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

From: Hans Schillstrom <[email-masked]>
Date: Wed, 4 Jan 2012 11:18:17 +0100

On Wednesday 04 January 2012 10:03:49 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.
> 

An other idea, turn off both conntrack and defrag
i.e. do like NOTRAC and rename the flag  ?

Quick example for IPv4:
--- a/net/ipv4/netfilter/nf_defrag_ipv4.c
+++ b/net/ipv4/netfilter/nf_defrag_ipv4.c
@@ -74,6 +74,14 @@ static unsigned int ipv4_conntrack_defrag(unsigned int hooknum,
...
+	const struct net_device *dev = (hooknum == NF_INET_LOCAL_OUT ?
+					out : in);
+
+	/* No defrag and not Previously seen (loopback)? */
+	if (dev_net(dev)->ct.sysctl_notrac_defrag && skb->nfct) {
+		/* Attach fake conntrack entry. as in NOTRACK */
+		skb->nfct = &nf_ct_untracked_get()->ct_general;
+		skb->nfctinfo = IP_CT_NEW;
+		nf_conntrack_get(skb->nfct);
+		return NF_ACCEPT;
+	}
...

-- 
Regards
Hans Schillstrom <[email-masked]>