[Bugfix][Router] Accept single-label static backend URLs

[spicysharkco]

This carries #737 by @osaimi forward. #737's current head, with the update from @ruizhang0101, already has the hyphen guard the automated review asked for. Its first commit has no sign-off, so the DCO check there still fails. This PR adds the tests and a signed-off commit, and drops the localhost alternative that the single-label pattern already covers. Happy to close it if you prefer to update #737 instead.

With --static-backends http://oss-20b:8000, the router logs Skipping invalid URL: http://oss-20b:8000 and drops the backend. validate_url() accepts dotted domains, localhost, and IPv4, but no other single-label host (src/vllm_router/utils.py:173-175). The new pattern accepts service names with ports or paths, including underscores, and rejects a leading or trailing hyphen on single-label hosts. localhost still matches it. Multi-label hosts keep the current pattern, so every URL main accepts today is still accepted. IPv6 validation is outside this change.

Startup and dynamic configuration both go through parse_static_urls() (src/vllm_router/app.py:202, src/vllm_router/dynamic_config.py:178). YAML and Helm pass backend strings to it; service discovery adds no second URL-format check.

Run from the repository root with router and test dependencies installed:

PYTHONPATH=src python -m pytest src/tests/test_utils.py -k 'validate_url or parse_static_urls' -q
PYTHONPATH=src python -m pytest src/tests/test_utils.py src/tests/test_parser.py src/tests/test_static_service_discovery.py -q
pre-commit run --files src/vllm_router/utils.py src/tests/test_utils.py

• With only the one-line fix reverted: 5 failed, 14 passed, 12 deselected.
• With the fix: 19 passed, 12 deselected; related suite: 56 passed.
• Pre-commit: 6 passed, 8 skipped because no files matched. git diff --check is clean.

Overlap checked: open and closed PRs and issues for validate_url, "single-label", "Skipping invalid URL", "static-backends" hostname, "docker compose" hostname, and "invalid URL". Only #737 addresses this bug. Main has no later fix, and there are no maintained release branches.

Not tested: deployed inference, end-to-end routing, or Helm functionality.

Related issues: N/A; this follows #737.

---

• Code changes pass the applicable pre-commit checks.
• Commit includes the DCO sign-off.
• PR title uses the [Bugfix][Router] prefixes.

[gemini-code-assist]

Code Review

This pull request updates the URL validation regex in src/vllm_router/utils.py to support single-label hostnames (ensuring they do not start or end with a hyphen) and removes the explicit localhost match since it is covered by the new pattern. It also adds comprehensive unit tests in src/tests/test_utils.py to verify this behavior. There are no review comments, so I have no feedback to provide.

[Mazyod]

Thanks for picking this up. We run the router with single-label backend names (http://vllm-worker:8000) in Docker and Kubernetes and carry an equivalent fix in our deployment. This PR carries forward #737's existing fix and adds tests and a signed-off replacement commit; we're happy to close #737 in favor of it.

@ruizhang0101 the workflow runs here currently show action_required; could you approve them?

[osaimi]

#737 is closed in favor of this pr.