OCPBUGS-87249: on AWS, select associated IPv6 CIDR block for egress IP subnet

[jechen0648]

On dualstack IPv4-primary AWS clusters, egressIPv6 addresses are never assigned to egress nodes even though the nodes report non-zero IPv6 capacity in their cloud.network.openshift.io/egress-ipconfig annotation.

The root cause is in getSubnet (pkg/cloudprovider/aws.go). AWS represents an IPv6 CIDR block association on a subnet through Ipv6CidrBlockAssociationSet, and each entry carries a state (associating, associated, disassociated, etc.). A subnet can accumulate multiple entries over its lifetime — for example, after an IPv6 CIDR block is replaced, the old entry stays in the list as disassociated ahead of the new associated entry.

The previous code unconditionally picked Ipv6CidrBlockAssociationSet[0] without checking state, if the first entry is stale (disassociated), one of two things goes wrong:

1. Wrong CIDR returned — the old, now-disassociated IPv6 CIDR is used as the node's IPv6 subnet in the annotation. OVN-Kubernetes stores this and then cannot find a node whose subnet contains the egress IPv6 address (which is from the current, correct CIDR), so it never assigns the EgressIP.
2. No CIDR returned — if the first entry has an empty Ipv6CidrBlock, the condition fails and v6Subnet stays nil. The annotation lacks an IPv6 subnet entirely and OVN-Kubernetes concludes the node has no IPv6 egress capability.
   In both cases, no CloudPrivateIPConfig is ever created for the IPv6 EgressIP, so CNCC is never asked to assign it.

Fix
Iterate through all entries in Ipv6CidrBlockAssociationSet and select the first one whose state is associated.

Two minor cleanups are included in the same change:

The empty-subnet guard is tightened from len > 1 to len != 1 so that a DescribeSubnets response returning zero subnets is also caught.
The IPv4 parsing block is cleaned up to avoid shadowing the outer subnet variable (_, v4Net, err instead of _, subnet, err).

Related
A companion fix in [openshift/ovn-kubernetes](openshift/ovn-kubernetes#3269) ensures OVN-Kubernetes re-evaluates unassigned EgressIPs when the cloud.network.openshift.io/egress-ipconfig annotation changes, so that any cluster where the annotation was previously incorrect (e.g., missing IPv6 subnet) gets healed automatically once CNCC updates it.

Testing
Pre-merge testing was performed with openshift/ovn-kubernetes#3269 on dualstack AWS IPv4 primary and dualstack AWS IPv6 primary cluster
Regression testing with openshift/ovn-kubernetes#3269 was performed on dualstack BM cluster

[openshift-ci-robot]

@jechen0648: This pull request references Jira Issue OCPBUGS-87249, which is invalid:

• expected the bug to target the "5.0.0" version, but no target version was set

Comment /jira refresh to re-evaluate validity if changes to the Jira bug are made, or edit the title of this pull request to link to a different bug.

The bug has been updated to refer to the pull request using the external bug tracker.

Details

In response to this:

On dualstack IPv4-primary AWS clusters, egressIPv6 addresses are never assigned to egress nodes even though the nodes report non-zero IPv6 capacity in their cloud.network.openshift.io/egress-ipconfig annotation.

The root cause is in getSubnet (pkg/cloudprovider/aws.go). AWS represents an IPv6 CIDR block association on a subnet through Ipv6CidrBlockAssociationSet, and each entry carries a state (associating, associated, disassociated, etc.). A subnet can accumulate multiple entries over its lifetime — for example, after an IPv6 CIDR block is replaced, the old entry stays in the list as disassociated ahead of the new associated entry.

