Here you can find two TCP proxies. The first one, written in C, allows to send an encapsulated packet (with the packet header hardcoded) to an host.

The second one, written in Python, is more flexible and it gives the possibility to forge the packet header directly from the program arguments. For example:

./tcpproxy.py --bind-address 0.0.0.0 --port 10125 --dst-ip 192.168.30.13 --dst-port 32444 --recv-ip 192.168.30.19 --recv-port 8080

This will launch a proxy, listening in every interface on the port 10125, with the destination of 192.168.30.13 and port 32444, encapsulating the data in a packet that it will have a destination ip of 192.168.30.19 and a destination port of 8080.

To run the python script you need Python3 and Scrapy.