ubuntu-cloud-init

cloud-init runs early in boot, reads configuration from a datasource, and applies it through modules across four boot stages. This skill is authoring-led (produce correct cloud-config from a description) with a strong validation/debug path, aimed at on-prem / air-gapped Ubuntu Server LTS using the NoCloud datasource.

Mapping: 24.04 LTS ≈ cloud-init 24.x; 26.04 LTS ≈ 26.1.

Authoring workflow

1. Identify the goal & datasource. On-prem/air-gapped → NoCloud (seed dir,

cidata ISO/USB, HTTP seed, or SMBIOS serial). Pin datasource_list: [NoCloud] to stop cloud-init probing cloud metadata services and timing out.

1. Choose the user-data format — almost always #cloud-config. Use MIME multipart

only to combine cloud-config with scripts. (Formats table below.)

1. Write the `#cloud-config` using the modules needed (see Quick reference).
2. Validate: cloud-init schema -c user-data --annotate.
3. Seed it (NoCloud) and set a unique instance-id in meta-data.
4. Test on a throwaway boot, then cloud-init clean --logs to re-run as if

first boot.

user-data formats

First line determines the format:

Any of these may be gzipped.

Quick reference — the modules that matter on-prem

users / ssh (created in the Network stage):

#cloud-config
ssh_pwauth: false
users:
  - default                          # KEEP this first to retain the distro user + cloud keys
  - name: deploy
    groups: [sudo]
    sudo: "ALL=(ALL) NOPASSWD:ALL"
    lock_passwd: true
    shell: /bin/bash
    ssh_authorized_keys:
      - ssh-ed25519 AAAA... deploy@site

passwd (a hash) applies only to new users; hashed_passwd applies even to existing ones. Generate a hash with mkpasswd --method=SHA-512 --rounds=4096.

write_files:

write_files:
  - path: /etc/myapp/app.conf
    owner: root:root
    permissions: '0640'
    content: |
      backend = db.internal:5432
  - encoding: b64                     # also gz, gz+b64
    content: aGVsbG8K
    path: /etc/motd.d/banner
    defer: true                       # write in Final stage (after users/packages)

runcmd vs bootcmd: runcmd runs once per instance in the Config stage (later); bootcmd runs early on every boot (guard once-only work with cloud-init-per once <name> ...).

runcmd:
  - [systemctl, enable, --now, myapp]

apt — local mirror + internal repo (the air-gapped essential):

apt:
  preserve_sources_list: false
  primary:
    - arches: [default]
      uri: http://mirror.internal/ubuntu
  security:
    - arches: [default]
      uri: http://mirror.internal/ubuntu
  sources:
    internal-app.list:
      source: "deb [signed-by=$KEY_FILE] http://mirror.internal/app stable main"
      key: |
        -----BEGIN PGP PUBLIC KEY BLOCK-----
        ...embed the key inline; do NOT rely on a keyserver air-gapped...
        -----END PGP PUBLIC KEY BLOCK-----
  conf: |
    Acquire::http::Proxy "http://apt-cache.internal:3142";

ca_certs — trust an internal CA (runs in the Network stage, before apt, so HTTPS to the internal mirror works):

ca_certs:
  trusted:
    - |
      -----BEGIN CERTIFICATE-----
      <internal root CA>
      -----END CERTIFICATE-----

ntp — point at local time source:

ntp:
  enabled: true
  servers: [ntp.internal]
  pools: []

Other on-prem-relevant modules: packages/package_update/package_upgrade (avoid package_upgrade without a full mirror), disk_setup+fs_setup+mounts, growpart/resize_rootfs, timezone/locale/keyboard, set_passwords/chpasswd, seed_random, power_state, hostname (hostname/fqdn/manage_etc_hosts). Avoid `phone_home` air-gapped — it only calls out to a URL and retries on failure. Full per-module detail (keys, snippets, stage, frequency) in references/modules.md.

