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

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Date:	Wed, 18 Feb 2009 11:49:58 -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 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?

Alan Stern

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