Bash Script to check and shift ISP if unreachable || linuxtopic

Bash script to check internet connection, This is a script to change ISP if it’s unreachable, i have 2 ISP and  3 interface eth0 eth1 and eth2, My eth0 is connected on Intranet and ISP connected to eth1 and eth2. This script ping my Primary ISP if its reachable exit if it’s not reachable  set secondary isp


vi  /usr/local/bin/shift_gateway

#!/bin/bash

# by lokesh carpenter #

# Change Gateway According to your ISP/Need

P_Gateway=182.152.162.xx

S_Gateway=179.158.21.xx

# Change DNS According to your ISP/Need

P_DNS1=8.8.8.8

P_DNS2=8.8.4.4

S_DNS1=8.8.8.8

S_DNS2=8.8.4.4

# Change Network According to your setup

MASQUERADE=172.17.0.0/16

Gateway=`/sbin/ip route | grep "default" | awk '{print $3}'`

/bin/ping $P_Gateway -w 30 > /dev/null

if [ $? -ne 0 ];then

       echo "Primary gateway $P_Gateway is not Reachable";

/bin/ping $S_Gateway -w 30 > /dev/null

       if [ $? -ne 1 ];then

    if [ $Gateway = $S_Gateway ];then

                   echo "Secondary Gateway $S_Gateway Already Added"

           else

                   /sbin/ip r d default ; /sbin/ip r a default via $S_Gateway dev eth2;

                   echo "Secondary Gateway $S_Gateway Successfully Added"

                   /sbin/iptables -t nat -D POSTROUTING -s $MASQUERADE -o eth1 -j MASQUERADE

                   /sbin/iptables -t nat -A POSTROUTING -s $MASQUERADE -o eth2 -j MASQUERADE

                   #echo "nameserver $S_DNS1" > /etc/resolv.conf

                   #echo "nameserver $S_DNS2" >> /etc/resolv.conf

           fi;

    else

          echo "$P_Gateway and $S_Gateway both are down, Please Contact to ISP"

    fi;

else

       if [ $Gateway = $P_Gateway ];then

               echo "Primary Gateway $P_Gateway Already Added"      

       else

               /sbin/ip r d default ; /sbin/ip r a default via $P_Gateway dev eth1;

               echo "Primary Gateway $P_Gateway  Successfully Added"

               #echo "nameserver $P_DNS1" > /etc/resolv.conf

               #echo "nameserver $P_DNS2" >> /etc/resolv.conf

               /sbin/iptables -t nat -D POSTROUTING -s $MASQUERADE -o eth2 -j MASQUERADE

               /sbin/iptables -t nat -A POSTROUTING -s $MASQUERADE -o eth1 -j MASQUERADE

       fi;

fi;

exit 0;

Explanation :

Gateway=`/sbin/ip route | grep "default" | awk '{print $3}'` : checking current system gateway,

/bin/ping $P_Gateway -w 30  : ping primary gateway for 30 seconds

if [ $? -ne 0 ]; : if  response is 0 then  

/bin/ping $S_Gateway -w 30  : ping secondary gateway

if [ $? -ne 1 ]; : set secondary gateway otherwise echo both gateway are down.

if [ $Gateway = $S_Gateway ];then :check  secondary gateway,  if already  found echo “Already Set S_Gateway, otherwise set Secondary Gateway

/sbin/iptables -t nat -D POSTROUTING -s $MASQUERADE -o eth1 -j MASQUERADE delete nat rules for eth1

/sbin/iptables -t nat -A POSTROUTING -s $MASQUERADE -o eth2 -j MASQUERADE

Set nat rules for eth2

#echo "nameserver $S_DNS1" > /etc/resolv.conf

#echo "nameserver $S_DNS2" >> /etc/resolv.conf : change dns entry as per ISP , here i comment because i used 8.8.8.8 & 8.8.4.4

else

if [ $Gateway = $P_Gateway ]  echo "Primary Gateway $P_Gateway Already Added"   

else set  $P_Gateway on dev eth1;

#echo "nameserver $P_DNS1" > /etc/resolv.conf

#echo "nameserver $P_DNS2" >> /etc/resolv.conf

change dns entry as per ISP , here i comment because i used 8.8.8.8 & 8.8.4.4

/sbin/iptables -t nat -D POSTROUTING -s $MASQUERADE -o eth2 -j MASQUERADE delete nat rules for eth2

/sbin/iptables -t nat -A POSTROUTING -s $MASQUERADE -o eth1 -j MASQUERADE Set nat rules for eth1

Set Execution Permission

chmod +x /usr/local/bin/shift_gateway

Set Cron for Checking Gateway in every minutes

vi /etc/cron.d/gateway_check

* * * * *  /bin/bash  /usr/local/bin/shift_gateway >> /var/log/shift_gateway-`date -I`.log