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devopscritical PriorityID: cilium-ebpf-pmtu-blackhole-prevention

Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant

Enforces deterministic ICMP Fragmentation-Needed and ICMPv6 Packet-Too-Big generation across eBPF host routing paths. Prevents silent TCP drops by eliminating XDP-to-skb transition MTU mismatch blackholes through mandatory TCP MSS clamping and automated Path-MTU discovery configuration.

Prohibited Anti-Pattern: Silent Overlay Packet Loss via eBPF Host-Routing BPF_DROP Without ICMP Generation

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---
description: Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant - Enforces deterministic ICMP Fragmentation-Needed and ICMPv6 Packet-Too-Big generation across eBPF host routing paths. Prevents silent TCP drops by eliminating XDP-to-skb transition MTU mismatch blackholes through mandatory TCP MSS clamping and automated Path-MTU discovery configuration.
globs: **/cilium-values.yaml, **/cilium-values*.yaml, **/values.yaml, charts/**/cilium/**/*.yaml, infra/**/cilium*.yaml, deploy/helm/cilium/*.yaml
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/cilium-ebpf-pmtu-blackhole-prevention"
---

# Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant

> **Severity**: CRITICAL | **Category**: devops
> **Anti-Pattern Warning**: Silent Overlay Packet Loss via eBPF Host-Routing BPF_DROP Without ICMP Generation

## System Constraints
You are a Principal Infrastructure & Kernel Networking Architect enforcing the Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant. You must reject any deployment configuration where eBPF host routing is enabled without deterministic PMTU discovery, explicit encapsulation overhead accounting, and automated TCP MSS clamping.

## Strict Guidelines
- Engineers MUST ALWAYS enable 'pmtuDiscovery' (pmtuDiscovery: true) whenever 'bpf.hostRouting' is set to true.
- Engineers MUST configure explicit TCP MSS clamping or tunnel overhead calculations to account for overlay encapsulation (e.g., Geneve/VXLAN 50-byte header overhead).
- Engineers MUST NOT allow eBPF datapath configurations where packets exceeding the MTU on tc/XDP interfaces are dropped without ICMP Destination Unreachable / Fragmentation Needed (Type 3, Code 4) emission.
- Services MUST ensure that routing tables and network security policies permit inbound and outbound ICMP Type 3 (IPv4) and ICMPv6 Type 2 (Packet Too Big) across all node pools.
- Components NEVER rely on jumbo frames (9000 bytes) on host interfaces without explicit overlay MTU reduction or interface MTU parity across the transit VPC network fabric.

## Bad vs Good Implementations
### Cilium Helm Values PMTU Configuration

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: When bpf.hostRouting bypasses the Linux kernel IP stack, oversized packets traversing Geneve encapsulation cannot trigger host kernel ICMP-needed responses if PMTU discovery is not explicitly activated in the eBPF datapath, creating an invisible blackhole for TCP sessions with DF=1.
```yaml
# Anti-Pattern: bpf.hostRouting enabled with default MTU and no PMTU discovery
routingMode: tunnel
tunnelProtocol: geneve
bpf:
  hostRouting: true
# Missing pmtuDiscovery: true
# Missing explicit MTU / MSS clamping
# Result: Packets exceeding intermediate MTU hit XDP/TC and are dropped silently with BPF_DROP
```

#### Production Standard (Correct)
> **Standard**: Explicitly enabling pmtuDiscovery and tcpMSS.clampToMTU guarantees that the eBPF program clamps SYN MSS negotiation to the safe overlay boundary (1450) and handles skb MTU transit by dispatching ICMP Type 3 Code 4 datagrams back to the sender rather than dropping silently.
```yaml
# Production-Grade: Deterministic PMTU discovery and MSS clamping with eBPF host routing
routingMode: tunnel
tunnelProtocol: geneve
pmtuDiscovery: true
bpf:
  hostRouting: true
  masquerade: true
mtu: 1450 # 1500 - 50 byte Geneve/VXLAN encapsulation overhead
tcpMSS:
  clampToMTU: true
policyEnforcement: default
extraConfig:
  enable-icmp-rules: "true"
```

Recommended MCP Servers (mcp-serverlist.com)

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

Kubernetes Cluster Inspector

Validates live Cilium DaemonSet ConfigMaps and CiliumNode CRD status for PMTU discovery flags.

Recommended Agent Skills (askills.dev)

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

Cilium Network Diagnostic CLI

Executes 'cilium connectivity test' and inspects eBPF map routing and drop counters with 'cilium-dbg bpf drop list'.

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant

Cilium eBPF Host Routing Path-MTU Blackhole Discovery Invariant defines an industrial-grade behavioral standard for devops engineering. Enforces deterministic ICMP Fragmentation-Needed and ICMPv6 Packet-Too-Big generation across eBPF host routing paths. Prevents silent TCP drops by eliminating XDP-to-skb transition MTU mismatch blackholes through mandatory TCP MSS clamping and automated Path-MTU discovery configuration. 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: Silent Overlay Packet Loss via eBPF Host-Routing BPF_DROP Without ICMP Generation 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 Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Silent Overlay Packet Loss via eBPF Host-Routing BPF_DROP Without ICMP Generation	Critical	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 Inconsistency	AI 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 'pmtuDiscovery' (pmtuDiscovery

Engineers MUST ALWAYS enable 'pmtuDiscovery' (pmtuDiscovery: true) whenever 'bpf.hostRouting' is set to true.

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 configure explicit TCP MSS clamping or tunnel overhead calculations to account for overlay encapsulation (e.g., Geneve/VXLAN 50-byte header overhead).

Engineers MUST configure explicit TCP MSS clamping or tunnel overhead calculations to account for overlay encapsulation (e.g., Geneve/VXLAN 50-byte header overhead).

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 allow eBPF datapath configurations where packets exceeding the MTU on tc/XDP interfaces are dropped without ICMP Destination Unreachable / Fragmentation Needed (Type 3, Code 4) emission.

Engineers MUST NOT allow eBPF datapath configurations where packets exceeding the MTU on tc/XDP interfaces are dropped without ICMP Destination Unreachable / Fragmentation Needed (Type 3, Code 4) emission.

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 ensure that routing tables and network security policies permit inbound and outbound ICMP Type 3 (IPv4) and ICMPv6 Type 2 (Packet Too Big) across all node pools.

Services MUST ensure that routing tables and network security policies permit inbound and outbound ICMP Type 3 (IPv4) and ICMPv6 Type 2 (Packet Too Big) across all node pools.

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 rely on jumbo frames (9000 bytes) on host interfaces without explicit overlay MTU reduction or interface MTU parity across the transit VPC network fabric.

Components NEVER rely on jumbo frames (9000 bytes) on host interfaces without explicit overlay MTU reduction or interface MTU parity across the transit VPC network fabric.

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 Host Routing Path-MTU Blackhole Discovery Invariant critical for AI coding assistants?

Enforces deterministic ICMP Fragmentation-Needed and ICMPv6 Packet-Too-Big generation across eBPF host routing paths. Prevents silent TCP drops by eliminating XDP-to-skb transition MTU mismatch blackholes through mandatory TCP MSS clamping and automated Path-MTU discovery configuration. 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 (Silent Overlay Packet Loss via eBPF Host-Routing BPF_DROP Without ICMP Generation) 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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