fix: set explicit hostname in Nova server create to prevent .novalocal suffix

[benedikt-haug]

Motivation

When Nova derives the OS hostname from the server display name, it appends the [api] dhcp_domain (commonly .novalocal). For Gardener shoot workers, the full machine name can exceed the 63-byte RFC 1123 label limit that Kubernetes enforces for the kubernetes.io/hostname node label, causing pod scheduling failures:

unable to parse requirement: values[0][kubernetes.io/hostname]:
Invalid value: "shoot--staging--kbo8cp525p-worker-ndrej-z3-58f8b-2t9lq.novalocal":
must be no more than 63 bytes

Additionally, the .novalocal suffix causes hostname instability across reboots (see gardener/gardener-extension-provider-openstack#569): cloud-init re-applies the FQDN on reboot, racing with the ExecStartPre workaround that strips the domain, leading to inconsistent hostnames and kubelet startup failures.

Background

Since Nova Wallaby (API microversion 2.90), the hostname field in POST /servers allows decoupling the OS hostname from the display name. When set, Nova uses this value directly in the metadata service — without appending dhcp_domain. The display name (Name) remains unaffected.

This was confirmed via a manual PoC against an OpenStack deployment (Nova max microversion 2.100):

• hostname field accepted with MV 2.90
• OS-EXT-SRV-ATTR:hostname stored the explicit value without .novalocal
• Normal VMs (without hostname field) remained unaffected

References:

• Nova spec: Configurable Instance Hostnames (Wallaby)
• API microversion 2.90 changelog
• gophercloud CreateOpts.Hostname field (available since v2.x, confirmed in v2.12.0)

Changes

1. pkg/client/nova.go: Pin the Nova compute client to microversion 2.90, enabling the hostname field in server create/update requests.

2. pkg/driver/executor/executor.go: Pass machineName as the explicit hostname in servers.CreateOpts, decoupling the OS hostname from the display name.

Effects

Aspect	Before	After
Nova display name	shoot--...-2t9lq	unchanged
OS hostname in VM	shoot--...-2t9lq.novalocal (potentially >63 bytes)	shoot--...-2t9lq (≤63 bytes)
kubernetes.io/hostname label	may exceed 63-byte limit	stays within limit
Hostname stability across reboots	unstable (race condition, see #569)	stable (short name from metadata)
Normal VMs (no hostname field)	.novalocal appended	unchanged

Compatibility

• Requires Nova ≥ Wallaby (microversion 2.90). Older deployments will reject the microversion negotiation.
• hostname validation by Nova: ≤63 chars, alphanumeric + dashes only, no trailing dash. Gardener machine names comply by construction (RFC 1123).
• Existing nodes keep their old hostname; only new VMs are affected.

Fixes gardener/gardener-extension-provider-openstack#569

🤖 Generated with AI assistance — the code changes, analysis, and PoC verification were performed by an AI agent. The approach was validated manually against a live OpenStack deployment.

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