The previous code unconditionally picked Ipv6CidrBlockAssociationSet[0] without checking state, if the first entry is stale (disassociated), one of two things goes wrong:

Wrong CIDR returned — the old, now-disassociated IPv6 CIDR is used as the node's IPv6 subnet in the annotation. OVN-Kubernetes stores this and then cannot find a node whose subnet contains the egress IPv6 address (which is from the current, correct CIDR), so it never assigns the EgressIP.
No CIDR returned — if the first entry has an empty Ipv6CidrBlock, the condition fails and v6Subnet stays nil. The annotation lacks an IPv6 subnet entirely and OVN-Kubernetes concludes the node has no IPv6 egress capability.
In both cases, no CloudPrivateIPConfig is ever created for the IPv6 EgressIP, so CNCC is never asked to assign it.

Fix
Iterate through all entries in Ipv6CidrBlockAssociationSet and select the first one whose state is associated.

Two minor cleanups are included in the same change:

The empty-subnet guard is tightened from len > 1 to len != 1 so that a DescribeSubnets response returning zero subnets is also caught.
The IPv4 parsing block is cleaned up to avoid shadowing the outer subnet variable (_, v4Net, err instead of _, subnet, err).

Testing

Related
A companion fix in openshift/ovn-kubernetes ensures OVN-Kubernetes re-evaluates unassigned EgressIPs when the cloud.network.openshift.io/egress-ipconfig annotation changes, so that any cluster where the annotation was previously incorrect (e.g., missing IPv6 subnet) gets healed automatically once CNCC updates it.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the openshift-eng/jira-lifecycle-plugin repository.

[coderabbitai]

Walkthrough

The getSubnet function in the AWS cloud provider was updated to change how subnet CIDRs are parsed. IPv4 CIDR is parsed directly from the subnet's CidrBlock, while IPv6 CIDR selection now iterates associations to find one in the "Associated" state, replacing the prior first-entry approach.

Changes

Subnet CIDR Parsing

Layer / File(s)	Summary
getSubnet IPv4/IPv6 CIDR parsing pkg/cloudprovider/aws.go	IPv4 CIDR parsed directly from subnet.CidrBlock with nil/empty checks; IPv6 CIDR now selected by scanning Ipv6CidrBlockAssociationSet for the first entry with Associated state instead of using the first entry unconditionally; parse errors now return explicit failures tagged by IP version.

Estimated code review effort: 2 (Simple) | ~10 minutes

Related PRs: None specified.

Suggested labels: None specified.

Suggested reviewers: None specified.

🚥 Pre-merge checks | ✅ 15

✅ Passed checks (15 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Stable And Deterministic Test Names	✅ Passed	Only pkg/cloudprovider/aws.go changed; no Ginkgo test definitions or test titles were added, so there are no unstable names to flag.
Test Structure And Quality	✅ Passed	No Ginkgo test code was changed; the PR only updates pkg/cloudprovider/aws.go production logic, so this check is not applicable.
Microshift Test Compatibility	✅ Passed	The PR only changes pkg/cloudprovider/aws.go; no new Ginkgo tests or MicroShift-unsafe APIs/features were added.
Single Node Openshift (Sno) Test Compatibility	✅ Passed	Only pkg/cloudprovider/aws.go changed; no new or modified Ginkgo tests or SNO-sensitive assumptions were added.
Topology-Aware Scheduling Compatibility	✅ Passed	Only pkg/cloudprovider/aws.go changed, and the diff updates AWS subnet parsing; no topology, affinity, replica, or nodeSelector scheduling logic was added.
Ote Binary Stdout Contract	✅ Passed	PR modifies only pkg/cloudprovider/aws.go in a non-test production controller; OTE Binary Stdout Contract check is inapplicable since this is not an OTE test binary and no process-level stdout writ...
Ipv6 And Disconnected Network Test Compatibility	✅ Passed	No new Ginkgo tests were added; the patch only changes pkg/cloudprovider/aws.go, so the IPv4/disconnected-network test check is not applicable.
No-Weak-Crypto	✅ Passed	Changed code only updates AWS subnet selection/parsing in getSubnet; no weak algorithms, custom crypto, or secret comparisons found.
Container-Privileges	✅ Passed	PASS: The PR only changes pkg/cloudprovider/aws.go; no K8s/container manifests are modified, and the edited file contains no privileged settings.
No-Sensitive-Data-In-Logs	✅ Passed	PASS: The changed getSubnet code adds no log statements; it only adjusts subnet parsing and error returns, with no new sensitive data exposed in logs.
Title check	✅ Passed	The title accurately summarizes the main change: selecting the associated IPv6 CIDR block for AWS egress IP subnet handling.
Description check	✅ Passed	The description is directly related to the code change and explains the bug, fix, and testing in detail.

