Hey everyone! I have a customer who needs help with a specific situation. They have several EC2 instances that must keep static private IP addresses (no Elastic IPs, just the same private IP assigned even after restarts or rebuilds). The subnet space is also quite limited. The main challenge we're facing is how to handle updates for the AMI (which gets a new release twice a month). These EC2 instances are set up through a CloudFormation stack, but we've encountered a problem: we can't detach the primary ENI (there's only one), as AWS blocks its removal. Does anyone have suggestions for how we might get around this? Thanks in advance!
4 Answers
You might consider using Launch Templates for these EC2 instances. In this setup, the association of a preallocated ENI is part of the template rather than the instance itself. There’s an example in a CloudFormation script you could check out. Just remember you'll need a separate template for each VM instance since the mapping of ENIs to instances is defined there.
When launching an instance, you can specify the private IP as long as it isn't already in use. This might help you in managing IP assignments during updates.
Is instance access really necessary for your use case? If not, setting up a NAT in front of these instances and whitelisting the NAT’s internal IP could be a simpler approach. This way, you can manage the EC2 instances more like disposable resources rather than things you need to maintain closely.
Yeah, unfortunately, the access is needed.
One option is to use a secondary private IP. You could create EC2 instance A with a secondary private IP assigned, then launch a new EC2 instance B with the updated AMI. After that, reallocate the private IP from instance A to B and decommission instance A. You'd need to adjust the OS routing rules to route outgoing traffic through the secondary IP, but it could work!
Thanks, I did not think about that! Will try that out if there will be no other ideas.
Thanks, that sounds like a solid solution for this use case.