An example failover script for dual WAN, using a ping healthcheck and managing default routes appropriately

failover.sh

#!/bin/bash
#
# Based on https://gist.github.com/Apsu/5021255 with two network interfaces,
# gateways can conneted on different network interfaces or in same network.

# Set defaults if not provided by environment
CHECK_DELAY=${CHECK_DELAY:-5}
CHECK_LOOPS=${CHECK_LOOPS:-3}
CHECK_IP=${CHECK_IP:-8.8.8.8}
PRIMARY_GW=${PRIMARY_GW:-192.168.0.1}
BACKUP_GW=${BACKUP_GW:-192.168.0.2}

# Compare arg with current default gateway address
gateway_ip() {
  [[ "$1" = "$(ip route list | sed -rn 's/^default.*via ([^ ]*).*$/\1/p')" ]]
}

trap "logger -t failover 'Terminated'; route del -host $CHECK_IP gw $PRIMARY_GW" INT TERM
logger -t failover "Started with route to `ip route get $INTERNET_IP | sed -rn 's/^.*via ([^ ]*).*$/\1/p'`"

# Setup host route to continuous check primary gateway
# Ignore error, if route always set
route add -host $CHECK_IP gw $PRIMARY_GW

# Cycle healthcheck continuously with specified delay
while sleep $CHECK_DELAY
do
  # If healthcheck succeeds from primary interface
  if ping -c 1 $CHECK_IP &>/dev/null
  then
    # Are we using the primary?
    if gateway_ip $PRIMARY_GW
    then
      LOOP=0
    else
      LOOP=$(( $LOOP + 1 ))
      logger -t failover "Ping to $CHECK_IP via $PRIMARY_GW is good ($LOOP/$CHECK_LOOPS)"
      if [[ $LOOP -ge $CHECK_LOOPS ]]
      then # Switch to primary
        logger -t failover "Switch to primary gateway $PRIMARY_GW"
        ip route del default via $BACKUP_GW
        ip route add default via $PRIMARY_GW
      fi
    fi
  else
    # Are we using the backup?
    if gateway_ip $BACKUP_GW
    then
      LOOP=0
    else
      LOOP=$(( $LOOP + 1 ))
      logger -t failover "Ping to $CHECK_IP via $PRIMARY_GW failed ($LOOP/$CHECK_LOOPS)"
      if [[ $LOOP -ge $CHECK_LOOPS ]]
      then # Switch to backup
        logger -t failover "Switch to backup gateway $BACKUP_GW"
        ip route del default via $PRIMARY_GW
        ip route add default via $BACKUP_GW
      fi
    fi
  fi
done