fix: IPv6-only fixes for listeners, ZMQ, SGLang KV events, and vLLM multinode

[jthomson04]

Summary

This PR fixes the remaining IPv6-only problems (#7619) on top of the resolver fix in #13126 and the IPv6 event plane in #15324.

Event plane (earlier commits):

• Set ZMQ_IPV6 on each bind or connect, and only for bracketed IPv6 endpoints. This restores the IPv4 behavior fix(runtime): support IPv6 ZMQ event paths #15324 changed:
  • IPv4 publishers bind plain IPv4 sockets again.
  • Hostnames resolve to IPv4 again, so broker URLs such as tcp://localhost:5555 work.
• Reject unbracketed IPv6 endpoints such as tcp://2001:db8::10:5555 with a clear error. Previously they retried forever without reporting anything.
• Add context to bind errors and a warning when the advertised address falls back to loopback. Skip the IPv6 tests on hosts without IPv6 loopback. Correct the docs.

IPv6-only follow-ups:

• Docs. IPv6-only hosts need DYN_HTTP_HOST=:: on the Frontend and DYN_SYSTEM_HOST=:: on workers. Both listeners default to 0.0.0.0, which accepts IPv4 only. The defaults are unchanged.
• HTTP host parsing. DYN_HTTP_HOST now accepts an unbracketed IPv6 address such as ::. Previously only [::] parsed.
• System status advertise. A :: system status listener also accepts IPv4 connections. On hosts with no non-loopback IPv6 address it now advertises a non-loopback IPv4 address instead of ::1. IPv4 wildcard binds are unchanged.
• Loopback warning. The system status server now warns once when it can only advertise loopback. That fallback was the silent reason the model never registered on IPv6-only hosts: the Frontend fetches the model card from this address.
• SGLang. The KV event subscriber connects over 127.0.0.1 when SGLang binds a wildcard address. SGLang's publisher listens on IPv4 only, so on IPv6-only pods the router received no KV events.
• Shared ZMQ helpers. The lib/llm and kv-router ZMQ helpers set ZMQ_IPV6 for each endpoint.
• Operator. The vLLM multinode wait-for-leader script tries an IPv6 lookup when gethostbyname fails. The script ships in a ConfigMap, so existing pods are not rolled.

This PR now also touches lib/llm, kv-router, the SGLang component, and the operator, so more code owners are involved.

Validation

IPv6-only end-to-end test (local Docker). The network was created with --ipv6 --ipv4=false, so containers have only IPv6 on eth0, plus 127.0.0.1 and ::1 on lo.

• Topology: a Frontend with --router-mode kv and two mocker workers, using file discovery and the default direct-ZMQ event plane.
• Workers get the same system env as the operator sets.
• The Frontend container has no local model files, so it must fetch the model card from each worker's advertised metadata URL.
• A separate container stands in for another pod to test reachability.

Network	Host settings	Result
IPv6-only	DYN_HTTP_HOST=::, DYN_SYSTEM_HOST=::	7/7 checks pass (see below). Metadata was served at http://[fd00:d1::2]:9090 and event-plane endpoints advertised as tcp://[fd00:d1::2]:….
IPv6-only	defaults (negative control)	Health and Frontend were unreachable from other containers, and the model never registered because the worker advertised http://127.0.0.1:9090. The new warning fired once per worker.
IPv4	defaults	7/7, no IPv4 regression
IPv4	::	7/7. Advertises 172.18.0.2; before the advertise fix it advertised [::1] and the model never registered.

The seven checks:

1. No resolver errors in any log.
2. Worker /health returns 200 from another container.
3. Frontend /v1/models lists the model from another container.
4. The model registered through the worker's metadata URL.
5. All 4 chat completions succeed.
6. The router applied stored-block KV events (dynamo_component_kv_cache_events_applied{event_type="stored",status="ok"}).
7. Repeated prompts produced non-zero router_kv_hit_rate.

The leader-wait resolver resolved an IPv6-only container name to its IPv6 address, and an IPv4 name to the same IPv4 address as before.

Unit tests:

• dynamo-runtime: utils::ip_resolver (16 tests), transports:: (66), and system_status_server.
• dynamo-llm --no-default-features: http::service::service_v2 (33) and utils::zmq.
• dynamo-kv-router: the zmq test with standalone-selection.
• Operator: go test ./internal/dynamo/....

Lint:

• cargo fmt --all -- --check and git diff --check pass.
• cargo clippy -D warnings is clean for dynamo-runtime, for dynamo-llm --no-deps and for dynamo-kv-router. The only exception is the existing macOS-only unused variable in quic_response.rs.
• Docs lint: 0 errors.

Earlier event-plane validation: all 49 event-plane tests passed, including on a host where localhost resolves to both 127.0.0.1 and ::1.

Not run locally:

• The SGLang publisher pytest, because it needs the sglang package. I ran the new helper directly against the test's cases instead.
• Real GPUs, an operator deployment, and EKS.

