NetCat (nc) pipe. Using coproc available in bash 4.

ncpipe

#!/bin/bash

server=$1
port=$2
lport=${3:=$port}

# stdout will be server
# stderr will be client

exec 3>&1

# use coproc instead of named pipe.
# bash 4
coproc { nc $server $port | tee >( cat >&3 ) ;}

{ nc -lk -p $lport | tee >( cat >&2 ) ;} <&${COPROC[0]} >&${COPROC[1]}