected by this sysctl.
> >> > > Default is 0 which is defrag.
> >> > 
> >> > Conntrack assumes that the packets are defragmented and will drop any 
> >> > unfragmented one. So your patch results packet drops.
> >> 
> >> Hmmm, more work...
> >> > 
> >> > Also, if you want to disable defragmentation then why don't you simply 
> >> > "mark" the packets with the NOTRACK target?
> >> 
> >> I don't think that will work since NF_IP_PRI_CONNTRACK_DEFRAG is -400
> >
> >Then change NF_IP_PRI_RAW so that it precedes NF_IP_PRI_CONNTRACK_DEFRAG. 
> >The raw table should be made possible to completely override conntack and 
> >defrag is implicit part of the latter.
> 
> We've been there (me in the thread even) - defrag is running before raw,
> because otherwise you could not select packets based upon L4 
> parameters for non-defrag in the first place:
> 
> 	-t raw ... -p udp --dport 53 -j CT --notrack
> 
> Not that I overly care about whether defrag is before/after raw..

You mean "for non-conntrack", but you are right. Nice cicle :-).

So we can sum up that either the system has got conntrack enabled which
requires defrag, or there's no defrag but then there's no conntrack at 
all.

Best regards,
Jozsef
-
E-mail  : kadlec@...ckhole.kfki.hu, kadlecsik.jozsef@...ner.mta.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : Wigner Research Centre for Physics, Hungarian Academy of Sciences
          H-1525 Budapest 114, POB. 49, Hungary
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