Script to check Internet connectivity and fail-over route table to standby

check_and_failover_NAT.sh

#!/bin/sh
. /home/ec2-user/.bash_profile
. /etc/profile.d/aws-apitools-common.sh
region=us-east-1

# these should be configured for each NAT instance
target1=8.8.8.8
target2=8.8.4.4
active_rt=rtb-ACTIVE
standby_rt=rtb-STANDBY
subnetids=(subnet-SUBNET1 subnet-SUBNET2)

# die with error if can't ping my target as I might have problem in Internet connectivity
ping -c 4 $target1 > /dev/null || exit -1

if ping -c 4 $target2 > /dev/null
then
	pingable=true
else
	pingable=false
fi

for subnetid in ${subnetids[*]}; do
	# check current route table and association
	eval `ec2-describe-route-tables --region $region -F association.subnet-id=$subnetid | egrep '^(ROUTETABLE|ASSOCIATION.*'$subnetid')' | awk '{print $1"="$2}'`

	if $pingable
	then
        	if [ "$ROUTETABLE" != "$active_rt" ]
        	then
			echo could ping $target2 via NAT instance in opposite zone. reverting RouteTable to active.
                	echo ec2-replace-route-table-association --region $region -r $active_rt $ASSOCIATION
                	ec2-replace-route-table-association --region $region -r $active_rt $ASSOCIATION
        	fi
	else
        	if [ "$ROUTETABLE" != "$standby_rt" ]
        	then
                	echo could NOT ping $target2 via NAT instance in opposite zone. swapping RouteTable to standby.
                	echo ec2-replace-route-table-association --region $region -r $standby_rt $ASSOCIATION
                	ec2-replace-route-table-association --region $region -r $standby_rt $ASSOCIATION
        	fi
	fi
done

can you give an example of what your network and route tables need to look like for this script to work?

Do you run this on each nat instance? or just the backup?