I'm implementing some kind of router which has multiple internet connections, based on current throughput of those connections it would forward packets/frames using faster connection.

In order to test my implementation I have to have some actual packets to route. In order to generate some traffic I implemented IP packet fragmentation. I almost found myself implementing TCP reliable connection to generate some traffic. I tried to look for a way to generate some traffic and thought I would open socket connection from IP A on local machine to IP B on remote host. I would send some data on that connection so I wouldn't have to bother with packet generation.

And here is the problem, I must intercept those packets in order to route them in my router. I initially thought I would create some rule to forward every packet to IP B back to localhost, I would open raw socket on localhost and filter out all packets with destination IP B. I tried to make iptables rule: sudo iptables -t nat -A OUTPUT -d some_ip -j DNAT --to-destination 127.0.0.1 but as I found out it was changing destination IP address of packet, and I just wanted to intercept original packet in order to route it.

Do you have any idea how to generate some traffic that I could capture or how to configure iptables to forward packets going to specified IP back to host but preserving original destination IP address?