linux-kernel - Re: NAT failure with TCP, too

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Date:	Sun, 22 Oct 2006 16:53:20 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Bernardo Innocenti <bernie@...eler.com>
CC:	Cristian Grigoriu <cristian.grigoriu@...vus.ro>,
	b.innocenti@...eler.com, lkml <linux-kernel@...r.kernel.org>
Subject: Re: NAT failure with TCP, too

Bernardo Innocenti wrote:
> 
> 
> Cristian Grigoriu wrote:
>> Hi,
>>
>> I can confirm the same bug you reported here 
>> http://www.ussg.iu.edu/hypermail/linux/kernel/0509.2/0279.html
>> This time it happens with TCP connections originating from the same 
>> TCP port (1234) from multiple machines. The SNAT simply doesn't take 
>> place and the normal routing occurs.
>>
>> Kernel is Debian stock 2.6.18-1.
>>
>> Please let me know if you have find a workaround.
> 
> It turned out that the real thing that was triggering the bug
> for me was unloading and reloading the ip_nat module without
> also reloading ip_conntrack.
> 
> The connection tracking tuple would remain in the kernel, visible
> in /proc/net/ip_conntrack, but no longer linked to the SNAT rule.
> I'd consider this a bug, but very few users will ever be affected.
> 
> The workaround for me was to remove my hand-cracted iptables
> rules from ppp's ip-up.local and move them to the distro-supplied
> iptables firewall instead.  The only downside is that I must now
> hardcode the destination ip of the SNAT rule because it's too early
> to read the interface address of ppp0.
> 

You may want to use the MASQUERADE target instead of the SNAT target.

	-hpa

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