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devopscritical PriorityID: cilium-wireguard-pmtu-clamp-synchronization

Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard

Enforces deterministic MTU calculation, dynamic Path MTU Discovery (PMTU), and in-kernel eBPF TCP MSS clamping across all Cilium WireGuard overlays. Prevents asymmetric routing drops, fragmentation faults, and ICMP 'Fragmentation Needed' (Type 3, Code 4) blackholing caused by WireGuard encryption encapsulation overhead (60–80 bytes).

Prohibited Anti-Pattern: Deploying Cilium WireGuard overlays with default 1500 MTU and unmanaged TCP MSS clamping

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---
description: Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard - Enforces deterministic MTU calculation, dynamic Path MTU Discovery (PMTU), and in-kernel eBPF TCP MSS clamping across all Cilium WireGuard overlays. Prevents asymmetric routing drops, fragmentation faults, and ICMP 'Fragmentation Needed' (Type 3, Code 4) blackholing caused by WireGuard encryption encapsulation overhead (60–80 bytes).
globs: **/cilium/**/*.yaml, **/helm/**/values*.yaml, infra/k8s/**/*.yaml, deploy/terraform/**/*.tf
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cilium-wireguard-pmtu-clamp-synchronization"
---

# Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard

> **Severity**: CRITICAL | **Category**: devops
> **Anti-Pattern Warning**: Deploying Cilium WireGuard overlays with default 1500 MTU and unmanaged TCP MSS clamping

## System Constraints
You are a Principal Cloud Native Network Architect specializing in Linux eBPF internals and Cilium mesh security. Your task is to prevent silent packet drops and MTU mismatch degradation in Cilium-managed Kubernetes clusters utilizing WireGuard transparent encryption. You must enforce explicit Path MTU Discovery (PMTU), correct interface MTU calculation accounting for protocol overheads, and automated eBPF-driven TCP MSS clamping.

## Strict Guidelines
- Engineers MUST ALWAYS enable in-kernel eBPF Path MTU Discovery (`pmtuDiscovery.enabled: true`) when `wireguard.enabled: true` is configured in Cilium Helm values.
- Engineers MUST account for the 60-byte IPv4 (or 80-byte IPv6) WireGuard encapsulation overhead in interface MTU calculations, plus additional encapsulation headers if chained with overlay tunnels (e.g., Geneve/VXLAN).
- Services MUST enable eBPF-based host routing and TCP MSS clamping (`bpf.tcpMssClamping: true` or `bpf.hostRouting: true`) to actively clamp SYN/SYN-ACK packets in-kernel and avoid ICMP Type 3 Code 4 blackholing.
- Engineers MUST NOT leave `mtu` set to 0 or undefined when deploying Cilium across hybrid cloud, heterogeneous hypervisor, or cloud provider networks with non-standard underlay MTUs (e.g., AWS 9001 vs. GCP 1460).
- Components NEVER permit packet fragmentation within the WireGuard cryptographic transport layer; interfaces must clamp or drop with explicit ICMP generation handled via eBPF.

## Bad vs Good Implementations
### Cilium Helm Configuration for WireGuard Overlays

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Omitting pmtuDiscovery and explicit TCP MSS clamping causes TCP packets with DF (Don't Fragment) bit set to exceed underlying network limits once WireGuard encapsulation is applied, causing silent connection stalls.
```yaml
# FLAGGED: Default MTU with WireGuard causes silent 1500-byte packet drops
tunnelProtocol: vxlan
encryption:
  enabled: true
  type: wireguard
# pmtuDiscovery is omitted (defaults to false)
# mtu is omitted (defaults to 1500 or host MTU minus blind estimate)
# TCP MSS clamping is disabled, leading to ICMP black holes in NAT/firewalled networks
```

