Simple script to change firewallrules in DD-WRT to handle if the main server is not accessible. Switches back again when the main server becomes accessible again.

simple_failover.sh

#!/bin/sh
MAIN_SERVER="192.168.1.8"
FAIL_OVER_SERVER="192.168.1.9"
PORT="80"
ACTIVE_SERVER=1
SAMPLING_TIME="5"

while :
do
  while ping -c 1 $MAIN_SERVER &> /dev/null
  do
    echo "Accessed"
    if [ "$ACTIVE_SERVER" -ne "1" ] ;
    then
      # MY enable firewall rule for server 2
      echo "enable 1"
      ACTIVE_SERVER=1
      iptables -D -t nat -I PREROUTING -p tcp --dport $PORT -j DNAT --to $FAIL_OVER_SERVER
      iptables -D -I FORWARD -p tcp -d $FAIL_OVER_SERVER --dport $PORT -j ACCEPT
      iptables -A -t nat -I PREROUTING -p tcp --dport $PORT -j DNAT --to $MAIN_SERVER
      iptables -A -I FORWARD -p tcp -d $MAIN_SERVER --dport $PORT -j ACCEPT
    fi
    sleep $SAMPLING_TIME # sleep 5 sec 
  done
  while ! ping -c 1 $MAIN_SERVER &> /dev/null
  do
    echo "Not Accessed"
    if [ "$ACTIVE_SERVER" -eq "1" ] ;
    then
      # MY enable firewall rule for server 2
      echo "enable 2"
      ACTIVE_SERVER=2
      iptables -D -t nat -I PREROUTING -p tcp --dport $PORT -j DNAT --to $MAIN_SERVER
      iptables -D -I FORWARD -p tcp -d $MAIN_SERVER --dport $PORT -j ACCEPT
      iptables -A -t nat -I PREROUTING -p tcp --dport $PORT -j DNAT --to $FAIL_OVER_SERVER
      iptables -A -I FORWARD -p tcp -d $FAIL_OVER_SERVER --dport $PORT -j ACCEPT
    fi
    sleep $SAMPLING_TIME # sleep 5 sec 
  done
done