fix: conditionally set explicit hostname in Nova server create to prevent .novalocal suffix by benedikt-haug · Pull Request #414 · gardener/machine-controller-manager-provider-openstack

benedikt-haug · 2026-09-09T14:54:34Z

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

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

gardener-prow · 2026-09-09T14:54:46Z

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gardener-prow · 2026-09-09T14:54:47Z

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federated-github-access · 2026-09-09T14:55:01Z

The PR needs to be labeled with ok-to-test by a maintainer to trigger the automated validation of the change

aaronfern · 2026-10-01T10:11:56Z

Thanks for the PR @benedikt-haug!

The only reservation I have is that this will now fail with any openstack deployment that does not have the 2.90 microversion supported.
Can we not use utils.GetSupportedMicroversions() to ensure that the required microversion is supported? If yes then Hostname can be set, else we can skip setting Hostname
wdyt?

benedikt-haug · 2026-10-01T12:12:00Z

@aaronfern
Fair point! I tried to let my agent write a fix that incorporates your feedback, please have a look <3

federated-github-access · 2026-10-01T12:12:18Z

The PR needs to be labeled with ok-to-test by a maintainer to trigger the automated validation of the change

federated-github-access · 2026-10-01T12:12:29Z

The PR needs to be labeled with ok-to-test by a maintainer to trigger the automated validation of the change

federated-github-access · 2026-10-05T11:09:29Z

The PR needs to be labeled with ok-to-test by a maintainer to trigger the automated validation of the change

benedikt-haug · 2026-10-05T11:25:05Z

@aaronfern Thanks for the review!
I hope I got my agent to incorporate your feedback, but given the risen complexity I don't think I still fully graph these changes myself :(

aaronfern

One last comment. Otherwise looks good to me

aaronfern · 2026-10-07T07:59:35Z

 			Expect(server.InternalIPs).To(ConsistOf(serverIPv4, serverIPv6))
 		})
+
+		It("should not set hostname when SupportsHostnameOverride returns false", func() {


Could this be renamed to be more accurate?

Suggested change

It("should not set hostname when SupportsHostnameOverride returns false", func() {

It("should succeed

when SupportsHostnameOverride returns false", func() {

aaronfern · 2026-10-07T08:21:02Z

@benedikt-haug, It also looks like the verify step in one of our workflows fails. Can you please run make generate and push those changes too?

AndreasBurger · 2026-10-08T14:22:32Z

+	// create requests is honoured by Nova. Without this, Nova derives
+	// the hostname from the display name and appends [api] dhcp_domain
+	// (e.g. ".novalocal"), which can push the OS hostname beyond the
+	// 63-byte RFC 1123 label limit enforced by Kubernetes for the
+	// kubernetes.io/hostname node label.
+	// Fall back gracefully if the Nova deployment does not support 2.90.
+	supported, err := utils.GetSupportedMicroversions(ctx, compute)
+	if err != nil {
+		klog.Warningf("failed to determine Nova microversion support: %v", err)
+	} else if ok, err := supported.IsSupported("2.90"); err != nil {
+		klog.Warningf("failed to check Nova microversion 2.90 support: %v", err)
+	} else if ok {
+		compute.Microversion = "2.90"
+	}


There is a function in the SDK, utils.RequireMicroversion() that does something very similar.

I suspect that it won't be much cleaner to use it, so I'll leave it up to you. Once @aaronfern's remarks are addressed lgtm

…vent .novalocal suffix Instead of hard-pinning microversion 2.90 unconditionally, use utils.GetSupportedMicroversions to check at runtime whether the Nova deployment supports 2.90+. Only then set the hostname field in server create requests. Fall back gracefully to the previous behavior on older Nova deployments that don't support 2.90.

federated-github-access · 2026-10-08T15:06:59Z

The PR needs to be labeled with ok-to-test by a maintainer to trigger the automated validation of the change

benedikt-haug requested review from a team as code owners September 9, 2026 14:54

gardener-prow Bot added the do-not-merge/needs-kind Indicates a PR lacks a `kind/foo` label and requires one. label Sep 9, 2026

gardener-prow Bot added the size/XS Denotes a PR that changes 0-9 lines, ignoring generated files. label Sep 9, 2026

benedikt-haug force-pushed the fix/set-explicit-hostname branch from 659a9c6 to 242f34f Compare October 1, 2026 12:11

benedikt-haug changed the title ~~fix: set explicit hostname in Nova server create to prevent .novalocal suffix~~ fix: conditionally set explicit hostname in Nova server create to prevent .novalocal suffix Oct 1, 2026

gardener-prow Bot added size/M Denotes a PR that changes 30-99 lines, ignoring generated files. and removed size/XS Denotes a PR that changes 0-9 lines, ignoring generated files. labels Oct 1, 2026

aaronfern reviewed Oct 5, 2026

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Comment thread pkg/client/nova.go Outdated

Comment thread pkg/client/nova.go Outdated

Comment thread pkg/driver/executor/executor_test.go

benedikt-haug force-pushed the fix/set-explicit-hostname branch from 242f34f to b031f9f Compare October 5, 2026 11:09

gardener-prow Bot removed the size/M Denotes a PR that changes 30-99 lines, ignoring generated files. label Oct 5, 2026

gardener-prow Bot added the size/L Denotes a PR that changes 100-499 lines, ignoring generated files. label Oct 5, 2026

aaronfern reviewed Oct 7, 2026

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AndreasBurger reviewed Oct 8, 2026

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benedikt-haug closed this Oct 8, 2026

benedikt-haug force-pushed the fix/set-explicit-hostname branch from b031f9f to 5cc97a1 Compare October 8, 2026 15:06

gardener-prow Bot added size/XXL Denotes a PR that changes 1000+ lines, ignoring generated files. and removed size/L Denotes a PR that changes 100-499 lines, ignoring generated files. labels Oct 8, 2026

benedikt-haug mentioned this pull request Oct 8, 2026

fix: conditionally set explicit hostname in Nova server create to prevent .novalocal suffix #426

Merged

	It("should not set hostname when SupportsHostnameOverride returns false", func() {
	It("should succeed
	when SupportsHostnameOverride returns false", func() {

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benedikt-haug commented Sep 9, 2026

Motivation

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benedikt-haug commented Oct 1, 2026 •

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