88 lines
4.8 KiB
Markdown
88 lines
4.8 KiB
Markdown
# Design v2 — implementation errata (RHEL/Rocky 9)
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Records where the as-built implementation diverges from
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`diskless-fleet-design-v2.pdf`. The PDF remains authoritative for the
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architecture; this note captures deltas discovered bringing the build up on
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**Rocky 9.8** until they are folded into a consolidated **v3**. Last updated
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2026-06-15.
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The architecture is unchanged: 802.1X EAP-TLS with a TPM-held key, MACsec on the
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access link, dm-verity-protected erofs root over NVMe/TCP, signed UKI, and the
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FROM-scratch OCI release artifact. Only the items below differ.
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## 1. Design-level change — writable root overlay
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**PDF says (§3 step 8, §5):** runtime state lives on a tmpfs overlay enabled by
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the kernel cmdline option **`rootovl`**.
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**As built:** the cmdline uses **`systemd.volatile=overlay`** instead.
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**Why:** `rootovl`/`overlayfs` is a Debian/third-party dracut module; RHEL/Rocky
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dracut does not ship it (`dracut: module 'overlayfs' cannot be found`).
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`systemd.volatile=overlay` is the RHEL-native, systemd-supported mechanism and
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is the recommended overlay for dm-verity-protected read-only roots. Behaviour is
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equivalent: the verity erofs root mounts read-only and a tmpfs overlay carries
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runtime writes; per-host `machine-id` is still generated into the overlay.
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**Caveat:** requires `systemd-volatile-root.service` in the initramfs (bundled by
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recent dracut; `install_items` it if the root stays read-only at boot). The known
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failure mode is NFS root — not applicable here (NVMe block root). Set in
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[../scripts/extract-artifacts.sh](../scripts/extract-artifacts.sh); module list
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in [../files/etc/dracut.conf.d/90-diskless.conf](../files/etc/dracut.conf.d/90-diskless.conf).
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## 1b. Design-level change — dm-verity uses two namespaces, not in-file offset
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**PDF says (§5):** the erofs data and the dm-verity hash tree share a single
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NVMe/TCP namespace, appended in one file and split on the host with
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`systemd.verity_root_options=hash-offset=`.
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**As built:** data and hash are **two separate namespaces** — `rootfs.erofs`
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(data) and `rootfs.hash` (hash tree) — each a file in the OCI artifact, served as
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its own NVMe namespace with a distinct pinned UUID. The cmdline uses
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`systemd.verity_root_data=` / `systemd.verity_root_hash=` by-id devices with no
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offset.
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**Why:** Rocky 9.8 `systemd-veritysetup` does not support `hash-offset=` — it
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logs `Encountered unknown option 'hash-offset=…', ignoring` then `Failed to load
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verity superblock: invalid argument`, so the single-namespace layout never maps
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`/dev/mapper/root`. Two namespaces is the design's own documented fallback and
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uses systemd's native verity (no custom activation code; keeps root-hash
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signature support for production). Devices are referenced by stable
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`/dev/disk/by-id/nvme-uuid.<uuid>` (not `/dev/nvmeNnM`) because controller
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indices shift with the transient discovery controller.
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**Touches:** [../scripts/extract-artifacts.sh](../scripts/extract-artifacts.sh)
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(emits `rootfs.hash`, two UUIDs, offset-free cmdline),
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[../scripts/build-oci.sh](../scripts/build-oci.sh) (artifact list + SHA256SUMS),
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[../serving/spdk-setup.sh](../serving/spdk-setup.sh) (2nd bdev/namespace), and
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CLAUDE.md invariant #9.
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## 2. Implementation notes (not architectural)
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These don't contradict the PDF (it doesn't specify them) but are worth recording.
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- **Network backend.** The dracut hook uses `network-legacy`/`ifup`, which needs
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`dhclient`; RHEL 9 deprecated it and the minimal ISO omits it, so the
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provisioner installs `dhcp-client` explicitly. dhclient is on its way out — the
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durable fix is porting the hook to the `network-manager` dracut module
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(tracked; see project todo).
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- **Build ISO.** Defaults to a version-pinned Rocky 9.x **minimal** ISO on the
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internal mirror (`iso_url` + `sha256` `iso_checksum`). `http/ks.cfg` installs
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only core+sshd+chrony from the ISO; the diskless tooling is pulled online by
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the provisioner, so the build VM needs outbound repo access.
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- **Reference serving stack.** §7's SPDK + nginx + DHCP/DNS serving stack has a
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concrete docker-compose implementation in [../serving/](../serving/), incl.
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Docker image-volume mounting of the OCI artifact image.
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- Assorted build-time fixes (HCL `%%{` escaping, file-provisioner staging,
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dracut module naming) are documented in
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[../CLAUDE.md](../CLAUDE.md) → "RHEL 9.x build notes".
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## 3. Temporary deviation — must be reverted before production
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- **802.1X/MACsec test bypass.** `disable_dot1x=true` bakes `rd.dot1x=0` into the
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cmdline so the host skips authentication/MACsec and DHCPs the NIC directly, to
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validate the UKI/rootfs/NVMe-oF path on an open switch port. The auth stack
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stays in the image (dormant). This is **lab-only**; a production build must
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drop the flag (`DISABLE_DOT1X: "false"` in CI) to restore the secure boot
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chain described throughout the PDF.
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