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devopscritical PriorityID: k8s-gateway-api-backend-tls-policy-standard

Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard

Enforces declarative, statically validated cluster-wide cryptographic routing configurations from the Gateway data plane directly to upstream backend workloads using Kubernetes Gateway API v1.2 BackendTLSPolicy. Replaces brittle, unvalidated ingress annotations with explicit CA certificate trust anchors and strict Subject Alternative Name (SAN) verification to guarantee zero-trust in-transit security.

Prohibited Anti-Pattern: Bypassing Upstream TLS Validation or Relying on Vendor-Specific Ingress Annotations for Backend Encryption

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---
description: Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard - Enforces declarative, statically validated cluster-wide cryptographic routing configurations from the Gateway data plane directly to upstream backend workloads using Kubernetes Gateway API v1.2 BackendTLSPolicy. Replaces brittle, unvalidated ingress annotations with explicit CA certificate trust anchors and strict Subject Alternative Name (SAN) verification to guarantee zero-trust in-transit security.
globs: **/*.yaml, **/*.yml, k8s/**/*.yaml, deploy/**/*.yaml, helm/**/templates/*.yaml
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/k8s-gateway-api-backend-tls-policy-standard"
---

# Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard

> **Severity**: CRITICAL | **Category**: devops
> **Anti-Pattern Warning**: Bypassing Upstream TLS Validation or Relying on Vendor-Specific Ingress Annotations for Backend Encryption

## System Constraints
You are a Principal Infrastructure and Security Architect enforcing the Kubernetes Gateway API v1.2 Declarative `BackendTLSPolicy` standard. Your mandate is to eliminate unencrypted pod-to-gateway transit and fragile, unvalidated ingress annotations (e.g., `nginx.ingress.kubernetes.io/backend-protocol`, `nginx.ingress.kubernetes.io/proxy-ssl-verify`) in favor of standard, statically verifiable `BackendTLSPolicy` resources.

Enforce strict validation of `targetRefs`, exact hostname verification (Subject Alternative Name matching), and explicit CA trust store references (`caCertRefs`) across all backend service definitions.

## Strict Guidelines
- Engineers MUST ALWAYS declare a `BackendTLSPolicy` (group `gateway.networking.k8s.io`, version `v1alpha3` or `v1alpha2` depending on cluster runtime, aligned with Gateway API v1.2) for any backend Service terminating TLS.
- Engineers MUST NOT rely on proprietary ingress controller annotations (e.g., `nginx.ingress.kubernetes.io/backend-protocol: HTTPS`, `haproxy.org/server-ssl`) for defining upstream transport encryption.
- Every `BackendTLSPolicy` MUST configure `spec.tls.hostname` with an explicit FQDN matching the backend pod certificate's SAN; wildcard matching SHOULD be minimized and unvalidated hostnames are STRICTLY FORBIDDEN.
- Services MUST define `spec.tls.caCertRefs` pointing to a valid ConfigMap (or Secret where permitted) in the same namespace containing the PEM-encoded CA bundle verifying the upstream pod certificates.
- Engineers MUST NEVER configure `spec.tls.wellKnownCACerts` to public CAs when validating internal cluster workloads using internal PKI (e.g., cert-manager, HashiCorp Vault).
- The `targetRefs` in `BackendTLSPolicy` MUST reference a core Kubernetes `Service` (group: `""`, kind: `Service`) within the identical namespace, matching the Service targeted by the corresponding `HTTPRoute`.
- Helm charts and manifests MUST statically fail validation or linting if an `HTTPRoute` routes to a port serving HTTPS without a matching `BackendTLSPolicy` definition binding the target Service.

## Bad vs Good Implementations
### Fragile Ingress Annotation vs. Declarative BackendTLSPolicy

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Uses non-standard Ingress annotations with upstream certificate validation completely disabled (`proxy-ssl-verify: 'off'`), exposing backend traffic to man-in-the-middle attacks within the cluster network.
```yaml
# INSECURE & OBSOLETE: Relies on proprietary ingress annotations with disabled verification
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: payment-service-ingress
  namespace: payments
  annotations:
    kubernetes.io/ingress.class: nginx
    nginx.ingress.kubernetes.io/backend-protocol: "HTTPS"
    nginx.ingress.kubernetes.io/proxy-ssl-verify: "off" # CRITICAL FLAW: Disables upstream trust verification
spec:
  rules:
  - host: payments.internal.domain
    http:
      paths:
      - path: /
        pathType: Prefix
        backend:
          service:
            name: payment-service
            port:
              number: 8443
```

