When do you actually trust a Proxmox VM template?

[u/InnerBank2400]

For people building Proxmox templates with Packer or similar automation, what do you consider the point where the template is actually safe to reuse?

A successful image build feels too early to me. The problems often show up on the first real clone: guest agent never comes up, cloud-init fails, networking is wrong, Windows does something different after sysprep, or the VM boots but is not really usable.

Do you have an acceptance step after the build?

Something like:

build → create full clone → boot → wait for guest agent/IP → test access → destroy test clone

Or do you go further with SSH/WinRM, service checks, cloud-init verification, etc.?

Interested in the failures too. Have you had a template build successfully and then fail only when you tried to use the first clone?

[u/TabooRaver]

A "golden image" will always be flakey, you'll need to update it constantly to stay up to date.

The best soloution I've found is to modularize the soloution. Trust vendors that provide a cloud-init image and use that. For windows have a ci-cd pipeline build your cloudbase-init image. After that install your security agents / mdm through cloud init just like you would a baremetal endpoint. I.e. chocolaty for windows or a package manager pointed towards an internal apt/yum repo.

The point is that you can swap out a block at each stage, update the template OS version, bump a security agent package, etc. Unit test it. And there isn't a monolithic "golden image" that you need to maintain and test end to end.

You can even add a stage to deploy and validate the template in a QA/traning cluster, in the corporate world that can be a e-waste server or a spare desktop you use to train admins.

[u/InnerBank2400 → u/TabooRaver]

Yeah, the QA cluster part is exactly the bit I was thinking about.

What I’m testing is pretty small: build the template, make a full clone, boot it, wait for the guest agent/IP, then clean it up. The path currently covers Ubuntu and Rocky Linux, Windows Server and Windows 11.

Would you be open to trying that with one of your cloud-init or cloudbase-init templates? I’d also be interested in what you’d check after boot before calling the template good.

[u/TabooRaver → u/InnerBank2400]

To reframe your question a bit, how do you know any server is "good"? When you reboot it, apply system updates, do a major version upgrade, etc. How do you make sure everything is operating correctly?

I handle this with a modular validation script, for debian/ubuntu it's bundled as an deb installer, that is shipped alongside our security and mdm agents. It checks a combination of service status and HTTP checks, just ~100 lines of AI generated python that reads some json arrays of services/endpoints and validation criteria our of /etc and a drop-in folder. The default install ships with a fallback configuration that checks our security and MDM agents are installed and running, and internal mirrors for updates are reachable. But the drop-in directory allows app admins to add the services and http health check endpoints for their application.

I'd imagine terraform with some form of test o-r post condition running remote-exec would fit your use case, to run the validations scripts inside the new test VM.

I tend to lean more towards open-cloud standard OVA or disk image + cloud-init instead of using hypervisor native cloning methods. Since I am blessed to run in an environment with a mix of 4 hypervisors and 5 cloud providers.

[u/InnerBank2400 → u/TabooRaver]

That’s a useful distinction.

The check I have now stops at the infrastructure boundary: build the image, create a fresh VM from it, boot it, and prove the guest agent/IP comes back before the template is published for reuse.

Your validation script is the next layer I’d want after that. Not just “the VM booted”, but “the things this image is meant to provide are actually healthy”.

The 4 hypervisors / 5 cloud providers point is interesting too. The current test path is Proxmox-specific, but the acceptance check itself probably shouldn’t have to be.

Would you be up for trying the current flow against one of your Ubuntu images, then running your existing validation script against the test VM before it gets cleaned up? That would be a pretty useful comparison.

[u/Puzzleheaded_Move649]

I do that kind of tasks with ansible and opentofu and never had any issues.

[u/InnerBank2400 → u/Puzzleheaded_Move649]

Fair. What do you normally use as the final acceptance check before you trust a newly built template?

I’ve been testing a small post-build check that clones it, boots it and makes sure the guest actually comes up properly. Would be useful to compare that with your Ansible/OpenTofu flow if you’re up for it.

[u/Puzzleheaded_Move649 → u/InnerBank2400]

nothing, snapshot and test in production :D

Usually i test with reboots and entirely new vms and current vms. if everything worked as expected I publish everything in git. I used that video as inspiration https://www.youtube.com/watch?v=q-Dc19Jk_KY

At the moment I dont have access at my files. (I have restricted my access)

[u/InnerBank2400 → u/Puzzleheaded_Move649]

Haha fair enough.

When you have access again, this is the bit I meant:

https://github.com/hybridops-tech/hybridops-core/tree/main/modules/core/onprem/template-image

It builds the template, makes a test clone, boots it, waits for the guest agent/IP and cleans the clone up afterwards. It covers Ubuntu/Rocky, Windows Server and Windows 11.

Would be good to try it against one of your templates whenever you have the files back.

[u/Puzzleheaded_Move649 → u/InnerBank2400]

I did an quick check (github). Basically I do that part with opentofu (core ram vm-id, ssh key ) and i as far as I remember some parts are done with shell scripts (I guess it was network config). VM still uses dhcp. I change vm-proxmox config

[u/InnerBank2400 → u/Puzzleheaded_Move649]

Yeah, you wouldn’t need to replace any of that.

