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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"test-net-listen-ipv6only is flaky due to IPv4 port collision","articleBody":"### Test\n\n`test/parallel/test-net-listen-ipv6only.js`\n\n### Platform\n\nEnvironment-independent root cause; observed on macOS (arm64) but can occur on any platform under high test parallelism.\n\n### Console output\n\nObserved once during a full `python3 tools/test.py` run (5521 passing, this test failing). The test passes 10/10 when run in isolation, which is the typical signature of a port-collision flake.\n\n### What is the problem?\n\nThe test verifies that `ipv6Only: true` disables dual-stack support. It listens on `::` with an ephemeral port (`port: 0`) and then connects to the **same port over IPv4** (`0.0.0.0`), expecting `ECONNREFUSED` — i.e. it assumes the IPv4 side of that port is free:\n\n```js\nserver.listen({ host: '::', port: 0, ipv6Only: true }, common.mustCall(() => {\n  const { port } = server.address();\n  const socket = net.connect({ host: '0.0.0.0', port });\n  socket.on('connect', common.mustNotCall());            // \u003c- fires under collision\n  socket.on('error', common.mustCall((err) => {\n    assert.strictEqual(err.code, 'ECONNREFUSED');\n    server.close();\n  }));\n}));\n```\n\nThe IPv4 side of the chosen ephemeral port is never reserved. Under heavy parallel\nexecution another test can occupy that same IPv4 port, so the connection succeeds\ninstead of being refused and `common.mustNotCall()` on `'connect'` fires (or the\n`ECONNREFUSED` assertion fails).\n\nThis is the same port-collision flake that previously affected the sibling cluster\nvariants — see #25813 / #29679, fixed in #26298 (\"eliminate port collision\"),\n#32381 and #32398, which moved the `ipv6Only: true` cases to `test/sequential/`\nwith a static port. The plain (non-cluster) test never received that treatment.\n\n### Fix\n\nMove the test to `test/sequential/` and use a fixed `common.PORT`, so it does not run\nconcurrently with other tests competing for the same IPv4 port. PR to follow.\n","author":{"url":"https://github.com/Han5991","@type":"Person","name":"Han5991"},"datePublished":"2026-06-27T13:10:12Z","url":"https://github.com/nodejs/node/issues/64172"}