Re: NAT in an already established TCP connection

[Julien Vehent <julien@linuxwall.info>]

On Tue, 28 Oct 2008 12:25:13 +0100, Diego Casado Mansilla
<diego.casadomansilla@gmail.com> wrote:
> Hello all!!!
> 
> This is my first mail in the list.
> 
> Hopefully the question is interesting and you can figure out how to help
> me.
> 
> I use iptables rules to manage the connections from internet to my local 
> network. I know how to filter, do nat, etc...
> But this days I'm trying to do NAT in connections that are already 
> established. The problem is (as far as I know) the packets which pass 
> throught the nat table are only the SYN packets (once), thus, the 
> packets that are used to perform a NEW connection.
> 
> After that the connection is created, the maintenance and the resolution 
> of the SNAT and DNAT are kept till the connection finish.
> What I'm wondering is: how can I change the ports or IPs of an already 
> established connection if my packets just go throught the nat table at 
> the connection time?
> 

What you want is to redirect an existing connection to a new destination.
If you use TCP protocol, the only way to do that is to record the current
connection and, in parralel, create a second connection to you new
destination and replay the payload on the packet in the new one.

If you use UDP, this doesn't apply since there's no connection tracking in
the UDP protocol. Netfilter, however, does some connection tracking on UDP
packets, so make some test to see if it's doable.

I had to solve the same problem in a honeypot project to redirect active
connections from low interaction honeypots to high interactions honeypots.
The solution I choosed was to queue the connections in userland using
netfilter_queue, process them and replay those I've selected to the new
destination in parrallel, and then drop the packet from the initial
connection.

It's tricky to do, and there's many issue to solve, but AFAIK this is the
most reliable solution.

> **** Maybe doing packets' replication since those ones are redirected to 
> annother machine?

Note : window-tracking is for the tracking of the window size in the tcp
header, it has nothing to do with this.


> 
> **** NAT TCP Extensions??Patch-O-Matic --> window-tracking??
> 
> **** I read this in an interntet site:
> 
> --- NEW (and RELATED non-icmp)
>     This is a very important part relevant for understanding the whole
NAT
>     subsystem. Only if the packet has the state NEW (i.e. it would
>     establish
>     a new connection, if we'd accept it), the NAT table is traversed by
>     calling ip_nat_rule.c:ip_nat_rule_find(), which in turn calls
>     ip_tables.c:ipt_do_table() for the actual IP table traversal. The 
> traversal
>     ends up in either ACCEPTing the packet as it is, or one of the nat 
> targets
>     (SNAT, DNAT and if loaded: REDIRECT, MASQUERADE) Please see
>     chapter FIXME for further description of those targets.
> 
> --- ESTABLISHED
>     This packet belongs to an already established connection. We don't
need
>     to traverse the NAT table again, as the necessary information
>     (struct ip_nat_info) was already gained Hello everybody,
> 
> 
> Thank you very much in advance and if my questions are not clear don't 
> doubt to send me a message.
> 
> Diego.
> 
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Regards,
Julien

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