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diff --git a/unix/iptables_portforwarding_nat.sh b/unix/iptables_portforwarding_nat.sh
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+#!/bin/sh
+#
+# Port forwarding from one address to another address in the same network,
+# using source and destination network address translation (SNAT & DNAT).
+#
+# The machine A performs this port forwarding to the target machine B,
+# which is in the same network as A.
+# The machine A behaves like a proxy, which allows e.g., external machine
+# access the services (e.g., SSH) on machine B which only allow access
+# from the internal network.
+#
+#
+# References:
+# [1] How to do the port forwarding from one ip to another ip in the same network?
+#     https://serverfault.com/a/586553/387898
+# [2] Source and Destination Network Address Translation with iptables
+#     https://thewiringcloset.wordpress.com/2013/03/27/linux-iptable-snat-dnat/
+# [3] How to List and Delete IPtables Firewall Rules
+#     https://www.digitalocean.com/community/tutorials/how-to-list-and-delete-iptables-firewall-rules
+#
+#
+# Weitian LI
+# 2016-11-29
+#
+
+
+# Enable IP forwarding
+sysctl net.ipv4.ip_forward=1
+
+# Save current rules
+iptables-save > iptables_rules.txt
+
+# Set default chain policy
+iptables -P INPUT ACCEPT
+iptables -P FORWARD ACCEPT
+iptables -P OUTPUT ACCEPT
+
+# Flush existing rules
+iptables -t nat -F
+iptables -t nat -X
+iptables -t mangle -F
+iptables -t mangle -X
+iptables -F
+iptables -X
+
+# Port forwarding using SNAT & DNAT
+THIS_IP="192.168.1.234"
+THIS_PORT="21127"
+TARGET_IP="192.168.1.248"
+TARGET_PORT="9999"
+echo "Port forwarding: ${THIS_IP}:${THIS_PORT} <-> ${TARGET_IP}:${TARGET_PORT}"
+iptables -t nat -A PREROUTING \
+         -p tcp --dport ${THIS_PORT} \
+         -j DNAT --to-destination ${TARGET_IP}:${TARGET_PORT}
+iptables -t nat -A POSTROUTING \
+         -p tcp -d ${TARGET_IP} --dport ${TARGET_PORT} \
+         -j SNAT --to-source ${THIS_IP}:${THIS_PORT}