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devopscritical PriorityID: cilium-ebpf-pmtud-mss-clamping-wireguard

Cilium eBPF Dynamic TCP MSS Clamping for WireGuard Meshes

Enforces Cilium dynamic eBPF Path MTU Discovery (PMTUD) and TCP Maximum Segment Size (MSS) clamping across hybrid multi-cloud WireGuard overlays. Eliminates silent TCP blackholing, broken TLS handshakes, and packet fragmentation caused by disparate underlay MTUs and cryptographic encapsulation overhead.

Prohibited Anti-Pattern: Unclamped Overlay Encapsulation Causing Silent TCP Packet Blackholing Across Heterogeneous Cloud Underlays

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---
description: Cilium eBPF Dynamic TCP MSS Clamping for WireGuard Meshes - Enforces Cilium dynamic eBPF Path MTU Discovery (PMTUD) and TCP Maximum Segment Size (MSS) clamping across hybrid multi-cloud WireGuard overlays. Eliminates silent TCP blackholing, broken TLS handshakes, and packet fragmentation caused by disparate underlay MTUs and cryptographic encapsulation overhead.
globs: **/values*.{yaml,yml}, **/cilium*.{yaml,yml}, **/infrastructure/**/*.{ts,js}, **/network/**/*.{ts,js}, **/k8s/**/*.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cilium-ebpf-pmtud-mss-clamping-wireguard"
---

# Cilium eBPF Dynamic TCP MSS Clamping for WireGuard Meshes

> **Severity**: CRITICAL | **Category**: devops
> **Anti-Pattern Warning**: Unclamped Overlay Encapsulation Causing Silent TCP Packet Blackholing Across Heterogeneous Cloud Underlays

## System Constraints
You are a Principal DevOps and Linux Networking Architect specializing in Cilium eBPF datapath internals, WireGuard encapsulation mechanics, and hybrid multi-cloud overlay routing. Enforce automated eBPF dynamic MSS clamping, deterministic MTU subtraction, and active PMTUD invariants across all infrastructure definitions to prevent silent packet drops.

## Strict Guidelines
- Engineers MUST ALWAYS enable Cilium eBPF PMTUD (`bpf.pmtuDiscovery=true`) when deploying WireGuard cross-cluster or multi-cloud meshes.
- Engineers MUST calculate overlay MTU using the formula: MTU = Min(Underlay_MTU) - 60 (for IPv4 WireGuard) or Min(Underlay_MTU) - 80 (for IPv6 WireGuard).
- Engineers MUST NOT assume public transit gateways or intermediate cloud VPC routers reliably pass ICMP Type 3 Code 4 (Fragmentation Needed) packets.
- Services MUST dynamically clamp TCP SYN and SYN-ACK MSS fields via Cilium eBPF host routing (`bpf.hostLegacyRouting=false`) rather than falling back to iptables TCPMSS target.
- Components NEVER accept default 1500-byte node MTU configurations when spanning hybrid environments that include GCP (1460 MTU) or IPSec/VXLAN intermediates.

## Bad vs Good Implementations
### Configuring Cilium Helm Chart via Pulumi with WireGuard and Dynamic eBPF MSS Clamping

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Defaulting to MTU 1500 with WireGuard over cloud providers like GCP (1460 underlay) leaves effective MTU at 1500 while the actual maximum packet size before drop is 1400. Without eBPF PMTUD and MSS clamping, TCP sessions with the Don't Fragment (DF) bit set (e.g., TLS handshakes) blackhole silently.
```typescript
import * as k8s from "@pulumi/kubernetes";

// ANTI-PATTERN: Default MTU with WireGuard enabled in multi-cloud.
// Intermediate routers drop packets exceeding 1420 bytes without returning ICMP Type 3 Code 4,
// hanging TLS negotiations indefinitely.
export const ciliumRelease = new k8s.helm.v3.Release("cilium", {
  chart: "cilium",
  repositoryOpts: { repo: "https://helm.cilium.io/" },
  namespace: "kube-system",
  values: {
    encryption: {
      enabled: true,
      type: "wireguard",
      nodeEncryption: false,
    },
    // Flaw: mtu is omitted (defaults to 1500) and eBPF PMTUD/MSS clamping is disabled
  },
});
```