#### Production Standard (Correct)
> **Standard**: Implements Gateway API v1.2 BackendTLSPolicy targeting the payment-service. It establishes static, cluster-wide cryptographic routing by validating the backend pod TLS certificate against the internal root CA and strictly matching the pod SAN hostname.
```yaml
# SECURE & STANDARDIZED: Gateway API v1.2 declarative BackendTLSPolicy
apiVersion: gateway.networking.k8s.io/v1
kind: HTTPRoute
metadata:
  name: payment-service-route
  namespace: payments
spec:
  parentRefs:
  - name: internal-gateway
    namespace: gateway-infra
  hostnames:
  - "payments.internal.domain"
  rules:
  - backendRefs:
    - name: payment-service
      port: 8443
---
apiVersion: gateway.networking.k8s.io/v1alpha3
kind: BackendTLSPolicy
metadata:
  name: payment-service-tls-policy
  namespace: payments
spec:
  targetRefs:
  - group: ""
    kind: Service
    name: payment-service
  tls:
    caCertRefs:
    - group: ""
      kind: ConfigMap
      name: internal-root-ca
    hostname: "payment-service.payments.svc.cluster.local"
```
Recommended MCP Servers (mcp-serverlist.com)

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

Kubernetes Resource Validator MCP
Validates Kubernetes manifests against OpenAPI v3 schemas and Gateway API CRD definitions.
Recommended Agent Skills (askills.dev)

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

Kubeconform & Gateway API Linter
Lints manifests against official Gateway API CRD schemas to enforce BackendTLSPolicy compliance prior to deployment.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard

Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard defines an industrial-grade behavioral standard for devops engineering. Enforces declarative, statically validated cluster-wide cryptographic routing configurations from the Gateway data plane directly to upstream backend workloads using Kubernetes Gateway API v1.2 BackendTLSPolicy. Replaces brittle, unvalidated ingress annotations with explicit CA certificate trust anchors and strict Subject Alternative Name (SAN) verification to guarantee zero-trust in-transit security. 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: Bypassing Upstream TLS Validation or Relying on Vendor-Specific Ingress Annotations for Backend Encryption 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 DriftBypassing Upstream TLS Validation or Relying on Vendor-Specific Ingress Annotations for Backend EncryptionCritical
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 declare a `BackendTLSPolicy` (group `gateway.networking.k8s.io`, version `v1alpha3` or `v1alpha2` depending on cluster runtime, aligned with Gateway API v1.2) for any backend Service terminating TLS.

Engineers MUST ALWAYS declare a `BackendTLSPolicy` (group `gateway.networking.k8s.io`, version `v1alpha3` or `v1alpha2` depending on cluster runtime, aligned with Gateway API v1.2) for any backend Service terminating TLS.

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 NOT rely on proprietary ingress controller annotations (e.g., `nginx.ingress.kubernetes.io/backend-protocol

Engineers MUST NOT rely on proprietary ingress controller annotations (e.g., `nginx.ingress.kubernetes.io/backend-protocol: HTTPS`, `haproxy.org/server-ssl`) for defining upstream transport encryption.

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

Every `BackendTLSPolicy` MUST configure `spec.tls.hostname` with an explicit FQDN matching the backend pod certificate's SAN

Every `BackendTLSPolicy` MUST configure `spec.tls.hostname` with an explicit FQDN matching the backend pod certificate's SAN; wildcard matching SHOULD be minimized and unvalidated hostnames are STRICTLY FORBIDDEN.

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 define `spec.tls.caCertRefs` pointing to a valid ConfigMap (or Secret where permitted) in the same namespace containing the PEM-encoded CA bundle verifying the upstream pod certificates.

Services MUST define `spec.tls.caCertRefs` pointing to a valid ConfigMap (or Secret where permitted) in the same namespace containing the PEM-encoded CA bundle verifying the upstream pod certificates.

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

Engineers MUST NEVER configure `spec.tls.wellKnownCACerts` to public CAs when validating internal cluster workloads using internal PKI (e.g., cert-manager, HashiCorp Vault).

Engineers MUST NEVER configure `spec.tls.wellKnownCACerts` to public CAs when validating internal cluster workloads using internal PKI (e.g., cert-manager, HashiCorp Vault).

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.
6

The `targetRefs` in `BackendTLSPolicy` MUST reference a core Kubernetes `Service` (group

The `targetRefs` in `BackendTLSPolicy` MUST reference a core Kubernetes `Service` (group: `""`, kind: `Service`) within the identical namespace, matching the Service targeted by the corresponding `HTTPRoute`.

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.
7

Helm charts and manifests MUST statically fail validation or linting if an `HTTPRoute` routes to a port serving HTTPS without a matching `BackendTLSPolicy` definition binding the target Service.

Helm charts and manifests MUST statically fail validation or linting if an `HTTPRoute` routes to a port serving HTTPS without a matching `BackendTLSPolicy` definition binding the target Service.

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 Gateway API v1.2 Declarative BackendTLSPolicy Upstream mTLS and SAN Validation Standard critical for AI coding assistants?

Enforces declarative, statically validated cluster-wide cryptographic routing configurations from the Gateway data plane directly to upstream backend workloads using Kubernetes Gateway API v1.2 BackendTLSPolicy. Replaces brittle, unvalidated ingress annotations with explicit CA certificate trust anchors and strict Subject Alternative Name (SAN) verification to guarantee zero-trust in-transit security. 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 (Bypassing Upstream TLS Validation or Relying on Vendor-Specific Ingress Annotations for Backend Encryption) 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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