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[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: jechen0648
Once this PR has been reviewed and has the lgtm label, please assign tssurya for approval. For more information see the Code Review Process.

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[jechen0648]

@coderabbitai review

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[jechen0648]

@coderabbitai full review

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

pkg/cloudprovider/aws.go (1)

268-277: 🩺 Stability & Availability | 🟠 Major | ⚡ Quick win

Handle zero subnet results before indexing.

Line 268 only rejects multiple results, so Line 277 can panic when DescribeSubnets returns zero subnets. This also conflicts with the PR objective to tighten the guard to exactly one subnet.

Proposed fix

-	if len(describeOutput.Subnets) > 1 {
+	if len(describeOutput.Subnets) != 1 {
 		subnetId := "<nil>"
 		if networkInterface.SubnetId != nil {
 			subnetId = *networkInterface.SubnetId
 		}
-		return nil, nil, fmt.Errorf("error: multiple subnets found for the subnet ID: %s", subnetId)
+		return nil, nil, fmt.Errorf("error: expected exactly one subnet for subnet ID %s, got %d", subnetId, len(describeOutput.Subnets))
 	}

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@pkg/cloudprovider/aws.go` around lines 268 - 277, The subnet lookup in aws.go
only checks for multiple results in the DescribeSubnets flow, so it can still
panic when zero subnets are returned before using subnet :=
describeOutput.Subnets[0]. Update the guard around the DescribeSubnets result in
the same block to require exactly one subnet, handling the zero-subnet case with
an error before indexing and keeping the existing error path for multiple
results. Use the existing networkInterface.SubnetId, describeOutput.Subnets, and
subnet variables to locate and adjust the logic.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@pkg/cloudprovider/aws.go`:
- Around line 268-277: The subnet lookup in aws.go only checks for multiple
results in the DescribeSubnets flow, so it can still panic when zero subnets are
returned before using subnet := describeOutput.Subnets[0]. Update the guard
around the DescribeSubnets result in the same block to require exactly one
subnet, handling the zero-subnet case with an error before indexing and keeping
the existing error path for multiple results. Use the existing
networkInterface.SubnetId, describeOutput.Subnets, and subnet variables to
locate and adjust the logic.

---

ℹ️ Review info

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Configuration used: Repository YAML (base), Central YAML (inherited)

Review profile: CHILL

Plan: Enterprise

Run ID: 85fa1e5c-85d3-4b26-9ac5-1f81e3d4ad6b

📥 Commits

Reviewing files that changed from the base of the PR and between 0b49df2 and b71c43e.

📒 Files selected for processing (1)

• pkg/cloudprovider/aws.go

[openshift-ci]

@jechen0648: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	b71c43e	link	false	/test security
ci/prow/e2e-gcp-ovn	b71c43e	link	true	/test e2e-gcp-ovn

Full PR test history. Your PR dashboard.

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[tssurya]

/assign @arghosh93 and @pperiyasamy

[openshift-ci]

@tssurya: GitHub didn't allow me to assign the following users: and.

Note that only openshift members with read permissions, repo collaborators and people who have commented on this issue/PR can be assigned. Additionally, issues/PRs can only have 10 assignees at the same time.
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Details

In response to this:

/assign @arghosh93 and @pperiyasamy

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[jechen0648]

@arghosh93 I pre-merge tested by fix by building pre-merged image using this PR + openshift/ovn-kubernetes#3269 which is same as u/s PR: ovn-kubernetes/ovn-kubernetes#6648, tests were performed on dualstack AWS for both IPv4 primary and IPv6 primary flavors.