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iptables DNAT reply packets with RST flag are sent using private ip?

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I'm looking at a strange phenomenon that occurs on an iptables firewall. There is a DNAT rule configured that maps a public IP to a private one where a web serve is listening. Normal request operate as expected that is the destination ip is modified to the private one when the request arrives at the firewall and on the response packet the private ip is mapped back to the public one.
What I noticed though is that for some response packets the source ip is *not* mapped back to the public ip and instead tcpdump shows that the packets are sent out with the private source ip. The thing all these packets have in common is that they have the RST flag set.

What could be the reason for this? Is there some particular iptables behavior that could explain this?

posted Sep 28, 2016 by anonymous

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1 Answer

0 votes

Try blocking --state INVALID packets.

I think RST retransmits would fall into your described case - the first RST has already removed the nat/conntrack entry, and so the second does not match on it anymore to rewrite.

(I have quite a lot of INVALID drops in my logs on a quite similar setup)

answer Sep 28, 2016 by Navneet
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