Here's a handy way to automatically update an A-Record in Amazon Route 53 whenever an EC2 instance changes IP address.

Scenario: You have a domain name in Amazon Route 53 pointing to an Amazon EC2 instance. However, if the instance is stopped and started, its public IP address changes. This breaks the A-Record since it is pointing to the wrong IP address.

"Wait!" you might say, "Why not simply use an Elastic IP address?"

(An Elastic IP address is a static IP address that can be assigned to resources in an Amazon VPC. The IP address stays the same until you return the Elastic IP address to AWS.)

Yes, using an Elastic IP address on the instance will prevent the A-Record from breaking. However, AWS gives a default limit of 5 Elastic IP addresses per region. You can request a limit increase, but what if you need lots of them?

As an example, a service might provide a separate Amazon EC2 instance for each customer, to ensure secure separation of data. Each customer is given a domain name to access their server (acme.example.com). The required number of Elastic IP addresses could be huge.

Fortunately, there is a fairly simple way to achieve the objective without needing any Elastic IP addresses.

Architecture

The architecture is quite simple:

• When the EC2 instance starts, it should get its new public IP address and update its own record in Route 53
• The DNS name to update is stored in a Tag on the EC2 instance
• The script should execute every time the EC2 instance starts (that is, every time it starts, not just the first boot)

Implementation

First, there should be a Record Set in Amazon Route 53 that defines the existing domain name.

It is currently pointing to the public IP address of an EC2 instance.

Next, add some tags to the EC2 instance that will be used by the script:

• DNS Name: The DNS Name to associate with the instance
• Hosted Zone ID: Uniquely identifies the Zone record in Route 53 that needs to be updated (get it from your Route 53 Hosted Zone record)

Whenever the EC2 instance starts, it will run a script that will:

• Grab the information from the above tags
• Retrieve the instance's current public IP address
• Update the Route 53 record set with the new IP address

#!/bin/bash
# Extract information about the Instance
INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id/)
AZ=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone/)
MY_IP=$(curl -s http://169.254.169.254/latest/meta-data/public-ipv4/)

# Extract tags associated with instance
ZONE_TAG=$(aws ec2 describe-tags --region ${AZ::-1} --filters "Name=resource-id,Values=${INSTANCE_ID}" --query 'Tags[?Key==`AUTO_DNS_ZONE`].Value' --output text)
NAME_TAG=$(aws ec2 describe-tags --region ${AZ::-1} --filters "Name=resource-id,Values=${INSTANCE_ID}" --query 'Tags[?Key==`AUTO_DNS_NAME`].Value' --output text)

# Update Route 53 Record Set based on the Name tag to the current Public IP address of the Instance
aws route53 change-resource-record-sets --hosted-zone-id $ZONE_TAG --change-batch '{"Changes":[{"Action":"UPSERT","ResourceRecordSet":{"Name":"'$NAME_TAG'","Type":"A","TTL":300,"ResourceRecords":[{"Value":"'$MY_IP'"}]}}]}'

To execute the script automatically each time the instance starts (as opposed to User Data scripts that only run on the first boot), put the above script in this directory:

/var/lib/cloud/scripts/per-boot/

Finally, the EC2 instance will need an IAM Role assigned that has permission to run the above commands:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "ec2:DescribeTags",
            "Resource": "*"
        },
        {
            "Effect": "Allow",
            "Action": "route53:ChangeResourceRecordSets",
            "Resource": "arn:aws:route53:::hostedzone/HOSTED-ZONE-ID"
        }
    ]
}

How to test

To test the script, simply Stop the instance then Start it again.

This will result in a new public IP address being assigned to the instance. The script will call Amazon Route 53 to update the record set. This might take a minute to update.

Then, return to Route 53 and look at the IP address assigned to the A-Record. It should be updated with the new IP address.