fix(kubernetes): set explicit MTU for Cilium in tenant clusters#2147
fix(kubernetes): set explicit MTU for Cilium in tenant clusters#2147kvaps merged 1 commit intocozystack:mainfrom
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Cilium's MTU auto-detection does not account for VXLAN overhead when running inside KubeVirt VMs. The VM interface inherits MTU 1400 from the parent OVN/Geneve overlay, and Cilium sets all interfaces (cilium_vxlan, lxc*, cilium_host/net) to 1400 without subtracting the 50-byte VXLAN encapsulation overhead. This causes intermittent packet drops for large packets (TLS handshakes, HTTP responses with data), resulting in timeouts and 499 errors for services running in tenant clusters. Set MTU to 1350 (1400 - 50 VXLAN overhead) explicitly in the default Cilium values for tenant Kubernetes clusters. Signed-off-by: IvanHunters <[email protected]>
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📝 WalkthroughWalkthroughA single configuration parameter was added to the Cilium HelmRelease template, specifying an MTU value of 1350 in the cilium values block. This minimal change affects the network settings applied during HelmRelease rendering. Changes
Estimated code review effort🎯 1 (Trivial) | ⏱️ ~2 minutes Poem
✨ Finishing Touches🧪 Generate unit tests (beta)
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Summary of ChangesHello, I'm Gemini Code Assist1! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed! This pull request resolves critical connectivity issues within tenant Kubernetes clusters by adjusting the Maximum Transmission Unit (MTU) for Cilium. By setting a specific MTU value, it prevents silent packet drops that occurred due to the overhead introduced by VXLAN encapsulation, thereby improving network stability and reliability for services running in these environments. Highlights
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Code Review
This pull request correctly addresses packet drops in KubeVirt-based tenant clusters by setting an explicit MTU of 1350 for Cilium. This change accounts for the VXLAN encapsulation overhead missed by auto-detection, resolving the described connectivity issues. I've added a suggestion to include a comment documenting the reason for this specific MTU value to improve long-term maintainability.
| k8sServiceHost: {{ .Release.Name }}.{{ .Release.Namespace }}.svc | ||
| k8sServicePort: 6443 | ||
| routingMode: tunnel | ||
| MTU: 1350 |
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While the value 1350 is correct according to the pull request description, it appears as a 'magic number' in the code. For better long-term maintainability, it's good practice to add a comment explaining how this value is derived (i.e., 1400 MTU from the host - 50 bytes for VXLAN overhead).
# Set MTU to 1350 to account for VXLAN overhead (50 bytes) in KubeVirt VMs,
# which inherit a 1400 MTU from the host cluster's overlay network.
MTU: 1350
kvaps
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Summary
Problem
Cilium's MTU auto-detection does not account for VXLAN overhead when running inside KubeVirt VMs. The VM network interface inherits MTU 1400 from the parent cluster's OVN/Geneve overlay (1500 - 100 Geneve overhead). Cilium detects this MTU and applies it to all tunnel interfaces without subtracting the 50-byte VXLAN encapsulation overhead.
This results in:
Fix
Explicitly set
MTU: 1350(1400 - 50 VXLAN overhead) in the default Cilium values for tenant clusters. This value can still be overridden viaaddons.cilium.valuesOverrideif needed.Test plan
Summary by CodeRabbit