aws-auto-healing-nat.sh

#!/bin/bash

# This script is to set a default route for route tables that need NAT in the same VPC.

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="network"
TAG_VALUE="private"

# Print help
function usage()
{
   echo ""
   echo "$0 [options]"
   echo "   --tag-key - route table tag key to indicate if it is a private subnet (default: $TAG_KEY)"
   echo "   --tag-value - route table tag value to indicate if it is a private subnet (default: $TAG_VALUE)"
   echo ""
}

# Get options
while [ "$1" != "" ]; do
    case $1 in
        --tag-key)      shift
                        $TAG_KEY=$1
                        ;;
        --tag-value)    shift
                        $TAG_VALUE=$1
                        ;;
        *)              usage
                        exit
                        ;;
    esac
    shift
done

# Determine route tables that need to use NAT for the same VPC
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 $2}'`

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
      # Create default route 
      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
      # Replace default route
      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

# disable source destination check
aws ec2 modify-instance-attribute --instance-id $INSTANCE_ID --no-source-dest-check --region $REGION