Follow-ups (not in this PR)

• Dual-stack :: defaults for the Frontend and system status listeners, so IPv6-only hosts need no settings.
• EKS IPv6: automatic address selection may prefer the pod's 169.254.172.x egress-only IPv4 address.
• IPv6 formatting for TRT-LLM multi-node streaming KV endpoints and the Mooncake bootstrap URL. This depends on whether those publishers listen on IPv6 at all.
• Hard-coded IPv4 listeners elsewhere: the KServe gRPC host, mocker disagg, multi-node KVBM, the metrics-only listeners, inference-gateway / EPP, and TokenSpeed.
• Recipes that set 0.0.0.0 explicitly stay IPv4-only.

Related Issues

Follow-up to #13126 and #15324. Addresses #7619.

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Walkthrough

The change configures ZMQ sockets according to endpoint address family, adds diagnostics for direct ZMQ host selection and bind failures, and updates documentation for reachable IPv4 and IPv6 addresses.

Changes

ZMQ address handling

Layer / File(s)	Summary
Configure ZMQ sockets by endpoint lib/runtime/src/transports/event_plane/zmq_transport.rs	ZMQ socket helpers select IPv6 for bracketed IPv6 TCP endpoints and IPv4 for other endpoints. Broker and subscriber connection paths use these helpers. Tests cover IPv4 hostnames and mixed IPv4/IPv6 endpoint pairs.
Report direct ZMQ host selection and bind errors lib/runtime/src/transports/event_plane/mod.rs	Automatic host selection warns when resolution falls back to loopback. Direct ZMQ bind errors include the endpoint and host-configuration details. Host-resolution and IPv6 publisher tests use updated resolver setup and skip when IPv6 loopback binding is unavailable.
Document reachable IPv4 and IPv6 addresses docs/fern/pages/reference/components/runtime-configuration.mdx, docs/fern/pages/kubernetes/kv-aware-routing/kv-dc-relay.md	The documentation describes automatic address selection, reachable host overrides, IPv6 listener behavior, and bracketed IPv6 PUB/SUB addresses.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 3d2cb

Failed multi-endpoint connections may take longer to diagnose. The issue is bounded and can be addressed as a follow-up.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 2 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main IPv6-related fixes across listeners, ZMQ, SGLang KV events, and vLLM multinode behavior.
Description check	✅ Passed	The description provides a detailed summary, implementation details, validation results, follow-ups, and related issue references. It does not use the exact Overview, Details, or reviewer-start headin…

Full details: Docstring Coverage

Explanation

Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 2 files. (2 skipped: 2 unsupported.)

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Actionable comments posted: 1

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Inline comments:
Review comments at @lib/runtime/src/transports/event_plane/zmq_transport.rs:
- Around line 140-141: Update connect_zmq_socket to add context containing the
endpoint and operation name when set_ipv6 or connect fails. Preserve propagation
of both errors while ensuring multi-endpoint callers can identify which
operation failed for the specific endpoint.

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📒 Files selected for processing (4)

• docs/fern/pages/kubernetes/kv-aware-routing/kv-dc-relay.md
• docs/fern/pages/reference/components/runtime-configuration.mdx
• lib/runtime/src/transports/event_plane/mod.rs
• lib/runtime/src/transports/event_plane/zmq_transport.rs

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[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: The multi-endpoint connection helper still propagates set_ipv6 and connect errors without endpoint-specific context, so failures from later broker/subscriber endpoints remain hard to attribute.
• Original discussion: Verified: connect_zmq_socket still propagates set_ipv6 and connect errors without endpoint context, so HA connection failures do not identify the failing broker or publisher endpoint.
• Original discussion: Verified: later connections in the multi-endpoint PUB/SUB paths still propagate set_ipv6 or connect failures without the affected endpoint or operation, making a failed broker/publisher connection difficult to identify.
• Original discussion: connect_zmq_socket still returns set_ipv6 and connect failures without endpoint context, so failures while adding a secondary broker or publisher endpoint cannot be attributed to the failing endpoint.

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[jthomson04]

/ok to test 78e63a1

[dynamo-review-agent]

Previously reported defects still present:

• Original discussion: Verified still present: connect_zmq_socket returns set_ipv6 and connect errors without endpoint or operation context, so failures while adding a later broker or publisher endpoint cannot be attributed to the failing endpoint.
• Original discussion: Verified still present: connect_zmq_socket returns set_ipv6 and connect errors without the endpoint or operation, so failed secondary HA broker/publisher connections cannot be attributed to the affected endpoint.
• Original discussion: Verified still present: connect_zmq_socket still propagates set_ipv6 and connect failures without endpoint or operation context, so failures while connecting additional broker or publisher endpoints remain hard to attribute.
• Original discussion: connect_zmq_socket still returns set_ipv6 and connect errors without endpoint or operation context, so failures while adding a later broker or publisher endpoint cannot be attributed to the affected endpoint.

[jthomson04]

/ok to test 9b4573c