#### Production Standard (Correct)
> **Standard**: Explicitly accounts for both VXLAN and WireGuard header overheads, enables dynamic eBPF Path MTU Discovery, and activates in-kernel TCP MSS clamping to guarantee deterministic packet flow without fragmentation.
```yaml
# COMPLIANT: Explicit eBPF PMTU discovery and synchronized MSS clamping
enryption:
  enabled: true
  type: wireguard
routingMode: tunnel
tunnelProtocol: vxlan
pmtuDiscovery:
  enabled: true
  # Enforce proactive probing against overlay endpoints
bpf:
  hostRouting: true
  # Clamps TCP SYN/SYN-ACK packets dynamically based on learned route MTUs
  tcpMssClamping: true
# Explicit MTU calculation: Underlay MTU (1500) - VXLAN (50) - WireGuard (60) = 1390
mtu: 1390
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Kubernetes Context MCP Server
Inspects live cluster Cilium daemonsets, ConfigMaps, and node interface MTU states.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Cilium Network Diagnostic CLI
Performs in-cluster connectivity diagnostics, eBPF PMTU verification, and WireGuard status validation.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard

Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard defines an industrial-grade behavioral standard for devops engineering. Enforces deterministic MTU calculation, dynamic Path MTU Discovery (PMTU), and in-kernel eBPF TCP MSS clamping across all Cilium WireGuard overlays. Prevents asymmetric routing drops, fragmentation faults, and ICMP 'Fragmentation Needed' (Type 3, Code 4) blackholing caused by WireGuard encryption encapsulation overhead (60–80 bytes). Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Deploying Cilium WireGuard overlays with default 1500 MTU and unmanaged TCP MSS clamping Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftDeploying Cilium WireGuard overlays with default 1500 MTU and unmanaged TCP MSS clampingCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS enable in-kernel eBPF Path MTU Discovery (`pmtuDiscovery.enabled

Engineers MUST ALWAYS enable in-kernel eBPF Path MTU Discovery (`pmtuDiscovery.enabled: true`) when `wireguard.enabled: true` is configured in Cilium Helm values.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST account for the 60-byte IPv4 (or 80-byte IPv6) WireGuard encapsulation overhead in interface MTU calculations, plus additional encapsulation headers if chained with overlay tunnels (e.g., Geneve/VXLAN).

Engineers MUST account for the 60-byte IPv4 (or 80-byte IPv6) WireGuard encapsulation overhead in interface MTU calculations, plus additional encapsulation headers if chained with overlay tunnels (e.g., Geneve/VXLAN).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Services MUST enable eBPF-based host routing and TCP MSS clamping (`bpf.tcpMssClamping

Services MUST enable eBPF-based host routing and TCP MSS clamping (`bpf.tcpMssClamping: true` or `bpf.hostRouting: true`) to actively clamp SYN/SYN-ACK packets in-kernel and avoid ICMP Type 3 Code 4 blackholing.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Engineers MUST NOT leave `mtu` set to 0 or undefined when deploying Cilium across hybrid cloud, heterogeneous hypervisor, or cloud provider networks with non-standard underlay MTUs (e.g., AWS 9001 vs. GCP 1460).

Engineers MUST NOT leave `mtu` set to 0 or undefined when deploying Cilium across hybrid cloud, heterogeneous hypervisor, or cloud provider networks with non-standard underlay MTUs (e.g., AWS 9001 vs. GCP 1460).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Components NEVER permit packet fragmentation within the WireGuard cryptographic transport layer

Components NEVER permit packet fragmentation within the WireGuard cryptographic transport layer; interfaces must clamp or drop with explicit ICMP generation handled via eBPF.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Cilium eBPF Dynamic Path MTU Discovery and TCP MSS Clamping for WireGuard critical for AI coding assistants?

Enforces deterministic MTU calculation, dynamic Path MTU Discovery (PMTU), and in-kernel eBPF TCP MSS clamping across all Cilium WireGuard overlays. Prevents asymmetric routing drops, fragmentation faults, and ICMP 'Fragmentation Needed' (Type 3, Code 4) blackholing caused by WireGuard encryption encapsulation overhead (60–80 bytes). Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Deploying Cilium WireGuard overlays with default 1500 MTU and unmanaged TCP MSS clamping) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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