top button
Flag Notify
    Connect to us
      Site Registration

Site Registration

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

+1 vote

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

Share this question
Facebook Share Button Twitter Share Button LinkedIn Share Button

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
Similar Questions
+1 vote

Recent versions of the Linux kernel and the libnftnl library define nft expression types with the names "match" and "target". However, I could not find any reference to these expression types in the code of the nft user space utility, but only in the code for iptables.

Is it possible to access iptables matches and targets from rules defined with nft, or is this not intended?

0 votes

I am trying to queue the packets of a process so I can use libnetfilter_queue to modify them.

I read in the documentation that I should use --pid-owner processid to filter packets of a process and iptables -I

  -j NFQUEUE --queue-num  to add them to the queue.

I read the documentation but I am still confused how to do this. If any one can help me to understand this command I would appreciate it a lot.

+1 vote

I have multiple wan port and multiple routing table in a box. The wan port should be the final destination of outside word. (eg: all the packets come from internet should have destination IPAddress of my wan port, there are no further routing beyond the wan IPAddress).

rp_filter seems not work correctly under multiple routing table situation, so I need to set rp_filter to 0.

I want to know if there are simple setup method to secure the wan port ? or I need to use iptables to drop the incoming packet not matching the wan interface IP?

The problem is I have dynamic PPP interface, so matching the ip via iptables is a little complicated. so I want to know if there are smarter ways to do the job.

0 votes

I have a setup* in which I have a server with multiple virtual network interfaces (vif1 .. vifX) that are connected (bridged) to one virtual machine each (vm1 .. vmX) which all have the same IP and MAC (let's say resp. 00:11:22:33:44:55). In addition to that my server also has one regular network interface (eth0) connected to the rest of my network (

Now I would like to be able to provide NAT-like capabilities such that every packet coming from vm1 reaches the regular network as coming from, every packet coming from vm2 as and so on. Of course, packets coming from the network destined towards 10.0.1.x should also be correctly translated by the server such that vmX will receive them.

From what I understand I could use ebtables with the snat-target to rewrite the MAC addresses for the vifX bridges, such that the remaining network would see at least a different MAC address per VM. However, I cannot see an easy solution to rewrite the IP. Using iptables nat-table and the snat-target will not work, since I would need to also filter on the source-mac or incoming device, but the snat-target is only available on the POSTROUTING chain.

One possible to solution would be to write a small program that listens on a rawsocket on every interface, rewrites packets and re-sends them on the correct interface, but I'd rather avoid that and use standard tools instead.

I would be very grateful for ideas/tips,

+1 vote

If you have an 'accept' rule for a service that is not currently running, is it possible to have iptables to simply not respond instead of reporting the port as 'closed'? During a port scan at, if the router doesn't reply the port will be reported as 'stealth'.