Loop back IP packets to a NIC?

beolsson · 12-15-2012, 01:42 PM

Hi Folks,

In order to do efficient bidirectional network testing, I would like to return all IP packets incoming to e.g. eth0, i.e. sapping source and destination address of all incomming packets and return them to eth0. Is this possible using iptables and NAT? Of course this can be done for a single port in user space but the gut feeling is that it should be possible and more efficient if done in the same way as conventional NAT.

Thanks

jefro · 12-17-2012, 02:30 PM

Create a loopback cable? Many loopback issues within a nic are not true full nic tests. The nic actually may never get the packet or only into half the nic and then returned to system.

beolsson · 12-19-2012, 10:18 AM

Well, a loopback cable is not really IP-savy.

I tried:

iptables -t nat -A PREROUTING -p tcp -m tcp -d 192.168.1.3 -j DNAT --to-destination 192.168.1.4

where 192.168.1.3 is the reflector machine (the one I am issuing the command on) and 192.168.1.4 is the machine to receive the redirected packets, i.e. the sender. However, no success... (to test I telnet 192.168.1.3 and would expect to connect to 192.168.1.4)

Any better ideas or do I miss something?

routers · 01-04-2013, 02:17 AM

IMO you need to do redirect port or call port forward for each program,
eg, telnet run on port 23 and create iptables rules this way

Code:

$IPT -t nat -I PREROUTING -p tcp -d 192.168.1.3 --dport 23 -j DNAT --to 192.168.1.4:23
or
$IPT -t nat -I PREROUTING -p tcp -d 192.168.1.3 --dport 23 -j DNAT --to 192.168.1.4

try it