This file should be installed on your router. In my case, I was testing with a VyOS router so I was able to easily extend it with this script. Just paste this into a new file and chmod +x to make it executable. Update the IP information below – most importantly, your LAN information. Then add it to a cron job and have it run once per minute. That’s it!
#!/bin/bash PATH=$PATH:/bin:/usr/bin:/sbin:/usr/sbin # LAN IPs of ISP Routers to Use as the Default Gateway PREFERRED="10.1.10.1" ALTERNATE="10.1.10.3" # Public Internet Hosts to Ping PUBLICHOST1="220.127.116.11" PUBLICHOST2="18.104.22.168" # Ping the first public Internet device first. See if it fails. RETURNED=`/bin/ping -c 5 $PUBLICHOST1 | grep 'transmitted' | cut -d',' -f2 | sed -e 's/^ *//' | cut -d' ' -f1` # If the first pings fail, check the secondary Internet device. if [ $RETURNED -eq 0 ]; then # Do a second check just to be sure, and to a different IP. Less pings, since we are already pretty sure of an issue echo "Failed ping test to $PUBLICHOST1. Trying against $PUBLICHOST2." RETURNED=`/bin/ping -c 2 $PUBLICHOST2 | grep 'transmitted' | cut -d',' -f2 | sed -e 's/^ *//' | cut -d' ' -f1` fi # If it still fails, assume that the Internet is down. if [ $RETURNED -eq 0 ]; then echo "Everything is not ok. Looks like the Internet connection is down. Better switch ISPs." date +%s > /tmp/cutover-start.log else echo "Everything is ok. Checking if we are using the preferred connection." CURRENT=`route -n | grep "^0.0.0.0" | cut -d' ' -f2- | sed -e 's/^ *//' | cut -d' ' -f1` if [ -f /tmp/cutover-start.log ]; then LASTSWITCHTIME=`cat /tmp/cutover-start.log | sed -e 's/\n//'` fi NOW=`date +%s` TIMEDIFF=$((NOW - LASTSWITCHTIME)) if [ "$CURRENT" == "$ALTERNATE" ] && [ $TIMEDIFF -gt 300 ]; then date +%s > /tmp/cutover-start.log echo "Testing if the primary gateway is back online." route add -host $PUBLICHOST2 gw $PREFERRED RETURNED=`/bin/ping -c 5 $PUBLICHOST1 | grep 'transmitted' | cut -d',' -f2 | sed -e 's/^ *//' | cut -d' ' -f1` if [ $RETURNED -eq 5 ]; then echo "Switching back now that primary ISP is back online." route del -net 0.0.0.0/0 route add default gw $PREFERRED else echo "The primary host is not back online yet." fi route del -host $PUBLICHOST2 fi exit fi # If the script does not exit before it reaches this point, then assume the worst. Time to cutover to the other ISP CURRENT=`route -n | grep "^0.0.0.0" | cut -d' ' -f2- | sed -e 's/^ *//' | cut -d' ' -f1` # Delete the existing default gateway route del -net 0.0.0.0/0 if [ "$CURRENT" == "$PREFERRED" ]; then echo "Switcing to the alternate ISP $ALTERNATE" route add default gw $ALTERNATE else echo "Switching to the default ISP $PREFERRED" route add default gw $PREFERRED fi