Use one AWS Elastic IP for several instances - amazon-web-services

Question

I need to provide my customers with fixed urls that don't change when the EC2 instances are stopped/started because sometimes we need to change the size of the EC2 and when we restart the instance the public IP has changed.
I thought on using Elastic IPs so I can keep the same public IP when the instance is rebooted, but I've seen that Amazon tells you that you only have 5 Elastic IPs. If you ask them they say that they can give you more, but I guess they're not giving you 10.000 of them.
How can I use a single public Elastic IP to give each user different URLs for out service?
It would be something like this, being 11.22.33.44 the Elastic IP and 192.168.0.X two EC2 instances:
11.22.33.44:**1000** --> 192.168.0.**1**:22
11.22.33.44:**1001** --> 192.168.0.**1**:80
11.22.33.44:**1002** --> 192.168.0.**1**:443
11.22.33.44:**1003** --> 192.168.0.**2**:22
11.22.33.44:**1004** --> 192.168.0.**2**:80
11.22.33.44:**1005** --> 192.168.0.**2**:443
I need to make it work programmatically, as I'm creating EC2 instances from the SDK as needed.
Another way I thought is using subdomains from my .com domain that points to the current public IP of each EC2 instance, but using the IP as I described before sounds better.

Answer 1

The issue is that instances are receiving new (temporary) Public IP addresses after they are stopped and started.
A simple way to handle this is to add a script to each instance that runs during every boot. This script can update a DNS record to point it at the instance.
The script should go into the /var/lib/cloud/scripts/per-boot directory, which will cause Cloud-Init to automatically run the script each time the instance is started.
# Set these values based on your Route 53 Record Sets
ZONE_ID=Z3NAOAOAABC1XY
RECORD_SET=my-domain.com
# 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=$(curl -s http://169.254.169.254/latest/meta-data/public-ipv4/)
# Extract Name tag associated with instance
NAME_TAG=$(aws ec2 describe-tags --region ${AZ::-1} --filters "Name=resource-id,Values=${INSTANCE_ID}" --query 'Tags[?Key==`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 $ZONE_ID --change-batch '{"Changes":[{"Action":"UPSERT","ResourceRecordSet":{"Name":"'$NAME_TAG.$RECORD_SET'","Type":"A","TTL":300,"ResourceRecords":[{"Value":"'$MY_IP'"}]}}]}'
The script will extract the Name tag of the instance and update the corresponding Record Set in Route 53. (Feel free to change this to use a different Tag.) The instance will also require IAM permissions for ec2 describe-tags and route53 change-resource-record-sets.
Update: I've turned this answer into a blog post: Amazon Route 53: How to automatically update IP addresses without using Elastic IPs