I started writing a TCP tproxy library for golang at:
https://github.com/elico/go-linux-tproxy
It works pretty well and there is an example for a simple TCP proxy
which just peeks a bit into the beginning of the connection and then
connects the client and the server.
The example server was written for http and there for would require the
connection to be a Client sends data first state.
Later I will maybe provide an example which will test for both client
and server connections for data and then decide on what to do.
Eliezer