netdev - Re: [BUG] SNAT sometimes allows packets to pass through unchanged

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Date:	Wed, 18 Feb 2009 12:41:52 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Patrick McHardy <kaber@...sh.net>
cc:	netdev@...r.kernel.org,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] SNAT sometimes allows packets to pass through unchanged

On Wed, 18 Feb 2009, Patrick McHardy wrote:

> Alan Stern wrote:
> > On Mon, 16 Feb 2009, Patrick McHardy wrote:
> > 
> >> The NAT table only sees the first packet of every connection
> >> and never INVALID packets. The mangle table should work fine.
> > 
> > I ended up adding a rule to the FORWARD chain of the filter table.  
> > The trick was to select based on the state.  That worked; it saw all
> > those un-NATed packets and was able to eliminate them.  In case you're
> > curious, the rule was essentially this:
> > 
> > iptables -A FORWARD -o eth1 -s 10.0.0.0/8 -m state --state INVALID -j DROP
>  >
> > Ideally, the rule should select all the packets which haven't been
> > altered by SNAT, not just the ones marked INVALID.  Is there any way to
> > do this?
> 
> Not in the sense that you could somehow catch valid packets "missed"
> by SNAT, that would be a bug. The conntrack match supports matching
> on whether the state says that a packet should be NATed.

You mean I should do something like this?

iptables -A FORWARD -o eth1 -s 10.0.0.0/8 -m conntrack --ctstate ! SNAT -j DROP

The man page says that --ctstate doesn't support using ! for negation,
but that detail is easily worked around.

Alan Stern

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