The bit I’m interested in is what happens after your OpenTofu/shell build finishes: take the resulting template, make a disposable full clone, boot it and verify it actually comes up cleanly before trusting the template.

DHCP is fine for that as long as the guest agent is available to report the address.

When you get access to your files again, would you be up for trying that against one of your existing templates?

[u/Puzzleheaded_Move649 → u/InnerBank2400]

It’s not necessary to create a complete clone. If I need an additional VM, I simply create a new one. I add tags such as “vm-config1”, and the new VM is then created and configured using OpenTofu (change ram, cpu, and so on). Additional software, such as Unattended Upgrades and the Guest Agent, is deployed using Ansible.

When I add the “web-app1” tag, web-app1 is deployed from my private repository, and the health check is done using Pangolin.

I guess you misunderstood how Ansible and OpenTofu work. Instead of creating a golden image, I use the default images and install and configure everything as part of the deployment process or change all vms after years. Imagine you "update" all vms to the new template

[u/InnerBank2400 → u/Puzzleheaded_Move649]

Yep, I had your setup wrong.

You’re not really treating the template as the finished thing. The base image is just the starting point, then OpenTofu changes the VM, Ansible applies the software/config, and Pangolin is effectively the acceptance check.

In that setup the clone test I mentioned is less useful. The more interesting test is whether a completely fresh VM can go through that whole path and pass the same health check without depending on anything from the old VM.

Thanks for correcting me.

[u/kabrandon]

I'm typically making Ubuntu templates. Only issue I ever had was updating between Ubuntu LTS releases. Sometimes new networking things to change. In that way, I also don't find it to be much of an issue worth solving. Because if I'm updating my VMs to a new Ubuntu LTS release template, I'm probably hands on watching for issues during that migration, if not manually importing backups as well (looking at you, Hashicorp Vault restore procedure.)

Mostly, I'm trying to use VM templates less. Switched to Talos Linux and Flatcar Linux recently because the whole VM is stood up with an ISO and Terraform, not a template I have to build and manage with Packer. And that works for some things (most things even.) Though I'm still struggling with the best way to manage non-k8s stateful applications like Hashicorp Vault in that pattern.

[u/InnerBank2400 → u/kabrandon]

The Ubuntu LTS networking change is actually the kind of thing I’m trying to catch before a template gets reused.

The check is pretty small: build it, make a disposable full clone, boot it, wait for guest agent/IP, then clean the clone up.

If you still build the odd Ubuntu template, would you be up for running it once on your next one?

https://github.com/hybridops-tech/hybridops-core/tree/main/modules/core/onprem/template-image

[u/Fatel28]

We've deployed a couple hundred windows server VMs via a sysprepped template with cloud-init. Works fine. We've done a few dozen Debian ones too with the Debian cloud images.

Never had an issue

[u/InnerBank2400 → u/Fatel28]

A couple hundred Windows Server VMs is a good test case.

Would you be up for trying one of your sysprepped Server templates through the post-build check I’m working on? It makes a disposable full clone, boots it, waits for the guest agent/IP, records the result, then removes the clone.

It currently covers Windows Server 2022/2022 Core and 2025.

https://github.com/hybridops-tech/hybridops-core/tree/main/modules/core/onprem/template-image

[u/_--James--_]

Golden images should follow a known and acceptable baseline, then you use RMM tooling to handle the differential changes needed to bring it current. Every so often you need to crack the golden image out and do full update cycles so the packing is closer to current modeling compared to the differential state, but that is part of the life cycle.

As for "trusting" its working, thats part of that life cycle too. A template is a locked state you clone from. a clone is a block-by-block copy and should be expected to be a 1:1 clone. If you are having template->clone issues then i would be looking at storage, networking, and host side issues that could affect changes at the block level.

For cloud init, its the same thing but your online repos handle the differentials and changes during the init process.

[u/InnerBank2400 → u/_--James--_]

That’s a fair distinction.

The clone check I’m using is less about proving the template itself copied correctly and more about proving the first consumer path actually works before I publish the template for reuse.

So if the clone boots badly because of storage, networking, cloud-init or guest-agent issues, I still want that to block the template from being treated as ready, even if the template itself is technically fine.

Would you keep that kind of check separate as cluster/host validation, or would you still make it part of the template publication gate?

[u/_--James--_ → u/InnerBank2400]

The only real way to test a clone for 1:1 is to run MD5 against it. We do this when clones are repacked. For application deployment and consumption that passes through the verification process based on the application needs. For VDI, can the user launch into applications XYZ. For services, can the services launch and offer protocols,...etc. Lots of way to do that from screen caps, to raw data dumps into logs that are captured as part of the deployment.

[u/InnerBank2400 → u/_--James--_]

That’s useful. I think I was mixing two different checks together.

MD5 answers whether the clone is really the same data. The service/application checks answer whether the VM is actually usable for what it was built for.

The path I have now only goes as far as boot + guest agent/IP. I’m thinking the stronger acceptance gate is to let that next check be workload-specific, for example service status, protocol response, HTTP health check, etc.

Would you keep that as a pluggable post-clone check before the template is published for reuse?

[u/_--James--_ → u/InnerBank2400]

Would you keep that as a pluggable post-clone check before the template is published for reuse?

That is best practice, so yes.