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What a “cloud image ISO” actually is

Ubuntu cloud images are designed to boot in cloud environments (AWS, Azure, OpenStack). It is a minimal, preconfigured base disk designed to be cloned thousands of times, with all customization deferred to cloud-init.
They expect configuration to arrive via cloud-init, which reads:

• user-data → user accounts, SSH keys, passwords, packages
• meta-data → instance ID, hostname

Cloud-init looks for these files on:

• a virtual CD-ROM
• a NoCloud datasource
• cloud provider metadata endpoints

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The structure of the seed ISO

The ISO contains exactly two files:

seed.iso
 ├── user-data
 └── meta-data

Cloud-init reads them on first boot and configures the VM accordingly.

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Step 1 — Generate an SSH key pair

You can generate the key on Windows, Linux, or WSL.
Here’s the Windows PowerShell version:

ssh-keygen -t ed25519 -C "ubuntu-cloud"

This produces:

• Private key → id_ed25519
• Public key → id_ed25519.pub

You will embed the public key inside user-data.

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Step 2 — Create the user-data file

This file tells cloud-init to create the ubuntu user and install your SSH key.

#cloud-config
users:
  - name: ubuntu
    ssh_authorized_keys:
      - ssh-ed25519 AAAA...yourpublickey...
    sudo: ALL=(ALL) NOPASSWD:ALL
    shell: /bin/bash

If you want a password instead (or in addition):

#cloud-config
password: ubuntu
chpasswd:
  expire: false
ssh_pwauth: true

You will need to generate the password hash using the command:

mkpasswd –method=sha-512

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Step 3 — Create the meta-data file

This file is simple:

instance-id: gns3-ubuntu
local-hostname: ubuntu-gns3

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Step 4 — Build the seed ISO

You have two main ways to do this on Windows:

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Option A — Native Windows tool (cloud-localds.exe)

If you installed Cloudbase-Init tools:

cloud-localds.exe seed.iso user-data meta-data

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Option B — WSL (Ubuntu on Windows)

Inside WSL:

sudo apt install cloud-image-utils
cloud-localds seed.iso user-data meta-data

Then copy it back to Windows:

cp seed.iso /mnt/c/Users/{user}/Desktop/

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Step 5 — Attach the ISO in GNS3

In GNS3:

1. Edit your QEMU VM
2. Go to Drives
3. Add a CD-ROM drive
4. Select seed.iso
5. Boot the VM

On first boot, cloud-init reads the ISO and:

• creates the ubuntu user
• installs your SSH key
• sets hostname
• configures sudo

After that, you can SSH in immediately:

ssh -i .ssh/key_name ubuntu@<vm-ip>

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Why this workflow is so powerful

It gives you:

• deterministic VM builds
• no manual password setting
• no copying keys by hand
• perfect reproducibility for labs
• ideal for automation (Ansible, Terraform, etc.)