#!/bin/bash
# isgate.sh -- The gateway disclosure script (2011/04/05)
#
# "THE BEER-WARE LICENSE" (Revision 42):
# <vladz@devzero.fr> wrote this file. As long as you retain this notice
# you can do whatever you want with this stuff. If we meet some day, and
# you think this stuff is worth it, you can buy me a beer in return. -V.
#
# Description:
# ------------
# Check if a remote host on a LAN is able to forward network packets.  On
# linux boxes, it means that the host has "ip_forward" set to '1' inside
# its kernel options.  In other words and in most of cases, that another
# network is behind that host.
#
# See technical details: http://vladz.devzero.fr/vladz-bash.php#isgate
#
# Scenario and usage:
# -------------------
# 1) Find a box that replies to ICMP requests:
#
#    # ping 192.168.0.16
#    PING 192.168.0.16 (192.168.0.16) 56(84) bytes of data.
#    64 bytes from 192.168.0.16: icmp_req=1 ttl=64 time=2.87 ms
#
# 2) As root, check targets (2nd arguments):
#
#    # ./isgate.sh
#    usage: ./isgate.sh <pingable-ip> <target-ip>
#    # ./isgate.sh 192.168.0.16 192.168.0.51
#    192.168.0.51: IP forward enabled

fatal(){ echo "${1}" && exit 1;}

if [ $# -ne 2 ]; then
   fatal "usage: ${0} <pingable-ip> <target-ip>"
fi

for ip; do
   ping -c 1 ${ip} &>/dev/null || \
      fatal "${ip} does not answer to ICMP packets"
done

mac=( $(awk -v p_ip="$1" -v t_ip="$2" '{
  if($1 == p_ip) p_mac = $4;
  if($1 == t_ip) t_mac = $4;
} END {print p_mac" "t_mac}' /proc/net/arp) )

if [ ${#mac[*]} -ne 2 ]; then
  fatal "One of both IP is not on the LAN or is local address"
fi

arp -s ${1} ${mac[1]}
ping -w 1 -c 1 ${1} &>/dev/null && status="enabled"
arp -s ${1} ${mac[0]}

echo "${2}: IP forward ${status:-disabled}"