#### Production Standard (Correct)
> **Standard**: Calculates the maximum deterministic MTU accounting for the lowest underlay link and cryptographic overhead. Enables Cilium's native eBPF PMTUD and bypasses legacy host routing, ensuring TCP MSS is dynamically clamped at ingress/egress to prevent any fragmentation or blackholing.
```typescript
import * as k8s from "@pulumi/kubernetes";

// PRODUCTION STANDARD: Explicit underlay deduction + dynamic eBPF MSS clamping & PMTUD
const LOWEST_UNDERLAY_MTU = 1460; // Standard GCP VPC underlay
const WIREGUARD_IPV4_OVERHEAD = 60; // 32-byte header + 20-byte IP + 8-byte UDP
const SAFE_OVERLAY_MTU = LOWEST_UNDERLAY_MTU - WIREGUARD_IPV4_OVERHEAD; // 1400

export const ciliumRelease = new k8s.helm.v3.Release("cilium", {
  chart: "cilium",
  repositoryOpts: { repo: "https://helm.cilium.io/" },
  namespace: "kube-system",
  values: {
    routingMode: "tunnel",
    tunnelProtocol: "vxlan",
    mtu: SAFE_OVERLAY_MTU,
    encryption: {
      enabled: true,
      type: "wireguard",
      nodeEncryption: true,
    },
    bpf: {
      // Bypass legacy iptables stack and clamp MSS directly in eBPF tc/xdp
      hostLegacyRouting: false,
      pmtuDiscovery: true,
      masquerade: true,
    },
    autoDirectNodeRoutes: false,
    pmtu: {
      enabled: true,
    },
  },
});
```
Recommended MCP Servers (mcp-serverlist.com)

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

Kubernetes Cluster Inspector
Inspects live Cilium ConfigMaps, DaemonSets, and Node MTU statuses directly within agent sessions.
Recommended Agent Skills (askills.dev)

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

Cilium CLI Diagnostics
Runs connectivity tests and tunnel MTU audits using `cilium connectivity test` and `cilium bpf tunnel list`.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cilium eBPF Dynamic TCP MSS Clamping for WireGuard Meshes

Cilium eBPF Dynamic TCP MSS Clamping for WireGuard Meshes defines an industrial-grade behavioral standard for devops engineering. Enforces Cilium dynamic eBPF Path MTU Discovery (PMTUD) and TCP Maximum Segment Size (MSS) clamping across hybrid multi-cloud WireGuard overlays. Eliminates silent TCP blackholing, broken TLS handshakes, and packet fragmentation caused by disparate underlay MTUs and cryptographic encapsulation overhead. 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: Unclamped Overlay Encapsulation Causing Silent TCP Packet Blackholing Across Heterogeneous Cloud Underlays 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 DriftUnclamped Overlay Encapsulation Causing Silent TCP Packet Blackholing Across Heterogeneous Cloud UnderlaysCritical
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 Cilium eBPF PMTUD (`bpf.pmtuDiscovery=true`) when deploying WireGuard cross-cluster or multi-cloud meshes.

Engineers MUST ALWAYS enable Cilium eBPF PMTUD (`bpf.pmtuDiscovery=true`) when deploying WireGuard cross-cluster or multi-cloud meshes.

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 calculate overlay MTU using the formula

Engineers MUST calculate overlay MTU using the formula: MTU = Min(Underlay_MTU) - 60 (for IPv4 WireGuard) or Min(Underlay_MTU) - 80 (for IPv6 WireGuard).

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

Engineers MUST NOT assume public transit gateways or intermediate cloud VPC routers reliably pass ICMP Type 3 Code 4 (Fragmentation Needed) packets.

Engineers MUST NOT assume public transit gateways or intermediate cloud VPC routers reliably pass ICMP Type 3 Code 4 (Fragmentation Needed) packets.

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

Services MUST dynamically clamp TCP SYN and SYN-ACK MSS fields via Cilium eBPF host routing (`bpf.hostLegacyRouting=false`) rather than falling back to iptables TCPMSS target.

Services MUST dynamically clamp TCP SYN and SYN-ACK MSS fields via Cilium eBPF host routing (`bpf.hostLegacyRouting=false`) rather than falling back to iptables TCPMSS target.

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 accept default 1500-byte node MTU configurations when spanning hybrid environments that include GCP (1460 MTU) or IPSec/VXLAN intermediates.

Components NEVER accept default 1500-byte node MTU configurations when spanning hybrid environments that include GCP (1460 MTU) or IPSec/VXLAN intermediates.

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 TCP MSS Clamping for WireGuard Meshes critical for AI coding assistants?

Enforces Cilium dynamic eBPF Path MTU Discovery (PMTUD) and TCP Maximum Segment Size (MSS) clamping across hybrid multi-cloud WireGuard overlays. Eliminates silent TCP blackholing, broken TLS handshakes, and packet fragmentation caused by disparate underlay MTUs and cryptographic encapsulation overhead. 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 (Unclamped Overlay Encapsulation Causing Silent TCP Packet Blackholing Across Heterogeneous Cloud Underlays) 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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This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

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