port forward, nat

port_fwd.sh

#!/bin/sh

# RUN THIS ON $ME

echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -F
iptables -t nat -F
iptables -X

ME=192.168.1.123 # local ip to "impersonate" as
MY_PORT=1111

TARGET=172.16.1.1
TARGET_PORT=4444

iptables -t nat -A PREROUTING -p tcp --dport ${MY_PORT} -j DNAT --to-destination ${TARGET}:${TARGET_PORT}
iptables -t nat -A POSTROUTING -p tcp -d ${TARGET} --dport ${TARGET_PORT} -j SNAT --to-source ${ME}

# test:
# on $TARGET: nc -lk 4444
# on 3rd host (other than ME/TARGET): nc $ME 1111