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

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Jan 24, 2012 at 06:56:10PM +0100, Hans Schillstrom wrote:
> On Monday 23 January 2012 18:01:50 Pablo Neira Ayuso wrote:
> > Hi Hans,
> > 
> > On Mon, Jan 23, 2012 at 10:49:16AM +0100, Hans Schillstrom wrote:
> > > 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.
> > 
> > OK, it would be great if we can provide a cleaner interface. The
> > current behaviour uses layer3-layer4 tuple hashing plus defaulting to
> > layer3 in case of fragments.
> > 
> > I'd prefer explicit configuration options:
> > 
> > --hashmark-method layer3
> >         use only address for hashing, this is fragment safe.
> > 
> > --hashmark-method layer3-layer4
> >         use addresses and ports for hashing, fragments not supported
> >         unless defrag is enabled.
> > 
> > Still, if you want to support the current behaviour, it should be
> > something like:
> > 
> > --hashmark-method layer3-layer4-fragments
> >         use addresses and ports for hashing, for fragments default to
> >         layer3 hashing. Document scenario in which hash consistency
> >         may break.
> > 
> > The behaviour of the target has to be specified by the configurations.
> > Defaulting to internal assumptions seems obscure to me.
> > 
> OK this is resonable, and it makes the fragment problem visible.
> 
> I'll make the changes to day and have a test run for a couple of days.

Fine, thanks Hans.