simplified-nat-route-setter.sh

#!/bin/bash
MAC_ADDRESS=`curl http://169.254.169.254/latest/meta-data/network/interfaces/macs/`
VPC_ID=`curl http://169.254.169.254/latest/meta-data/network/interfaces/macs/${MAC_ADDRESS:-1}/vpc-id`
INSTANCE_ID=`curl http://169.254.169.254/latest/meta-data/instance-id`
REGION=`curl http://169.254.169.254/latest/dynamic/instance-identity/document|grep region|awk -F\" '{print $4}'`
TAG_KEY="Reach"
TAG_VALUE="private"
ROUTE_TABLES=`aws ec2 describe-route-tables --filters "Name=tag:$TAG_KEY,Values=$TAG_VALUE" --region $REGION --output text | grep ROUTETABLES | grep $VPC_ID | awk '{print $3}'`
for ROUTE_TABLE  in $ROUTE_TABLES
do
   TARGET=`aws ec2 describe-route-tables --filters "Name=route-table-id,Values=$ROUTE_TABLE" --region $REGION --output text | grep "0.0.0.0/0" | awk '{print $3}'`
   echo "Checking $ROUTE_TABLE"
   if [ "$TARGET" = "" ]; then
      echo "No default route is detected. Creating default route for $ROUTE_TABLE"
      aws ec2 create-route --route-table-id $ROUTE_TABLE --destination-cidr-block 0.0.0.0/0 --instance-id $INSTANCE_ID --region $REGION
   elif [ "$TARGET" != "$INSTANCE_ID" ]; then
      echo "Default route is set to $TARGET. Replacing default route to $INSTANCE_ID"
      aws ec2 replace-route --route-table-id $ROUTE_TABLE --destination-cidr-block 0.0.0.0/0 --instance-id $INSTANCE_ID --region $REGION
   else
      echo "No change is required"
   fi
done
aws ec2 modify-instance-attribute --instance-id $INSTANCE_ID --no-source-dest-check --region $REGION