Loopback an Interface

[tm37]

I was wondering if anyone knew how to make a network interface into a loopback. I want one network interface to simply repeat everything it recieves back out. Any ideas?

[amitsharma_26]

I want one network interface to simply repeat everything it recieves back out. Any ideas?

Can you explain it with more verbosity ?
Do you want to echo everything you are recieving at any ethernet interface ? or do you want redirect packets you are recieving at any ethernet device into some kind of loopback ?
I am still confused about your query.

[tm37]

I have a NIC in server running gentoo. I want that NIC to repeat everything that it recieves from the cable plugged into it back out that same cable. I am finding this very hard to explain. I am sending packets at high speed to this NIC and I want the exact same packets returned to me.

[amitsharma_26]

iptables -t nat -PREROUTING -p ALL -d <thix-box-ip> -j DNAT --to <me(the box you are sending packets from)>
iptables -t nat -POSTROUTING -p ALL -d <me> -j SNAT --to <this-box-ip>

Initially we were getting packets as,
Source = Me & Destination = this-box
& now we have changed them to
Source = this-box & Destination = Me.

But i am still not impressed about why do you want to do that.

[amitsharma_26]

You can get a good idea of port forwarding, packet forwarding with iptables & SNAT, DNAT at http://amitsharma.linuxbloggers.com/portforwarding.htm

Amit..