Allow opt-in private endpoints for remote URLs

[premctl]

Summary

ValidateRemoteURL unconditionally rejects private/loopback IPs and internal hostnames (cluster.local, kubernetes.default*, localhost, metadata.google.internal) with no opt-in. That blocks a legitimate topology: a Virtual MCP reaching a co-located in-cluster backend in-mesh, where going through an external address would strip the Istio/SPIFFE workload identity the backend's authorization policy keys on. The design (per-resource opt-in field + exact carve-out below) was agreed with @aponcedeleonch in #5784.

• Add allowPrivateEndpoint (bool, default false) to both MCPServerEntry and MCPRemoteProxy specs, and change ValidateRemoteURL to take a ValidateRemoteURLOptions carrying the flag, threaded from both controllers.
• With the flag on, only the private-network allowance widens: RFC-1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), IPv6 ULA (fc00::/7), and hostnames ending in cluster.local (.svc names were never blocked, since no DNS resolution is performed).
• The SSRF protections stay unconditional regardless of the flag: link-local (169.254.0.0/16, fe80::/10), loopback/unspecified (127.0.0.0/8, ::1/128, ::/128, 0.0.0.0/8), localhost, metadata.google.internal, and all kubernetes.default* variants — including kubernetes.default.svc.cluster.local, which is checked before the cluster.local relaxation can apply. IPv4-mapped IPv6 forms are normalized before matching on both the blocked and relaxed sides.
• With the flag off (the default), behavior is byte-for-byte identical to today.

Fixes #5784

Type of change

• Bug fix
• New feature
• Refactoring (no behavior change)
• Dependency update
• Documentation
• Other (describe):

Test plan

• Unit tests (task test, task operator-test) — table-driven coverage of every carve-out category with the flag on and off, including the never-relaxed set staying blocked with the flag on and IPv4-mapped IPv6 forms; controller tests for both MCPServerEntry and MCPRemoteProxy proving the CR field flows through to validation (private URL rejected with flag unset, accepted with flag true)
• E2E tests (task test-e2e)
• Linting (task lint-fix)
• Manual testing (describe below)

API Compatibility

• This PR does not break the v1beta1 API, OR the api-break-allowed label is applied and the migration guidance is described above.

Additive optional field only (allowPrivateEndpoint, defaults to false).

Does this introduce a user-facing change?

Yes. MCPServerEntry and MCPRemoteProxy gain an optional spec.allowPrivateEndpoint field (default false). When set, the remote URL may point at a private/in-cluster endpoint (RFC-1918, IPv6 ULA, cluster.local/.svc hostnames) — intended for reaching a co-located in-cluster backend in-mesh so workload-identity authorization still applies. Loopback, link-local, cloud-metadata, and kubernetes.default* endpoints remain blocked regardless of the flag.

Implementation plan

Approved implementation plan (agreed in #5784)

• Add allowPrivateEndpoint bool (default false) to both MCPServerEntry and MCPRemoteProxy CRD specs.
• Change ValidateRemoteURL(rawURL string) error to take an options parameter (rather than the bare string) carrying this flag, and thread it through from both mcpserverentry_controller.go and mcpremoteproxy_controller.go.
• Exact carve-out:
  • Relaxable when allowPrivateEndpoint: true: RFC-1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), IPv6 ULA (fc00::/7), hostnames ending in cluster.local or .svc.
  • Never relaxed, regardless of the flag: link-local (169.254.0.0/16, fe80::/10), loopback/unspecified (127.0.0.0/8, ::1/128, 0.0.0.0/8), localhost, metadata.google.internal, kubernetes.default*. Loopback staying blocked is explicitly the point that matters most — a pod reaching loopback hits itself, which is never a legitimate co-located backend.
• Regenerate CRD schema/deepcopy/YAML and docs.

Special notes for reviewers

• Hostname check ordering is load-bearing: the always-blocked names (including kubernetes.default.svc.cluster.local) are evaluated before the cluster.local relaxation, so opting in cannot expose the API server's DNS names.
• One inherent consequence of the agreed carve-out worth stating explicitly: with the flag on, the kubernetes.default* hostname block cannot protect the API server from being addressed by its ClusterIP directly (e.g. 10.96.0.1 is an RFC-1918 address the flag allows). The hostname block should not be read as API-server protection once a resource opts in — the flag is a deliberate, per-resource trust statement about the private network.
• .svc-suffixed hostnames (other than kubernetes.default.svc) were already allowed before this change (the validator does no DNS resolution), so the flag does not change their handling; the "relaxable" list documents intent rather than adding a new block.
• zz_generated.deepcopy.go is unchanged — a plain bool needs no generated deepcopy code. CRD YAML and docs/operator/crd-api.md are regenerated.
• A stale comment in test/e2e/thv-operator/virtualmcp/mcpremoteproxy_scaling_test.go claiming MCPRemoteProxy "never" targets in-cluster servers is updated (comment-only).

Generated with Claude Code

[amirejaz]

Reviewed with a security focus and cross-checked against the carve-out agreed in #5784 — the never-relaxed set, the ordering (kubernetes.default* blocked before the cluster.local relaxation), the IPv4-mapped IPv6 normalization, and the byte-for-byte-unchanged flag-off behavior all check out. Looks good, approving.

One check for @aponcedeleonch before merge: you'd listed .svc as relaxable (blocked unless opted in), but the PR leaves bare .svc allowed in both flag states (documented as never-blocked since no DNS resolution happens). The reasoning is sound — gating it would break the flag-off-unchanged contract and the existing .svc e2e workaround — but are you good with that, or do you want .svc genuinely behind the flag?

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.46%. Comparing base (aa3f5b3) to head (3e53d21).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/thv-operator/pkg/validation/url_validation.go	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6195      +/-   ##
==========================================
- Coverage   72.51%   72.46%   -0.06%     
==========================================
  Files         739      739              
  Lines       76719    76722       +3     
==========================================
- Hits        55634    55595      -39     
- Misses      17107    17161      +54     
+ Partials     3978     3966      -12     

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