An IP address of EC2 instance gets changed after the restart

Question

I am running an EC2 Linux instance. For some maintenance purposes I shutdown the instance and started it again. However, EC2 IP has changed now.

How to keep an IP address of Amazon EC2 instance unchanged after stop and start it again?

Answer 1

Actually, When you stop/start your instance, the IP address will change. If you reboot the instance, it will keep the same IP addresses. Unfortunately, it is not possible for us to reassign the address to your instance as that address would have been released back into the pool used by other EC2 instances.

If you want to avoid this issue in the future, depending on your needs:

• If you only need a fixed public IP address, you can assign an Elastic IP address to your instance.
• If you need both public and private IP addresses to remain the same throughout the lifetime of the instance, you can launch your instance in VPC instead. The private IP address assigned to an instance in VPC remains with the instance through to termination.

To learn more, see the aws documentation to assign elastic ip.

Answer 2

Elastic IP has its limitations.

If you have reached the maximum number of Elastic IP addresses in a region, and all you want is a constant way to connect to an EC2 instance, I would recommend using a route53 record instead of using IP address.

I create a route53 record that points to the IP address of my EC2 instance. The record doesn't get changed when the EC2 is stopped.

And the way to keep the record pointing to the address of the EC2 is by running a script that changes the route53 record when the EC2 launches.

Here's the user data of my EC2:

Content-Type: multipart/mixed; boundary="//"
MIME-Version: 1.0

--//
Content-Type: text/cloud-config; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="cloud-config.txt"

#cloud-config
cloud_final_modules:
- [scripts-user, always]

--//
Content-Type: text/x-shellscript; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="userdata.txt"

#!/bin/bash

# get the public ip address
# Ref: https://stackoverflow.com/questions/38679346/get-public-ip-address-on-current-ec2-instance
export public_ip=$(curl http://169.254.169.254/latest/meta-data/public-ipv4)

cat <<EOF > input.json
{
  "Comment": "optional comment about the changes in this change batch request",
  "Changes": [
    {
      "Action": "UPSERT",
      "ResourceRecordSet": {
        "Name": "my-domain.my-company.com",
        "Type": "A",
        "TTL": 300,
        "ResourceRecords": [
          {
            "Value": "${public_ip}"
          }
        ]
      }
    }
  ]
}
EOF

# change route53 record 
/usr/bin/aws route53 change-resource-record-sets \
  --hosted-zone-id <hosted_zone_of_my-company.com> \
  --change-batch file://input.json >
--//

Here I use my-domain.my-company.com as the route53 record for my EC2.

By using this method, you get a route53 record that points to your EC2 instance. And the record does not change when you stop and start the EC2. So you can always use the route53 record to connect to your EC2.

Remember to assign an IAM role that has route53 permissions to the EC2 instance so that you can run the user data without errors.

And remember that the user data I provided is intended for use with Amazon Linux 2, and the commands may not work for other Linux distributions.

Answer 3

Yes, you can.

Go to Elastic IP and Create one.

Then select the newly created Elastic IP and Select Associate Elastic IP and select the Instance ID to which you want to associate it with.

That's it. The IP will now be assigned even if you reboot your EC2.

Do note that AWS charges $0.005/hr for each Elastic IP, And $0.10 per Elastic IP address remap for additional remaps over 100 per month

Answer 4

It depends on which IP you speak about:

• Public IP: try to use an Elastic Ip, then you will not have this problem anymore. You can allocate an new one to your instance directly on AWS Console or programmatically. But if your are using an autoscaling-group you will have to do it on your user-data or cloud-init process.

• Private IP: Unfortunately you cannot fix a private Ip address to an instance. The only way is to use DNS and in that case a private DNS zone for you VPC (https://docs.aws.amazon.com/fr_fr/vpc/latest/userguide/vpc-dns.html). No need to buy a domain in that case.

I would also recommend to use DNS on the first case with maybe a domain you have rather than using an IP address

Answer 5

If you don't want your IP adddress to be changed after stopping and starting your EC2 Instance, use Elastic IP. For more reference on how to use Elastic IP:
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html