netdev - Re: [PATCH 2/3] NETFILTER module xt_hmark, new target for HASH based fwmark

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Date:	Mon, 23 Jan 2012 10:49:16 +0100
From:	Hans Schillstrom <hans.schillstrom@...csson.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
CC:	Hans Schillstrom <hans@...illstrom.com>,
	"kaber@...sh.net" <kaber@...sh.net>,
	"jengelh@...ozas.de" <jengelh@...ozas.de>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/3] NETFILTER module xt_hmark, new target for HASH based fwmark

On Monday 23 January 2012 10:12:41 Pablo Neira Ayuso wrote:
> On Mon, Jan 23, 2012 at 12:20:15AM +0100, Hans Schillstrom wrote:
> > The text should clarify that this is valid for the fragments not the "flow"
> > 
> > > I've got one scenario that may break with this assumption:
> > > 
> > > 1) your traffic follows one path over router A and B to reach your
> > >    firewall F which requires no fragmentation at all.

I missed the last part here  "requires no fragmentation at all"

> > > 2) path to router B becomes broken while there are established flows
> > >    with firewall F.
> > > 3) router A decides to forward packets to router C, which fragment
> > >    packets because it is using smaller MTU than router A.
> > > 4) packets arrive to firewall F, then hashing is calculated based on
> > >    addresses, not ports, and you load-sharing becomes inconsistent.
> > > 
> > > This can rarely happen, but it does, it would break.
> > > 
> > > To fix this, I think that HMARK requires that you have to specify the
> > > hashing strategy. If you want to support fragments, use only
> > > addresses. If you're sure you will not get fragments, use layer 3 and
> > > layer 4 information.

This can be acomplished by setting --hmark-sp-mask and --hmark-dp-mask to Zero
Then you don't use port in the hash calc.

> > 
> > I know but if you use conntrack, fragments will not be seen by HMARK
> > (except for IPv6 until Patric has fix the IPv6 defrag)
> 
> Please, read the scenario, I'm not talking about conntrack this time.
> 
> > We handle this by not having stateful FW:s when connected to external routers.
> > Fragments will take an extra turn to a container with conntrack and there
> > HMARK works as on the unfragmented packets in the flow.
> 
> Yes, I got the idea. Indeed HMARK can be very useful in other situations,
> like cluster-based OSPF setups with stateful firewalls following a
> similar approach.
> 
> However, you don't reply to my scenario. What I'm telling is that,
> even with conntrack disabled, HMARK is not consistent if you start
> receiving fragments at some point.
 
Yes, after reading once again I see what you mean.
I think that masking src & dst ports will be sufficient, 
i.e. no new param will be needed for hashing strategy.

> [...]
> > > > +/*
> > > > + * ICMP, get inner header so calc can be made on the source message
> > > > + *
> > > > + * iphsz: ip header size in bytes
> > > > + * nhoff: network header offset
> > > > + * return; updated nhoff if an icmp error
> > > > + */
> > > 
> > > Please, remove these comments:
> > 
> > No problems
> 
> Thanks.
> 
> > > > +struct _icmpv6_errh {
> > > > +	__u8		icmp6_type;
> > > > +	__u8		icmp6_code;
> > > > +	__u16		icmp6_cksum;
> > > > +	__u32		icmp6_nu;
> > > > +};
> > > 
> > > Interesting, by quick search, I don't find this structure defined
> > > elsewhere, why?
> > > 
> > I have no idea ...
> > the closest is "struct icmp6hdr" but it contains everythingi
> 
> have a look at offsetof, you can use the existing structure but tell
> skb_copy_header to copy only the part you're interested. Add a comment
> telling what you're only copying part of the header to warn others (in
> this case, the comment becomes useful since it clarifies something
> that you may not notice at a first glance by looking at the code).
> 

OK I'll do that.

> [...]
> > > > +	if (!info->hmod)
> > > > +		return XT_CONTINUE;
> > > 
> > > why this? check in user-space that libxt_HMARK does not send this to
> > > kernel-space and check it again in checkentry().
> > 
> > Well, better safe than ... divide by zero
> > 
> > OK, it very very unlikely that it becomes zero
> > so if you want I can remove that check.
> 
> *Extremely unlikely*, I'd say :-). If you double check that hmod is
> non-zero in user-space and checkentry(), we will not hit that branch
> ever. Moreover, that branch is in the hot path while the others are
> only configure-time paths.

Got the message :-)
I'll remove it.

-- 
Regards
Hans Schillstrom <hans.schillstrom@...csson.com>
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