netdev - Re: Why can't we SNAT (or inverse DNAT) in PREROUTING?

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Message-ID: <20150627085007.GA4910@electric-eye.fr.zoreil.com>
Date:	Sat, 27 Jun 2015 10:50:07 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Network Development <netdev@...r.kernel.org>
Subject: Re: Why can't we SNAT (or inverse DNAT) in PREROUTING?

Andy Lutomirski <luto@...capital.net> :
> On Fri, Jun 26, 2015 at 3:48 PM, Francois Romieu <romieu@...zoreil.com> wrote:
> > Andy Lutomirski <luto@...capital.net> :
[...]
> >> Shouldn't the order of operations be:
> >>
> >> 1. Check rp_filter.
> >> 2. Handle NAT.
> >> 3. Routing decision.
> >> ?
> >
> > The admittedly painful fwmark part would still be needed for pre-NAT
> > source address based policy routing (assuming SNAT loses valuable policy
> > information). Life would be easier for your current requirements but
> > some different policy requirements would be unable to avoid the
> > fwmark/mangle style stuff.
> 
> What kind of policy routing would care about the pre-NAT source
> address?  AIUI, the usual use of policy routing is to *route*, not to
> filter. But maybe I'm missing something.

?

The same could be said regarding your (post NAT) source address policy
routing requirement.

> > Btw, the suggested scheme implies that filtering between SNAT and DNAT
> > would be done before routing, thus without INPUT vs FORWARD tainting.
> 
> What do you mean by "filtering between SNAT and DNAT"?

I had Jan Engelhardt's netfilter packet flow diagram in mind.

Replacing it with a 3 steps packet processing conceptual flow does not
seem trivial to me.

-- 
Ueimor
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