NoCloud seeding (the air-gapped core)

cloud-init finds NoCloud config from (in precedence order) SMBIOS serial → kernel cmdline → seed dirs → labeled block device. The common methods:

• Seed directory: drop meta-data + user-data (and optionally vendor-data,

network-config) into /var/lib/cloud/seed/nocloud/.

• `cidata` ISO/USB: a vfat/iso9660 filesystem labeled CIDATA (case-insensitive)

containing those files in its root. genisoimage -output seed.iso -volid cidata -joliet -rock user-data meta-data network-config (or cloud-localds seed.iso user-data meta-data).

• HTTP seed: kernel cmdline ds=nocloud;s=http://10.0.0.1:8000/ (the scheme

decides local vs network; trailing slash required — files are appended as <uri>/user-data etc.).

• SMBIOS serial (QEMU/libvirt): -smbios type=1,serial=ds=nocloud;s=http://10.0.0.1:8000/.

Required files: meta-data and user-data. Optional: vendor-data, network-config. Since cloud-init 24.3, an HTTP/seed source that omits network-config triggers a boot retry/timeout — ship an empty `network-config` for back-compat, or a real netplan-v2 one.

`instance-id` re-run rule: cloud-init only re-applies user-data when the instance-id in meta-data changes (or after cloud-init clean). Bump it whenever the config changes.

Full seeding detail (cmdline grammar + aliases, DMI variable expansion, FTP, GRUB escaping, seedfrom, dsname deprecations) is in references/nocloud-and-airgapped.md.

Network config (hand-off to netplan)

cloud-init network-config has v1 (its own schema) and v2 (which IS netplan format). On Ubuntu the renderer is netplan; cloud-init writes /etc/netplan/50-cloud-init.yaml. user-data cannot set network config — networking comes from the datasource, system config, or kernel cmdline.

To author the network block, or to hand network control back to netplan entirely (network: {config: disabled} in /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg), use the ubuntu-netplan skill.

Validation & debugging (fast path)

cloud-init schema -c user-data --annotate     # validate a file, errors inline
sudo cloud-init schema --system --annotate     # validate the live system's user-data
cloud-init status --long --wait                # 0=ok, 1=error, 2=recoverable error
cloud-init query --all                         # inspect instance-data
sudo cloud-init clean --logs                   # wipe state → next boot is "first boot"

Logs: /var/log/cloud-init.log, /var/log/cloud-init-output.log, /run/cloud-init/ (incl. ds-identify.log, result.json, instance-data.json). Config: /etc/cloud/cloud.cfg + /etc/cloud/cloud.cfg.d/*.cfg. Full CLI, the boot stages, and re-run/golden-image notes in references/cli-and-debugging.md.

Air-gapped checklist

• Pin datasource_list: [NoCloud] (or [NoCloud, None]) in a cloud.cfg.d drop-in

— stops cloud probing/timeouts.

• Local apt mirror via the apt module; embed repo signing keys inline.
• Internal CA via ca_certs.trusted.
• Local NTP via ntp.servers.
• Omit phone_home; don't package_upgrade without a full mirror; avoid snap

unless a local store / pre-acked assertions are available.

• Use vendor-data for site-wide defaults (mirror, CA, NTP, base users); per-host

user-data overrides it.

Boundaries (sibling skills)

• `network:` → ubuntu-netplan skill (v2 is netplan format).
• Ubuntu Server install automation → ubuntu-autoinstall skill. The installer

is itself driven by a NoCloud user-data carrying a top-level autoinstall: key; cloud-init ignores autoinstall and passes it to Subiquity. Inside that document, the nested user-data: is cloud-config for the installed system (first boot).

Reference files

• references/nocloud-and-airgapped.md — every NoCloud seeding method, cmdline

grammar, SMBIOS/DMI, instance-id, datasource_list pinning, DataSourceNone, vendor-data, deprecations.

• references/modules.md — full cloud-config module reference (on-prem modules in

depth; the rest listed by stage). Has a TOC.

• references/cli-and-debugging.md — CLI, boot stages & module ordering, re-run /

golden-image, version notes (24.x vs 26.1).

• references/examples.md — minimal and realistic air-gapped configs with matching

meta-data.