Magic Hosts Switcher (Bash Script)

magic_hosts

#!/bin/bash

# Magic Hosts Switcher
# Automatically detects a target network and turns on/off LAN routing in
# your /etc/hosts file accordingly.
#
# Setup: In your root crontab, add and edit the following...
#     * * * * * /path/to/magic_hosts
#
# AUTHOR(S): Jason C. McDonald
# VERSION: 1.2
# LAST UPDATED: 27 April 2018

# Copyright (c) 2018 Jason C. McDonald
# All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#     * Redistributions of source code must retain the above copyright
#       notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above copyright
#       notice, this list of conditions and the following disclaimer in the
#       documentation and/or other materials provided with the distribution.
#     * Neither the name of the <organization> nor the
#       names of its contributors may be used to endorse or promote products
#       derived from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
# DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT HOLDER> BE LIABLE FOR ANY
# DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
# SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

# CHANGE ME: Watch for a network with this name
NETWORK="mcdhome"

# Store a sentinel file here
SENTINEL='/tmp/.magic_host_run'

# If we're not root, just abort.
if [ "$EUID" -ne 0 ]
then
    echo "Please run as root"
    exit
fi

# Check if we're on the target network
athome=`/sbin/iwconfig 2>&1 | grep -c ${NETWORK}`
nowifi=`/sbin/iwconfig 2>&1 | grep -c 'ESSID:off/any'`

# If the senintel file exists...
if [ -e ${SENTINEL} ]
then
    # Load our previous value from it
    stored=`cat ${SENTINEL}`
else
    # Otherwise, store our new value to it...
    echo ${athome} > ${SENTINEL}
    # ...but make sure we DO make the change below.
    stored='-1'
fi

# If we haven't changed networks since last time...
if (( athome == stored ))
then
    # Report that all is well and exit.
    echo "No change made"
    exit
# If we don't have a network connection...
elif [ ${nowifi} -eq 1 ]
then
    # Do nothing and exit
    echo "No wifi; no change made"
    exit
# Otherwise, if we changed networks...
else
    # Backup our /etc/hosts file
    cp /etc/hosts /etc/hosts.bak
    # If we're on the target network...
    if [ ${athome} -eq 1 ]
    then
        # Turn on our routing by LAN IP
        echo "Enabled LAN routing"
        sed -i 's/#192/192/' /etc/hosts
    # Otherwise, if we're on another network...
    else
        # Shut off our routing by LAN IP
        echo "Disabled LAN routing"
        sed -i 's/192/#192/' /etc/hosts
    fi
    # Store whether we were on target network
    echo ${athome} > ${SENTINEL}
fi