linux-kernel - [BUG] SNAT sometimes allows packets to pass through unchanged

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Date:	Wed, 11 Feb 2009 12:37:25 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	netdev@...r.kernel.org
cc:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: [BUG] SNAT sometimes allows packets to pass through unchanged

I'm seeing some strange behavior on my firewall, which is running 
Fedora 8's version of 2.6.26.  Every so often a packet with a private 
source address is sent out the public interface unchanged, when it 
should be dropped.

This happens when internal hosts are slow to close their end of a TCP 
connection.  For example:

	Internal host A (using a private address) initiates a TCP
	connection to an external server B.

	Data is sent back and forth.

	External host B sends a FIN and host A responds with ACK.

	Several minutes later (after the tracking for this connection
	has expired), host A sends a FIN to host B.  This packet
	goes through the firewall unchanged and is sent out the
	public interface with the private source address intact.

Now I would expect that such packets would be dropped, because they 
don't belong to an existing connection and they can't be the start of a 
new connection.  The fact that this doesn't happen indicates there is a 
bug in the netfilter code somewhere.

For reference, here is a lightly-edited extract from my iptables 
script on the firewall:

------------------------------------------------------------------------
# Use SNAT for new outgoing connections from private addresses.
OUTIP=...  # The IP address of the firewall's public interface, eth1
iptables -t nat -N startnat
iptables -t nat -A startnat -p tcp -j SNAT --to-source $OUTIP:7000-65500
iptables -t nat -A startnat -p udp -j SNAT --to-source $OUTIP:7000-65500
iptables -t nat -A startnat -p icmp -j SNAT --to-source $OUTIP
iptables -t nat -A startnat -j DROP

iptables -t nat -A POSTROUTING -o eth1 -s 10.0.0.0/8 -j startnat
------------------------------------------------------------------------

Can anyone help figure out what's wrong?

Alan Stern

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