Quick GNU/Linux Masquerade Script

quick-masq.sh

#!/bin/bash

###########################################
# Quick GNU/Linux Masquerade Script       #
# John Lettman <jlettman@openmailbox.org> #
###########################################

# Help text
read -d '' HELP <<- EOF
Usage: quick-masq [-h/--help | [IN_PORT] [OUT_PORT]]
Quickly initialize network masquerading (network gateway) via iptables and procfs.
  
Report bugs to: John Lettman <jlettman@openmailbox.org>
Source: 
EOF

IN_PORT=$1
OUT_PORT=$2

# Detect help condition
if [ $# -lt 2 -o "$1" == "-h" -o "$1" == "--help" ]; then
  echo "$HELP" # Print usage and help information if help condition is met
  exit
fi

# Check if running as root
if [ "$EUID" -ne 0 ]; then 
  echo "Please run as root."
  exit -1
fi

echo "Masquerading packets: [$IN_PORT] -> [$OUT_PORT]."

# Flush old entries from iptables (DESTRUCTIVE!)
iptables --flush
iptables --table nat --flush
iptables --delete-chain
iptables --table nat --delete-chain

# Add NAT and FORWARD table entries to enable masquerade per user configuration
iptables --table nat --append POSTROUTING --out-interface $OUT_PORT -j MASQUERADE
iptables --append FORWARD --in-interface $IN_PORT -j ACCEPT

# Enable kernel IPv4 packet forwarding functionality through procfs
echo 1 > /proc/sys/net/ipv4/ip_forward