Fix get nexthop IP address for OVN mgmt interface

[yboaron]

The getNextHopOnK8sMgmtIntf() function would return Dst.IP when no explicit gateway was set on a route. However, Dst.IP is the network address (the base of the CIDR), not a valid host IP.

This PR adds validation to skip routes where Dst.IP equals the network address (IP & Mask == IP). This ensures only valid host IPs are used as gateways.

Also added test case to verify network addresses are rejected and only valid host addresses are used as gateways.

Fixes: #4121

Summary by CodeRabbit

• Bug Fixes

  • Fixed route selection to avoid using a network address as a gateway when no explicit gateway is configured.
  • Preserved valid gateway-less host routes during host-network reconciliation.
  • Ensured routes with valid gateways are preferred during route selection.
  • Improved host-network routing reliability when multiple routes are available.

• Tests

  • Added coverage for network-address routes, host routes, and gateway selection scenarios.

[submariner-bot]

🤖 Created branch: z_pr4131/yboaron/fix-ovn-gateway-network-address
🚀 Full E2E won't run until the "ready-to-test" label is applied. I will add it automatically once the PR has 2 approvals, or you can add it manually.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 49cc4f0e-8820-4637-b370-a2f3edcfcaea

📥 Commits

Reviewing files that changed from the base of the PR and between 216fa46 and 4f5ccce.

📒 Files selected for processing (1)

• pkg/routeagent_driver/handlers/ovn/handler_test.go

🚧 Files skipped from review as they are similar to previous changes (1)

• pkg/routeagent_driver/handlers/ovn/handler_test.go

---

Walkthrough

OVN host-network route selection skips gateway-less network-address routes while retaining gateway-less host routes. Regression tests verify valid gateway selection and host-route handling during reconciliation.

Changes

OVN gateway route selection

Layer / File(s)	Summary
Gateway validation and regression coverage pkg/routeagent_driver/handlers/ovn/host_networking.go, pkg/routeagent_driver/handlers/ovn/handler_test.go	getNextHopOnK8sMgmtIntf skips gateway-less network-address routes and accepts /32 and /128 host routes. Regression tests verify both behaviors and valid gateway selection.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Mergeability Score: ⚪ Minimal · up to 4f5cc

This localized fix rejects invalid network-address gateways and adds coverage for the behavior; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested labels: bug, ready-to-test

Suggested reviewers: skitt, vthapar, oats87

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the fix to next-hop selection for the OVN management interface.
Linked Issues check	✅ Passed	The changes address [#4121] by rejecting network addresses as gateways while preserving valid host-route selection.
Out of Scope Changes check	✅ Passed	The implementation and regression tests remain within the scope of the gateway-selection fix described in [#4121].
Actionable Comments Resolved	✅ Passed	The PR page marks the check passed: the sole actionable CodeRabbit comment is resolved; the diff adds /32-/128 handling and a Gw=nil host-route regression test.

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[coderabbitai]

Actionable comments posted: 1

🤖 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.

Inline comments:
In `@pkg/routeagent_driver/handlers/ovn/host_networking.go`:
- Around line 183-187: The route filtering in getNextHopOnK8sMgmtIntf must skip
masked network-address routes without rejecting valid /32 or /128 host routes;
update pkg/routeagent_driver/handlers/ovn/host_networking.go#L183-L187
accordingly. Add a Gw == nil host-route case in
pkg/routeagent_driver/handlers/ovn/handler_test.go#L822-L895 and verify its
Dst.IP is selected after a network-address route is skipped.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 0b10259b-a5d4-4dfa-bc9f-10f20aff2dc4

📥 Commits

Reviewing files that changed from the base of the PR and between a5d2aba and e456685.

📒 Files selected for processing (2)

• pkg/routeagent_driver/handlers/ovn/handler_test.go
• pkg/routeagent_driver/handlers/ovn/host_networking.go

[yboaron]

cc @tpantelis

[submariner-bot]

🤖 Closed branches: [z_pr4131/yboaron/fix-ovn-gateway-network-address]